Cooper,

Belatedly... Let's be a bit more precise here...

In reply to:

Sure, those curves are flat because they go into a non-reactive test load. And speakers measure awfully flat when tested at a fixed volume (1W ?). However, impedance curves are far from flat, on any speaker. So what would happen if I took a fixed voltage and fed actual music to a speaker who's impedance varies from 4 ohms to 20 ohms (rated with a nominal impedance of 8 ohms)? Well the high impedances are pretty easy to deal with linearly, but as the impedance drops, the speakers demand much more power. To perform linearly, the amp must provide exactly double the power from 8 ohms to 4 ohms given a constant voltage. And quickly.


So far, the argument is accurate.

In reply to:

But wait a minute. Notice that the curves for the different resistive loads, while individually flat, are not at the same absolute level. 2 watts at 4 ohms will virtually always produce less than double the power than 1 watt at 8 ohms; the magnitude of the difference depends on how well-equipped the amp is to handle high-current / low-impedance. Okay so there's maybe a few-decibel difference at 1W. How about comparing 10 watts @ 8ohm to 20 watts @ 4ohm? What happens if a peak in the music demands instantaneous power of 300 watts for a tiny fraction of a second?


If the power (or voltage) output of a solid-state amp decreases by as much as a few dB within audio frequencies when driving a 4-ohm load, the amp should be considered defective (unless it is a tube amp with a very low damping factor, i.e., high output impedance). This aspect is covered by the output impedance measurements over the frequency range.

The vast majority of today's solid-state amps have output impedances of well below 0.1 ohm, which stays low and ruler-flat up to (at least) 20kHz, where it slowly starts to rise. When driving a 4-ohm load, this translates to a power loss of less than ~0.1dB, NOT "a few dB" loss. And this is maintained until the maximum available power output (for the given load impedance) is approached, where the sonic degradation and its differences between amps becomes very obvious anyway.

In reply to:

Additionally, Stereophile also tests amps into a "dummy loudspeaker load," which has a typical loudspeaker impedance curve. Amps always produce the same general freq. response curve, dipping where impedance drops, with the degree of interaction being dependent partly on the amount of amp output impedance. The slope of the loudspeaker impedance curve also has an effect on its load to the amp.


Yes, and as stated above, those "dips" are only ~0.1dB deep or less. Even if one can reliably hear a 0.1dB dip in frequency response (highly unlikely), this does NOT provide a physical explanation for the alleged "brighter" sound of the Yamaha receivers on many different speakers (with wildly different impedance curves from each other). If you have any logical idea here, please tell me...

In reply to:

Back to transients. This is partly why a square wave is often used in testing amps. When there's a rapid change in the voltage signal, the amp has to quickly supply the current as well. Does the amp just go straight to that voltage/current, especially at high wattages? Depends on the amp. It may overshoot the increase to the top of the wave, then undershoot the drop to the bottom.


Again, if a solid-state amp exhibits an overt overshooting/ringing under a resistive or resistive-inductive (speaker) load, the amp should be considered defective today, period. Under a realistic load conditions (resistive/inductive combined with some capacitative components), the vast majority of current solid-state amps reproduce "text-book" square waves.

In reply to:

Tube amps round off these square waves substantially, leading to a softer, warmer sound that many people appreciate or prefer.


That is simply because, in many tube amps, the frequency response starts to roll off at 10-15kHz. No magic here -- and some tube amps have a definitive sonic character precisely because of this and several other reasons (whether it is subjectively good or bad). Anyway, my initial post deals with modern solid-state amps.

In reply to:

Regarding distortion. Decades ago, amp manufacturers engaged in a THD war. Solid state was in, and so was negative feedback. Problem is that extreme use of negative feedback can have a very detrimental impact on sound quality. Sure, the 1W frequency response looks great though! Some amps run with little or no feedback, though the middle road today is to simply use it in moderation.


I am not aware of a modern solid-state amp that runs with "little or no" NFB, say, less than a total of 10dB feedback (please show me if you know any such commercial designs). The open-loop characteristics of solid-state op-amps and power stages have come a long way in the last 3 decades, so that today's amps achieve, with moderate levels of NFB, a very low THD+N and output impedance over and well beyond audio frequencies, while maintaining a robust stability and load tolerance.

Bottomline: Your "technical" arguments still do not provide a good explanation for the alleged sonic characters of solid-state amps.


curtis,

Yes, I had instantaneous A/B compared, in several occasions, two current models of Yamaha receivers (the flag-ship RX-Z1 and a $1000-ish model, of which I don't remember the model number) in the direct stereo mode, with various other receivers including the Pioneer, Denon, H/K, Onkyo, and Marantz. After carefully ensuring that every processing is defeated and the listening level is within +/- 0.5dB, I did not reliably hear a sonic difference among them. This wasn't even a blind test -- I knew what I was listening to and was looking for the alleged, published "characters."

I personally suspect that, if any real sonic difference exists among these receivers, it resides in the digital-domain algorithms and/or the ADCs/DACs. I believe that it is fairly hard to hear the sonic difference among modern solid-state amps (or the amp portion of the receivers), although I do not want to claim or prove that all SS amps do or should sound identical. To my ears, they certainly do not provide a strong enough sonic signature to affect/modify the timbral characters of loudspeakers, which are, of course, highly audible in most cases.