AudioCircle
Audio/Video Gear and Systems => Owner's Circles => Bent Audio Owners => Topic started by: John Chapman on 9 Jun 2003, 03:22 pm
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Hello!
There was a thread posted over at HD that was interesting and since there is more activity over here these days I thought I'd move it over here as well.
It was about using a transformer as an attenuator for headphones. The idea was to connect directly to your source and then to the transformer attenuator and after that to the headphones. This would eliminate the headphone amp but as the thread shows the TX102 is not a candidate for this - but it may be worth exploring a different unit that would work in this place.
Thread below....
Thanks!
John
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Czilla9000 wrote:
Hello, I am looking to build a device which will allow me to passively attunuate a signal for headphones. Since you guys tend to be the leaders in this stuff, I was wondering how I would go about wiring a transformer (like the B&S one) for this use.
Thank you.
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I wrote:
Hello!
Thanks for the post! I hate to answer with a question but need a bit more info first. What is the source driving the headphones?
The TX102 transformer is used for level (volume) control and I have a few in headphone amps but they attenuate the signal at line level before the amp's output circuits. They can handle a pretty high signal (like >7 Vrms) but I don't think they'd be the trick to kick down the headphone signal in-line with the phones - which have a much lower impedance than the TX102 would normally see.
Send a bit more info about the setup you have and we'll see what might work.
Many Thanks!
John Chapman
www.bentaudio.com
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Czilla9000 wrote:
I am trying to build, eccentually, a passive headphone amplifier. The idea of building a passive headamp is very contraversial with headphone audiophiles, despite the fact most sources give out more than enough power to power any phone (leaving only attenuation neccesary). I don't know much about my source, but I have heard that using a transformer would be better than a stepped atennuator.
The reason I want a passive headamp is because I don't want coloration. To me simplicity is better, and an active amp is certainley not simpler than a simple resistor or transformer.
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I wrote:
Hello!
It's true that a transformer would do a better job of using the energy from a source to drive headphones directly but I have to question if most sources give out enough total power. They may give out enough voltage (2V typical and many higher than that) but most would not handle the low impedance of headphones well at all. Most sources might have an output impedance of say 100 or 200 ohms for example. This would be a typical solid state CD players output impedance. Some would be lower but much below 100 ohms would be rare. If you hook up a 10K to 50K amp load most of the signal will get to the amp with a very small amount lost via the source/transformer output impedance (which can be thought of as a resistor in series). With a headphone load fo maybe 40 ohms or so (I don't follow this stuff that close so correct me if I am off on this) most of the signal will be lost across the sources output impedance.
As you can tell from the products I carry I love passive stuff and I appreciate what you are trying to do. I think you'd get some sound out but I have to think there would be improved quality with a buffer stage to drive the headphones easier - even though you'd be adding additional stuff to the signal path. I agree that simpler is better but this setup may be pushing it a bit far.
If you want to give it a go look for a source with a very high voltage out and - most important - an extremely low output impedance. Some early Theta DAC's and as I recall some Wadia's had killer output stages and level controls. Basically in these cases the buffer stage is built into the source. A transformer after an output stage like this would lower the sources output impedance even more - which is a good thing! and we could play with the TX102.
If you can let me know what the impedance of the headphones you have in mind are and what the sources you'd think of using are I'll try and re-create the setup here with my AP system 2 analyser and see how the TX102 would behave with those conditions.
Many Thanks!
John Chapman
www.bentaudio.com
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Czilla9000 Wrote:
To be honest I am not an expert yet on this impedence mismatching stuff. Could you be so kind to explain what all of this impedence stuff is?
So I take it you believe that a transformer would be better than a ladder type stepped attenuator?
Headphones have a wide variety of output impedences. Grados, for instance, have an impedence of 32 ohms, while the most popular audiophile phone, the famed Sennheiser HD600 has an impedence of 300 ohms.
PS - Thank you for all of your help.
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I wrote:
Hello!
While I would not call myself an expert either the work with the TX102 (and the way it 'translates' impedance) has got me thinking about it quite a lot. I'll run through it as best I can but please post more questions as they come up.
Everytime you connect one component to another you have 2 impedances to think about - the output imp of the 'source' and the input impedance of the downstream component. An exapmple would be the output impedance of a pre-amp deefing into the input impedance of an amp. Another would be the output imp of that amp feeding into the input impedance of a speaker. In every case it's desireable to have a low output impedance feeding a higher input impedance. Here are some typical values:
CD Player with a 100 ohm output Imp feeding a pre-amp with a 40K input Imp.
Pre-amp with 50 ohm output impedance feeding an amp with a 20K input Imp.
A solid state amp with a .05 ohm output Imp feeding an 8 ohm speaker.
A single ended tube amp with a 2 ohm output imp feeding a 16 ohm speaker.
These are kinda typical values - if there is such a thing!
In each case you can see that the input impedance is many times more than the impedance feeding it. I spotted a good article on impedance here:
http://www.transcendentsound.com/amplifier_output_impedance.htm
The ratios range quite a bit but 100 to 1 is kinda the middle ground - with many cases typically higher. The triode amp is included in the list on purpose. It's ratio is much lower (although still 8 to 1) and many folks would think this would not work. In practice if the following impedance is somewhat constant that can work well. The article referenced above is a good description of this from the amps point of view. Basically a high ratio makes the system more immune to variations in impedance of the downstream component and also has other benifit's to the signal transfer as well.
Finally - lets look at the headphone case. If you have a source with a 200 ohm output impedance (typical CD player but impedance figures vary wildly) and headphones like the HD600's with a 300 ohm impedance we have a 1.5 to 1 ratio - many many times lower than the triode amp to speaker ratio that would freak out a lot of 'measurement types'. These amps can work very well and I am a bit of a fan. I included them in the list to show that exteme ratio's like 400 to 1 are not needed to get good results.
Our 1 to 1.5 ratio would be pushing it too far I think and so if you were to want to do a good job of driving the phones I'd guess a ratio of more like 20 or 30 to 1 would be a good target. This would lead us to be looking for a source with a 10 or 15 ohm output impedance. THis is low and it's rare to find a source with this low an output impedance.
Now on to transformers as a level control.... We'd need to test if the TX102 would behave at all into such a low load - I have no idea what will happen! You have me curious now so the next time I crank up the test gear I am going to play with it and see.
The TX102 steps the voltage down to lower level but the bonus is how it lowers the output impedance as you step down further and further. The cool thing is that the impdance goes down really fast because it is related to the square of the turns ratio. Once you get down below about half way it is way way lower than the source feeding it. This - if it would work - might be the savior of this passive headphone plan. A source with an output Imp of 10 ohms is rare - very rare. If we can use a transfomer to lower voltage we could get away with a higher output impedance at the source - as long as the normal listenning was done at a level well down from the full level the source puts out. We'd still be looking for a low output impedance and a higher than typical output voltage from our source - so that we can step it down and still have sufficient level. You can see from this that it looks like it might just work! - but only if we pay careful attention to the source and the phones impedances and voltage requirements.
Long post - sorry! Next steps:
1- Could you dig around and see what a voltage would be required accross a pair of HD600's for a typical listening level.
2- I'll test the TX102 here and when I next talk to S&B bounce it off him and see what his thought are....
Many Thanks!
John Chapman
www.bentaudio.com
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I wrote again....
Hello!
Moving too fast again. Lots of typo's in the post above and I missed the 'edit window'. Sorry guys....
Thanks!
John
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Czilla9000 wrote:
Thank you.....do you mind if I relay your posts on this issue to another disscussion forum. I know a headphone forum which would be interested in this.
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I wrote:
Hello!
No problem relaying the posts - but please let then know I am not a headphone guy so I have been making a lot of guesses when it comes to headphones!
I spoke to S&B today and I had asked Jonathan about this. His first reaction was like mine - not likely to work well. With very high impedance phones like the HD600's 300 ohms he started to think about it a bit more. I have leasured the TX102 into a 600 ohm load and it stays well behaved.
I had missed one thing that he brought up - resistive losses. When a TX102 is connected to a high impedance load (like it usually is) the resistive losses are not a factor as the TX102's internal resistance is only a small fraction of the resistance of the load. When we move the load inpedance down then the losses at the transformer become a factor.
What this means is that we'd loose a big chunk of the signal accross the transformer - this makes the source output voltage requirement even higher than we'd thought. I think we may be reaching the point where you have added so much restriction on the source (likely needing a source with extra gain stages and buffers inside) that it may defeat the purpose of the passive level control - which is to simplify the circuit! I am thinking you'll find that a simple output stage from the source with a level control followed by a gain stage / buffer circuit (ie. a nice headphone amp!) would perform better than the gain stage / buffer in the source followed by the level control.
Since we've come this far I'd still like to find out what the voltage level at the phones would be for a typical 'high' listenning level and maybe the folks at the other forum could help with that.
Many Thanks!
John Chapman
www.bentaudio.com
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Czilla9000 wrote:
Thank you for all of your help. So basically it is impractical and worse than using an active headamp? Correct?
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I wrote:
Hello!
I think it comes down the the fact the source you use (cd player / dac, etc) would have to have an output stage that was the equivilent of a headphone amp in it! My thinking is at that point why not just use the source you want (instead of having to choose it primarily on it's output stage) and then use a heaphone amp after that. There is such a variety of headphone amp circuits that you could pick anything from a solid state unit to a tube unit depending on what would best match your phones and your listenning preference.
I was starting to think it may work but the TX102 is just not made for this low an impedance. I think there may be ways to get the passive control just ahead of the phones to work. An autoformer along the lines of the 'zero' autoformer for speakers (but scaled down) with many taps like a TX102 so you could select levels might work - again given the right source. If a unit were made especially for this application then the transformer losses could be minimized by keeping the winding resistance's low. The transformer (or autoformer) could solve the impedance mis-match issue - much like the 'zero autoformers' that are popular with OTL amp guys solve the amp to speaker impedance mis-match.
Did you ever spot what a typical voltage would be to get the HD600's to a somewhat loud listenning level?
Thanks!
John
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Czilla900 wrote:
Yes, about 1 volt (I have had people at another forum try this and say 1.2 volts is very loud, so I assume 1 volt would be nessecary.
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Jazz (good handle!) wrote:
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john chapman wrote:
...Finally - let's look at the headphone case. If you have a source with a 200 ohm output impedance (typical CD player but impedance figures vary wildly) and headphones like the HD600's with a 300 ohm impedance we have a 1.5 to 1 ratio... Our 1 to 1.5 ratio would be pushing it too far I think and so if you were to want to do a good job of driving the phones I'd guess a ratio of more like 20 or 30 to 1 would be a good target. This would lead us to be looking for a source with a 10 or 15 ohm output impedance. This is low and it's rare to find a source with this low an output impedance.
...I'm coming from the same forum (Head-Fi) as Czilla9000. We have discussed a lot - basically with other people who rate the direct-path idea as obsolete and impracticable, roughly spoken with merely the thought in mind that «line-out amps aren't designed to drive complex and low-impedance loads». Of course this argument has to be taken into consideration. Otherwise it's easy to try it, in the first stage by just plugging a headphone into your soundcard's line-out and thus avoiding any additional resistors in the signal path. This shows that it principally works, at least in most cases, I guess. The other question is about the impedance ratios. I have to agree that the majority of the common CDPs and DACs have output impedances around 100 or 200 ohm. Which is a bad thing in view of adequately driving headphones without serious colorations due to the interaction with their impedance curves and also due to the low amount of remaining current.
But in my case it's a Theta Pro basic II and a Bel Canto DAC2 with 6 and 20 ohm output impedance. Whereas it's not a really great value looking at the 250-300 ohm of the concerning headphones (Beyerdynamic DT 880 and Sennheiser HD 600) with their bass-resonance induced impedance amplitudes towards 500 ohm or so, it seems to work flawlessly! I come to this conclusion after some tests: how does the complex, low-impedance load affect the line-out amp's signal? For normal listening I use dedicated headphone amps. I had one switched to the DAC's line out, serving as a monitor. Additionally I switched a 500-ohm potentiometer with a plugged-in headphone onto the line out, parallel to the amp. Then I listened to some music samples through the monitor amp with my reference headphone, alternatingly plugging the potentiometer/headphone system out and in... to verify to what extent the sound would change. Result: there was barely any sound change noticeable. Even a Philips DVD 963 SA with its 200 ohm caused only negligible sound changes (hard to rate because of the considerable volume change in its case).
So my conclusion is: At least some low-impedance line-out stages can drive headphones directly (or via potentiometer, resp.) in an adequate manner, without causing any serious sound degradation. This in contrast to all headphone amplifiers I have tried - and that's the actual motivation behind my direct-path idea. The main reason for being bashed therefor by the majority of the forum members is my experience - and theirs, particularly - that the direct path sounds somewhat less appealing (mostly called «worse») than ordinary headphone amps, although it excels in the main criteria: dynamic, transient speed, focus, resolution, transparency, clarity, cleanness... The main concern from my side is a certain lack of coherency and color(ations).
Based on my experiments my standpoint now - in contrast to others - is that amps (be it tubes or solid-state; I own both) generally color the sound in a euphonic manner. This idea is broadly and passionately rejected. Preferably by just ignoring my tests...
After my experiences I'm rather optimistic that the theoretically superior transformer operation will offer similar results, besides avoiding the damage potential provided by the high current flow with the potentiometer operation (could you tell something to this?). Since your theoretical considerations to the subject, cited by Czilla in the Head-Fi forum, already have lead to the proclamation of the death of the direct-path idea, I hope to have given you some useful information in view of your own standpoint. I'm not an electronics expert, BTW, and my findings are solely based on my listening experiences.
JaZZ
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I wrote:
Hello!
Thanks for the post! I have far from given up on the idea and sure don't want to be responsible for it's death! I had thought in my last post about it that I was stating that given the right transformer (or autoformer) and given a low output impedance source it could work well. Most of the comments about it not working were with reference to the TX102 - which was designed to feed loads more like 10K and above (typical amp inputs). This would however never be the kind of thing you could just plug-n-play without considdering the source and the impedance of the phones.
To answer your question about the potentiometer:
If you introduce a potentiometer into the signal path between the source and the phones you wildly change the impedance situation - for the most part for the worse! Even a low value pot (what value did you use?) will introduce a series resistance in all but the ful volume setting. As you adjust the level of the pot you are changing this series resistance - from 0 ohms when wide open to a quite high value as the pot lowers the volume. This is the area where the transformer/autoformer may show promise. With a transformer as you lower the level the output impedance drops - and it drops with a realtionship following the square of the turns ratio. Every time the turns ratio is changed down 2:1 (6db drop in volume level) then the output impedance is 4x lower than before. The winding restance and some other things come into play but that's the general idea. There is a table at S&B's TX102 page showing output impedance for a 1K source here:
http://www.stevens-billington.co.uk/page102.htm
This table (and page) are for the TX102 transformer but they do show how as the level is lowered the output impedance goes down and after a few steps down from the top it goes down drastically fast. The TX102 is not made for driving the low impedance headphone load but that does not mean the same effect can not be used to advantage is a new transformer or autoformer were made that was designed for this. The 'zero autoformer' that the OTL amp guys use for lower impedance speakers uses the same principle to translate impedance. Both these impedance translations are after the same thing you are with the direct headphone drive - to take advantage of the sources output impedance and lower it even more so it can better cope with lower impedance loads. This is pretty much the oposite effect that a pot has - where you lower the pot's level the output impdance is going up!
Good thread and thanks for your comments!
John Chapman
www.bentaudio.com
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[Copied from the Harmonic Discord Forum]
John...
I'm glad the direct-path idea isn't dead in your eyes...
john chapman wrote:
If you introduce a potentiometer into the signal path between the source and the phones you wildly change the impedance situation - for the most part for the worse!
I'm aware of this. BTW the one I use is a 500-ohm type. Fortunately the sound is virtually unaltered by the direct-path system it is part of. This is a surprise to me too. (I hope you have noticed my test procedure and configuration.) So the conclusion that amps cause clearly more important sound alteration is justified from my point of view.
JaZZ
...I should have mentioned that I've also tried the "direct" direct connection without the potentiometer (with low-level music passages) to verify the change of sonic character by the serial resistance.
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Hello!
500 ohms is a lowish value - and that's what I hoped you'd played with. Do you find you have the level turned down quite a bit for 'normal' listening levels? What kind of phones are you using and what is their impedance.
John Chapman
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John...
...first of all: thanks for your replies! :)
My previous potentiometer was a 1 kOhm type. It worked equally well, but sounded somewhat darker than the 500 ohm type I'm using now - a logical consequence of the then mainly used HD 600's impedance curve. The 500-ohm type provides a sonic balance comparable to the headphone amps I use (Earmax Pro and Corda HA-2). Anyway, I wouldn't call the sonic signature between 500 and 1000 ohm principally different. The commonness they share is the relative dry, unvarnished sound compared to the more colorful presentation from the amps.
The average effective serial impedance resulting from the 500-ohm potentiometer's voltage-divider function in cooperation with the 300-ohm Sennheiser or the 250-ohm Beyer DT 880 should be in the range of OTL tube amps.
For normal listening levels the settings are between 1/3 and 2/3 (BTW it's logarithmic) with the DAC2 (max. 2 volt); with the Theta (max 5 or 6 volt) it was a bit less.
BTW, what do you think: do such low load impedances (= such high currents) represent a serious damage risk for normal (CDP/DAC) line-out stages?
(http://www.handykult.de/plaudersmilies.de/nosmile/peacesign.gif) JaZZ
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Hello!
Thanks again to you Jazz and to Czilla9000 for all the info - this has been a really fun thread!
With settings typically from 1/3 to 2/3 on a log taper pot the way the transformer or autoformer would be in the range where it would significantly lower output impedance - which would be a big help in finding sources that'd drive the phones directly. I would suggest that the 250 or 300 ohm phones you have been using would also be a requirements. 32 ohm Grado's would be out!
As far as damage to output stages I think we are pretty safe. As long as you were picking a source with a fairly low output impedance to start with - say less than 100 ohms you should be fine. At 2V out feeding into 300 ohms (or accross 400 ohms if we include the sources output impedance) we'd be asking the source to feed about 6mA or so - certainly no chance of damaging a typical op-amp based output stage at that level. Most could easily feed 5 to 10 times that level and an output stage like the Theta (which i think used the BUF03 chip as an output buffer) would be happy feeding even more current. These calc's assume the 300 ohm impedance is flat and does not dip. If phones behave as speakers (like I'd guess they do) this is not likely the case! I have to think though that there would not be any significant dips down to a really low impedance - has anyone measured the impedance curve of headphones?
You guys have me curious now so I am going to get a pair of HD600's to mess with and test. Maybe we'll wind up something in a while to try this out!
Thanks!
John Chapman
www.bentaudio.com
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Hello!
I spotted the impedance curve for the HD600's here:
http://www.headphone.com/layout.php?topicID=3&subTopicID=26&productID=0020080600
The curve does not get down to 300 ohms and so it never goes lower.
Thanks!
John
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John...
...yeah, you got the impedance curve I otherwise would have picked up. Of course a transformer would do a better job than my 500-ohm potentiometer in view of the 540-Ohm resonance peak. But believe me, even though there may be a certain predictable hump in the electrical frequency response, about 1/2 or 1 dB I guess (whereas a dedicated amp would produce a perfect straight line), this is negligible (to my ears!) compared to the sound degradation/coloration caused by amps. You just can't measure the latter. BTW, the 32-ohm Grados do work quite well, surprisingly, or less surprisingly when you look at their impedance curves:
(http://www.headphone.com/graph.php?graphID=115)
The loss of efficiency caused by the line-out's output impedance and the potentiometer is undisputed, though.
Unfortunately Czilla9000 is out of play. He has completely misinterpreted your comment concerning the TX102's possible inability to work with headphones and now thinks that the direct-connection idea must be buried, not least due to the massive pressure from the majority of the Head-Fi people who feel attacked by it – because for them amps are like a holy cow and nobody should dare to call them effect devices... :lol: BTW: I also think that headphones sound better through amps than through direct connection (haven't I already mentioned this?) – but not because they're more accurate, but because they're more euphonic.
Do you know this line-stage transformer (http://www.divertech.com/aqreferencetools.html)[/url] specially designed for headphones?
(http://www.divertech.com/images/aqreferencetools.jpg)
I'm looking forward to your headphone direct-drive experiments and am very curious. Are you also considering to try a (500-ohm) potentiometer? And finally: Be warned: you may first be shocked by the unvarnished direct-path sound. I hope you don't confuse it with inaccuracy. :| IMO it's as accurate as can be.
(http://www.handykult.de/plaudersmilies.de/nosmile/peacesign.gif) JaZZ
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Hello!
I am quite familiar with the ASL stuff - I own 2 of their amps and kind of act as a dealer locally here now. I had not seen that device - not a lot of info but I will be talking to Divergent and will try and get the scoop on it. Does it do just what we are talking about here?
I spoke with S&B today and he was going to mess around with some things like maybe an autoformer with low winding resistance (to minimize losses) and maybe come up with something we might offer.
Your experience is great and a transformer or autoformer should do a good job of doing the impedance translation. If we are down at say -12 db for a typical listenning level then that would turn a 200 ohm CD players output impedance into a about a 20 ohm output impedance driving the phones.
Thanks again for all the info.
John Chapman
www.bentaudio.com
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Jazz,
That ASL box you link above might be just the trick. It does just what we have been talking about. At it's price I have to think we might be able to make a no holes barred version that would use better quality switches and transformer core materials, etc but the general principle is the same. I'll have one on the way to me soon as I have a couple amps being shipped here anyway. I'll mess with it here and then send it down to you to play with and try if you'd like.
Many Thnaks!
John Chapman
www.bentaudio.com
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John...
...that's a great offer! I'd be glad to accept it. But are you aware that I live in Switzerland? (Shipping charge!) :?
I've already tried to contact Divertech via e-mail – without success, their account seems to be out of action. Do you know a valid e-mail address of them? I was about to order one of the Reference Tool boxes, but now that you plan to design an improved and maybe more esthetic version, I'm hooked on your virtual product.
BTW, are you a professional manufacturer and/or representative of an enterprise (Bent Audio?)? How would you rate the sonic differences between resistor- and transformer-based passive pre-amps in not too critical cases, e.g. DAC2 out 20 ohm, Metaxas Solitaire in 100 kOhm? I use a simple Alps 10 kOhm pot, and it seems to work fine... (after having used a switched attenuator) but I'm interested in a transformer approach for the speaker rig, too. But don't the immense cable lengths in such devices make them sound a bit like long cables? I mean: do you have to choose between cable-induced smearing effects and resistor-induced impedance/capacitance effects (frequency response!)?
(http://www.handykult.de/plaudersmilies.de/nosmile/peacesign.gif) JaZZ
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Jazz,
I do this audio stuff full time (and then some!) and all the products and parts I sell through my company Bent Audio. I also do it for fun. Can't get enough of it. I usually bow out of talking online about how stuff I make sounds. Since most likely everyone but us has left this monster of a thread by now I'll make an exception.
The TX102 does have lots of wire in it. I don't understand why but it does not seem to be any problem at all. You are likely getting good results with the pot you have and resistor stepped attenuators can work quite well. All the work I did on my TAP system originally was with resistor based passives. Where I find the transformer to be ahead of the resistors is in terms of the body of the music. The resistors can sound a bit thin. Always extremely clear and transparent but sometimes lacking body. My first reaction on listenning to the TX102 was that it had a more organic quality to it. It very well could be that this is a distortion but the transformers measure extremely well (Dead Flat to about 100Khz and THD numbers like < 0.005%) so on the bench they are very close to the resistor based units. This is a surprise to many who expect transformers to measure awful! It was a pain for me to change from the resistors I'd spent years with but the transformers have been a lot of fun.
You are I'm sure aware of it but with a 10K pot the low output impedance of your sources is only really there from the amps 'point of view' when the pot is all the way up. As the pot is turned down the series resitance introduced becomes the source impedance the amp see's and it will be very much higher than the source's low output impedance.
I was just sent a link to a short write-up about a new product using the TX102. It is here:
http://www.ultraaudio.com/home.shtml
Caught my eye as he uses the same dac I use and I too find the connection through the transformer better then direct.
I did not spot that you were over there but I think I have a plan for the ASL box. Shoot me an e-mail at john@bentaudio.com and I'll explain....
BTW - Divergent's page has been down for a day or two - some net hosting trouble....
Many Thanks!
John
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John...
...it's highly interesting and a pleasure to talk to you. Instead of answering my question concerning the sonic impact of the long wires the way you did, you could have stated: «The bandwidth of the TX102 is ..., so within the audio band it's perfectly linear, and there's no degradation of sound.» But you seem to know exactly what I'm talking about; I appreciate that a lot.
As to the «more organic» sound of the transformer or the thin sound of resistors, resp.: what do you think is the cause, especially for the latter? When I hook up my HD 600 or my DT 880 to the DAC2's line-out without any resistor in between, I get the same sound as with the potentiometer. Doesn't this mean the transformer adds some euphonic coloration to it? Don't get me wrong: I'm ready to accept this, as long as it sounds better, and I don't expect it to cause such important sound alterations as from amplifiers. But still there's the mystery why the direct connection (be it with speaker amps or headphones) doesn't automatically sound more credible and natural, rather the opposite. Do you think, apart from this consideration, that resistor-based attenuators do something fundamentally wrong to cause this typical thin and incoherent sound, exaggerately said? Or would you agree with the assumption that there's something fundamentally wrong with the signal from high-level output stages, in other words: today's state of digital technique? It's easily imaginable how amps can mask the «digital» aftertaste of digital sources, the same way CDs, heard through a radio tuner, sound nothing similar to the sterile digital sound, due to the masking of the digital artifacts (or maybe the effects from the sharp low-pass cut-off, resp.!) by the produced distortion.
Yes, I'm aware that the 10 kOhm pot increases the actual source impedance seen by the amp, but it should be far beyond any critical value anyway (say 5 kOhm to 100 kOhm), although I know nothing about the impedance curve of my Metaxas nor the capacitance of my cables.
(http://www.handykult.de/plaudersmilies.de/nosmile/peacesign.gif) JaZZ
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Jazz,
The resistors and transformers measure very close so I don't know if one can be said to be particularly more 'accurate' than the other. I love my test gear and believe stuff has to behave pretty well but as everyone knows that does not tell much of the story when it comes to how something sounds. The organic quality may be a form of inaccuracy but it seems to restore something that is missing sometimes with straight resistor passives (at least in my playing around). Even with resistor passives the type of resistor has an effect on the sound. Vishay's for instance are very transparent and clear but could end up a bit edgy if the system had a tendency to be that way. Tantilum films are more forgiving and a lean a bit toward the transformers sound. Which resistor might fit best would be a system / listenning preference kind of thing. I don't think it is that resistors do anything wrong at all - it's just that in most systems I have played with the transformers seem to result in a bit more body to the sound that I like. . This organic quality could also be related to the impedance situation being so different from resistors to transformers.
Your amp will have a flat input impedance (just a resistor) so there would not be an impdance 'curve' like the headphones (and speakers) have.
Thansk!
John
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John...
The organic quality may be a form of inaccuracy but it seems to restore something that is missing sometimes with straight resistor passives...
[/i]...the question is: what is it that is missing and has to be «restored», and where has it gotten lost? Do you think, like me (after my corresponding experiences), that the perfectly accurate signal doesn't sound appealing enough? And why is it so? Of course I don't really expect the final answer from you, but maybe you have an idea...
BTW: «organic quality» sounds quite appealing... even more so than «accuracy»...
(http://www.handykult.de/plaudersmilies.de/nosmile/peacesign.gif) JaZZ
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JaZZ,
These are tough questions! As I mentioned I really like stuff to behave pretty well on the test bench but I find I am less and less concerned with have the most 'accurate' sound. I don't know what exactly is at work with the parts of the system but everyone (well - almost everyone) would agree that the goal is to have the sum of the be a pleasure to listen to! I have heard truly great systems with resistor passive pre-amps and I also am like'n what the transformers do in many systems so I don't think it's a 'contest' between them so much as picking what best fits your system and your listening preferences. In many systems there seems to be a slight thinness and a like of rythmic drive that the transformers (or a really nice tube line stage) seems to restore. If the system has that body and drive in spades then a passive resistor pre-amp (matched to a source and amp that fit the needed impedances) can do a good job I think.
Going back to the headphones we might find that the transformer behaves the same way in that enviroment - giving back a bit of the body and drive that folks like in their active headphone amps. This would be the only case for taking that route over the resistor based units you have built. The transformer passive headphone arrangement will place some restrictions on which sources and which headphones will best suit this arrangement but should be much more forgiving than the resistor passive circuit. This is similar to the transformer pre-amp which behaves from an electrical point of view very much like an active pre-amp - having a high input impedance and a low output impedance.
Thanks!
John
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John...
....I find I am less and less concerned with have the most 'accurate' sound.
Does this mean you're gonna built active amps in the near future? :) But seriously: a certain interest in what's real and what's fake you surely have, no? I have to confess that I even prefer headphone amps to the direct drive, just because the missing «organic» quality is restored with them. But that doesn't keep me from being curious about what's wrong with the pure original signal. You have no idea?
Going back to the headphones we might find that the transformer behaves the same way in that enviroment - giving back a bit of the body and drive that folks like in their active headphone amps.
I like your wording! It's true, «folks» do want some meat on the bones...:lol:, me included. But again: how come that it's missing in the original signal (to a certain degree)?
I have managed to compensate for the dryness «caused» by my passive pre-amp by fine-tuning my speaker's crossover network – something that usually can't be done with headphones. Interestingly I don't have to fine-tune them to my tuner or turntable sound; those are much less critical in this regard.
(http://www.handykult.de/plaudersmilies.de/nosmile/peacesign.gif) JaZZ
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JaZZ,
I have no plans to build any active pre-amps! Nothing against them as some are very good - just too busy with all the passive stuff on the go!
I still don't think with the passive resistor pre-amp there is anything 'wrong' with the signal as such. I think the situation is different in different systems and the fact that sometimes they work very well follows from this. The only things that I can think of causing this tendency toward thinness with them is that:
1- Impedance issues are causing it. The large series impedance (compared to transformers or active pre-amps) could be an issue some of the time. These series impedances are not helping in any stage of the signal path and particularly where they are followed by cables and an input stage of the amp to drive. This, however, is not what is at work all the time because a direct connection from a source with a built in level control (and presumably a buffered output with constant and lowish impedance) can still sometimes display this tendency - although not to the same degree in my experience.
2- Some systems have this 'sound' and so choosing a component that adds a bit of body or organic quality (like an active curcuit with tubes or a transformer passive) helps to ballance things out and results in a more real sounding and fun to listen to system. The same could be said for going with a tube amp vs. a solid state amp and there are many fans of both - neither is really right or wrong. Which fits best depends on the tendency of the rest of the system and, most important, the system owner's preference.
3- A combination of #1 and #2 - likely to be the case sometimes (may much of the time??) as well.
Many Thanks!
John
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Hi, it is me.
I am confused. I still listen to my headphones passively but I am confused to whether or not it is better (objectively).
So basically, what conditions must be met inorder for a passive headamp to be better than an active one? How does one make sure it is better?
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Hello!
Welcome back with us! Much of this thread has talked about how we can not really predict if a passive headphone level control will be better than an active one. Our next step in this is to build some stuff and play with some stuff and see how it is working. JaZZ is ahead of us as he has already been experimenting with this stuff!! If it turns out we prove out that a passive control can be made to work well it does not mean at all that it will be better than an active one in all cases. For one thing you can be sure that some setups and some listeners will prefer active headphone amps - For them the active units will do what it takes to make them happy with the sound. Also we can pretty much predict that for high output impedance sources (almost certain) OR low impedance headphones (maybe) that an active circuit would be prefered. That likely leaves a small group of users that the passive solution might be best for.
I think there is not a way to analyze it and come to a conclusion. It also seems that there are a lot of opinions but little real experiments happening so far. For yourself I'd suggest getting a modest active unit (used) or maybe scooping one of these little ASL passive transformer headphone units (called "Reference Tools"). Then you'll have some stuff to listen to and play with and compare to the passive unit you have now. If our testing develops into something we think is working really well then if your system suits the passive system then we can get one down for you to play with. The only way to 'make sure' it is better for you is to experiment with some of the alternates and see which works best. in your setup
Sorry for all the confusion but we are just at the front end of this thing and don't have a lot of answers yet!
Many Thanks!
John
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Has the impedence problem been solved?
How do I build something of the like?
Have you come to any sort of conclusion to whether or not a transformer is better than a resistor?
Thank you.
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Hello!
These are easier questions! They are kind of related but I'll take each one at a time.
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>Has the impedence problem been solved?
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Sort of... Given high (or highish) impedance headphones and a lowish impedance source (say 200 ohms or less) we can with a transformer (or autoformer) get impedance ratio's that are comparable to what many amp/speaker ratio's are like (for things like triode amps and OTL's). Since many folks including myself have had good results with these kind of ratio'd in-room I'd think that they'd work pretty well for headphones.
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>Have you come to any sort of conclusion to whether or not a transformer is better than a resistor?
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From an impedance point of view this is a 'no brainer'. The transformer solution killls the resistor and this is why we even bother considdering the Transformer/autoformer solution. Once we step down a few ticks from the top level the output impedance goes down very fast. This impdeance does not go down in a 'linear'way because it's related to the sqaure of the turns ratio on the transformer. The table at the S site referenced earlier in the thread shows this impedance for a typical pre-amp application so the absolute numbers in the table don't fit what we'd do for headphones. The rate they drop off would be the same though.
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>How do I build something of the like?
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A bit tricky to home brew right now - since no-one I know of makes transformers or autoformers for just this application. It could be that a speaker level autoformer (radioshack?) might be a way to play with it and I'll check the next time I am near one. I don't know if these things would have the inductance to give an easy to drive load to the source though... If you want to play with something along this line soon get an ASL box. For $150.00 or so you could not buy decent parts to home brew a similar unit.
Whatever Bent Audio might have to offer will come after months of testing and only if we think we have come up with something that does something better than the other options out there (active or passive).
Many Thanks!
John Chapman
www.bentaudio.com
[/quote]
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Hi... and welcome back, Czilla!
(Czilla9000) «Have you come to any sort of conclusion to whether or not a transformer is better than a resistor?»
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(John Chapman) «From an impedance point of view this is a 'no brainer'. The transformer solution kills the resistor and this is why we even bother considering the resistor/autoformer solution...»
I guess this applys primarily to the attenuators used instead of preamps. IMO, for headphone direct-path application you don't have to apply the same rigid measure. Even a 100- or 150-ohm line out can provide a sufficiently appropriate source for direct drive – apart from the reduced sensitivity. You may know that transformerless tube amps have output impedances around 100 ohm, and they work well, at least with high-impedance cans. My Earmax Pro even is designed to drive low-impedance headphones as well.
Nevertheless, the high serial resistance has in fact a certain impact on the frequency response, though, dependent on the headphone's impedance curve; but it's all but dramatic – otherwise you had to remove OTL tube amps from the list of usable amps. There are even certain headphones which benefit from high output impedances and are recommended to be used with 120 ohm serial resistance (Beyer DT 931, 831), and BTW 120 ohm are an official norm for headphone amps (don't ask me where this is codified).
Futhermore keep in mind that an additional voltage-divider attenuator in the signal path on the one hand indeed decreases the load impedance «seen» by the line-out amp, but on the other hand also causes a considerable smoothing of the impedance curve – minimizing the corresponding coloration potential. So in reality the problems with the resistance approach are far less important than they appear initially.
So the main issue is its (overrated) frequency-response twisting potential. But such a behavior is something that can't clearly be rated as a sound degradation in the context of the whole system – it may just as well be in favor of the final frequency response provided by the headphone. This in contrast to amps: the signal degradation they cause has more to do with harmonic distortion and transient-response corruption – and is irreversible by nature.
So from this point of view, John, with reference to your statement that the transformer approach is the more forgiving and possibly less accurate variant, its theoretical advantage over the resistor approach in terms of the impedance proportions has to be put into perspective.
That said, like you I'm mostly interested in a credible and natural presentation and less in a theoretically correct sound. So if the transformer provides the «better» result, I'll favor it over my potentiometer solution. And I would be glad if it could combine the euphonic («organic») qualities of amps with the direct path's transparency and transient speed.
After all there's just one question left. You answered that «...there's nothing wrong with the signal as such in a passive preamp». Whereas my question actually was: «What's wrong with the original signal that it needs some subtle modification to restore its "organic quality"?» Well, that's my standpoint after my experiences with different sources and headphones as well as from different preampless speaker rigs. And this mystery – why a degraded signal sounds more appealing and natural than the accurate one – is my main motive for my occupation with the direct path. Could the digital technique bear the blame? With analog sources I never had the same impression.
(http://www.handykult.de/plaudersmilies.de/nosmile/peacesign.gif) JaZZ
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Hello!
I had a typo in that quote and have changed the original post - It should read "Transformer/autoformer" instead of "resistor/autoformer".
Good point with the OTL amps - that example and the SE amp stuff is why I thought the tranformer might be able to get impedance down to acceptable levels. One of main diferences between active and passive solutions to this is the impedance issue and the fact that transformers have an impedance behavior more in-line with active headphone amps make me have some hope for this approach meeting some success and possibly acceptance. For example when we are driving a pair of headphones from a 2V source with a 200 ohm output impedance (typical CD player or DAC) the output impedance at a 0.5V listening level would be in the order of 15 ohms or so I'd estimate.
I am afraid I can not anwer to the question of what might be missing or 'wrong' with the original signal - other than I don't think it really can be talked of in terms or right and wrong only in terms of what fits a particular system/listener and what does not. I think maybe a good way to think about it is if you hooked up a system without the pre-amp and listen to it that'd be it's sound. Then from that sound you can choose a level control (pre-amp) that best fits what's needed to ballance out the sound. Some will need a dynamic kick and some, for instance, might need some of this organic stuff....
Many Thanks!
John Chapman
www.bentaudio.com
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John...
...thanks for your comments so far! See you later...
(http://www.handykult.de/plaudersmilies.de/nosmile/peacesign.gif) JaZZ