A listening test of two power amps

@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.

Thank you very much budjoe for taking the time to listen to the recordings and for the detailed account. Yes, it’s a very good point about environmental sound affecting the recording and the listener’s ability to distinguish between the two. There was perhaps a bit more wind in the first recording too along with those angry birds at the end. I think the Tascam and the valve mic had been on for over half an hour and the second test was done shortly after the first. So probably not much of an imbalance there. But good point, also.

jji666. I agree!

Thanks too, erik_squires. I think it’s a great idea to do this test bi-passing the speakers and consequently any unwanted ambient noise and irregular room interactions. I’m hoping I can do this later today or on Wednesday. I’ll likely do the recording at the amp end of the speaker wire. Some recent surgical work has left me a bit shy of shifting equipment!

Back with a new test as per erik_squires’s suggestion

This time the tannoys high and low frequency terminals were jumper-ed and there was no bi-amping or bi-wiring done.

Three recordings were made of three pieces of music with two amps. One amp has two speaker out options, so that’s why there’s three recordings. The three amp configurations are name "j", "y" and "p". The three recordings are of a sparse piano track with a change in dynamic in the middle: "piano". A samba jazz trio: "trio". And a voice with orchestra track (great production but the master recording could be better, i think): "voice". So the 9 edited recordings are:

https://escuta.org/webtmp/j-trio.flac

https://escuta.org/webtmp/y-trio.flac

https://escuta.org/webtmp/p-trio.flac

https://escuta.org/webtmp/j-piano.flac

https://escuta.org/webtmp/y-piano.flac

https://escuta.org/webtmp/p-piano.flac

https://escuta.org/webtmp/j-voice.flac

https://escuta.org/webtmp/y-voice.flac

https://escuta.org/webtmp/p-voice.flac

Steps.

1. A 60Hz sine tone was recorded at approximately -1.0dB

2. The tone was played with each amp setup with the voltage of the amp adjusted with the preamp to measure 0.5V. Fine adjustments of the voltage were made with the volume control of the digital source (Logitech Media Server)

3. For each amp, the three musics were played at the adjusted amplitude.

4. Recordings were made at 48kHz with a Behringer UCA222 interface on a laptop running the DAW Ardour.

5. Nine edited ecordings exported as 48kHz 24bit Flac files.

If anyone has time to listen and give some feedback, please do!

Listening to these now, I think I’ve found an unpleasant problem and not just a difference, in one of the amps....

The problem is in the recordings j-piano.flac and y-piano.flac which are recordings of the Sunfire’s "Voltage source" and "Current source" speaker outs respectively. The piano, in its quieter passages, produces a distinct distortion, especially on certain notes. The recording p-piano.flac, that of the Quad 303, produces no such distortion.

I have a theory on what the problem is:

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.

My theory is that when the piano is played softly, certain notes seem to interact with the 60Hz hum, producing what seems like distortion, so if the hum can be removed, perhaps that distortion will disappear.

Is some kind of a mains filter likely to solve the problem or is there something that can be done to the Sunfire to make it less susceptible to this 60Hz hum?

And yes, I do hear the distortion through the speakers. I had noticed this sound before on occasions, but since I was using the Sunfire in my studio, I always suspected the speakers (a pair of JBL 4312A monitors), which have had a long hard life.