Measuring Performance?

[mwh-eng]

I know that measuring the performance of a SS power amp has been discussed and over-discussed. Judging performance with our ears is very important, but somewhat subjective.

If our goal is to re-produce the original production without adding anything to it or taking anything away, can that be measured?

If our goal is to color the original production, can that be measured?

Has anyone here tried the following:

Use a digital phosphor oscilloscope to measure the difference between the signal output of a turntable or CD player (input to audio amplifier(s)) and the output of the audio amplifier(s). In the case of a separate pre-amp and power amp use 3 channels or compare the source to the output of the pre-amp and power amp separately. One piece of equipment for this measurement for example, could be the TDS 3054. For the source; CD or record use something that has a lot of dynamics with fast rise times, or try several music sources.

If the bandwidth of the amplifier(s) is sufficient as to not cause significant phase shift, a difference between a sampled portion of the input and output waveforms could be performed. This is not a real time test, but a test on the digitally stored (saved)waveform samples. Of course, I'm assuming the ouput magnitude can be scaled to equal magnitude of the input. Taking the difference or subtracting these waveforms may show something significant or maybe not. Looking at (visual study) the saved waveforms on dual trace at different time bases may provide some good information. If the test even works, performing it on one amplifier may not be useful. However, comparing the results for several amplifiers may show some understandable differences. Test results could show effects of THD and/or IMD and/or variations in amplifier magnitude versus frequency, for example.

I choose actual music for the source instead of sine waveforms, triangular waveforms, or square waveforms because in most cases this is what we want to amplify. There may be too much phase shift (delay) from input to output for this test to yield useful results. It may be a wasted effort. I've not tried it since I do not yet own anything like the TDS3054. I suspect some have performed this test. I did search for this type of test, but did not spend a lot of time on it, as I live in the boonies and can only get slow Internet access. If this method of testing has already been discussed, please excuse me.

Thanks

[SY]

Variations of this go back to Hafler and earlier. The results are hard to correlate with subjective impressions gained through uncontrolled listening tests. And in controlled listening tests, amps with low source Z and moderately low (or better) distortion that aren't clipping or unstable with a given load tend to be indistinguishable by ear.

You can draw your own conclusions from that.

[mwh-eng]

Hard to draw an objective conclusion with the information provided.

Low, moderately low (or better) seems relative to me.

Controlled listening could have been 10 people listening to 5 amps from different manufacturers, but of similar design and quality. Or controlled listening could have been with more than 50 people listening to several amps of different design and quality but still achieved low source z and low distortion without stability problems or clipping.

Assuming that an adequate audio amplifier just needs to have a low source z, low distortion without stability problems or clipping, proves to me there is a lot of money driven marketing hype going on. Electronic components are very inexpensive in general.

Six years ago I walked into a high end audio store. The owner rushed me over to an Italian audio system with a $15,000.00 plus, price tag and gets me to listen for a few minutes. I didn't say much and left pretty soon. I didn't want to hurt his feelings, but my inexpensive home system sounded as good or better to me. However, if he had listened to a system like mine he might have said it sounded very bad. Maybe there is some placebo effect taking place.

[SY]

Assuming that an adequate audio amplifier just needs to have a low source z, low distortion without stability problems or clipping, proves to me there is a lot of money driven marketing hype going on.

To a large extent, yes. For some loads, heroic measures are needed to achieve these. But for most reasonable speakers, it's just not all that difficult to build a box of gain that will be audibly transparent in a properly controlled blind test. Past that, it's image, build quality, service, adherence to an esthetic that lies beyond the realm of the provable, and... yes, often hype.

A wonderfully fun exercise is to take any high-end design shibboleth (Class A or no loop feedback or silver wire or tubes-only or FETs-only or whatever) and list how many well-regarded amplifiers do NOT incorporate that concept. The very diversity of these "philosophies" without any indication of convergence is suggestive.

[mwh-eng]

Yes, what you say makes good sense. Thanks for the reply.

I've read so many claims, most of which are hard to believe based on science, education and experience. If it sounds too good to be true, it's probably not true, is my experience with about everything in life, especially these days.

Some of the audiophiles that write amp review magazine articles must be paid to write nice things about an amp they are given to demo. They describe the sound of the amp like it's a gourmet dinner at a fine restaurant. To me, the sound should be somewhat related to the amps true specifications or just be transparent. The color and/or taste should be in the music, it seems.

I believe my old B&W speakers were tested to have something like 2 percent distortion. If that's true then I don't think an amp needs to have .001 percent distortion. Seems like .1 percent is plenty good enough. Something like 50 milliohms output impedance should be acceptable for most speakers.

I do think that too much global feedback can contribute to transient distortion. Combined local and global feedback for a THD of about .05 to .1 percent should be adequate.

Probably a solid power supply, the LM3886, LM3875 or similar and OPA2134 or similar would make a decent audio amp. I suspect many of the mass marketed home theater amps use similar components.

[traderbam]

mwh,
Very interesting. You are not just a good cook.

I believe my old B&W speakers were tested to have something like 2 percent distortion. If that's true then I don't think an amp needs to have .001 percent distortion.

Caution. This statement assumes that all amp distortion is swamped by the speaker distortion. Are you sure?

Something like 50 milliohms output impedance should be acceptable for most speakers.

Yes. IME 200m-ohms is quite acceptable with normal speakers.

[SY]

Some of the audiophiles that write amp review magazine articles must be paid to write nice things about an amp they are given to demo. They describe the sound of the amp like it's a gourmet dinner at a fine restaurant.

Well, I know a few magazine writers (audio and otherwise) and what they get paid for is writng reviews that help sell more magazines and advertising. That doesn't necessarily mean "good" reviews, but it means reviews that affirm the notion of the industry which supports their magazine. Some of them are cynical about what they do, some of them really believe what they do, but none of them actually critically examine the basis of what writes the checks (at least they don't in public).

The chip amp phenomenon that you see here is a reflection of the fact that indeed one can build very transparent electronic channels without much expense or effort (if one is inclined to do so). Then there are nuts like me who take the trouble to do tubes and more exotic circuits, just for the esthetic pleasure.

[jackinnj]

Originally posted by SY


Well, I know a few magazine writers (audio and otherwise) and what they get paid for is writng reviews that help sell more magazines and advertising. That doesn't necessarily mean "good" reviews, but it means reviews that affirm the notion of the industry which supports their magazine. Some of them are cynical about what they do, some of them really believe what they do, but none of them actually critically examine the basis of what writes the checks (at least they don't in public).

SY -- some of the reviewers are best termed (seen on this BB previously) "para-physicists" --

[mwh-eng]

Originally posted by traderbam

Caution. This statement assumes that all amp distortion is swamped by the speaker distortion. Are you sure?

Yes, I agree with you! No I'm not sure. My statement was way too general. I need to define the type of distortion. The speaker can pass some nasty sounding amp distortion. Amp IMD or transient distortion may not be swamped at all or very little by the speaker. Depending on nature of THD (magnitude of even and/or odd harmonics), they may not be swamped by the speaker.

Thanks for catching my mistake!