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

If you people would read the measurements section of Stereophile, you would see that I’m correct.

Look at the frequency response for the Moon 861. Remember, we are talking power amps here, so no preamp functions (e.g., loudness) enter into the discussion.

And the discussion:

The output impedance in stereo mode was extremely low, at 0.008 ohms at 20Hz and 1kHz, rising slightly to 0.03 ohms at 20kHz. As the two output stages are in series in mono mode, the output impedances were twice the stereo values. In both stereo and mono modes, the variation in the frequency response with our standard simulated loudspeaker (fig.1, gray trace) was negligible.

Now look at the BAT REX 500 frequency response:

And the discussion:

The REX 500 amplifier’s output impedance, including the series impedance of 6’ of spaced-pair cable, was relatively high, at 0.45 ohms at low and middle frequencies and 0.6 ohms at the top of the audioband. As a result, the variation in the frequency response with our standard simulated loudspeaker (fig.1, gray trace) was ±0.3dB.

So while the response is flat into a load resistor, it varies by ~+-0.3 dB into a real life speaker load. That’s enough of a difference to hear as a difference. And of course every speaker is going to have a different load, so the amp is going to have a different response for each speaker dependent on that load.

Know that a lot of amps show considerably more variation than this.

So the second graph is better with respect to the line that’s more wavy. Which means it’s more responsive with a more accurate delivery of Dynamics as the music demands. A lower level of Dynamics, flatter line, is expected when impedance is lower?

No the first graph is better as the power level, and hence volume, of the sound does not change with frequency.

In the second graph, the power goes up and speakers will get louder ~75 Hz and 1-2kHz and have a dip at ~5kHz so you would not attain a flat frequency response with that amp with that speaker load.

Note that the same "real life" speaker load is used for both amps.

The peanut gallery chiming in with my $0.02... I’m going to say power cord and power conditioning. The Audioquest Niagara 7000 is no joke in the way it makes the background black, and the transient in all its variable forms, leap out at you.

Next, the Niagara is sensitive to power cords. I use an AQ Hurricane HC (high current) power cord, it has 3 loosely braided shielded conductors. I **think** the Hurricane "high current" cord is the top range in the Storm series of cords, and then next comes the BIG bucks cord series in the "Mythical Creatures," category consisting of (again, I think) Thunderbird, Firebird, and the big daddy, Dragon; these have copper + silver blends for the first two, and the Dragon uses all silver conductors.

The Hurricane HC (high current) is good. It’s very sopen and smooth. Then I inserted an older Shunyata power cord in the Python Zitron HC. With this cord, all the sudden, it sounded like bass got added and there was greater "jump" to music. Then I tried a Anaconda Zitron HC, which yielded an even warmer tone and gave everything greater separation, shape and dimensionality. My last cord is a more current cord in the Sigma XC (same as HC) V2 power cord. I think, Sigma is on top, then Alpha V2 and then Delta on the bottom; after that are the big bucks cords in the Omega XC and QR cords. Greater clarity and for the most part, bit better bass except a bit of warmth (which I do like) was lost. There something to those older Shunyata Python/Anaconda/King Cobra cords and the bit of warmth they add that I find welcoming.

Not only does the main power cord to the entire rig make a difference as the first cord in the chain, that changes everything up stream. It makes everything sound better, not surprisingly. If had throw away cash, I run out and buy a Shunyata Omega XC (QC?) cord or that really killer Synergistic Galileo SX or SRX power cord that is nose bleed costly, at least for humble me. Have a prosperous and Happy New Year’s all! 😁

Peter Walker → straight wire with gain (QUAD)

John Farlowe → an amplifier is only as good as it’s power supply (Exposure)

Julian Vereker followed their advice! (Naim)