Bent Audio's John Chapman comments on direct connection.

[Czilla9000]

My quest to solve the riddle of the direct connection lead me all over the globe .... and eventually ended at the deep depths of "Harmonic Discord" in the "Bent Audio" forum. In there I found the likes of Bent Audio's John Chapman, the leader (I think he is) of a company which solely builds/sales passive pre-amps and passive DIY parts (including the TX102 passive volume transformer). He was very enthusiastic about the idea, despite the eventually conclusion.


I perposed the idea of direct connection in this thread:

http://www.harmonicdiscord.com/forum...?p=82747#82747


I asked permission to reproduce the thread here. John is also going to reproduce the thread at audiocircle.com. He wants me to warn you however that he is not 'a headphone guy'.


Quote:

Czilla9000: 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. john chapman: 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 Czilla9000: 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. john chapman: 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 Czilla9000: 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. john chapman: 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/amp..._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 john chapman: Hello! Moving too fast again. Lots of typo's in the post above and I missed the 'edit window'. Sorry guys.... Thanks! John Czilla9000: 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. john chapman: 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 Czilla9000: Thank you for all of your help. So basically it is impractical and worse than using an active headamp? Correct? john chapman: 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 chapman: Hello! czilla9000 - This is a cool thread and I'd like to copy it over to the new forum at audiocircle.com - would that be o.k. with you? Thanks! John Czilla9000: 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. Oh....and yes you can copy it over. I assume that this thread has reached its climax. I will now post it on the board I asked you to allow me to post it on as well.

The thread may continue, but the conclusion so far, from John's best efforts and according to what S&B says about it, passive attenuation is impractical. MacDef and crew were probably right.

 

[markl]

I also want to point out as a footnote that Sennheiser themselves recommend that even the high-impedence HD600s be powered by an amp with as close to 0 ohm output as possible for best results. So if we are to believe the manufacturer, they also would not recommend direct path for their cans either (unless you happen to have a CDP with a 0 ohm output which as we see above may exist but is probably very very rare).

Somehow, I still don't think this will sway the true believers in direct connection, and probably won't stop the weeekly posting of new "wonders of direct path" threads.

 

[dhwilkin]

Interesting discussion, the link on impedence was also helpful.

 

[MacDEF]

Interesting exchange; thanks!

 

[Dan H]

I’ve met John two or three times through the local Bottlehead groups. He is a very nice guy as you can see from his posts. I’ve heard his preamps with 300B SET amps and other higher power tube amps. They sound great! He also sells some pre-amps in kit form.

 

[minya]

Hahaha. You lose.

- Chris

 

[Budrew]

Certainly you need something between the source and headphones just as you need something between a source and speakers. (Soemthing more than just attenuation.)

 

[Czilla9000]

Quote:

Originally posted by minya Hahaha. You lose. - Chris

At least I admitted it. I could of hid this conversation if I chose to.

 

[dd3mon]

Quote:

Originally posted by Czilla9000 At least I admitted it. I could of hid this conversation if I chose to.

That means you're not a sore loser

Good work, I'm glad someone finally took an intelligent approach to investigating this issue. Now we know.

My take on this is, that the best headphone experience would be one driven by a source that was also a headamp (truly eliminate the double amplification problem). A fully balanced source with a 0ohm, extremely hot output stage and transformer attenuated signal would be nothing short of spectacular

-dd3mon

 

[Czilla9000]

Anyway...cheers

 

[acs236]

Thanks for posting that thread, it was quite informative and interesting.

 

[DanG]

Great discussion, thanks for posting it.

 

[Solude]

Quote:

Originally posted by minya Hahaha. You lose. - Chris

Hmm although I agree with the above thread it doesn't change that some sources are that 'damn hard to find, built to headphone req' source. The M-Audio Revo is one of them. If I had to guess I'd say that most headphone jacks on pro-sumer equipment was up to the task.

But yes it takes just the right source to make the direct connection work well. Enough about this though. Someone go try the ASL HB1 buffer already

Interested to see how it works for those not looking for ear bleeding levels from their amp.

 

[JaZZ]

You have to take into consideration that John Chapman hasn't tried what he's talking about; he even doesn't state the direct path won't work with headphones, he's just considering theoretical objections which are justified by logics: The requirement of low output impedances and if possible high load impedances was conceded from the beginning of the direct-path discussion. As is the requirement that the concerned line-out stages are able to supply enough current. He even admits that he doesn't know if that's the case in the individual configurations.

In turn I would rather rely on somebody who has tried if it works. I have, and it really works in my case - with 6 ohm and 20 ohm output impedances and 250-300 ohm loads. It even works with 200 ohm output impedance - as to the sonic result! -, but with restricted volume, and it works with Grados because of their flat impedance curves, but - again - with considerable losses with power.

I have showed that the complex load and its low impedance doesn't cause any clear sound change (by alternatingly switching a direct-path system composed of a potentiometer and a headphone onto the line out in parallel to the monitoring amp), thus any serious signal degradation from the line-out amp's side. So there's absolutely no reason to state that the direct-path idea has died. But admittedly the transformer variant is the theoretically superior one as to high output impedances - and potential damage due to the high currents (which so far is not more than a possiblity in view of the lacking experience). That doesn't necessarily mean it's the more accurate one in each case.

Cilla, pity you base your new opinion on just one person's opinion which is of just theoretical nature. (And this applies to all the people who are willing to ignore serious testings.)

 

[taoster]

most interesting!

thanks Czilla9000, go JaZZ!