OTL Amps - "Direct-Coupling to Music"?

[watchnerd]

I was reading a review of the Feliks Audio Euforia tube headphone amp and ran across this line:

"A really good OTL design makes your transducers sound like they are direct-coupled to the music, and that is precisely what the Feliks Audio Euforia does to your headphones."

I'm not really trying to start a debate about this particular headphone amp, but what does that line by the reviewer even mean?

Or, if we just assume that it's audio reviewer gibberish...

What are the alleged benefits of OTL amps?

And are those benefits different for headphones vs speakers?

 

[Wombat]

I was reading a review of the Feliks Audio Euforia tube headphone amp and ran across this line:

"A really good OTL design makes your transducers sound like they are direct-coupled to the music, and that is precisely what the Feliks Audio Euforia does to your headphones."

I'm not really trying to start a debate about this particular headphone amp, but what does that line by the reviewer even mean?

Or, if we just assume that it's audio reviewer gibberish...

What are the alleged benefits of OTL amps?

And are those benefits different for headphones vs speakers?

I have always thought that 'music' is coupled to transducers via the air they both inhabit.

 

[watchnerd]

I have always thought that 'music' is coupled to transducers via the air they both inhabit.

Well, yeah.

Unless one considers electrical signals themselves to be music?

 

[Wombat]

Well, yeah.

Unless one considers electrical signals themselves to be music?

Just electrical representations. No transducers, no sound(music).

 

[PaulD]

It's reviewer gibberish...

 

[JohnYang1997]

It's output transformerless. Most headphone amps you see in this forum don't have transformer at output. Only some tube amplifiers do. Tube have high output impedance. Output transformer trades output voltage swing for higher output current and lower output impedance. But in general tubes have higher distortion to begin with and due to the lack of high open loop gain, and are sensitive to supply noise. OTL means really nothing here.

 

[watchnerd]

It's output transformerless. Most headphone amps you see in this forum don't have transformer at output. Only some tube amplifiers do. Tube have high output impedance. Output transformer trades output voltage swing for higher output current and lower output impedance. But in general tubes have higher distortion to begin with and due to the lack of high open loop gain, and are sensitive to supply noise. OTL means really nothing here.

Yeah, I understand the electrical implications of OTL with regard to current and impedance and the nature of tubes (not currently being used, but I have 2 tube phono stages, 1 hybrid tube headphone amp, and lots of musical instrument amps that use tubes).

What I don't understand is why being OTL is considered necessarily superior by some, as opposed to just another design trade off that *might* make it perform differently with lower impedance phones.

Maybe all the solid state amps should market themselves as "Now OTL!" in much the same manner as "Gluten Free" pork rinds.

 

[PaulD]

Yes, you are correct, it's reviewer gobbledygook based on the misconception that transformers are "flawed" in some way that other components or topologies are not. Transformers perform a useful impedance matching function at the output of valve amps. Transformers can be *very* well designed and engineered, but that of course makes them expensive. Nelson Pass has used transformers in the VAS position of 2 of his high-end amps (F6 and M2), and of course transformers are typically involved in studio electronics as either mic input transformers or line output transformers. It's just another component with its own set of limitations to work around, some are poorly engineered and implemented, some are well engineered and implemented and basically transparent. Microphone electronics went through a phase of being advertised as "transformerless" some time ago.

 

[watchnerd]

Yes, you are correct, it's reviewer gobbledygook based on the misconception that transformers are "flawed" in some way that other components or topologies are not. Transformers perform a useful impedance matching function at the output of valve amps. Transformers can be *very* well designed and engineered, but that of course makes them expensive. Nelson Pass has used transformers in the VAS position of 2 of his high-end amps (F6 and M2), and of course transformers are typically involved in studio electronics as either mic input transformers or line output transformers. It's just another component with its own set of limitations to work around, some are poorly engineered and implemented, some are well engineered and implemented and basically transparent. Microphone electronics went through a phase of being advertised as "transformerless" some time ago.

Maybe it's just me, but the OTL fetish seems correlated with minimalist / hobbyist designs that emphasize simple circuits and low parts counts.

"The lack of a transformer isn't a bug -- it's a feature!"

 

[PaulD]

You are probably correct, and the unwillingness of some hobbyists or manufacturers to pay for quality transformers. It is possibly to build truly excellent performance audio electronics both with and without transformers.

 

[SIY]

Maybe it's just me, but the OTL fetish seems correlated with minimalist / hobbyist designs that emphasize simple circuits and low parts counts.

"The lack of a transformer isn't a bug -- it's a feature!"

Except that OTL amps are almost invariably more complex. And they're generally not direct coupled unless you're happy with the risk of a circuit fault and high voltage applied across your temples.

 

[watchnerd]

Except that OTL amps are almost invariably more complex. And they're generally not direct coupled unless you're happy with the risk of a circuit fault and high voltage applied across your temples.

So if it's more complex, why bother?

Or is it a case of more complex, but cheaper, because transformers are expensive?

 

[SIY]

So if it's more complex, why bother?

Or is it a case of more complex, but cheaper, because transformers are expensive?

And a story. In this market niche, you need a story.

 

[Panelhead]

Most are OTL amps are capacitor coupled. Like large value electrolytic.
I much prefer direct coupled amplifiers. A direct coupled tube amp is rare.
Most bipolar rail solid state amps are direct coupled.
Pick your topology, transformer coupled, capacitor coupled, or direct coupled.

 

[anmpr1]

Julius Futterman is the name most people who know OTL, know. Futterman made his amps using TV tubes as output tubes; hand made everything in his Manhattan apartment. Shellaced hand wound transformers in a pressure cooker. I have no idea how his wife put up with it. A couple of outfits attempted to market his designs commercially, the last being P.T. Barnum...um...er, I mean Harvey 'Dr. Gizmo' Rosenberg of New York Audio Labs.

Actually, Harvey's NYAL OTL creations were monsters (some on four chassis), costing monster dollars, looking like gear you might see in a 1950s Ed Wood flying saucer movie. Futtremans always received good reviews in the subjective press, and his early designs (1960s) were favorably reviewed in mainstream publications such as Hi Fidelity and Stereo Review. I have a copy of Rosenberg's 'book', Understanding Tube Electronics. It's worth it if you can track down a copy, just for Harvey's reminisences on Julius Futterman's life work.

 

[anmpr1]

PS: for anyone interested in OTL and Futterman's history/legacy, the following site has some information, photos, and a review from the old Audio Critic magazine.

https://jacmusic.com/techcorner/ARTICLES/English/Harvey-Rosenberg/futterman.htm

 

[sergeauckland]

So if it's more complex, why bother?

Or is it a case of more complex, but cheaper, because transformers are expensive?

And a story. In this market niche, you need a story.

OTL amplifiers exist entirely because of marketing. Tube amplifiers these days exist almost entirely because of marketing.

There's no technical benefit to an OTL amplifier, whether for loudspeakers or headphones. I've never seen an OTL amplifier that had an output impedance as low as is trivially easy for any SS amplifier, so the frequency response will be dependent on the impedance characteristics of the load. This may (just) be acceptable for headphones where impedances tend to be higher than loudspeakers, but wholly inadequate (as are most tubes) although a few get just about low enough. Futterman amplifiers had large amounts of feedback, possible because of the lack of phase shift through not having an output transformer, but even so, if I recall correctly, their output impedance was still somewhere around 1-2ohms or at least 10x more than any competent SS amp.

I can understand a manufacturer wanting to be different, indeed needing to be different, but technically it makes no sense.

S.

 

[Panelhead]

The OTL amps that do not use a coupling cap use some variation of the Circlatron circuit to balance out the DC. They are the ones that sometimes became unstable, dumped a lot of DC on the outputs and blew up.
It is like SET. No technical reason for existence. But a definite client base.

 

[watchnerd]

Futterman amplifiers had large amounts of feedback, possible because of the lack of phase shift through not having an output transformer, but even so, if I recall correctly, their output impedance was still somewhere around 1-2ohms or at least 10x more than any competent SS amp.

1-2 ohms output impedance should be okay for most headphones, though, I would think, if one believes in the 'rule of 10'.

(35 ohm is the lowest impedance set of cans I own)

 

[sergeauckland]

1-2 ohms output impedance should be okay for most headphones, though, I would think, if one believes in the 'rule of 10'.

(35 ohm is the lowest impedance set of cans I own)

Yes, as I said above, should be just OK for headphones, although I have had some 8 ohm headphones in the past. Nevertheless, why have a hugely expensive, power guzzling, heavy box to do what a 5532 can do better?

Sometimes modern life makes no sense.
S