AudioCircle
Industry Circles => Tortuga Audio => Topic started by: tortugaranger on 30 Jun 2015, 03:27 pm
-
As a manufacturer I am obviously biased towards our products - LDR Passive Preamps. That said, I'm also a life-long audiophile looking for what works best together, sounds better, and along the way trying to separate the hype from the reality.
I got into the business of designing and manufacturing LDR Passive Preamps because I was blown away by the sound quality relative to all other options and thought we could do a better engineering job of taming the wild LDR - which I think we've done!
Now that we have a few years of business under our belt and have accumulated a decent size customer base there's little doubt (in our minds at least) that LDR passive preamps are the best sounding volume control (attenuation) devices available today. This isn't just our opinion it's the overwhelming conclusion of our customer base. Several of our customers owned exceptional tube and SS preamps costing upward of $10k and after auditioning the Tortuga pre, sold off their hi-end active preamps and stayed with the Tortuga. Customers tell us we bested their stepped attenuators and even their transformer based volume controls costing way more than our models.
Yet even a casual exploration of online audio forums reveals a fairly consistent picture where passives are generally viewed as inferior to conventional active preamps. The active vs. passive debate may rear it head now and get rehashed but the prevailing view remains that actives trump passives - generally speaking. And since LDR based passive preamps are a subset of passive preamps they get painted with the same inferior brush - however unfair and inaccurate that might be.
The case for LDR passives is both simple and compelling:
- Excellent overall sound quality including remarkable bass - open, articulate, uncolored, transparent
- Great dynamics - punch, slam & pluck
- The vast majority of audio systems do not need preamp gain (a.k.a. "active" amplification of the audio signal prior to the amp)
- Lowest possible noise floor (no power supply interaction with audio signal)
- Lower cost
- Adjustable input impedance to optimize bridging impedance and performance between source and amp.
And yet, if you lined up any 10 audiophiles and read off this list of substantiated audio goodness I would argue that most (8 out of 10? ) would assume this was questionable marketing hype and refer you to the assumption that you need a quality conventional active (powered) preamp to get the best sounding audio system. And of course none of them will have actually tried (listened to) an LDR passive yet be firm in their convictions.
So my collection of interrelated questions for the audiophile community at large are as follows:
1) How do you change the minds of informed audiophiles with information contrary to their prevailing views?
2) Is framing this as a passive vs. active debate even useful or effective?
3) What are the key issues with preamps and does the LDR preamp address these or not?
-
My questions about passive devices. I need, or rather like, a remote for volume and sources. I'd like a balance control too.
That said, the one thing I require that is non existent in passive devices is very low output impedance.
I've tried passive devices, not yours, and found them lacking. These were older devices. For comparison, my favorite preamps to date are the Horn Shoppe Naked Truth, Morrison ELAD (discontinued) and for tubes and just a step lower in my opinion only Transcendent Sound Grounded Grid.
I do agree that most devices have enough gain to drive a power amp. I am also convinced that those devices need a buffer too.
steve
-
I have owned many high dollar preamps, and several types of passive devices. I think if your system is suited for a passive, the LDR just can't be beat. That said, there are some systems that just have to have additional gain. For the most part, I think these are systems that weren't well planned from the start. But hey, that's just me.
However, there will always be the guys that have passive friendly systems that won't even consider a passive of any pedigree. They are fully wed to active preamps and there's no changing their minds. Added to that are the guys that would never consider any device in their systems that cost so little, no matter how good the performance.
Lastly, there are people who don't realize how bad their systems are because the preamp is masking the flaws. Put a truly transparent preamp in the system and it's a wake up call they aren't willing to receive.
So that's my take on why passive devices, LDR or othewise, will never suit everyone.
Shakey
-
1. The only way to change minds is to have people try them risk free ; The audio equivalent of The Pepsi Challenge.
2. No, it doesn't. If anything, the whole passive vs active debate will only fuel preconceived notions or experiences based on lesser passives. Perhaps a better discussion should revolve around this question : Should a preamp color the signal or not, and why?
3. Passive or active, the key issue with preamps is coloration (in my opinion). Ideally, a preamp should just pass the signal and provide attenuation (and perhaps other conveniences like source selection) and provide the best electrical signal (impedance) to the amp. Because all systems are different, it seems that without a buffer of some sort, performance of a pure passive will always vary.
So, with anything, there are tradeoffs. A buffer will make things simpler / easier and more universal, but will color the sound (more than a pure passive). A pure passive can be the most transparent, but this assumes factors of voltage, impedance, cable length etc are always accounted for by the user. Even some seasoned audiophiles don't quite grasp all of the latter. We all want the best sound, but not all of us care to have to consider or even know to consider all the factors involved with passive preamp implementation.
-
RD, I tend to agree with most of your post.
Point of difference. A good buffer is absolutely transparent, and will avoid many problems with interconnects.
If you need long interconnects, you need very low impedance, 50 Ohms or less works well. You also need some drive current. In some systems a passive can work very well. Short, decent quality interconnects, and an easy to drive amp. I think we're pretty much in agreement.
How about a passive pre tour? I'm all in for that. It doesn't have to be a top of the line new unit. It doesn't have to be pretty either. It just has to show Tortuga's ability.
steve
-
Well....having nothing in the signal path is more transparent than having something in the signal path, which is why I don't think a buffer is absolutely transparent regardless of how good it is.
-
My questions about passive devices. I need, or rather like, a remote for volume and sources. I'd like a balance control too.
The Tortuga pres have both of those.
That said, the one thing I require that is non existent in passive devices is very low output impedance.
The output impedance of any passive attenuator (voltage divider) will vary as you run it from 0 (zero voltage/volume) to 100% (full voltage/volume). This is a matter of physics really - the law of resistors in parallel. This is exactly what happens with any potentiometer. An LDR passive more or less emulates a potentiometer so it behaves the same way. If you want fully resistive/passive, you don't get low/fixed output impedance.
However, the assumption that a low (or fixed) output impedance is a requirement for an effective passive preamp is generally not the case. To make it an absolute requirement is to effectively close off the possibility of using any resistive passive. Such a requirement is synonymous with requiring a buffer stage on the output of the "passive" and the moment you do this it ain't no passive no more - it's an active preamp with no gain. So we are back full circle with the "you don't need a conventional active preamp to have excellent audio when an LDR passive can more often than not to a better job of it."
I've tried passive devices, not yours, and found them lacking.
I've tried a few myself and agree with you. And others have had similar experiences. It's really tough convincing someone who's already tried something and didn't like it to try it again. What became clear to me when I first started working with LDR attenuation is the simple fact that LDR's just sound different. Or perhaps more correctly - they don't sound like anything at all! :green:
I do agree that most devices have enough gain to drive a power amp.
Most but not all. We did run across a couple of instances where the customer had a low wattage SET tube amp connected to relatively low efficiency speakers and concluded after trying our preamps that they simply needed a little upstream gain to get the volume level up. But most of our customers running Tortuga preamp into tube amps have found there's no need for additional gain. We've been slowly developing an output buffer option for just such a situation but release of that is still a ways off.
I am also convinced that those devices need a buffer too.
We've found that as long as long as your amp has sufficiently high input impedance relative to the output impedance of your source, that LDR passives do a fantastic job without a buffer. Of course there will always be exceptions but based on our experience and that of our customer base those exceptions have turned out to be quite rare.
-
However, there will always be the guys that have passive friendly systems that won't even consider a passive of any pedigree. They are fully wed to active preamps and there's no changing their minds. Added to that are the guys that would never consider any device in their systems that cost so little, no matter how good the performance.
Such is life. A friend of the family where I grew up when asked what he wanted would always answer..."I don't care. Just make it big, heavy and expensive." He owned a car dealership, had the cash, and had a philosophy. Worked for him.
Lastly, there are people who don't realize how bad their systems are because the preamp is masking the flaws. Put a truly transparent preamp in the system and it's a wake up call they aren't willing to receive.
I'm of the view that most are in that boat. I know I was. It was a revelation when I first started working with LDRs. I could hardly believe my ears. I hauled in family members and did some blind A/B testing. These were not audiophiles. The results were black and white. I wasn't imagining it. The fact that a preamp/attenuator could make such a difference changed my whole outlook on the relative importance of various audio gear. But you have to be at least open to the idea of trying it.
-
Just a couple of quick comments. And posted with friendly intent.
Yes a well designed passive can be excellent with most equipment. Agreed.
The very best buffer type preamps have noise measurements at the residual noise of the test equipment used to measure them. A passive can't sound more quiet. Disagree.
So how about a tour unit? Here's a chance to prove that a passive device can equal/exceed the best active devices. It might generate some sales too. I keep a tube, a solid state, a photo optic, and soon a buffer only unit. ( I keep a variety of pre and power amps as I design speakers ) If you can convince me, I'll sing your praises, and buy one too.
-
So how about a tour unit?
I thought this was something that was already happening?
Shakey
-
You are correct. I'm a newcomer to Tortuga, and I missed that. Too late to join in?
steve
-
You are correct. I'm a newcomer to Tortuga, and I missed that. Too late to join in?
steve
That tour has left the station and is full up - with a long delay - but is getting rolling again. And is only for our LDRxB - Balanced Passive Preamp.
I'm willing to start a separate tour for our LDR3.V2 singled ended unit and will set that up outside of this thread.
-
Thank you. I would love to join that one.
steve
-
After I got my LDR3.V2, I sold my $3750 tubed preamp. It sounded "thick & fuzzy" in comparison. I like "warm", but who needs "thick & fuzzy"?
I don't know how you're going to convince someone that it's better, until they've heard it themselves.
For me, the "LDR" may be a bigger aspect than the "passive".
I used the original LDR3X.V1 to fix my broken Melos SHA Gold. That Preamp has both passive & active outputs. Switching between those two outputs showed subtle differences in my system. The differences between the LDR and conventional volume controls though, is huge!
You've got a great product Morten!
I don't know how you get the word out. I would be afraid of the mainstream audio press, so many politics at some of those places.
-
Steve,
I don't want to hijack this thread, but I just want you to know what other active preamps I have owned.
BAT VK3i
BAT VK30
Audible Illusions Modulus 3a
Cary SLP-98
CJ Premier 17LS
deHavilland Ultraverve
Art Audio DPS
Audio Research SP9 II
Antique Sound Labs Flora
JJ Electronics 234
There were others, but those are the heavy hitters. What took me away from all of them were transformer/autoformer preamps, of which I've owned a few. All of these were better than any of the active preamps by a fair margin. I settled on the Sonic Euphoria AVC and nothing seemed to be able to best it. Until I heard an LDR.
I just wanted to put things in perspective for you and encourage you to try an LDR if you get the chance. You'd be welcome to come listen to mine if you were close enough.
Shakey
-
Thought I'd mention this just in case folks don't pick up on this from our website.
We have a 30 day buy-try-decide audition policy. What this means in practical terms is you have 3-4 weeks to try out our preamps at home in your own system. If things don't work out, you ship the unit back and we refund you 100% of the cost of the unit - no restocking fee. You pay the return shipping cost.
So at least in the US, for somewhere in the range of $20-35 net worst case (return shipping), you get to try out our preamps. Sure, you do have to lay out the cash up front but you get that back if you don't keep it. About as risk free as it gets.
It's a bit costlier for international buyers due to higher shipping costs plus duties/taxes etc.
Here's the audition policy details but it's basically as outlined above: http://www.tortugaaudio.com/policies/30-day-audition/ (http://www.tortugaaudio.com/policies/30-day-audition/)
-
Thought I'd mention this just in case folks don't pick up on this from our website.
We have a 30 day buy-try-decide audition policy. What this means in practical terms is you have 3-4 weeks to try out our preamps at home in your own system. If things don't work out, you ship the unit back and we refund you 100% of the cost of the unit - no restocking fee. You pay the return shipping cost.
So at least in the US, for somewhere in the range of $20-35 net worst case (return shipping), you get to try out our preamps. Sure, you do have to lay out the cash up front but you get that back if you don't keep it. About as risk free as it gets.
It's a bit costlier for international buyers due to higher shipping costs plus duties/taxes etc.
You can't get more fair than that.
Shakey
-
Because you are asking:
1) How do you change the minds of informed audiophiles with information contrary to their prevailing views?
If I knew the answer to this! Audiophile dogma dies hard, I think you are already doing what you can, allowing for fairly painless home auditions. Very precise, clear, measurements can help (see 3 below).
2) Is framing this as a passive vs. active debate even useful or effective?
This depends on what you want to make/sell. Of course LDR does not have to mean passive. I suspect that you would find a very good market for an LDR preamp with a really good, discrete, MOSFET unity gain buffer at its output (some might be OK with a diamond buffer IC). There is no question that some source/amp combos need additional current drive (not gain though) to achieve good dynamics. The problem is a passive will not suit all systems. I prefer to view gain as a separate issue, too many audiophiles do not address proper gain matching in their systems and are throwing away performance.
3) What are the key issues with preamps and does the LDR preamp address these or not?
Inserting any component results in losses. I prefer source-amp direct connection as long as the source has enough current drive to achieve good dynamics. As I DIY, I just make sure my DAC has a robust enough output stage, and good gain matching, and I can then use digital volume control and eliminate a box and interconnect. I also prefer single source systems, eliminating any switches from the signal path, as switches can be heard, as can an extra interconnect.
Volume control implementation is a critical design element of preamps, no question. So, if LDR really is better, than say, Shallco switch stepped attenuators with 0.1% TX2575 resistors, then you have a benefit-but without publishing very precise measurements from an AP sys 2 of a passive with the above two volume controls there is little evidence to prove that the LDR is better. Let's see a comparison of noise and distortion spectra at: 0 dB, -4 dB, -8 dB, -16 dB, -32 dB, -64 dB.
In the best preamps, active or not is not really the question, as the best active stages can be so close to fully transparent (distortion levels below the threshold of audability) that there is no downside to having an active circuit. Implementaion of PCB layout, input switches, and volume control are going to be the defining aspects of preamp "sound".
-
Well....having nothing in the signal path is more transparent than having something in the signal path, which is why I don't think a buffer is absolutely transparent regardless of how good it is.
R: the above is only true as a thought experiment, or technically. From the standpoint of audability, it is entirely possible to design and implement a buffer circuit which will be below the threshold of audability, and hence, be transparent.
-
barrows,
Why don't you participate in the Tortuga LDR tour? Do your tests if you want, listen to how it sounds compared to your "fully transparent" active pre and come to your conclusions first hand.
-
So, if LDR really is better, than say, Shallco switch stepped attenuators with 0.1% TX2575 resistors, then you have a benefit-but without publishing very precise measurements from an AP sys 2 of a passive with the above two volume controls there is little evidence to prove that the LDR is better. Let's see a comparison of noise and distortion spectra at: 0 dB, -4 dB, -8 dB, -16 dB, -32 dB, -64 dB.
Thanks for your comments.
If specs and measurements were all it took to line up audio gear on a continuum from worst to best then audioland, as we know it, would be a very different place indeed. No measurement is going to convey the benefits of LDRs over other alternatives. I've not exhaustively measured all aspects of the LDR performance but our LDR manufacturers have done quite a bit of work and it's not all that impressive on paper. Distortion specs are middling. I've had people tell me point blank the LDRs will not, can not, and do not work in audio applications and would sound like crap if you tried to use them. Which of course is totally ridiculous because the proof is in the listening and not in the measurements. I realize and fully accept that there's a segment of potential buyers for whom lack of detailed measured data is a non-starter. I'm an engineer and I love hard data. But at the end of the day all I really care about is what does it sound like.
When I first started working with LDRs back around 2009 I was shocked at their sound quality relative to pots and stepped attenuators. I had a few friends try out early prototypes and got nothing but rave responses. Frankly I was worried I was deluding myself but with rare exception most everyone who gets an earful of LDR attenuation has a similar reaction.
Eventually we will get around to offering a buffer option which will take care of those few cases where additional current drive is needed. The trick will be to do no harm in the process which should be doable. And by harm I mean add noise and/or coloration.
-
R: the above is only true as a thought experiment, or technically. From the standpoint of audability, it is entirely possible to design and implement a buffer circuit which will be below the threshold of audability, and hence, be transparent.
I understand. I was objecting to your statement about a buffer being "absolutely" transparent. It can't be. No component is 100% transparent. But for the sake of the topic, we can limit the range of the concept of signal transmission transparency to the source output signal and components following the source. In this case, I still say anything in the signal path all the way to the sound produced by the transducers will affect the signal to some degree (including the transducers themselves). The degree of audability that a component in the chain affects transparency, would assume one has heard and possibly measured the source signal directly for comparison.
To keep discussion on track, I like zero gain, buffered, preamps. I think an LDR with a really good class A (maybe JFET) buffer, would be AMAZING. Absolutely transparent? No. Perhaps the least compromized preamp solution? Yes, quite possibly. Would I be very interested in trying such a preamp and possibly purchasing it? Yes. Absolutely. :thumb:
-
barrows,
Why don't you participate in the Tortuga LDR tour? Do your tests if you want, listen to how it sounds compared to your "fully transparent" active pre and come to your conclusions first hand.
My time for testing audio gear is quite limited, I would rather spend my free time for pleasure listening rather than testing. I help develop products (no preamps) and test those, and do my own DIY stuff as well, so my plate is pretty full.
I do not have a preamp at all, nothing sounds better than no preamp, I make no claim to having a transparent active pre.
I build my own DACs, make sure I am well matched for gain, and use digital volume control, which as long as it is used within reasonable attenuation limits is totally transparent (within the limits of audibility). I use a single input DAC (USB or Ethernet, I am working with both) to avoid degrading switches in the digital signal path, and have a single balanced interconnect pair going to my amp. I am big believer in less being more.
I find the thread a little confusing, as on the one hand it appears to be about the benefits of LDRs for volume control, and on the other hand it is also a discussion of passive preamps.
Since I have no need for an additional (signal degrading) component, there would be no point in joining the tour.
I do have experience, in the past, with passive and active preamps of course. But I have moved beyond their capabilities in my set up. Next thing to try will be the Ethernet DAC approach, with an I/V consisting of only 4 MOSFETS and 8 resistors for balanced output, further simplifying the signal path.
-
I build my own DACs, make sure I am well matched for gain, and use digital volume control, which as long as it is used within reasonable attenuation limits is totally transparent (within the limits of audibility).
Yeah, but how do you spin yer rekkids? There is nothing more important than that......
Shakey
-
To keep discussion on track, I like zero gain, buffered, preamps. I think an LDR with a really good class A (maybe JFET) buffer, would be AMAZING. Absolutely transparent? No. Perhaps the least compromized preamp solution? Yes, quite possibly. Would I be very interested in trying such a preamp and possibly purchasing it? Yes. Absolutely. :thumb:
What this tells me is we better finish getting that buffer designed, tested and launched. If having a buffered LDR attenuator is what people want then we should deliver that option. What will be interesting is the little bypass switch in the back that will allow you to hear it with and without the buffer. I'm quite curious myself. :green:
-
"What will be interesting is the little bypass switch in the back that will allow you to hear it with and without the buffer. I'm quite curious myself."
Me to!
But I would suggest, that if one hears a degradation in sonics with the buffer switched into the circuit, the correct conclusion to draw will be that the buffer design (including its power supply, in fact the power supply may be more important than the buffer circuit itself) is not good enough. But, it is a shame to have those switches in the signal path. I have participated in listening tests (and seen measurements, even measurements show distortion of the best relays vs a straight wire) of switches vs a wire, hence my preference to get as many junctions out of the signal path as possible (including jacks, relays, switches, solder joints, etc)
As to the poster who asked about LP playback in my system: I do not do it. I prefer a single source for the aforementioned reasons, and my focus for the past 6 years or so has been to get my digital performance as high as possible. I do appreciate the sound of vinyl, but it is not transparent (this is another discussion, OT, and hence I will not address it here again). That said, I have been collecting LPs over the last few years, and there will be a time when I have a turntable and phonostage in my system again (I will probably switch interconnects though, rather than add a signal degrading preamp with source switching).
-
My time for testing audio gear is quite limited, I would rather spend my free time for pleasure listening rather than testing. I help develop products (no preamps) and test those, and do my own DIY stuff as well, so my plate is pretty full.
I do not have a preamp at all, nothing sounds better than no preamp, I make no claim to having a transparent active pre.
I build my own DACs, make sure I am well matched for gain, and use digital volume control, which as long as it is used within reasonable attenuation limits is totally transparent (within the limits of audibility). I use a single input DAC (USB or Ethernet, I am working with both) to avoid degrading switches in the digital signal path, and have a single balanced interconnect pair going to my amp. I am big believer in less being more.
I find the thread a little confusing, as on the one hand it appears to be about the benefits of LDRs for volume control, and on the other hand it is also a discussion of passive preamps.
Since I have no need for an additional (signal degrading) component, there would be no point in joining the tour.
I do have experience, in the past, with passive and active preamps of course. But I have moved beyond their capabilities in my set up. Next thing to try will be the Ethernet DAC approach, with an I/V consisting of only 4 MOSFETS and 8 resistors for balanced output, further simplifying the signal path.
So your schedule is too busy for testing gear. Why don't you sign up for the tour and just listen, don't measure, to the Tortuga in your own highly revealing system? Surely that wouldn't take much time and then you'd be able to comment on the Tortuga with first hand knowledge and your opinion would have more substance.
-
What will be interesting is the little bypass switch in the back that will allow you to hear it with and without the buffer. I'm quite curious myself. :green:
Hmmmm......maybe rather than a switch you could have internal jumpers? I know this would take away the convenience factor, but I think performance would be less compromized. Another thought is to use external jumpers, as commonly seen on NAD integrated amps.
OR........and I'm not sure how difficult this would be, but could you have 4 outputs (2 buffered and 2 straight)? What's cool about this is it would have no compromizing switches or jumper connections, AND it'd allow the user to mix and match outputs as needed. Maybe one could use a straight output to their amp, and a buffered output to a sub. That'd be sweet! :o
-
I think zero gain preamps are a good thing for most systems these days but am also of the opinion a buffer is a good thing, otherwise the output section of your source would have to be a prime consideration. I am also of the opinion, based on my own experience, that gain devices (like a source's output) should not simultaneously have to provide current, so they should be buffered. I do think an LDR volume control paired with a tube based white follower would be close to an ideal preamp while a non-buffered attenuator's performance is going to vary depending on the system it's used in.
I also agree with barrows about leaving out as many parts as possible, good switches are expensive and if you can do without that would be best.
-
What this tells me is we better finish getting that buffer designed, tested and launched. If having a buffered LDR attenuator is what people want then we should deliver that option. What will be interesting is the little bypass switch in the back that will allow you to hear it with and without the buffer. I'm quite curious myself. :green:
I agree, and expect a better sounding preamp once you're done, along with a much higher price tag. :icon_lol:
My experience with three different passives, including an LDR preamp, is the need for power. I don't care what the specs say, I came to the conclusion that a powered preamp simply sounds more, ummm... powerful. The "fewest parts in the signal path" approach has little merit, IMO. I'd suggest adding a buffer to the LDR attenuator that incorporates a well designed, overbuilt power supply.
-
My experience with three different passives, including an LDR preamp, is the need for power. I don't care what the specs say, I came to the conclusion that a powered preamp simply sounds more, ummm... powerful. The "fewest parts in the signal path" approach has little merit, IMO. I'd suggest adding a buffer to the LDR attenuator that incorporates a well designed, overbuilt power supply.
Then I would suggest you evaluate your system. You probably have an amp with low input sensitivity and low input impedance. That situation surely requires more drive than a passive device will offer. I don't want transparency at the expense of body, weight and drive, but I give up none of these with the LDR. Then again, my system is more suited to this situation.
Shakey
-
I also agree with barrows about leaving out as many parts as possible, good switches are expensive and if you can do without that would be best.
Perhaps add a switch to the prototype so you, as the designer, can know exactly what the buffer is doing. I don't see much benefit for customers to be able to switch back & forth between active and passive.
-
I just looked at the specs on your amp and it's not surprising that you need an active preamp.
Input sens 1.5v
Gain 26db
Input Z 47K
Shakey
-
Then I would suggest you evaluate your system. You probably have an amp with low input sensitivity and low input impedance. That situation surely requires more drive than a passive device will offer.
When I said, "I don't care what the specs say...", I meant that my amps and sources at the time all conformed to the input sensitivity and impedance specs for all of the passive preamps I've tried, but specs alone are insufficient in determining the resulting sound.
-
Understood.
But if you were using the amp listed in your system while trying any of these passive devices, you weren't doing them any justice. Now if you had a different amp, well, that's different isn't it?
-
I just looked at the specs on your amp and it's not surprising that you need an active preamp.
Input sens 1.5v
Gain 26db
Input Z 47K
Shakey
Those are specs from the MK3 model. I have the MKI. The input sensitivity on my amp is 1.1v. Besides, I didn't own this amp when I used the other passive preamps I've owned.
-
Ok. But just remember that even though listening is the final arbiter of good sound, in this instance the specs do mean something. I am not a specs/graph whore by any means. But when mating amps to preamps, they tell quite a story.
Just sayin......
Shakey
-
I am a little confused:
"Input sens 1.5v
Gain 26db
Input Z 47K"
The above specs do not describe an amplifier which would be challenging to drive. There are many amplifiers with much lower input Z than this.
-
Perhaps add a switch to the prototype so you, as the designer, can know exactly what the buffer is doing. I don't see much benefit for customers to be able to switch back & forth between active and passive.
There will be a buffer bypass switch in the prototype and maybe even in a tour unit but not in a final commercial unit. And I will definitely convey my impressions of performance with and without the buffer in the circuit path.
However, I confess there's a part of me that would love to ship a tour unit around with a big fat red bypass switch on the front panel with a sign under it that reads: "Caution: Bypassing the buffer may disrupt you're preconceptions regarding the necessity of a buffer in an LDR preamp." :thumb:
-
I can tell you from experience that amplifiers with sens. of <1v and input impedance of 100K are FAR easier for a passive linestage to drive. You start straying too far away from these specs and it's a crap shoot.
What I am saying is that if you don't need additional gain, the additional gain itself won't make a preamp sound "more powerful". Ultimately, when you are using 1/4 of the volume control, you are throwing away unnecessary gain. You add it, only to attenuate it. How does that make sense?
Shakey
-
So your schedule is too busy for testing gear. Why don't you sign up for the tour and just listen, don't measure, to the Tortuga in your own highly revealing system? Surely that wouldn't take much time and then you'd be able to comment on the Tortuga with first hand knowledge and your opinion would have more substance.
+1.
I used to prefer minimalist approach, abandoned an Adcom active preamp to go with CD direct variable gain, then EVS attenuators, then a TVC (transformed volume control). My honeymoons with those passive devices were not very long, due to lifeless, weak bass presentation in trade of lower noise floor and a perceived transparency.
For the last few years I've been fairly happy with a Dodd Audio variable gain tube preamp. It seems responsive well to the changes in the setup. But perhaps like many audiophiles here, I keep my ears and eyes open. I've found this forum discussing the LDR preamp and ordered one unheard based on positive reviews and what it promises to offer. I hope it will beat or at least equal my current active preamp.
-
I am a little confused:
"Input sens 1.5v
Gain 26db
Input Z 47K"
The above specs do not describe an amplifier which would be challenging to drive. There are many amplifiers with much lower input Z than this.
I agree and would like to steer us around this topic detour.
Amp sensitivity per se has very little to do with compatibility with a Tortuga LDR passive or passives generally. Input sensitivity is the input voltage level (from the source/preamp) which drives any given amp to its full output. Typically these range from 0.7 to 2.0 with most somewhere in the middle. Reason being that most source components have a nominal line stage output voltage within this same range.
What is far more relevant is the combination of amp power output (at full voltage input) together with the sensitivity of the speaker. A very low wattage SET tube amp with highly sensitive speakers works great with our LDR passive - plenty of volume available at mid range attenuation levels. But hook up a low sensitivity speaker to that same low wattage tube amp and you will likely discover you need some premap gain (more voltage) in which can no passive will suffice.
What is most relevant is the bridging impedance ratio between the amp's input impedance and the output impedance of the source. If this ratio is big enough, the source doesn't have to work very hard to deliver its audio energy (minimal current demand).
What we've found in practice based on customer experience is most combinations of sources and amps are well suited for LDR passives and very few are not.
-
Just for giggles, I hooked my LDR3.V2 to a Dynaco ST-150.
Zin = 35K, Input sensitivity = 1 volt.
I was surprised how good the combination sounded.
-
There will be a buffer bypass switch in the prototype and maybe even in a tour unit but not in a final commercial unit. And I will definitely convey my impressions of performance with and without the buffer in the circuit path.
However, I confess there's a part of me that would love to ship a tour unit around with a big fat red bypass switch on the front panel with a sign under it that reads: "Caution: Bypassing the buffer may disrupt you're preconceptions regarding the necessity of a buffer in an LDR preamp." :thumb:
IME if you build a good tube buffer it will sound better in a large majority of circumstances, and not just because of pleasant coloration.
-
IME if you build a good tube buffer it will sound better in a large majority of circumstances, and not just because of pleasant coloration.
And there you have the crux of the LDR passive challenge boiled down into one sentence or even part of a sentence. The perception that with a "buffer it will sound better". In the end it may simply be easier and more productive to add a buffer than to try to convince an informed skeptic that it's not necessary.
The irony here (and it is truly ironic) is that we arrived where we are (passive LDR with no buffer) by working backwards from a tube preamp with pot, to a tube preamp with an LDR attenuator, to no tube/buffer and only a passive LDR attenuator and at each step the sound quality improved. Whereas many here are saying, no, go back, put the tube buffer back in because it will sound better. :duh: :roll: :? :lol:
-
Morten,
As much as you try, you can't convince everyone. As the saying goes, "save the saveable".
Some don't qualify......
Shakey
-
I'd say, whatever well executed will sound good. And "Less is More" is not always true. Recently I added a Cisco switch in between audio PC and control PC, and the set up sounds better, more to my liking. Back to this thread discussion, a very good active pre is not likely going to beat a very good passive pre (such ad the LDR). Here I think "Less is More" does work although there are additional ingredients (LDR, design, execution). In a nutshell, I think Tortuga LDR is a breakthrough with superb sound quality, remote control, upgradability, and passionate service. When mine arrives in a few days, will find out if I'm wrong.
Congratulations Morten. I think you're onto something special.
-
I just don't understand why the skeptics don't sign up for the tour? :scratch: It's free except for postage.
-
I just don't understand why the skeptics don't sign up for the tour? :scratch: It's free except for postage.
RJ perhaps the smartest thing to do is to have multiple tours running in parallel.
-
There are always skeptics, and there are always ones who leap on faith. I'd let the time and the words of mouth do the work.
-
And there you have the crux of the LDR passive challenge boiled down into one sentence or even part of a sentence. The perception that with a "buffer it will sound better". In the end it may simply be easier and more productive to add a buffer than to try to convince an informed skeptic that it's not necessary.
The irony here (and it is truly ironic) is that we arrived where we are (passive LDR with no buffer) by working backwards from a tube preamp with pot, to a tube preamp with an LDR attenuator, to no tube/buffer and only a passive LDR attenuator and at each step the sound quality improved. Whereas many here are saying, no, go back, put the tube buffer back in because it will sound better. :duh: :roll: :? :lol:
The operative word here is a GOOD tube buffer. Many preamps do not have good driver sections and many simple buffers are compromised in their design and implementation. Indeed, if you put in a crappy buffer it will sound crappy. I think you have not experienced or built a decent buffer yet, and you do not seem open to the idea this is possible. Your anecdotal experience with one tube preamp means very little to nothing. Anyway, you seem pretty set on your ideas... and belittling of mine, so I am out.
-
"The operative word here is a GOOD tube buffer. Many preamps do not have good driver sections and many simple buffers are compromised in their design and implementation. Indeed, if you put in a crappy buffer it will sound crappy. I think you have not experienced or built a decent buffer yet, and you do not seem open to the idea this is possible. Your anecdotal experience with one tube preamp means very little to nothing. Anyway, you seem pretty set on your ideas... and belittling of mine, so I am out. "
Please see my post earlier in this thread regarding the tube preamps I have owned. I'm sure at least some of these qualify as "good".
Shakey
-
You might consider having a few basic units available to potential customers, not just those on the tour. Then if they like it, they buy it or return it for a model with more features. Several companies are doing variations of that, though most want payment first and then refund all or part of it. This should bring in customers faster than waiting to hear it on a tour.
For myself, as I have said here before, a few seconds of hearing this product in my system was all it took. Later I became tempted by the active passive argument again, so I had Roger Modjeski build me a small preamp with just a switch for no gain/ +6 dB gain and no other controls. I placed it in my system between the Tortuga and my amps and heard no change, other than the gain when I flipped the switch. So for me pure passive remains the way although it's nice to know I can add some gain if needed. I think this also paints of good picture of Roger's abilities to design good gear.
In short, try it and you will probably prefer it to whatever you have now. And the prices are great. And no, Morten doesn't pay me to say this stuff.
-
The operative word here is a GOOD tube buffer. Many preamps do not have good driver sections and many simple buffers are compromised in their design and implementation. Indeed, if you put in a crappy buffer it will sound crappy. I think you have not experienced or built a decent buffer yet, and you do not seem open to the idea this is possible. Your anecdotal experience with one tube preamp means very little to nothing. Anyway, you seem pretty set on your ideas... and belittling of mine, so I am out.
Dave, No belittling intended certainly. That's not my style. Sorry if I offended. The premise of this thread is that LDR Passive Preamps sound better than they have a right to according to conventional preamp design. That has been demonstrated over and over. Not to you, but to enough other folk that my personal anecdote isn't relevant. The conundrum here is I'd like to convince a broader audience of the merits of our LDR preamps but am confronted with the presumption that because the design doesn't conform to conventional preamp design (buffer - and maybe gain too) that it can't possibly be as good as we say and so that segment of potential buyers are closed off to trying it...unless or until I but a buffer back in...yes, a good one, but it's GOT to have one! A maddening bit of go around and around you have to admit. :D
-
I own a Tortuga LDR and I own a Concert Fidelity LSX. Which do I like more? The Concert Fidelity. However, the retail price is 10x the Tortuga on a good day. LDR's are great, but they do have some quirks in implementation and, because it is DIY, design choices. That said, it is hard not to like them if built right. OK, but I have to say that I hate the encoder dial with a passion. It eventually drives me nuts every time I try to change a setting for one reason or another. Regardless, the sound is stellar. It will never replace the Concert Fidelity. And maybe that is because I love the tube sound/distortion it provides (and I used the d word), but that is just my opinion. I don't feel I miss any details in the music in either case, just find the CF to be more engaging to my ears.
Shawn
-
I own a Tortuga LDR and I own a Concert Fidelity LSX. Which do I like more? The Concert Fidelity.
At $24,000 list I would hope you like it better! :green:
-
At the risk of complicating the issue, why does anyone here want a tube buffer? Solid state please.
-
At the risk of complicating the issue, why does anyone here want a tube buffer? Solid state please.
I've had a few interesting discussions with some audio friends who feel quite strongly that the place to put tubes is in the preamp - for that tubey goodness - and keep the amps SS for all the reasons SS is good. I'm currently using a slightly modified Music Hall 25.2 DAC with a tube output stage, a Tortuga passive pre, and SS amps. The presence of that tube does add a nice quality to the sound. Just my 2 cents.
-
A couple of thoughts since this thread has been quite active. There's a lot we could be doing in terms of product development but there's only so many hours in the day. So this type of feedback and discussion is very useful.
We've been working on two separate approaches to a buffer design/option for some time now. Frankly it's been an on again off again process because I've been waffling on whether it makes sense to ultimately pursue a buffer given how well the LDR Passives perform without one. At present I remain agnostic as to whether a really well designed buffer will improve performance - I just don't know. Early results with prototypes have been inconclusive. Feedback such as Glynn's experience with Modjeski's build only adds to that uncertainly.
In parallel we've also been using variations of our buffer design work towards developing an LDR headphone amplifier. An LDR headphone pre/amp simply must have a gain/buffer stage so one way or another we are going to end up with a buffer design - perhaps both a SS and a tube version.
As some point I suspect we'll make the decision to add a buffer option to our existing LDR design or launch a separate parallel LDR active preamp product....with the only caveat being .... do no harm. Maybe, just maybe, it will even be a bit better. We shall see.
-
Dave, No belittling intended certainly. That's not my style. Sorry if I offended. The premise of this thread is that LDR Passive Preamps sound better than they have a right to according to conventional preamp design. That has been demonstrated over and over. Not to you, but to enough other folk that my personal anecdote isn't relevant. The conundrum here is I'd like to convince a broader audience of the merits of our LDR preamps but am confronted with the presumption that because the design doesn't conform to conventional preamp design (buffer - and maybe gain too) that it can't possibly be as good as we say and so that segment of potential buyers are closed off to trying it...unless or until I but a buffer back in...yes, a good one, but it's GOT to have one! A maddening bit of go around and around you have to admit. :D
Ok, thanks. The issue for me is a passive pre's performance without a buffer is going to depend on the source and amp, a buffer removes that dependency. The lack of standards wrt audio components means every system is different. I do maintain that a good buffer will be better in most cases, my view on the subject is mostly learned from John Broski, who is one of the most talented electronics designers around imo. His theory that a gain stage should be followed by a buffer with extremely high impedance is absolutely correct IME, which means the performance of a passive pre is largely dependent on the design of the source's output stage, and can be made even worse by an amp with low input impedance.
For example, if your DAC's tube output stage is a single triode without being follower by a buffer, it's performance driving an amp is going to suffer. In this case a buffer will make a huge improvement. Don't believe me, try it with a tube based white follower and see what happens. :wink:
-
At present I remain agnostic as to whether a really well designed buffer will improve performance - I just don't know.
The sound is in the power!
The outcome will depend largely on your business plan, design goals and pricing strategy. If you build a super beefy power supply with high quality parts, you'll likely achieve a better sound. The downside is that the active LDR preamp will be several times the price of your current offerings, and many of your existing customers will be unable to afford it. Then you'll have two different market segments to deal with. It's an important business decision. The question is...do you wanna go down that rabbit hole?
-
I've had a few interesting discussions with some audio friends who feel quite strongly that the place to put tubes is in the preamp - for that tubey goodness - and keep the amps SS for all the reasons SS is good. I'm currently using a slightly modified Music Hall 25.2 DAC with a tube output stage, a Tortuga passive pre, and SS amps. The presence of that tube does add a nice quality to the sound. Just my 2 cents.
The idea of using tubes for a zero gain output buffer is contrary to my understanding of what tubes are really good at: tubes are good for voltage gain, they are not so good at driving current (same with JFETs). A unity gain buffer circuit is used to drive current; my experience suggests that MOSFETs would be the way to go, or (hate to say it) you might want to experiment with some of the diamond buffer IC chips as well. As mentioned before, to make a really good buffer stage you are going to need superb power supply rails, virtually zero noise, and very low output impedance, if you want the buffer to perform at a really high level.
-
I'm willing to be a tester/reviewer for the buffer. :D I'll build it into my current diy tortuga. I'm definitely curious about what a tube buffer will do. I have tubed monoblocks (rebuilt Heathkit W5m monoblocks with some mods) so I already have some tube goodness. :wink: I could also run it to a couple of small digital amps.
-
I'm willing to be a tester/reviewer for the buffer. :D I'll build it into my current diy tortuga. I'm definitely curious about what a tube buffer will do. I have tubed monoblocks (rebuilt Heathkit W5m monoblocks with some mods) so I already have some tube goodness. :wink: I could also run it to a couple of small digital amps.
I would say if you need all that, then stay with a conventional preamp.
Shakey
-
The idea of using tubes for a zero gain output buffer is contrary to my understanding of what tubes are really good at: tubes are good for voltage gain, they are not so good at driving current (same with JFETs). A unity gain buffer circuit is used to drive current; my experience suggests that MOSFETs would be the way to go, or (hate to say it) you might want to experiment with some of the diamond buffer IC chips as well. As mentioned before, to make a really good buffer stage you are going to need superb power supply rails, virtually zero noise, and very low output impedance, if you want the buffer to perform at a really high level.
Tubes and JFETs have plenty good enough current delivery for purposes of a preamp buffer. Moreover the job of preamps isn't so much to deliver current as to deliver the voltage signal (the music). With amps having input impedances of 10k or more even a full volume (unattenuated) line stage signal of say 2 V would only have to deliver 2/10,000 = 0.2 ma of current. It's ain't nothing, but it ain't much either.
Tubes can and are used quite often as buffers when configured as unity gain cathode followers as opposed to their more common configuration as voltage amplifiers on their plate side. John Broskie (widely recognized as a tube design expert) has quite a lot to say about cathode followers. Here's one of his articles on the subject: http://www.tubecad.com/2011/08/blog0212.htm (http://www.tubecad.com/2011/08/blog0212.htm)
And then there's Nelson Pass with his JFET B1 Buffer where much like tube buffers the JFETs are used as followers in a unity gain buffer. This is a very effective preamp buffer design and has been widely adopted within the DIY community. https://www.passdiy.com/project/preamplifiers/b1-buffer-preamp (https://www.passdiy.com/project/preamplifiers/b1-buffer-preamp)
I do agree that in the end it's all about the power supply. That's what all audio preamps (active) and amps are all about really; their outputs are modulated power supply signals. Garbage in, garbage out. That is perhaps the most compelling reason to avoid introducing a buffer into a passive preamp which has proven it doesn't need one to deliver awesome performance.
-
The sound is in the power!
The outcome will depend largely on your business plan, design goals and pricing strategy. If you build a super beefy power supply with high quality parts, you'll likely achieve a better sound. The downside is that the active LDR preamp will be several times the price of your current offerings, and many of your existing customers will be unable to afford it. Then you'll have two different market segments to deal with. It's an important business decision. The question is...do you wanna go down that rabbit hole?
Red pill or blue pill? :thumb:
-
Ok, thanks. The issue for me is a passive pre's performance without a buffer is going to depend on the source and amp, a buffer removes that dependency. The lack of standards wrt audio components means every system is different. I do maintain that a good buffer will be better in most cases, my view on the subject is mostly learned from John Broski, who is one of the most talented electronics designers around imo. His theory that a gain stage should be followed by a buffer with extremely high impedance is absolutely correct IME, which means the performance of a passive pre is largely dependent on the design of the source's output stage, and can be made even worse by an amp with low input impedance.
You've arrived at the essence of our design rationale. Our LDR passives are not a universal fit between every source/amp combination. We are very up front about that. And yes, adding a buffer would mitigate that lack of universal fit. However, what we've discovered is that it does indeed fit well with most systems for the simple reason that most amps do have high enough input impedance and most sources do have low enough output impedances (and sufficient line stage current drive capacity). And by eliminating an unnecessary subsystem (buffer/power supply) between the sources and the amps where the LDR preamp is a good fit we are able to deliver amazing sound quality that exceeds the performance of most conventional preamps - for way less $ I might add. That, together with the inherent transparency and clarity of LDRs, is the whole point a nutshell.
For example, if your DAC's tube output stage is a single triode without being follower by a buffer, it's performance driving an amp is going to suffer. In this case a buffer will make a huge improvement. Don't believe me, try it with a tube based white follower and see what happens. ;)
I'm fairly certain that it is a tube cathode follower output stage. I rolled a few tubes through it and stuck with a $200 NOS French Mazda that sounds very nice. I'm more than willing to try adding a buffer and assessing the impact. I'm only asking for folks to remain open to the possibility that it may not be necessary and ..... first just try it the way it is! :thumb:
-
I'm willing to be a tester/reviewer for the buffer. :D I'll build it into my current diy tortuga. I'm definitely curious about what a tube buffer will do. I have tubed monoblocks (rebuilt Heathkit W5m monoblocks with some mods) so I already have some tube goodness. ;) I could also run it to a couple of small digital amps.
When we get that far, you're on!
-
I would say if you need all that, then stay with a conventional preamp.
Shakey
I didn't say that I needed a buffer. :scratch: I just volunteered to test the prospective tube or ss buffer. Do you have a closed mind? Also I didn't ask for your advice.
I already have the Tortuga and sold my very nice active hybrid pre several months ago.
I noticed that your power amp is tubed and you don't have a Tortuga LDR.
-
You've arrived at the essence of our design rationale. Our LDR passives are not a universal fit between every source/amp combination. We are very up front about that. And yes, adding a buffer would mitigate that lack of universal fit. However, what we've discovered is that it does indeed fit well with most systems for the simple reason that most amps do have high enough input impedance and most sources do have low enough output impedances (and sufficient line stage current drive capacity). And by eliminating an unnecessary subsystem (buffer/power supply) between the sources and the amps where the LDR preamp is a good fit we are able to deliver amazing sound quality that exceeds the performance of most conventional preamps - for way less $ I might add. That, together with the inherent transparency and clarity of LDRs, is the whole point a nutshell.
Earlier, perhaps at the beginning of the thread, you posed the question (not word for word) : "What would it take to convince (active preamp fans) that LDR passives can be great in their systems?" We've come full circle. The lack of being a universal fit IS the big issue. Making an LDR passive more universal via a good buffer, would very nearly mitigate the problem, as you are aware. Until you have a buffer, you'll always have those few people that have a system where a pure passive won't work, and they'll always hold to their guns that passives are lacking in some way (even if their experience is limited to other passive types).
It boils down to, do you want to hold your spot doing what you do now, or do you want to also have a more "inclusive" product option also? Both options have their merits and complications.
-
If I'm understanding this correctly it seems to come down to a couple of things..
A) Do you want the customer to change or adjust their system to get the full benefit and performance from a pure passive like the LDR and therefore less likely to return a unit that might not be to their liking or...
B) Do you as the manufacturer change and adjust by offering the customer another option in your lineup with a buffer so that a broader audience can now experience your preamp's abilities.
Then I guess it boils down to is it worth the time,effort, and money to create a buffer that you're happy with versus the amount of new customers it may bring.
If I'm way off please help me understand..
-
Red pill or blue pill? :thumb:
I prefer the one that doesn't do anything at all.
-
And by eliminating an unnecessary subsystem (buffer/power supply) between the sources and the amps where the LDR preamp is a good fit we are able to deliver amazing sound quality that exceeds the performance of most conventional preamps - for way less $ I might add.
After hearing this quality of sound for such a modest investment, it would be difficult to even consider going back to a "conventional" preamp when I consider that money could go to improve something else in the system.
Or to even buy more music! :)
-
If I'm understanding this correctly it seems to come down to a couple of things..
A) Do you want the customer to change or adjust their system to get the full benefit and performance from a pure passive like the LDR and therefore less likely to return a unit that might not be to their liking or...
Thank you for framing your comment this way. It's helpful.
The simple answer is NO. We do not want, nor do we ask, nor do customers have to do anything different to get the full benefit of our LDR preamps. What we've observed based on 100's of customer experiences with all manner of source-amp combinations is that with very few exceptions our LDR passives are a great fit with customers systems AS IS. The simple reality we are trying to convey is that in the vast majority of cases, preamps with active buffer/gain are not necessary and that our LDR passives will do an excellent job. What does "vast majority" really mean? Conservatively, I'd say well over 90%. :o
B) Do you as the manufacturer change and adjust by offering the customer another option in your lineup with a buffer so that a broader audience can now experience your preamp's abilities?
Then I guess it boils down to is it worth the time,effort, and money to create a buffer that you're happy with versus the amount of new customers it may bring.
I think the wiser choice is yes, offer customers a buffer option. It's probably a far more productive path than trying to change someone's mind. 8)
-
I'm skeptical that a buffer will improve the Tortuga. The adjustable impedance function optimizes the bridging impedance between the source and amp makes the Tort adaptable to almost any amp. Nevertheless, a tube buffer may do something and you don't know for sure until you try it.
-
I didn't say that I needed a buffer. :scratch: I just volunteered to test the prospective tube or ss buffer. Do you have a closed mind? Also I didn't ask for your advice.
I already have the Tortuga and sold my very nice active hybrid pre several months ago.
I noticed that your power amp is tubed and you don't have a Tortuga LDR.
Not built by Tortuga, but uses a Tortuga board.
And don't get so butt hurt. "You"was meant as a generic reference, not YOU in particular.
Shakey
-
To Shakeydeal - I don't seem top know enough about Audio Circle to be able to find out what amp you are using that includes a Tortuga Board. Would you mind sharing the brand name with me? So far the only company I know of using LDRs is Dar T Zeel and I can't even afford to say that name in public.
-
Abacus Music in Charlotte NC. The builder is a member here.
Shakey
-
What does "vast majority" really mean? Conservatively, I'd say well over 90%. :o
That's pretty high! ...But I'd expect there to be a range of compatibility, it's not a black and white issue. For me, the problem is I'd have to consider the factors we've talked about when selecting an amp and source... To the point they would be priority factors when choosing equipment and in/out impedance specs don't tell the whole story. You can have low ouput impedance without the ability to supply much current. It might be difficult to get the necessary information from all the manufacturers you are considering. It's frustrating because there are no specs you can count on, no accepted standards for preamp/amp ins/outs.
I think offering the pre without a buffer is a good idea too as a buffer does add more complexity and cost to the system. I also think you can build a tube buffer that you will find to be an improvement in most cases.
-
That's pretty high! ...But I'd expect there to be a range of compatibility, it's not a black and white issue. For me, the problem is I'd have to consider the factors we've talked about when selecting an amp and source... To the point they would be priority factors when choosing equipment and in/out impedance specs don't tell the whole story. You can have low ouput impedance without the ability to supply much current. It might be difficult to get the necessary information from all the manufacturers you are considering. It's frustrating because there are no specs you can count on, no accepted standards for preamp/amp ins/outs.
All good points. Recognizing that lack of info and acknowledging the potential for the occasional misfit we put all of that aside, focus on the key criteria of impedance bridging ratio and suggest people have a go at it since the probability of success has proven to be "pretty high" in actual practice. For those rare times it doesn't work out, we offer 100% refund so the risk to try and find out is de minimis. The upside is possibly the best sounding - and best bang for your buck - preamp most people are likely to ever own.
I think offering the pre without a buffer is a good idea too as a buffer does add more complexity and cost to the system.
Exactly.
I also think you can build a tube buffer that you will find to be an improvement in most cases.
We shall see. We are currently slowly moving along with a hybrid cathode follower design using dual 6SN7's with a split (+/- 100V) voltage supply using one SET in place of the conventional cathode resistor. This results in the buffer output having almost zero DC offset and with an upstream op amp servo we can literally totally zero out the offset and thus eliminate the need for a sonics robbing coupling capacitor; i.e. plan to make it dc coupled. The trade off is we are putting in a safety disconnect relay tied to a DC offset protection circuit in case anything wonky happens it will disconnect the output. If testing shows we can't guarantee harmful DC bumps we'll dump the servo and go with a coupling cap.