Merits of LDR Passive Preamps - A Discussion

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Early B.

Re: Merits of LDR Passive Preamps - A Discussion
« Reply #60 on: 2 Jul 2015, 09:43 pm »
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?   

barrows

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Re: Merits of LDR Passive Preamps - A Discussion
« Reply #61 on: 2 Jul 2015, 11:53 pm »
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.

rajacat

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Re: Merits of LDR Passive Preamps - A Discussion
« Reply #62 on: 3 Jul 2015, 12:24 am »
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.

Shakeydeal

Re: Merits of LDR Passive Preamps - A Discussion
« Reply #63 on: 3 Jul 2015, 10:32 am »
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

tortugaranger

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Re: Merits of LDR Passive Preamps - A Discussion
« Reply #64 on: 3 Jul 2015, 01:48 pm »
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

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

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.

tortugaranger

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Re: Merits of LDR Passive Preamps - A Discussion
« Reply #65 on: 3 Jul 2015, 01:53 pm »
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:

tortugaranger

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Re: Merits of LDR Passive Preamps - A Discussion
« Reply #66 on: 3 Jul 2015, 02:36 pm »
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.

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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:

tortugaranger

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Re: Merits of LDR Passive Preamps - A Discussion
« Reply #67 on: 3 Jul 2015, 02:41 pm »
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!

rajacat

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Re: Merits of LDR Passive Preamps - A Discussion
« Reply #68 on: 3 Jul 2015, 02:47 pm »
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.

RDavidson

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Re: Merits of LDR Passive Preamps - A Discussion
« Reply #69 on: 3 Jul 2015, 04:26 pm »
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.

Tomy2Tone

Re: Merits of LDR Passive Preamps - A Discussion
« Reply #70 on: 3 Jul 2015, 04:58 pm »
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..

konut

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Re: Merits of LDR Passive Preamps - A Discussion
« Reply #71 on: 3 Jul 2015, 04:58 pm »

Red pill or blue pill?  :thumb:

I prefer the one that doesn't do anything at all.

33na3rd

Re: Merits of LDR Passive Preamps - A Discussion
« Reply #72 on: 3 Jul 2015, 05:05 pm »
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!  :)

tortugaranger

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Re: Merits of LDR Passive Preamps - A Discussion
« Reply #73 on: 3 Jul 2015, 05:51 pm »
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

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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)

rajacat

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Re: Merits of LDR Passive Preamps - A Discussion
« Reply #74 on: 3 Jul 2015, 05:53 pm »
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.

Shakeydeal

Re: Merits of LDR Passive Preamps - A Discussion
« Reply #75 on: 3 Jul 2015, 07:35 pm »
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

glynnw

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Re: Merits of LDR Passive Preamps - A Discussion
« Reply #76 on: 3 Jul 2015, 08:07 pm »
 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.


Shakeydeal

Re: Merits of LDR Passive Preamps - A Discussion
« Reply #77 on: 3 Jul 2015, 09:10 pm »
Abacus Music in Charlotte NC. The builder is a member here.

Shakey

DaveC113

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Re: Merits of LDR Passive Preamps - A Discussion
« Reply #78 on: 3 Jul 2015, 09:49 pm »
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.

tortugaranger

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Re: Merits of LDR Passive Preamps - A Discussion
« Reply #79 on: 4 Jul 2015, 02:08 pm »
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.

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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.

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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.