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A listening test of two power amps

Hello, 

It's my first post here. I've been using two power amp setups for my main stereo and I've been curious to see if I can really discern any acoustic difference between the two. One setup involves a bi-wired high-powered stereo power amp and the other uses a pair of identical lower-powered amps with which the speakers, a pair of Tannoy System 12 DMT II monitors, are vertically bi-amped.

I decided to devise a listening test involving a mono acoustic recording made with a valve-condenser mic positioned at my usual listening position. I've used a relatively simple method to ensure that the recordings are level-matched. I've chosen a mono recording method since my goal is, principally, to evaluate the "tone" of the two recordings. I've been inspired to do this test after reading W. A. James' eBook "High end audio on a budget". The aim of the listening test is to try and discern which power amp setup provides the most realistic rendering of acoustic instruments. I thought that a mono recording might help the listener concentrate on the tone. After listening, I think it does. It's less distracting, especially on piano, where stereo or other multi-mic recording setups tend to splay out the notes across the stereo field.

I made two recordings for the test and will place links below so that the audio can be downloaded. I won't at this point give the make and model of the power amps involved, but this is the method used:

Method

1. I created an audio file with white noise at -10dB RMS and put the file on a Logitech Media Server so that I could play it on my stereo using a Raspberry Pi 3 with Audio Injector Pro card and RCA interface (192kHz 24bit DAC).

2. I then put on an LP on a Pro-ject 1.2 and set the volume to my usual listening level on a Quad 34 preamp. Following this, I then played the white noise and used a decibel meter, positioned next to the mic, to measure the level. It measured 67.3 dB.

3. Still playing the noise, I set the record level on a portable Tascam digital recorder arbitrarily to somewhere above -15dB. The microphone used was a large diaphragm valve condenser mic. The Tascam was set to record at 192kHz 24bit.

4. I then recorded the first track of the LP on the Tascam.

5. After that, I wired up the other amp configuration. I played the white noise and adjusted the volume of the preamp such that the decibel meter again measured 67.3dB at the position of the mic. The volume control on the Quad 34 is stepped, so I was lucky it matched!

6. I then recorded the same track on the LP as before, leaving the Tascam record levels unchanged.

7. I tidied the two recordings in Ardour (trimming start and finish only) and exported each as a 192kHz 24bit Flac file. I did not adjust the gain on either recording.

8. I listened to the recordings on the computer with a pair of AKG K501 headphones and Focusrite Scarlett interface.

Results

At first, I could distinguish a marked difference between the two. But now, I'm uncertain of the first qualitative difference that I'd noticed but I have noticed other more subtle differences (for the moment anyway). And that's why I'm here!

It would be wonderful if some people here could listen to the recordings and say which recording produces the most realistic rendering of the three instruments therein, and why. The instruments being piano, drums and string bass.

I've given the two files nondescript names: e.flac and t.flac. If anyone needs a different format or for me to down-sample, please let me know.

Finally, here are the files:

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

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

Cheers,

128x128surdo
surdo's avatar
surdo OP
22 posts
 

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.

 

budjoe
24 posts
 

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.

 

surdo's avatar
surdo OP
22 posts
 

@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,

 

budjoe
24 posts
 

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.

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