andy_c said:
Haven't had time to address this and the issue Scott raised about Geddes doing the tests at low levels:
Geddes:"Its NOT the level of the harmonics that matter but where the nonlinearity occurs - at low levels or high levels and the order, 2nd, sixth, etc. Low level nonlinearity is by far the most insidious especially if high order - like crossover distortion in an amp. This is why an amp with extremely low levels of THD can still sound terrible. But loudspeakers, on the other hand, tend to have nonlinearities that increase with level and are likely very low level like second or third. This makes them fairly benign. In fact, for the most part, nonlinearity in a loudspeaker (as long as its not broke) is a non-issue. In a test of compression drivers we had twenty five people evaluate distortion levels up to 25% and statistically noone could detect it at those levels."
and: "What's basically missing in most distortion studies is how the distortion goes as the signal drops - not how it grows with signal level. The later is almost irrelavent while the prior is critical. When I looked at the harmonics for amps as the signal went down into the noise floor I found significant differences that correlated very well with the subjective perception. In a poor amp the levels of the harmonics rise as the signal falls and it can be very high order. A clean amp shows nothing as the signal falls. What it does near clipping is almost unimportant and in fact, I claim that a soft clipped amp sounds better."
http://www.diyaudio.com/forums/showthread.php?s=&threadid=139046&perpage=35&pagenumber=1
Perhaps I am daft, but I think the way I summarized his findings is well correlated to the way he summarizes his findings?
It is increasingly bizzare that I quote Geddes... heh. Nvm.
_-_-bear
bear said:
That quote supports exactly what I said. I don't mean to be difficult but I think you are missing an important point.
>>In a poor amp the levels of the harmonics RISE as the signal falls and it can be very high order.<<
The THD at low levels is very high but THD at max power as people put on data sheets is low. He says that a device, speaker or amplifiers that has monotonically decreasing distortion will sound better. Mechanical transducers inherently behave that way, that's why vinyl is tolerable even though it has several percent distortion at high levels.
My comment stands, when all distortion is <.01% (random number) the exact spectra probably does not matter. I simply asked Mr. Geddes if he ever repeated his perceptual tests where all the DISTORTION was at very low levels not the signal.
Yes it is strange you quote him, didn't he say something about not having to listen because the design process was "perfect"...
bear said:
Hi bear,
When I said "Earl's distortion criterion", I meant specifically "The GedLee metric as described in his two papers about it". That may be how we got to be at cross-purposes here. I wasn't aware of any comments he'd made in the speaker forum as I generally don't read it.
Earl's statement, "What's missing in most distortion studies..." appears to apply to his own papers on the GedLee metric. The THD number he uses in that paper does not appear to be a worst-case number over all signal levels below clipping. That dead-zone circuit he uses will have undefined (infinite?) distortion at low signal levels, because there will be no output signal at all, even for a non-zero input signal. But his THD numbers don't reflect that.
A while back, he had some WAV files on his site for download. One of these was supposedly for "0.01 percent distortion". The thing was grossly distorted and completely unlistenable such that there's no way that was the worst case over all signal levels. So I agree with Scott's concern that the THD should be taken over all signal levels. If he's going to compare the GedLee metric with a THD number, that THD number should be the worst case over all signal levels of ordinary operation.
At any rate, you mentioned that his criterion was for the spectrum of the output, and I want to reiterate that the GedLee metric is not signal-based, but based on the nonlinear transfer characteristic.
syn08 said:Only if you promise to avoid PMSing again
There is a slight variation of the phase margin with the signal level. The resistor broadens the phase margin peak at 8MHz, providing an almost constant phase margin for all inputs.
Okay, understand.
BTW, what the heck is PMS? Pre-Menstrual Syndrome?
If so, don't worry, I'm not a woman.
BTW2: did you read my e-mail?
Now, who is acting 'out of control' and rude? A (hint) Edmond, Prozac will remove PMS like symptoms in men. That is what I use. The last time I tried to get off it, I got binned. It really works! It is why I normally can laugh here, rather than really get angry at the implied insults and petty gossip.
Bear, what you brought out from Geddes, is VERY insightful, (and obvious to you and me). OF COURSE, I want to listen to my music it the most linear mode possible. I also know that I can REDUCE my quiescent current and get essentially the SAME rated distortion. I have a switch that allows this on the JC-1 power amp.
I make a number of amps. I now use a 250W/channel with perhaps .7A Iq. It is OK, but nothing special. I make amps that range from about 100W/channel to 800W/channel into 4 ohms for Parasound. Most of my amps are really marginal when it comes to Iq, but they are still relatively OK, because the amps are fast, some Iq is allowed, due to a reasonable sized heatsink, and the topology is essentially linear.
However, for my BEST efforts, I prefer listening for the majority of time in the Class A region of the amp. Charles understands this, Nelson Pass understands this, Bear understands this, and so does Dimitri, AudioWolf, PMA, and many others. I compete with true competitors, and they are NOT afraid to use quiescent current, in fact, Nelson and Bear use MORE quiescent current than I do in their best designs. They know something that I know, and the rest of you should just accept. It sounds better.
I make a number of amps. I now use a 250W/channel with perhaps .7A Iq. It is OK, but nothing special. I make amps that range from about 100W/channel to 800W/channel into 4 ohms for Parasound. Most of my amps are really marginal when it comes to Iq, but they are still relatively OK, because the amps are fast, some Iq is allowed, due to a reasonable sized heatsink, and the topology is essentially linear.
However, for my BEST efforts, I prefer listening for the majority of time in the Class A region of the amp. Charles understands this, Nelson Pass understands this, Bear understands this, and so does Dimitri, AudioWolf, PMA, and many others. I compete with true competitors, and they are NOT afraid to use quiescent current, in fact, Nelson and Bear use MORE quiescent current than I do in their best designs. They know something that I know, and the rest of you should just accept. It sounds better.
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