A listening test of two power amps

"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."

Thanks a lot budjoe. I really appreciate your comments. Yes, please send the analysis. I’m interested to see exactly what you’re looking at.

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

I don’t think the phones were overdriven. I hear this sound through the speakers and on headphones (AKG K501 and K246). What I don’t hear on the speakers is the 60Hz buzz. This I only hear when "tapping" the speaker output of the amp and listening on headphones, either directly or on a recording. Although that doesn’t necessarily mean that the sound is not there - there’s a bit of other noise here always.

The more I listen to the sound, the more it sounds like a type of ring-modulation of the piano and I wonder if some sort of cross-modulation is occurring. If this is actually the case, there would be additional side bands appearing in the analysis and not necessarily a distortion.

Listening now to the 3 piano files on phones and at the same volume, p-piano.flac does not have this ring-modulation sound in the first half of the recording, but y and j certainly do.

The original recording is of tracks 8 and 9 ("Var. V. Lento" and "Var. VI Poco movendo") of Alessandro Simonetto’s album "Erik Satie: Works for Piano". I streamed them into a DAW from Qobuz and used this recording for the three amp tests.

budjoe, I normalised the peak of the j-piano and p-piano recordings to -0.3dB (which is what you did?). The above is Ardour’s spectrum analysis of the 1st note of the two recordings. The j-piano recording seems to have a lot more noise. The p-piano part seems to have stronger high frequency transients between 1 and 5kHz.

Also, just above the 1st orange band above 100Hz, there’s what seems like a pulsing partial that’s not present in the p recording. This was evident before normalising both recordings and in which there was less apparent noise but still very weak upper partials (between 1 and 5kHz) in the j recording.

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.

@budjoe Thanks again for your help with this. Yes, the last set of recordings was done with the same cables, speakers (Tannoy), preamp (Quad 34) and recording device. Only the power amps were changed.

I wonder if normalising the peak values is the right way to go. I imagine that the two amps will potentially render peaks quite differently. And if that’s the case, wouldn’t that produce audio mismatched for loudness and with the background noise exaggerated in the recording with the lower peak?

I’ve gone back to my Tannoy/Quad34 recordings and normalised the files, constraining the RMS to -5.0dB. There was no clipping and the two files sound reasonably equal in loudness to me in headphones.

Yes the Quad recording has noise and a thin, grainy sounding hum, but the hum of the Sunfire is louder and deeper, to my ears at least. On the Quad the hum is louder in the right channel. On the Sunfire the hum is louder in the left but more balanced.

All the same...

More important to me than the noise, is the distortion of the piano sound in the Sunfire recording. Are you (or anyone else) able to hear this distortion?

Here are the two files normalised for loudness:

https://escuta.org/webtmp/QuadPiano.flac

https://escuta.org/webtmp/SunfireVoltagePiano.flac

Cheers,

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.