OTL or not OTL (That's the question)
Aug 14, 2009 at 1:27 PM Thread Starter Post #1 of 26

Denys

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This should be my last question before I made my choice for an headphone amp.

So far I learned that with an OTL amp you get the sound directly from the tube, therefore it should sound more natural. It is also made for headphones with high impedance like the Senn that I use.

However, as you get the sound directly from the tube, I read that it might also damage the headphone should the tube "burns".

So, should I consider more an amp with transformer and sacrifice the sound quality or still go with an OTL amp ?

By the way, if I was wrong with what I just mentioned, please let me know. This will help me in choosing between the WA2 or the WA6SE..

Thanks for your time..

Denys
 
Aug 14, 2009 at 1:47 PM Post #2 of 26

tintin47

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Not much of that is right. If you use a good trafo, there is no degredation in SQ. The point of using a transformer coupled to the output is to lower the output impedance for better impedance matching with the load, be it speakers or phones. You use a transformer to avoid the problems of impedance matching, such as the midbass boost that you get and a loss of power transfer.

Also, what do you mean by sounds more natural? That doesn't make much sense. Transformers do impart a sound to music, but certainly no more than tubes IMO.

If you are using high Z phones, then buy either an OTL or trafo coupled amp. Don't worry about it. They should both sound good, so just turn to reviews or personal experience to make the choice. As a general rule, you really only need to worry about output impedance and transformers if you are using low Z phones, and your post makes it seem as though you are not.
 
Aug 14, 2009 at 3:33 PM Post #3 of 26

Uncle Erik

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No, you do not get sound directly off a tube with an OTL.

The sound typically goes from the tube through an output capacitor before reaching the headphone jack.

So you're either going to be listening to your sound through a capacitor or through a transformer. Both capacitors and transformers can be excellent, but I've been tending more towards the transformer camp these days.

Oh, and OTL amps can have DC offset, which will cook your headphones. Most well designed and well built amps will not have this problem.

The reason OTLs are more prominent is solely an economic one. A quality pair of output transformers begins around $200 and goes up from there. That's about the cost to manufacturers - you can find quantity price breaks on transformers and they're not discounted that much for large orders. A pair of decent output caps can be had for $20 or $30. So if you're shooting for a $300-$500 price point, it's pretty obvious why you won't find output transformers in budget amps.

But what's the difference? Well, you're not going to get a good, low output impedance without using transformers. There's the rare exception, like the Zana Deux, but that's only because the 6C33C has a low output impedance. Otherwise, you have to apply a ton of negative feedback to an OTL circuit to get the output impedance down. Negative feedback can (in my opinion) strangle the sound of the music if overused. A little bit isn't necessarily a bad thing, but there's a reason why some audiophiles won't touch a piece of gear that uses any negative feedback.
 
Aug 14, 2009 at 3:37 PM Post #4 of 26

FrankCooter

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An OTL tube amp does not connect the output tube of the amp directly to the headphone. Most OTL amps are variations of what's called a cathode follower circuit. A cathode follower uses a capacitor to couple the headphones to the output tube. This capacitor, especially if it's of large value and poor quality, can degrade sonics much more than any transformer. As mentioned above, OTL designs really aren't suited for low z loads such as Grados. Everything else being equal, a quality transformer is usually superior as an output coupling device in a tube cicuit.
 
Aug 14, 2009 at 4:05 PM Post #6 of 26

cswann1

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If you are using a 300ohm Sennheiser, the WA2 would be a good choice. If you want to drive a 25ohm Denon the WA6 is the way to go.



BTW the WA6SE is really praised highly on the Woo Audio owners thread, relative to the WA6.
 
Aug 14, 2009 at 4:32 PM Post #7 of 26

UglyJoe

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Erik makes a lot of good points. OTL amps suffer from two major drawbacks: Output capacitors and lots of global negative feedback. I have seen some designs which use a servo to set the DC offset to zero and therefore don't require a coupling capacitor, but I'd be very nervous about going that route. There are many failure modes that could result in your headphones seeing lots of destructive DC voltage, so an output capacitor is a must. The second problem with the capacitor is that the voltages the capacitor will see are very large and the value of the capacitor is quite large as well, which pretty much requires an electrolytic or an ENORMOUS film cap. Neither option appeals to me. Finally, even with high impedance headphones like Senns usually OTL designs require some amount of global negative feedback which reduces gain and might have a detrimental effect on the sound. According to specs, the WA2 can drive 30-600 ohm headphones, and I don't see any adjustment for NFB, which means that there is probably a lot of global NFB applied to the amplifier, something I would also not find ideal, especially for Senns.

On the other hand transformer coupled designs have problems of there own. Fortunately, for the headphone world, power isn't nearly as much of an issue as it is for speakers, so SET designs can work remarkably well for headphones. Transformer design keeps getting better, and the frequency response for most headphone output transformers is pretty good nowadays - the WA6 for example has a frequency response of 20-30kHz, -3dB. This is pretty good for a SET design.

I have a question for any Woo Audio owners out there. The transformer coupled designs at WA typically are rated for a headphone impedance of 8-600 ohms. This seems extremely large to me. I know at least some of them (maybe all of them) have a high/low impedance switch, but something about the switch tells me that the switch isn't changing the tapping point of the transformer secondary (like changing between 16-8-4 ohm taps on a transformer coupled speaker amp). The only way I can see this working is that the transformer has a 4 or 8 ohm resistor in parallel with the headphone load and that most of the output power is dropped across this resistor, with the switch basically diverting the input signal through another resistor such that it effectively reduces the overall gain of the amplifier for lower impedance phones. I find this to be a little bit inelegant and certainly a waste of amplifier power and efficiency. Does anyone know how the WA amps drive such varied loads from it's output transformer?
 
Aug 14, 2009 at 6:27 PM Post #8 of 26

Denys

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Great !!! Thanks a lot guys !!!

I already own an Antique Sound Lab OTL headphone amp...

However, as I just bought Senn HD800 I want to have an headphone amp to fit with them.... I also have a pair of HD424 (600 ohms) bought 30 years ago and still working fine...hence my question about OTL....

But you just worried me a bit....I have a DIY tube amp at home and also asked the guy who built it for me if he could build an OTL headophone amp... He never did it before...

If I do not go with WooAudio then I might choose to go with the transformers and buy Tango transformers etc....Mundorf capacitors etc....I am willing to spend $1,500 cos I know I will keep it for many years....

I would not want to take the chance to burn my HD800 !!!

Would you ??

Tanks again guys for your clear advices

Denys
 
Aug 14, 2009 at 7:09 PM Post #9 of 26

Uncle Erik

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If you have access to a builder, take a look at the projects over at HeadWize. There are a number of excellent designs (both transformer couled and OTL) to consider. Your builder would also be able to visit the forums there and ask for advice on building the amp of your choice. Usually, your money goes much further with a DIY design and you can pick the parts that you want.

I'd also like to know if Woo's output transformers have multiple taps or if they use a resistor to change the impedance. If I recall, Woo has in-house magnetics, so you don't know what its specs are. Since output transformers are so critical, I prefer ones that are made available to the DIY community. If the hobbyists get a chance to kick a transformer around, you can find out a lot about them. Not so with proprietary iron. And no, that isn't a knock against Woo. For all I know, their output iron could be excellent. I just wish they'd start selling them to hobbyists, too.
 
Aug 14, 2009 at 8:00 PM Post #10 of 26

Donald North

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As has been mentioned, nearly every OTL headphone amp has output coupling capacitors. The can be electrolytic type (as in some of the Singlepower amps and ASL MG Head DT OTL MkIII) or film capacitors. I would not want a OTL/OCL tube amplifier powering my headphones. If something goes wrong, your headphones are cooked.

More importantly, you need to look for the right sonic synergy between your headphone(s) and amplifier. Since you recently bought the HD800, I recommend choosing an amp which drives them best and can do double duty powering your HD424.
 
Aug 14, 2009 at 8:29 PM Post #11 of 26

UglyJoe

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Quote:

Originally Posted by Uncle Erik /img/forum/go_quote.gif
If you have access to a builder, take a look at the projects over at HeadWize. There are a number of excellent designs (both transformer couled and OTL) to consider. Your builder would also be able to visit the forums there and ask for advice on building the amp of your choice. Usually, your money goes much further with a DIY design and you can pick the parts that you want.

I'd also like to know if Woo's output transformers have multiple taps or if they use a resistor to change the impedance. If I recall, Woo has in-house magnetics, so you don't know what its specs are. Since output transformers are so critical, I prefer ones that are made available to the DIY community. If the hobbyists get a chance to kick a transformer around, you can find out a lot about them. Not so with proprietary iron. And no, that isn't a knock against Woo. For all I know, their output iron could be excellent. I just wish they'd start selling them to hobbyists, too.



Speaking of, I'm currently thinking about a SET transformer coupled design that I'm getting ready to present in the DIY forums. There's a dearth (IMO) of this kind of DIY design - probably because of the high voltage PS and the expense of output transformers. Still, I think there ought to be a vetted design by a lot of the DIYers around here that falls into the class of an M^3 or something like that. Hopefully I get enough of the base goals and details together to post it in the next week or so...
 
Aug 15, 2009 at 5:28 PM Post #12 of 26

KZEE

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There are great sounding examples of both transformer and OTL amps available on the market, so I wouldn't get too hung up about it.

I also wouldn't get too hung up on whether an OTL amp will cook your headphones - although I'm sure it has at some point happened, I can't recall anyone ever posting here on the boards about an OTL amp taking out a set of headphones, so it would seem that it's pretty much a non-issue.

IMO the Woo WA3 OTL and the Senn 580/600/650 series of headphones have great synergy together, so if I owned that combo then rather than fretting over whether the WA3 may possibly to take out my headphones if it fails, I'd rather just enjoy the music and not worry about it.
 
Aug 15, 2009 at 5:37 PM Post #13 of 26

Donald North

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Quote:

Originally Posted by KZEE /img/forum/go_quote.gif
I also wouldn't get too hung up on whether an OTL amp will cook your headphones - although I'm sure it has at some point happened, I can't recall anyone ever posting here on the boards about an OTL amp taking out a set of headphones, so it would seem that it's pretty much a non-issue.


This is because OTL amps are usually capacitor coupled on the output.

OCL means output capacitorless. To do this with tubes, you need split rail power supplies and/or servos. There have been hi-fi amplifiers of this design, but I don't know of a commercial tube OTL/OCL headphone amp.
 
Aug 15, 2009 at 6:05 PM Post #14 of 26

Uncle Erik

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Quote:

Originally Posted by UglyJoe /img/forum/go_quote.gif
Speaking of, I'm currently thinking about a SET transformer coupled design that I'm getting ready to present in the DIY forums. There's a dearth (IMO) of this kind of DIY design - probably because of the high voltage PS and the expense of output transformers. Still, I think there ought to be a vetted design by a lot of the DIYers around here that falls into the class of an M^3 or something like that. Hopefully I get enough of the base goals and details together to post it in the next week or so...


smily_headphones1.gif
smily_headphones1.gif
smily_headphones1.gif


You have my attention. Any hints as to the tube compliment?
 
Aug 15, 2009 at 8:48 PM Post #15 of 26

Skylab

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Quote:

Originally Posted by Denys /img/forum/go_quote.gif

I already own an Antique Sound Lab OTL headphone amp...



Is your amp the ASL OTL Mk III? If so, it has BOTH an OTL mode *and* a Transformer-coupled mode. If you haven't tried it yet with the HD800's, you might try it before buying a new amp. With a nice set of tubes, the ASL amp sounds pretty darned good, and the availability of the 2 modes can be quite handy.
 

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