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Old 6th June 2013, 08:52 PM   #1
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Default Amplifier distortion with music signal (measurements inside)

(since I've taken a vow not to post again in the sound vs measurements thread which I find to be an utter waste of server space).

I've made measurements of this type before but never bothered to investigate too much or post them.
here's the thing. 2 amps with pretty different sound. the measurement is made at the level I normally listen at, that is ~9V RMS with full-scale sine input (4 ohm, 83 dB W/m speakers, impedance equalized, 4x5 m room, listening position at ~2.something meters away from the speakers plane). I've used a song which is especially revealing wrt sonic differences.
one of the amps has average harmonic distortion (3rd order harmonic @-50dB, the rest vanishing pretty rapidly) but I've tried to simulate something similar in software and it doesn't seem to explain the differences (quick math confirms it, at 20W in 4 ohms THD is ~0.2%). the other one has considerably lower distortion.
oh, and the load was the actual speaker playing music.

so, here are the 3 time-domain pics. amp input vs outputs of both amps. the outputs were low-pass filtered at 250Hz to eliminate phase-shift issues at high frequencies due to different corner frequencies.

if it's not already obvious, both amps are flat in that band (<250Hz).

first interesting question is, which of the two is the better sounding one?

(try to view the files with IrfanView or a similar viewer, and switch from one to another in order to see the differences more easily)
Attached Images
File Type: png amp_input.png (49.5 KB, 199 views)
File Type: png amp_A_output.png (51.2 KB, 197 views)
File Type: png amp_B_output.png (49.0 KB, 196 views)
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Last edited by mr_push_pull; 6th June 2013 at 09:22 PM.
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Old 6th June 2013, 09:11 PM   #2
DF96 is offline DF96  England
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Your low-pass filter may have wiped most differences. In any case, time-domain differences will be mainly due to slightly different frequency responses, which is why people use FFT to get a spectrum instead.
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Old 6th June 2013, 09:20 PM   #3
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Quote:
Originally Posted by DF96 View Post
Your low-pass filter may have wiped most differences. In any case, time-domain differences will be mainly due to slightly different frequency responses, which is why people use FFT to get a spectrum instead.
sorry, but I don't see how that is possible. I agree, maybe I'm missing something but at this point I don't know what that may be. "slight" (how slight?) FR variations translating to huge time-domain variations? are you sure? have any quantitative example? 0.5dB means a 5% difference on a linear scale.

first, I would say the time-domain differences are huge, I didn't need a looking glass
second, there are no significant freq. response variations (mentioned in the original post).

and, speaking of which, guess which of the 2 amps has the FR that most deviates from flat?

are you sure you switched from one pic to another like I described? (amp in vs A, amp in vs B) using the forum pic viewer is useless unless you're very patient.

PS: IrfanVIew is free

later edit: the level difference seen in the first few negative spikes is ~1.5dB, I think that would be pretty oticeable in a FR plot
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Last edited by mr_push_pull; 6th June 2013 at 09:45 PM.
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Old 6th June 2013, 09:48 PM   #4
Pano is offline Pano  United States
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It can be done in windoze if you save the pics.

Anyway, it's an interesting approach, I just don't know how to interpret the results. Is there some software that would allow an FFT of that difference signal? Basically, I'd like to see the spectrum of the I/O difference with a musical signal. I know that's not all there is to it, but I would be a help to get started.
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Old 6th June 2013, 10:11 PM   #5
DF96 is offline DF96  England
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No, I didn't compare the pics as I don't see the point. It is easy to produce a big time domain difference which is almost inaudible - just change the phase a little. On the other hand, some small time domain differences can be quite audible - perhaps a bit of crossover or peak clipping but then smoothed by a low pass filter.

Time domain pics are good for debugging/repairing faults but poor for determining sound quality.
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Old 6th June 2013, 10:11 PM   #6
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the problem with difference signal (null testing) is that you need to make sure there's no phase shift involved. which is practically never true and even pretty severe with some amps.

I'll give you a hint: one of the amps is bass shy and pretty uninvolving

DF96, don't get me wrong, I'm sure what is seen in the pics above is pretty simple to explain, but I dare you to come up with a frequency response that is +/- 0.05 dB 10Hz-3kHz but gives time-domain variations of ~1.5dB. of course, non-linearities aside, since it's obviously the only thing that can explain those differences, otherwise we're into twilight zone
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Old 6th June 2013, 10:18 PM   #7
DF96 is offline DF96  England
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Are you saying that the amps have the same measured frequency response into the same real speaker load? Or the same response into resistors?

'Bass shy' means what, given the same frequency response?

You can get phase changes without frequency changes if the circuit includes an all-pass filter or some other non-minimum phase filter (typically, any circuit where there are two or more parallel signal paths feeding the output - can be deliberate or accidental).
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Old 6th June 2013, 10:20 PM   #8
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Quote:
Originally Posted by DF96 View Post
No, I didn't compare the pics as I don't see the point. It is easy to produce a big time domain difference which is almost inaudible - just change the phase a little.
who said there's any phase shift involved? a time domain difference being almost inaudible has nothing to do with my original post, not sure what's your point.
I'll summarize again:
both amps have flat FR responses, actually the worse time-domain plot is of the amp which is the flattest (+/- 0.05 dB).

I'll do some frequency response measurements tomorrow. at least the ones published by a well-known magazine would not explain it.
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Old 6th June 2013, 11:03 PM   #9
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Quote:
Originally Posted by DF96 View Post
You can get phase changes without frequency changes if the circuit includes an all-pass filter or some other non-minimum phase filter.
maybe it's exactly something along those lines that this topic aims at? do I generally strike you as the type who expects that there is some "obscure" or undiscovered part of audio engineering?
actually, I had suspected that the amp becomes highly non-linear above a certain level (and that is intentional). which is not visible is the publised plots because the measurement is done ~2W in 4 ohms.

meanwhile I did FFTs of the pictured time-domain data. in the one displaying larger spikes, there is actually a pretty consistent difference as compared to the input signal and the other amp, as expected. but the difference is pretty localized at ~120 Hz and is ~4dB high (!). I'll do FR measurements tomorrow with the real (speaker) load. I trust my drivers will withstand short sweeps of ~20W
at any rate, there's something interesting to be learned. maybe I have a defective (but nevertheless good sounding) unit, maybe the published data is intentionally misleading... we'll see.
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Old 6th June 2013, 11:09 PM   #10
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IMO, if the amps are flat they probably don't have phase differences if the signal is well in the passband. Few amps include all-pass filters! Have you done any square wave testing into speaker loads? They might well have different stability or zobels or something that explains the differences, even though steady state signals look identical. I'd also look for stray RF from the PS or stability related as I've seen RF oscillations on the peaks of waveforms change the average path of the waveform. This is the sort of test I like because it goes after real world differences. OTOH, I've never heard two amplifiers that sounded different unless something was wrong with one or both. Correctly designed and functioning power amps operated at moderate levels are and should be indistinguishable. Any amp that's different from the average is suspect.
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