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transformers/output impedance

Hello,
I'm confused, or is it a question accepted that the use of output transformers for tube amplifiers, aims to lower the output impedance of the same, to match with the impedance of the speakers? However, how can a passive device (transformer) influence the output impedance of an active amplifier unit? I say that the output impedance is a characteristic of the amplifying machine, and transformers only optimize the ratio of its ability to deliver current, with the current requirements of the speaker, easily translated by its impedance
160562
bifwynne
2,206 posts
 
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,
kijanki's avatar
kijanki
4,450 posts
 
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.
bifwynne
2,206 posts
 
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,
georgehifi's avatar
georgehifi
10,312 posts
 
160562
You also need to consider the negative feedback if any around the amp, it too will lower the the output impedance and speed up it's precieved HF bandwidth.

Feedback can make a very average quality output transformer look reasonable, at the expense of less gain and some stablity, also some believe that the lush tube midrange can suffer as well, but maybe it's just the tighter bass because of the lower output impedance that takes away a bit of that midrange bloom.

A good test for an output transformer is to see it's bandwidth and damping factor (output impedance) with the feedback removed, this btw also increases the gain and can also make the amp more stable.
Really well made output transformers don't need feedback unless they have to drive harder speaker loads.

Cheers George
charles1dad's avatar
charles1dad
10,250 posts
 
I agree with George's point in regard to output transformers. The really good ones can avoid the need for introducing NFB, if one choses to use easy to drive speakers. While I appreciate the sound reasoning and theory of eschewing the output transformer, the very best sounding tube amplifiers I've heard all utilize them! So definitely "art" plays a large role in designing audio components.
Charles,
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