A listening test of two power amps

Haven’t listened to the files yet, but hope to later today.

I find your wiring configuration of the Sunfire amp confusing. To my understanding, current drive would have the most effect on the lower frequencies. The tweeters should not need for much current regulation or drive. Have you tried the Sunfire with the current/voltage switched. I think that might change the sound presentation significantly.

@erik_squires "1. Match level by multimeter instead of SPL. Get a 60 Hz signal and check the output on either speaker. You can get really accurate this way."

I am a little ambivalent about using 60Hz sine and voltage to verify output level balance. It can tell you something about what an amplifier is doing, and something about speaker efficiency, but I am not sure it can translate into anything about sound quality. There is also the problem of ensuring the meter you are using is measuring true RMS versus average AC voltage. If you are trying to use that method to balance output level, I think you would be better to use pink noise which has a full frequency range. Measurement becomes more of a problem, but it more closely resembles what you may hear when converted to sound using real music. In other words, since speakers are a reactive load, frequency and duration affect response and the amplifier’s drive of voltage across a frequency and time response will vary and may affect actual perceived sound volume.

I don’t really understand why SPL would be frowned upon. It is specifically an agreed measurement of perceptual sound volume reference. It is tailored for a specific frequency range and can be A or C balanced for perceptual preference favoring broad frequency response (A) or lower frequency weighted (C). The only caution for use in this testing is that the ambient noise level is maintained the same, and the SPL is taken from the same location in the same environment. If you are within 0.1 or 0.2 db that is considered well matched.

@surdo 

Finally got to due a little more compare of the e & t files. My first try was at a lower volume than I think should be used for comparison. First pass at that low volume, file e was much better than t, but on replay, t was closer to e than first pass of either. Maybe my electronics were not fully warmed up.

Today played at more reasonable listening level, and also played through headphones. Both files very close overall. Occasional portions sound a little better on one or the other. In general, I like the piano sound (tibre, resonance, attack) better on e than on t, but that is mostly from middle C and above. Lower octaves may be slightly better on t. Bass very close on both. Drums and Drum kit too low in volume and too far in behind piano and bass to judge.

In general, I think most of the differences may be more environmental and microphone, rather than effects of either amplifier and connection. Note especially squeaky wheel/bird song audible on file e from about 4’43’’ to 4’51”, but missing on file t.

So in all, I think you tried an interesting experiment, but I can only tell you about what I can hear differently on the 2 files, and I think there may be more environmental artifacts in the two files than you were hoping for. Some may even be from the microphone and tascam reaching a thermal equilibrium, as well as the electronics and the room. Not sure I would hear the same things if I were in the room listening without the microphone and Tascam involved, so I think any of my comments about the files should be discarded.

@erik_squires "It’s your experiment, but as someone who worked quite a bit with microphones and speakers and electronics, the advice I gave you is solid."

As I said, I am ambivalent regarding matching amplifier out at speaker via 60HZ for amplifier compare. 60Hz sine voltage matching thru the rest of the audio chain is a really good way to compare different pieces of the chain. It is also a good reference for speaker efficiency. For all that is going on between and amplifier and speakers I am not sure it takes everything into account. 

I would love to see a graphing/charting of 60Hz voltage matched amps to the same speakers done in an anechoic chamber with dbA and dbC with some wide frequency music to see the active perceptual effects. Know anyone that can perform this kind of testing? Will see if I can find anything regarding similar testing.

Listened to all nine as originally provided, and with a modification (more on that later).

In general across the 3 different versions of each I liked j the most, y second, and p the least. Differences are slight. I like the attack on j and y the best, and think that p tends to round the sound off more; both on the attack and the decay. Most differences showed up in the trio recordings where the lead guitar has a much more natural sound to me. The plucking of the strings and the resonance of them is better and more natural. Also most noticeable in the voice where her voice cracks in the beginning at ”and you know darn well”.

Had a hard time telling much difference in the piano samples except that once again I felt the p was more rounded. That is the only way I know how to describe it. Bass was also very hard to tell across all samples. You may need someone with better sense of piano and bass to hear the differences in those samples.

Here is the more later.

I loaded all the samples in an app that lets me analyze the waveform, and also see there that the j and y samples are very close to each other, and the p ends up being pretty close to 1 dB lower in both average RMS, max RMS, and peak amplitude. across all samples. Within that app I normalized all the samples. Used -3.0 dB peak for the voice and piano and -0.3 dB peak for the trio as that was the max peak in the originals.

I played both the normalized and originals thru 2 different playback apps and my observations hold across both the original and normalized thru both apps.

If you want the full analysis values from the originals and the normalized files, I will send them in a direct message. Let me know.

Just read your post about distortion on the j and y piano files. I think you may have something else going on. Looking at the wave form of the j and p files, there is almost no ripple in the quiet parts on either file. Certainly not enough to be audible. (Wish I could record something from lp to digital with that low a signal, but maybe your source was not lp). I pushed those files (j and p) to the -0.3db normalization and again see almost no ripple in the quiet parts and very little difference between the files.

Were listening to these files through the speakers or headphones? Possible you were hearing issues with the headphones and possibly being overdriven?

In the samples I sent to you, the waveforms were normalized to -3.0 dB, then the zoomed portion was normalized to -0.3 dB.

In the zoomed section, the noise on the quiet part of the j-piano-zoom was very similar on both the left and right channel. Some high frequency noise, but very low level. In the p-piano-zoom, the left channel (top) has almost no high frequency noise, but the right channel has the most noise of all the waveforms. That is what leads me to believe there is something else going on.

Connections, grounding, or impedance, or some problem in one of the amps. I can’t say from here. I think you should explore some tests with the minimum number of components in each chain. Try with all components in the same power circuit. If any component used has only a 2 conductor power cord, try turning that cable over to see if it changes the waveform and/or the sound. Swap left and right channel interconnects one at a time and see if the noise moves from one channel to the other. Swap the speaker wires from left to right again to see if the noise moves from one channel to the other.

From what you are hearing, and the wave forms; i don’t think you need to have any input to chase the issue. I would use headphones at the amplifier headphone jack (if there is one) and with just the minimal signal path connected and no audio playing turn up the volume and listen just for the noise and try to swap interconnects and speaker cables to look for any changes. You can add in your recording set up to see if the noise you are hearing shows up in the waveforms. That’s how I chase noise.

@surdo "The Sunfire’s speaker outputs produce a low level, but audible on headphones, 60Hz hum (or a harmonic of 60Hz). The Quad, while perhaps more noisy (with hiss) has no 60Hz tone. I tested both the Sunfire’s unbalanced inputs with the Quad 34 preamp and the balanced inputs with the sound coming from a mixing desk. Both produce the hum and the amp produces a hum even with no inputs connected and with the laptop, that’s monitoring the terminals, running on batteries. I’ve tried disconnecting the Sunfire’s earth and reversing the pins, but the hum is unchanged.”

Just went back and read this part again. I don’t see much 60Hz on any of the waveforms, but as I said, there is 120Hz on all of them. Using just the Quad 34 preamp and connected to the 2 amps and no input look for the noise. I am also not clear if your tests were done with 2 different sets of speakers and speaker cables, but to compare the amps, it should be done with the same speakers and speaker cables from either amp. If that is not the case, the speakers and cables cannot be separated from what the amps are doing.

Normalizing should be done to peak amplitude, not RMS. Both of your files have a peak amplitude that is clipped at between 1’08”  and 1’09”. I generally normalize to -1.0 dB of peak. I used values in your original samples to come up with the values I used when initially normalizing them since I wanted to add a little variable as I could to the samples. 

The rest of the values from the waveform indicate the min and max of both the RMS absolute dB are very closely matched.

Haven’t listened to them yet, but noticed that the sonogram for the Quad has some strong tones around 8KHz that is much less in the Sunfire.