OTL amplifiers

OTL amps are a subject I really can get excited about in this hobby.

Advantages include, top to bottom clarity, resolution and tonal naturalness more akin to real music. The reason being there is no transformer that CAN add or detract from the signal. These amps when matched properly to the speaker they will be used with can take you to sonic nirvana like no other amplifying device I have heard including transformer coupled tubes, SET with transformers which typically don't offer the frequency extension of the best OTL designs and ss which with few exceptions doesn't offer the immediacy and presence that seems exclusive to tubes. So the best way to describe OTL's are the best world of both tubes and ss with few if any of the weaknesses with the possible exception of bass "slam" which seems more exclusive to ss designs. Bass resolution and natural impact is of much greater interest to me and is harder to get right than slam (dynamic impact, often exaggerated).


Disadvantages of OTL's are as long listed as their advantages:

Since the output tubes provide all the current to drive the speakers without use of an output transformer, it is important that the impedance characteristics not drop below 4 ohms in any case and ideally should be between 8 and 16 ohms at all frequencies without compromising the performance they are capable of. Unfortunately there are few speakers with these characteristics so use of an OTL is not ideal in most instances. Heat is another problem. They run very hot because many tubes are used in parallel to produce the current necessary. They can be designed for high watts but the price is many tubes and a lot of heat, up to 1,500-2,000 watts for a big pair of mono's.

Dependability has always been an issue with these designs but it seems this has become less so with current designs than in the past. The players include Transcendent, Atmas-phere, Joule, Tenor, Graff and the Berning Quasi OTL. They are all great performers with the right speakers.

...so basing on pricing of OTL amps you can easily get an output transformer with ferromagnetic core taken from the Moon(99.9% carbon-free iron only currently found on Moon) and brought to the Earth to make tube amp much more efficient and cooler running.

bottom line is large buck for the small watt.

Hi DFrigovt:

OTL

Output TransformerLess

They name it for what it does not have! Sounds stupid but the main advantage of an OTL is that it does not have an output transformer.

A transformer is used in most tube amps because there are major problems matching tubes to speakers. A tube is a high voltage, high impedance thing (thousands of ohms) and a speaker needs current and has a nominal impedance of 8 ohms or so. These are real problems. You hear folks talk about "power tubes" whcih usually means current. But even many "big tubes" do under .5 amp or so. The output circuit and the transformer creates the power. A transformer steps down voltage and steps up current and matches the amp's and speaker's impedances. Thus it solves some problems. You can see from Tubegroover's answer re speaker selection that the OTL, while gettin rid of hte tranny, has not solved all these issues either.

But the tranny creates problems too. John Broskie over at Tube Cad Journal says "The best transformer is not as good as the worse tube." Basically transformers inevitable cause distortion for a number of reasons. I'd be happy to outline the basics but I think I'd bore you to death.

OTL's tend to be expensive as Mark notes. Bruce Rozenblit's are not to bad though. They tend to be made in small shops so the economy of scale isn't to good and by people who are quite smart (Berning) and believe they deserve to make a decent living.

Sincerely
I remain,