AudioCircle
Audio/Video Gear and Systems => Owner's Circles => Bent Audio Owners => Topic started by: John Chapman on 5 Apr 2013, 11:05 pm
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Hello!
This has come up on and off over the years. Finally a year or so back I had a customer who was driving headphones right off the autoformers (with no headphone amp) and loved it so I did some testing here. Here is the setup
Source ---> Autoformer Pre-amp ---> Headphones.
Nice and clean. There are a couple of rules of thumb that I discovered in testing.
1- To drive a low impedance like headphones you need to be running with the volume down somewhat from the top level. I found once you were under 40 or so it did a really good job. This was with a 600 ohm source setting on the analyser. With a lower source impedance you could go higher.
2- Related to '1' - Ultimately the source is driving the headphones. It gets a big help from the impedance lowering affect as you turn down the levels but a really 'delicate' low voltage or high output impedance source would not be ideal.
I have tested with some Sennheisers that are about 300 to 600 ohms impedance and some Fostex phones that are very low impedance - rated at 25 ohms. It was remarkable how well it did with the really low impedance. What saved the day there though was that they are more sensitive and therefore volume levels were no higher than the mid 30's on the pre-amp - lower is better for driving low impedances. There will be combo's that are not ideal but it seems it might work well in many situations. Don't bother hooking up your K-1000 phones for instance!
I've not compared this to a top flight heaphone amp - don't have a headphone amp of any kind here..... Still given it is a freebie of sorts it really did sound very good and the bench tests on the gear showed things were behaving when I simulated the impedances and level settings.
Just wanted to throw this out there in case others wanted to experiment with it. I used a little dual (male) RCA to 1/4" (female) phone jack adapter cable. I keep meaning to make up a better quality adapter but have yet to get to it......
If you do play with this please do report back.
Thansk!
John
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This make sense as many headphones amps also are advert as preamps.
I would like to test a headphone direct at the CD player, but I yet had to find some P2/RCA adapter plugs.
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Hello!
It's true that an active pre-amp stage is often able to drive headphones (and that a headphone amp circuit might also make a fine active pre-amp stage) but for a passive device to pull it off is not so obvious. The autoformers do 'look' a bit like an active stage from an impedance point of view - with a high input impedance and a low output impedance (at typical listening levels). Before my testing I thought maybe the higher impedance phones would be attainable but it was surprising how well it worked even driving the low impedance Fostex TH-900 phones.
Driving straight from the CD player (I'm guessing with a built in level control) will depend on it's output impedance and the phones impedance. Feel free to post details and I'll take a stab at how it would work out. Note that with the autoformer pre-amp in-line the driving impedance gets better and better (than the sources original impedance) as you turn down the level. For instance a combo of the 600 ohm source output impedance and the 25 ohm headphones would not work at all without the help of the autoformers kicking that source's impedance down.
Thansk!
John
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Thanks for remember this impedance details. A CD player usually had a hi impedance output, what is a prob.
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Hello!
The output impedance value for different CD players (or any other source) will vary a lot so don't give up on going direct! Look for a value of 100 ohms or lower (the lower the better in general) if your phones are 600 ohms. Note that this is for direct drive from CD player to the headphones without any help from autoformers in-line. The autoformers at typical settings here in my system lower that driving impedance a lot which lets you get away with higher source impedances and also lower impedance phones.
The efficiency of the headphones play a big role too - if it takes a high voltage to drive the phones then you'll tend to have the level up higher not getting that impedance lowering 'kick' as much. The Sennheiser's are higher impedance but less efficient than the low impedance Fostex headphones here so it kinda works out. For the fostex the volume level is typically set lower on the autoformer which gives a better driving impedance to cope with the Fostex's low impedance. For the Sennheisers the volume is higher to get the same listening level but fortunately the higher impedance of those phones is less demanding of the feeding source's impedance so the high levels work out ok impedance wise. I chose both these headphones specifically because they were opposite in those respects. I can picture situations involving low impedance headphones which are not efficient making a bad combo that might not work out - you need to turn the level way up so you no longer have the help of the autoformers impedance wise while at the same time turning levels down to get that impedance nice and low would result in volume levels lower than you want to listen at..... Still from my testing so far lots of combinations would indeed work out rather well.
Thansk!
John
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Hi there,
I think I am the customer you are referring to John. Yes, I have driven headphones very successfully with the TAP-X.
For context, I use a Metric Halo ULN-8 DAC. It's output impedance is 5 ohms. I have driven headphones directly from the DAC's output, through the headphone output of the DAC (which adds a driver stage to the regular 5-ohm output), and through the TAP-X connected to the DAC. I achieved the best results through the TAP-X. Clarity, space around the notes, and low-end drive all improved. I was driving a pair of Ultrasone Edition 9 headphones, whose impedance is somewhere in the 30s (from memory).
I also drove these headphones through a Yamamoto HA-02 headphone amp, and I also preferred the TAP-x. The TAP-X was transparent as water, and the Yamamoto clearly voiced and less detailed.
Finally, I also used the TAP-X to drive headphones with a pre-amp. I used a Coincident Statement Line Stage and an LFD Mistral. Both worked great. The TAP-X interfaced perfectly between the pre-amps and the headphones. This time I was using a higher impedance headphone of 250-ohms. I no longer had the Ultrasone ED 9s.
The point is that, as John stated, there is no reason why a pre-amp cannot drive headphones. I never liked the idea that virtually all pre-amps in the market that offer both functions do not offer the same sound for each of these functions. One must usually choose between a better pre-amp with a good headphone amp, or a great headphone amp with a decent preamp functionality.
In addition, there are plenty of great preamp topologies that have been developed over time, but the burgeoning headphone amp market was offering new solutions, less tested by time. I did not see the need to compromise in this way. And the TAP-X provides a great solution to this dilemma.
It reminds me of the Speltz autoformers applied to low-end signals.
I told John at the time that I was surprised no one was using his autoformers for headphone amps. Use them on their own if you like your source and want to hear it in all its glory. If you want more tube goodness, put them in the signal path with the autoformers for a killer combo.
I just saw on hifi heroin that Dave Slagle has created a headphone interface using autoformers. Hopefully the idea will gain more ground. I've also seen on the Experience Music website that he has developed some custom headphone amps. Wonder if he incorporated autoformers in them.
Lastly, a word about John Chapman. I recently damaged my TAP-X while transporting it, and John, as always, was very generous with his time and components helping me repair the damage I had done. He is a pleasure to deal with, and deserves all the best. (John, it's me Paul, from California :)
Thanks!
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Hey guys, great idea! I never thought of trying this with my TVC. Maybe I can whip up a cable this week and give it a try. I'm not sure if a TVC will behave the same as an autoformer, but I should give it a shot anyway and see what happens. Glad I saw this post. Thanks. :thumb:
One must usually choose between a better pre-amp with a good headphone amp, or a great headphone amp with a decent preamp functionality.
What I do with my TVC is connect the single ended output jacks to the speaker amps, and the balanced XLRs out to the headphone amp. It works fine connected to both amplifiers at the same time since I am always in the friendly output impedance zone of the TVC (more than 24dB of attenuation). Probably not the ideal purest situation with two amplifier loads on the output of one TVC, but it sounds fine to me. Gonna try it passive and see what happens. Hope I can find the time this week.
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Portos and QE,
Thanks for your input fellas. QE, let us know what headphones you are playing with.
John,
I think it is plainly obvious how much value is integrated into even a fully balanced TAP-X. Not only is it probably one of the finest passive preamps (with none of the attendant disadvantages of a traditional passive pre), it also has an extremely slick interface. If you have a true balanced source with balanced circuitry inside, it's a no brainier. If you have headphones, the likelihood is high that you can work it well, and even better from a balanced source with balanced headphone connectors.
So a handsome pre with headphone driving capability as well as an option to have a 'little gain' for those fickle situations and configurations?
A reference for all others to be judged!
Best,
Anand.
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Hello!
I tried and tried to recall just who that was that had done that testing a few years back - I get so mixed up with forum names and e-mail names and occasionally even real names..... Or maybe (more likely) it is just my bad memory. Thanks very much for the additional info Paul and I am glad the repairs went well.
For quiet earth - can't hurt to try with your TVC. If you want to post your source and headphone info I can see if any obvious roadblocks are in the way.
And for Anand - thanks for the kind words. I have yet to try it running balanced to my headphones but intend on making up some cables to test that. Only one of my two pairs of headphones has the ability to wire balanced - the Fostex would require a bit of a rebuild to get balanced cables on them....
Thansk!
John
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Well, I gave it a quick try yesterday with a cable and adapter that I already have on hand. I listened to my old Grado RS-2 open back headphones, and newer Beyer Dynamic DT 770s using my CD transport and DAC as a source. The headphones were plugged into the single ended RCA outputs of the TVC. The Grados are fairly efficient headphones at 98dB and 30 ohms. They have a steep bass roll off below 100Hz. They also have a very clear and forward sound. The Beyers are closed back headphones at 250 ohms, and less efficient than the Grados. The Beyers have much deeper bass than the Grados but overall they sound less refined and lower fi due to their plastic body and lower price point. I still like them though. I use them when I need sound isolation from the family. I'm not really a headphone guy,,, I much prefer my SET amps and speakers. I use headphones when I don't want to bother anyone. Here's what I got out of my experiment though:
Neither headphone could get very loud driven by the TVC, but certainly loud enough for normal listening. The Beyers sounded lifeless and anemic with none of the deep bass they normally have. The more I tried to turn them up, the less volume I actually got out of them. In fact, they were distorting in the low end unless they were kept relatively quiet. The Grados on the other hand sounded pretty good. They didn't have that forward sound that they normally posses, and they actually sounded more refined and sophisticated than usual. There was no deep bass but there never is any with these phones anyway. Again, cranking up the volume control only gave me a small increase in volume. I guess my TVC doesn't like that 30 ohm load . . . . :wink:
Oh yeah, I did try it with the +6 dB switch egaged too.
This was a fun experiment for me and it gave me a better appreciation and understanding for what's going on here. I have no regrets that it didn't work out since I still enjoy my TVC very much using it as it was intended. I do want to try an autoformer volume control some day though. Not because it might drive my headphones, but because I would just like to hear what it does differently than my TVC. ( I actually have both Stevens and Billington and Silk TVCs.)
Thank you guys for posting this thread! I am learning a little bit all the time. I love this hobby!
:D
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Hello!
Thanks for giving it a go. The fact you had trouble raising the volume means it was indeed struggling. No sign of that here with either headphones I play with. It seems either the sources output impedance is higher than needed or the TVC's are just not happy with the loads. I never did give it a go with the S&B units - wish I had now. If you could let me know your DAC model I'll have a peek at that impedance number. Also if you could report what levels you were running at that would be interesting.
Thansk!
John
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I'm not sure about the volume levels measured with a decibel meter if that's what you mean. I don't think I could get an accurate measurement anyway, since you really need to have the headphone in place on your head to get an apples/apples comparison. Let's just say it was clearly and obviously not as loud and controlled as when driven by a dedicated headphone amp.
I used the single ended outputs of this DAC :
http://www.audionote.co.uk/products/digital/dac_3.1x_bal_01.shtml (http://www.audionote.co.uk/products/digital/dac_3.1x_bal_01.shtml)
I think the output impedance is 600ohms using either the balanced or single ended taps. I suppose I could have used the balanced outs of the DAC because that output is much stronger (12dB). But I was just doing this for fun and didn't really want to make it too complicated by rewiring everything. Maybe I should go back and try and also check phono and tape. Maybe later. FWIW, the balanced output of my DAC almost sounds a little too strong (congested or pinched) in my set up. I think a TVC starts to overload/saturate around 7 volts input..... :scratch: I don't remember. Anyway, the single ended outputs sound much better and plenty strong.
Perhaps the autoformer is the way to go if you want to do headphones amp-less. Maybe other people will give it a shot and report back. I think if I had more efficient sealed headphones, or only listened to the Grados it would be OK for quiet listening.
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Hello!
Coincidentally when I did my initial testing on the bench here I used an output impedance of 20 ohms and 600 ohms. Wth the 600 ohm source you needed to be down at about -15 db or lower to get it to have enough drive - even for the higher imoedance headphones. Ideally you'd want a source with a bit lower output impedance for sure.
I should have been clearer when asking about levels - I was wondering how far down from the top volume setting you were listening at? To get that impedance kick ( certainly enough to drive headphones ) you need to be down at least a handful of steps from the top. I am guessing you were indeed down quite a bit.
As a side note I'd expect that not using +6db mode would be the way to go. I your case likely it was struggling enough that it might have been hard to tell.
From your description I think the culprit was mostly the sources output impedance being higher than needed - just not enough grunt to pull it off.
Thanks again very much for posting the test results.
John
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QE,
Not sure about using a dac with a 600 ohm output impedance with 300 ohm and 30 ohm cans. Even though you are using a TVC, one should still stick with a source of lower impedance. Although a 10:1 ratio is ideal, I've still had great luck with even 3:1. For your 300 ohm cans that would be a dac with 100 ohm output impedance and 10 ohms for your 30 ohm cans. Whether you are using TVC's or AVC's shouldn't change this too much I think.
My dac has a 42 ohm output impedance on the balanced outs, and 21 ohms SE...
Best,
Anand.
PS: in other words, you may not have enough grunt from your higher output impedance source...
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Hello!
The impedance lowering action of the autoformers ( or tvcs ) does act as a big helper - but only as you turn the levels down. The output impedance of the preamp will be way way lower than the sources output impedance once you are down quite a bit from the top settings. Its that 'post preamp' output impeance that is driving the phones. You are right though that even with the helper I line ultimately the sources output impedance is a factor.
As a side note you should be in good shape to play with that nice low output impedance of your dac.
Thanks!
John
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And another thing that John touched on. The +6dB switch is a double edges sword. Yes you get more gain, but if the rules for regular transformer coupling apply: When you double the voltage ratio (2X or 6dB) you will QUADRUPLE the impedance ratio. Suddenly your low output impedance dac isn't so, and it's even worse at higher volume levels as the impedance lowering ability of the TVC or AVC is minimized.
That's why whether you are playing with headphones or using the TAP-X in a regular 2 channel speaker system you must have some excess gain, either on the amp side or source side or with highly efficient speakers, etc...
Yes, my dac should do fine, and on the output end we have 4V RMS balanced and 2V RMS single ended.
It's all about compatibility...
Best,
Anand.
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Wth the 600 ohm source you needed to be down at about -15 db or lower to get it to have enough drive
Thanks Guys. Yes, this is always the case with a TVC. I remember that -14dB is the the most amount of attenuation you would ever want to use with the older Stevens and Billington transformers. And that is with the gain switch set to 0dB as you guys have correctly pointed out. I would imagine that all of the other brands of TVCs have a similar friendly impedance zone due to the nature of the beast.
So yeah, I used the gain up +6dB with lots of attenuation (-34 to -24) and it sounded better than 0dB gain at -28 to -18 dB of attenuation (driving headphones). It all fell apart near the -18dB mark as would be expected.
FWIW, I can never play my DAC louder than the -22dB attenuation mark while using a 8 watt SET and a pair of speakers. And that is with the gain switch at 0dB. It would be crazy loud unless it is a very old '80s disc where they did not record peak levels above -20dBu. (Remember those old discs? Yes, I still enjoy them.) So, I guess what I also learned is that my DAC does very well driving what it was built to drive - a high impedance line stage of some sort (passive or active). I have no problem with that since that is what I bought it for.
But this thread is about driving headphones from a source and autoformer. That's quite different than driving a high gain tube amp with 100K+ input impedance. Hopefully this will be a helpful data point for those that are interested in doing that.
Cheers guys! Thanks for the fun. :thumb:
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I want to see about reviving this thread because I am currently deciding between an AVC or an LDR volume control. What could tip me over to the side of an AVC is the option of using headphones. I read through the thread and am beginning to understand. I thought I could reach out and ask if this source and headphones combo would work with an AVC headphone amp:
Source: Schiit Bifrost 2v at 70ohms
Headphones: Fostex t50rp at 92db at 50ohms
I would be buying the Slagle autoformers that have 42db of attenuation in 23 positions
Any help would be awesome!!
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Regardless of what the TVC or AVC are doing, you cannot violate the laws of physics and power in = power out. What you are doing here is effectively presenting a varying load to the source which is trying to pump out the required current to achieve the desired volume (power) output. If your source can't provide that much current, then it's not gonna work.
That all said, I have not tried this particular combination so can't answer you directly.
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Wrong post. Whoops.
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I have done some more reading and realized that this combination will not work. I need something with high impedance and sensitivity in regards to headphones and low impedance in my source. I looked and realized that the Sennheiser HD600/650 or that Beyerdynamic DT-880/T1 are the best candidates (although I am concerned about the Beyer's 96db sensitivity).
I will have to contact Dave at Intact to double check. I just sold my preamp, so I am ready to take the leap.
The information contributed is much appreciated.
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You'd be better of getting a headphone amp with no volume control and feeding that into the autoformer. Preamps just aren't string enough.
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So I have tried two sets of headphones with a newly made AVC module and I am not sure if there is a wiring issue, but both sound terrible. Fostex tr-50 and a Beyerdynamic dt880 were the two headphones used and they both sound similarly bad. Bad in the sense that the vocals were recessed and the overall sounds had the cupped hands delivery. One peculiar thing that makes me think that it might be a wiring mishap, is that the volume is balanced and improved when one channel is turned up and the other is turned down? I can't get my head around that because when playing through speakers the AVC sounds fine. Go figure?
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Can you post a schematic of how you have it wired up? Your description almost sounds like your ground connection is broken.
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Here's the best way I could draw the schematic:
(http://www.audiocircle.com/image.php?id=161574)
I only have the first input wired to the avc's because I could not get the inputs to wire up to the Grayhill rotary switch I have installed.
The outputs from the avc's are wired to a switched headphone jack before going to the outputs. Like I wrote earlier, when the headphones are not inserted and I run the avc to the tube amp everything works fine.
Here are some photos of the case I made and some internal shots:
(http://www.audiocircle.com/image.php?id=161575)
(http://www.audiocircle.com/image.php?id=161576)
(http://www.audiocircle.com/image.php?id=161579)
(http://www.audiocircle.com/image.php?id=161577)
(http://www.audiocircle.com/image.php?id=161578)
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Um... you need ground on your headphone jack. Otherwise you're effectively bridging the L/R channels through the sleeve of the headphone TRS plug and what you're hearing is echo because your main signals have effectively cancelled out (which is also why turning up only one channel sounds louder, because then you're only listening to one side-ish and they aren't cancelling out as much)
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Where is the best place to ground the headphone jack? Do I ground from both avc modules to the one ground on the jack?
Sorry for the simple questions, I am new to this and thanks so much for the help!!!
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Um... you need ground on your headphone jack. Otherwise you're effectively bridging the L/R channels through the sleeve of the headphone TRS plug and what you're hearing is echo because your main signals have effectively cancelled out (which is also why turning up only one channel sounds louder, because then you're only listening to one side-ish and they aren't cancelling out as much)
I wired up ground connections from the avc's to the headphone jack, from the headphone jack to the outputs and that has corrected the problem. MAJOR HATS OFF TO YOU ARMAEGIS!!!!!
Now, I'm going to evaluate the headphones for their sonic qualities. The Fostex can be driven by the avc even though it is 92db at 50ohms. It is only has a little less gain than the Beyers with 96db at 600ohms. The fostex sounds muffled, but I think that is the nature of the headphone rather than the reaction to the avc.
Anyone have a good experience with any other headphones?
Thanks again for your help!
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So, I got the selector switch to work. The only thing is I'm getting some bleeding from other inputs if two sources are active, but that is ok for now.
I didn't mean to hijack this thread, but if there is anything I can contribute to this thread related to bent modules and headphones is that it should work. After fixing my wiring issues, with the help of armaegis, I got both pairs of headphones: the Beyerdynamic dt880 and the Fostex trp50mkiii, to work. I realized why the Fostex sounded so muffled and inefficient compared to the Beyerdynamic, which were these custom earpads I put on distanced my ears too far from the drivers which caused them to sound less loud and clear and I think the padding caused some muffledness. After swapping the pads for the originals they played as loud as the Beyers.
Am I not sure about sensitivity and impedance with autoformers now? The Fostex are 92db at 50ohms, but they play as loud as the Beyerdynamics dt880 that are 96db at 600ohms which are, on paper, a better match. This somewhat mimics John Chapman's experience. Is impedance irrelevant with autoformers and all you needs is a baseline sensitivity? I'll post more findings soon.
In summary, autoformers work with headphones. You need 2 volts and a source under 100 ohms for sure, but the headphone match is still a mystery.
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You need to doublecheck those sensitivity specs on the headphones thought. Usually they are rated at 1mW, but sometimes they are rated at 1V input. The distance at which they measure this figure is also not standardized (and is just lumped in at where they dummy head ear or measurement mic is located) so it's a bit of a crapshoot anyway you look at it.
Regardless of all that, the impedance is in theory irrelevant if the sensitivity rating is given for a specific power at a specific distance.