What contributes most to a change in how an amplifier sounds?

What is it about the transformer?? What is it about the wiring? What is it about the chassis? What is it about the chips? What is it about the capacitors? What is it about the power of the amplifier? etc

@emergingsoul Which transformer- the power transformer or output?

Steel chassis are effective shielding at audio frequencies; not so good at RF frequencies. If there are tubes you don’t want the chassis to be resonant. This is less important with solid state. If the circuit is fully balanced and differential you may not need the chassis to shield anything. But with single-ended this can be important.

The insulation of the wire is probably the thing that affects it the most, since you can interchange the word ’insulation’ with ’dielectric’ which is something found in capacitors to insulate the plates from each other. Put another way, wires cause stray capacitance. They also have an inductive property but that isn’t an issue at audio frequencies.

Chips- that’s a tricky one. Since opamps are chips I’ll just say that opamps from 40 years ago are not nearly as good as opamps are today. So older opamps have a ’sound’ which is why you have to be careful changing them in guitar effects pedals because you can alter the sound of the pedal. Guitar players are very picky about that sort of thing.

Capacitors are affected by dielectric absorption of which Teflon is one of the better insulators, followed by polystyrene (which is a lot harder to find these days), polypropylene and mylar. Paper capacitors are pretty terrible but they have a following because of the distortion they make. Another factor is ESR (equivalent series resistance) which should be kept low; the higher this value the longer the cap takes to charge. Caps can also change value with temperature; this is called temperature coefficient. There are caps nowadays called ’NP0’ whose value do not drift at all.

Really if you want to learn more there are books for this sort of thing. Norman Crowhurst wrote some wonderful stuff 60-70 years ago and his books are worth finding. Many are on http://www.tubebooks.org/technical_books_online.htm

A lower level of Dynamics, flatter line, is expected when impedance is lower?

@emergingsoul Dynamics comes from the signal not the amp. When it seems like the amp is more 'dynamic' the chances are extremely high that what you are hearing is actually just distortion masquerading as 'dynamics' due to how the distortion interacts with the human ear.

Class D operation sidesteps the annoying Class AB artifacts, but then you get deep into designing pulse-width modulators that are unconditionally stable, resist transient upsets, have good phase margin, and also have low distortion, even under dynamic conditions. Basically all the challenges of designing a state-of-the-art ADC and DAC that can also deliver power into complex and nonlinear loads.

@lynn_olson If you design a self-oscillating class D amp then you satisfy all these requirements. In a self oscillating amp you intentionally exceed the phase margin by adding so much feedback the amp goes into oscillation as soon as its powered up. The feedback loop is designed to only allow one solution for the oscillation, which is used as the switching frequency. This has the benefit of allowing much higher feedback without the problems caused by lessor amounts and having it poorly applied. It also solves the problem of noise caused when the switching frequency drifts. So this allows the amp to be dead silent even on horns.

@emergingsoul The secondary thing is the distortion signature of the amp, which audiophiles call the 'sonic' signature. This is in turn caused by something called the 'transfer characteristic' of the amp. The transfer characteristic has something to say about how the amp responds to transients, in particular those that overload the amp. It also says what sort of distortion will be produced, for example if based on a quadratic exponent, the 2nd harmonic or a cubic exponent, resulting in a dominant 3rd harmonic.

These two harmonics are treated by the ear much the same way in that they are innocuous. But intermodulation distortion is not and also plays an important role that is defined by the transfer function.

If you want to know more about this, read this article which starts at page 35 at the link. It might be more than you want to know, but it does answer your question correctly.