transformers/output impedance

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

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,

Thank you all,
Is nice that when I start this thread I was thinking in preamplifiers, even if I did not say, because it was easyer to introduce the subject talking about amplifiers.
Now I can say, The more we learn, the long we know the way is
Jorge

To clarify a few points made immediately above:

Negative feedback has only a minor effect on high frequency bandwidth. This is because HF bandwidth is a function of the risetime (or slew rate) of the circuit, which will not be affected by bandwidth. However, there is a *perception* that it has this effect, because feedback will add trace amounts of odd ordered harmonic distortion, which is perceived as brightness by the human ear.

Transformers do indeed transform impedance, and in *both* directions. So if for example a 16 ohm tap is loaded with a 4 ohm load, the power tubes will see a load that is considerably lower than they will be able to drive properly. Distortion will be higher and output power will be lower. This is why its important to select the right tap.

The use of feedback has little to do with the transformer, and everything to do with the linearity of the amplifier circuit that precedes it. Usually the driver sections are fairly linear, but output sections may not be if tetrodes or pentodes are used. It is this reason that feedback is used rather than the choice of output transformer.

Almost any transformer currently in use in a hifi amp is capable of operating in a design without feedback. Again, feedback will not have a large effect on improving bandwidth of the transformer due to the demands that would be made on the preceding circuitry.

IOW, the reason you see zero feedback tube designs is not because a transformer is particularly suited for it, more so it has everything to do with the linearity of the circuitry preceding. In Charles1dad's case, his amps have triode power tubes which are inherently linear- no need for feedback.

To be clear, the use of a transformer can indeed reduce the output impedance, but there is a myth that adding loop negative feedback will too. It doesn't. I'll explain that further if anyone asks.

To be clear, the use of a transformer can indeed reduce the output impedance, but there is a myth that adding loop negative feedback will too. It doesn't. I'll explain that further if anyone asks.

That is what I always felt and have taken a lot of flack for it. I think a little further explanation would help a lot of us Ralph.