What do OTL and transformer-coupled mean?

[Asr]

I've been curious about this for a while and would like to get educated on the subject.

What exactly does "OTL" mean? And what's the difference compared to "transformer-coupled"? Can someone explain it in newbie terms?

And what makes transformer-coupled amps more well-suited for driving low-impedance headphones, is it higher current output?

 

[PFKMan23]

OTL = output transformerless

Transformer coupling means that there are output transformers used (along with other transformers).

 

[Asr]

Sure I got that part, but I want some more detail than that - how are they implemented? Transformer refers to the power transformer, right? How exactly can an amp not use a transformer?

 

[itsborken]

My CDP uses a pair of transformers instead of opamps, etc. to boost the output signal.

 

[d-cee]

Actually it doesn't refer to power transformers

OTL usually applies in tube applications. the output of a tube is generally quite high impedance and low current and as such is more suited to high impedance phones.

driving low impedance phones may lead to distortion/clipping

placing a transformer behind the tubes allows it to drive low impedance headphones

it also is generally accepted that the sound signature is affected as well

edit: just in case it wasn't obvious, the above are generalisations and there are always exceptions to the rule. eg. single power's amps are supposed to be OTL but certain configurations. ie SLAM are capable of driving low impedance phones.

 

[cclragnarok]

Power = voltage * current.

Transformers basically allow you to trade voltage for current or vice versa, hopefully without losing too much power.

Tube amps in general tend to be able to output a lot of voltage, but not enough current. Output transformers are used to trade some of that excess voltage for current, which gives the amp enough headroom current-wise to drive low impedance headphones better.

Tube amps also tend to have high output impedance, and the output transformer lowers the output impedance. This is also more important for low impedance headphones.

Feedback can also be used to lower the effective impedance of an amp, but it does not improve the maximum current output of the amp. Using an output transformer achieves both, but transformers do add some distortion to the sound. You can also try to avoid this issue by using output tubes that have relatively (for a tube) low output impedance and high current capability to begin with, and/or by using lots of output tubes in parallel.

By the way, Stax transformer boxes do the opposite. They trade current for voltage since electrostatic drivers need a lot of voltage, but very little current.

 

[cyd2025]

to put it simply, an OTL tube amp bypasses an output transformer
to match the impedance of the headphone or speaker being used...
because output tubes generally have the characteristics to
match a high impedance can or speaker it doesn't need an output
transformer to alter the current, voltage and output impedance...

but in order to drive a lower impedance headphone or speaker,
an OTL amp has to use very specific tubes and high current bias
of the tube plates to match the lower impedance...or implement
a parallel circuit of tubes to reduce the plate resistance of the
output tubes...

on the other hand, the output transformer is used to
alter the characteristics of a typical tube, which has a high out
impedance, high voltage and low current, to the opposite using
what is known as a "negative feedback" method...this will
translate the output impedance of the tubes to match closely
the lower impedance of the can or speaker...at the cost of weight,
higher cost and some instability of the signal, theoretically...

it's a completely different approach to matching impedance...thus
you hear a difference in sound signature between these two types
of amps...

 

[penger]

Nice information to have. Thanks to asr for posing this question and to those who answered.

 

[Asr]

Ahh, exactly what I wanted to know, thanks for the info!

 

[Skylab]

One of the coolest things about the ASL OTL MG-Head III I have is you can switch between OTL and transformer mode on the fly.

 

[slwiser]

One of the coolest things about the micro-ZOLT is that it has a totally different design to match it output tubes to the impedance load. It uses no output transformer.

Very few people actually understand the design of the Berning ZOTL patient,http://www.davidberning.com/patents.htm .

Anyone want to take it on?

There was a thread about what is your dream tube design and I made this suggestion but others never responded to it.

I think it must be simply to complicated for the average "joe." I don't have a clue myself.

 

[vcoheda]

Quote:

Originally Posted by Skylab /img/forum/go_quote.gif One of the coolest things about the ASL OTL MG-Head III I have is you can switch between OTL and transformer mode on the fly.

yes. that is a neat feature. i wonder why more OTL amps don't do that.

 

[Skylab]

Quote:

Originally Posted by vcoheda /img/forum/go_quote.gif yes. that is a neat feature. i wonder why more OTL amps don't do that.

Or, perhaps, that more transformer-coupled amps don't offer an OTL mode

 

[Podster]

Quote:

Originally Posted by Skylab /img/forum/go_quote.gif Or, perhaps, that more transformer-coupled amps don't offer an OTL mode

Exactly what I was thinking

 

[jcx]

Quote:

Originally Posted by slwiser /img/forum/go_quote.gif One of the coolest things about the micro-ZOLT is that it has a totally different design to match it output tubes to the impedance load. It uses no output transformer. Very few people actually understand the design of the Berning ZOTL patient,http://www.davidberning.com/patents.htm . Anyone want to take it on? ...

the Berning patent describes a switching converter between the tube output and the load - switching powersupplies are regarded with suspicion in the audiophile world (some are OK if designed for low noise and plenty of filtering)
I would be a lot more careful when the suggestion is to chop the analog signal at Hundreds of KHz and stick it in your ear right after the rectifier

Quote:

Originally Posted by jcx /img/forum/go_quote.gif interesting that you can attach a 250 KHz switching converter to a tube amp output and it sells as a "mysterious improvement" - "otl" is a very finely parsed lie, the "impedance converter" shown in the patent incorporates a high frequency transformer - just not an audio band "big iron" output transformer "Toward the fulfillment of these and other objects, the output transformerless amplifier impedance matching apparatus and method has at least one switching power converter bridge having opposite terminals connected between a fixed source of voltage and the load. Two bridges and two power supplies are used for bi-polar or push-pull applications. The returns for the power supplies are connected to the return for the load. The other two terminals of a given bridge are connected across the primary winding of a conversion transformer."