OTL amplifiers

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,

Detlof. You had a Futterman blow on you?

I am reading through the Rosenberg book (from the Sherod thread) on Tube Electronics the last part of which is a long presentation (ad for?) Futterman amps. There was a recent thread here re warranties and everyone was impressed with Bryston's 20 years. That's small change.

The good news is that Rosenberg's book clearly states that the Futterman amps carry a "lifetime warranty!"

The bad news is I'm not sure if they meant your life of Harvey's.

You'll enjoy the book. It has a picture of the pressure cooker Futterman used to prepare the varnish to varnish his power transformers which he then "put on the window sill to cool."

Times have changed.

Sincerely
I remain,

>>I disagree with tubegroover about a disadvantage being that they run hot. That' a characteristic of Class A operation, not the OTL topology.

I like your posts GS (cuz I usually lern somethin) but tis a bit form over substance no?

OTLs do, as you say "have many tubes" and they do, as you say, "consume a lot of power" which they dissipate as a lot of heat. That is.... they run hot.

I remain,