transformers/output impedance

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.

Ok Ralph .. you offered. Please do. :)

Perhaps you might also touch on the what is meant in plain English when ARC says its amp's "[o]utput stage coupling is a combination of “ultralinear” and Audio Research’s patented 'partially cathode-coupled' topology ...." From what little I can glean from the description, it looks like some variant of local feedback. That's just a guess.

Gentlemen, what Ralph appears to be referring to regarding the relation between feedback and output impedance was discussed extensively in this thread, beginning with the post that the link opens at.

Best regards,
-- Al

P.S. to my last post. Was thinking to myself that if zero negative feedback is so disadvantageous, why would top flight companies like ARC use it???

This is just a guess ... so here goes. Perhaps simply stated ... NF is used to reduce the output impedance and increase DF for **marketing** purposes.

As Ralph and Al have explained many times already, the world of tube friendly, true Power Paradigm speakers is not as large as Voltage Paradigm friendly speakers. So .. using NF to reduce output impedance and increase DF increases the playing field of speakers that may be compatible. That's my thesis.

Perhaps a good case in point is the match with my amp and speakers. Technically speaking, my speakers are poster-child "SS-amp" friendly speakers because they have rough impedance and negative phase angle functions. Yet my ARC amp manages to muscle its way through the tough spots with a modicum of aplomb.

I am somewhat dubious that a true Power Paradigm amp having a "high'ish" output impedance would do as good a job. Not because of design, build or sonic deficiencies. But just because of the "high'ish" output impedance. At the very minimum, all other factors being equal, there will be a tipping point where acoustic coloration will outweigh reduced NF induced distortion (e.g., TIM and odd-ordered harmonics).

I don't know how most Power Paradigm amps address power supply issues. I suspect that access to lots of joules can only be a good thing when the amp is being tasked to deliver current when called upon.

This post is just a bunch of guesses. So please don't come down hard on poor little me. Perhaps Ralph who is in the business can speak to the bona fides of my surmises.

Cheers,