Thought on OTL tube amps

Hi Psag,

I had looked at the writeup at his site, and also at the patent which it references. There is a transformer between the outputs of the power tubes and the output of the amplifier, along with some solid state switching devices. As I read the writeup, what he said is that there is no AUDIO output transformer, but "the high-voltage, low-current tube impedance-plane is re-mapped to the high-current speaker impedance-plane through a special transformer at a constant RF carrier frequency of 250 kHz."

So while his very innovative approach avoids the use of an AUDIO output transformer, the design is not Output Transformerless (OTL).

Best regards,
-- Al

The fact that the Berning amplifier (while an outstanding and very innovative design) does not fall within the definition of an OTL is something that has been put to bed many times. While I respect David as one of the top designers in the audio world, I do find it inexplicable that his website makes this claim.

OTLs by definition do not have an output transformer; that much should be pretty clear. 'OTL' refers simply to 'Output TransformerLess. Over time (the last 50 or 60 years) the acronym generally has referred to a *tube* amplifier without an output transformer; in that regard having semiconductors in the output section would not qualify the amplifier as an OTL. And of course the Berning amps all include semiconductors as an essential part of the output circuit.

Yes, there's a tranformer in the Berning amp. The real issues are what its doing there, and how one wants to define OTL. The 'traditional' definition of OTL seems to require parallel tubes, and no transformer anywhere near the output. Ok fine, but what is the Berning transformer doing as compared to the traditional output transformer? Without knowing the answer, the question is moot.

I read the patent many years ago. I suspect Al has too.

The transformer is impedance matching. It is air core, as it is operating at RF frequencies to modulate the output of a switching power supply. The output of the switching supply then has its RF component filtered out and the result is the audio signal. So the output transformer and the switching supply work in tandem to do the impedance conversion.

A conventional output transformer does impedance matching as well only it does it at audio frequencies.

Either way there is a transformer that represents the load to the output tube or tubes. It can only be called an OTL when a transformer can no longer be called a transformer. Since these terms have been with us a long time and will continue to be with us for decades to come, the result is that the acronym 'OTL' is misapplied to this amp.

Ralph is from Minnesota(I might be wrong) and in Minnesota you need lots of heat in the winter. OTL amps partially solve the heat problems.