Electronics > Repair
Audio… (Amps, THD, channel separation) with analog equipment (scopes, etc.)
TimFox:
With an “H”, the load is located on the horizontal line between the two uprights, where each upright has a “pull-up” and a “pull-down” device. Neither end of the load is grounded, so you can’t put the ground lead of the ‘scope on one side of the load. The two sides move up and down with respect to the power supply, with 180 deg phase difference.
My “low DC voltage” comment refers to the power supply. The peak-to-peak output voltage is twice the DC supply voltage, while the usual circuit gives a pk-pk voltage equal to the DC supply.
This is handy for annoying other drivers when blasting audio from a 12 V automobile supply.
Fried Chicken:
I got my amp out of the system and hooked up. I carefully set up my signal generator and at its lowest output it's within a safe regime, but still slightly high lol.
I turns out I have two 8 Ohm 200W dummy loads, so perfect!
I carefully got everything wired up, here's my setup (apologies for the out of focus picture):
Immediately I notice the right channel has a lower p-p than the left channel, which is unusual because I swear the soundstage is shifted to the right. I can now start dicking around. I'm just getting my feet wet here, so I'm being stupid cautious.
Fried Chicken:
It's pouring rain outside (actually thunderstorming so I've had to pause my dicking around).
The left channel is definitely slightly stronger than the right channel, as reported by both the scope and the lights on the front of the amp (left rises first). I swapped the scope probes as a test, and indeed, the left channel on the amp reports stronger. I didn't measure the resistance of the loads I'm using, maybe I should do that. This is weird because the soundstage is off to the right when listening.
I drove the amp into clipping, and.. oh boy those power resistors got hot fast. (I really am doing this for the first time)
The front of the amp reported somewhere between 30 and 90 watts into clipping.
The p-p voltages measured on the scope are ~110-120V since I have the probes attenuating 10x... This doesn't sound right to me, that's almost line voltage??!
I guess this would make sense for power at a single frequency?
Fried Chicken:
So now I'm dicking around and things are starting to get interesting.
I inverted the B channel signal and set the scope to add... and the difference was quite stark. This difference changed significantly as output on the function generator was increased, but remained almost identical as the frequency was changed: it was like a sine-wave squishing and expanding apart.
I disconnected the power resistors and measured their resistance with the Fluke.
I came up with 8.22Ω and 8.20Ω so they're decently close to one another, then I swapped them around Left/Right, cut off a bit of wire and reattached the cleaned wire to the binding post yielding these new different results:
Weirdly, after cleaning and swapping around the resistors, the outputs look almost identical, Vp-p wise. The behavior of the difference of signals remains the same (changes on output from signal generator, squishes with changes in frequency).
The probes are on 10x for this. The I'm thinking it might not be stupid to open the amplifier and start probing to find where that difference of signal might be coming from, but first I think it would be smart to set the biases appropriately.
One final note worth adding (I am writing notes to myself as well as everyone else, I'm not expecting everyone absorb and interpret every detail on my behalf!), depressing the "Auto Class-A" button makes no difference in the signal. None. I tried at different power levels, low, high, different frequencies, measuring the subtracted signals, I saw zero change to the signal.
I think I'm going to open the amp up and set the adjustments according to the manual... One final thing I've noted, I need longer probe cables for my fluke multimeter!
Kleinstein:
The observed 120 V peak to peak are some 43 V RMS. So still quite a bit lower than mains voltage. Higher power amplifiers, especially when made for 8 ohms can indeed provide quite high voltages. They may have a warning about potentially hazardous voltages at the speaker output.
The amplifier output is normally low impedance. So the difference in the loading should not be that relevant.
An amplitude dependent effect would point to a linearity problem - maybe also loudness enabled.
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