transformers/output impedance

That's right, except for the losses in transformer. This impedance cannot be simply measured with ohmmeter because transformer does not operate at DC but behaves according to ohm laws for the CHANGES in voltage. It is called reflected impedance. Transformer has to be large enough to carry desired power. Cross section of the center of the core is proportional to square root of power and is roughly 1 square inch for 50W (for 200W it will be 2 sq inch etc.). We want as few turns as possible to avoid losses in copper but we have to provide minimum number of turns per volt. This number is inversely proportional to square area of the core and is smaller for large transformers. Knowing maximum voltage and turns per volt we can establish absolute number of turns. We select core material, size and number of turns to prevent core saturation, when magnetic flux is higher than max flux density rated for given core.

All this is also related to frequency. It is easier to saturate transformer at lower frequencies and it has to be taken into account for size of core and number of turns per volt.

Absolutely right Bifwynne. That's why audio is more of the art than science. We can calculate but have to match and listen.

Thanks Kijanki. As I have come to appreciate, there are so many design considerations and trade-offs going on, at some point, amp design is about acoustic philosphy and art. Ergo why Ralph (Atmasphere) opted for the OTL amp topology.

As a consumer, though difficult, IMO, it is incumbent on me to at least understand enough of the theory so I can make as an informed a choice as possible when matching components. Admittedly, it has been a verticle learning curve, marked with many missteps and dead-ends.

Cheers,

Bifwynne, some people believe that transformers and capacitors have no place in signal path. Eliminating output transformer is nice but requires some descent output impedance to drive speakers and it is usually achieved by placing a lot of large tubes in parallel. Unfortunately heaters take a lot of power. 6AS7G that Atmasphere was using takes 2.5A at 6.3V =15.75W per tube. 20 tubes will dissipate 315W. If you prefer stereo listening it will be 630W. Adding main circuit will result in more than 1kW - a space heater (nice in the winter). On the other hand we audiophiles are eager to sacrifice for great sound, and I've heard it is great.

Gotcha Kinjanki. I'd love to hear one of Ralph's OTL amps, but part of my verticle learning curve of bad choices is that I bought speakers that were designed and voiced to be driven by a low output impedance, high power, high current SS amp. I'm dumb-arsed lucky that my ARC Ref 150 has a low'ish output impedance, especially off the 4 ohm tap, and has a large power supply (1040 joules), so it can deliver some serious juice when needed.

Unfortunately, I am dubious (albeit not certain) that one of Ralph's amp would be a good match for my speakers. If Ralph thinks differently, he should of course chime in and correct my misconception.

Cheers,