Have you tested isolating the amplifier and preamplifier to find out where the problem is located? (remove the bridge on the back)
@Jakobsson, Good idea. I just did and it turns out that the startup/shutoff noise is coming from the preamp.
Illustrating that conclusion, let's start with the right speaker output at startup when the bridge between the preamp and the power amp has been removed:
The startup noise seems to be absent. (although that right spike looks like the intermittent noise I'm also experiencing but I'm hoping that fixing the startup/shutoff noise will fix that as well or perhaps adjusting the DC offset. In any case, one issue at a time)
Conversely, here's the right preamp channel at startup:
We can see by zooming in that it's a 50 Hz sine:
Interestingly, measuring the same right preamp channel at shutoff shows the same sine but it lasts about 2 minutes before tapering off like this:
which feels a bit like a capacitor discharging.
Here's the waveform zoomed in in the middle of the taper:
Not sure what more to make of this but I'm looking forward to hearing your thoughts on this @Jakobsson @kapitiaudio
Illustrating that conclusion, let's start with the right speaker output at startup when the bridge between the preamp and the power amp has been removed:
The startup noise seems to be absent. (although that right spike looks like the intermittent noise I'm also experiencing but I'm hoping that fixing the startup/shutoff noise will fix that as well or perhaps adjusting the DC offset. In any case, one issue at a time)
Conversely, here's the right preamp channel at startup:
We can see by zooming in that it's a 50 Hz sine:
Interestingly, measuring the same right preamp channel at shutoff shows the same sine but it lasts about 2 minutes before tapering off like this:
which feels a bit like a capacitor discharging.
Here's the waveform zoomed in in the middle of the taper:
Not sure what more to make of this but I'm looking forward to hearing your thoughts on this @Jakobsson @kapitiaudio
So you get a burst of 50Hz out of the right channel preamp at startup. Was the original problem only on the right channel? It could be a muting issue since a power supply issue would usually affect both channels. But I'd go back to connecting the preamp back up and looking at both speaker outputs.
The first set of scope shot's on the speaker outputs weren't that useful - too much zooming in. What you need to do is set the amplitude to say 200mV/div and the timebase pretty slow (seconds) (since this noise lasts for 10s at startup). Then hook up both scope channels, ch1 to the left speaker output and ch2 to the right speaker output. Then you should be able to see how both channels behave at turn on at the same time and get a long enough history to further fine tune the scope settings. I would have the scope set to DC coupling so we can see output offset behavior at startup too.
You could connect the speakers to make sure it's actually faulting at the time you're doing the measurement as that's what we are interested in. This may also reduce the measured noise on the output which would be helpful.
The first set of scope shot's on the speaker outputs weren't that useful - too much zooming in. What you need to do is set the amplitude to say 200mV/div and the timebase pretty slow (seconds) (since this noise lasts for 10s at startup). Then hook up both scope channels, ch1 to the left speaker output and ch2 to the right speaker output. Then you should be able to see how both channels behave at turn on at the same time and get a long enough history to further fine tune the scope settings. I would have the scope set to DC coupling so we can see output offset behavior at startup too.
You could connect the speakers to make sure it's actually faulting at the time you're doing the measurement as that's what we are interested in. This may also reduce the measured noise on the output which would be helpful.
The opamps have a very basic powersupply construction limiting voltage from +/-20v to +/- 14.3v only by a resistor and capacitors. My idea is that if the capacitors go bad (high esr)(C311/C312) the opamp may act up a bit before it stabilize. Check that the current limiting resistors is not out of spec.(R311/R312). The muting circuit have some startup delay but maybe its not long enough to coverup the noise.
Attachments
I just remembered that there is a switch built in to main powerswitch that is very important for the muting/startup function.
@Jakobsson, I changed out C311 and C312 after you first suggested they might be the cause and I checked R311 and R312 as well and they show 1k in-circuit.
@kapitiaudio, A strange thing happened. I went out to the electronics supply store to get another probe for the scope so that I could measure both the left and right speaker outputs at the same time as you suggested. I rebridged the preamp and power amp and hooked up the speakers as well and measured. For some reason unknown to me, the startup/shutoff noise has now all but disappeared and is only faintly audible now if put my ear right up to the speakers. The only thing I can think of that could explain the improvement is the extra cleaning I did on the brown capacitor goop - I still can't get it all off without removing a lot of components but I did try to break possible shorts it was making by carefully scoring and chipping at it with small screwdriver.
I also suspect that the old probe I was using is quite noisy due to this startup measurement where the old probe is on ch1 below and the new one on ch2 above.
I confirmed that this difference is actually in the probes and not the amp channels by switching them and confirming that the ch1 (old probe) was still noisier than ch2 (new probe).
Anyways, with the startup/shutoff noise miraculously out of the way (hopefully for good), I thought I'd try aligning the DC offset and idling current using a multimeter to see if that helps with the intermittent noise. The left channel DC offset was <10 mV already so I moved on to the right channel where I hit a bit of a roadblock. When trying to adjust VR402 to get the right channel DC offset down from 150 mV I noticed that the variable resistors have all been fixed in place with some kind of white glue:
Despite this, before realizing I somehow made it jump to -230 mV while trying to turn it without really managing to turn it at all.
Measuring the speaker output at startup again I now see and hear a lot of the same kind of intermittent noise right away on the right channel (lower side cut off a little bit in the measurement):
Interestingly, the DC offset looks more like -70-80 mV on the scope but in any case, the sudden increase in noise coinciding with the unfortunate DC offset adjustment (more of which I'm afraid to attempt with the VR's glued up) leads me to believe they're connected.
So, that leaves me wondering: Do I try to manually remove the glue on the VR's enough to be able to turn them and perform the alignment procedure or should I look for replacements instead? Am I wrong in thinking the DC offset and noise are connected and Is there some other probable cause for the noise that I should be looking into instead?
@kapitiaudio, A strange thing happened. I went out to the electronics supply store to get another probe for the scope so that I could measure both the left and right speaker outputs at the same time as you suggested. I rebridged the preamp and power amp and hooked up the speakers as well and measured. For some reason unknown to me, the startup/shutoff noise has now all but disappeared and is only faintly audible now if put my ear right up to the speakers. The only thing I can think of that could explain the improvement is the extra cleaning I did on the brown capacitor goop - I still can't get it all off without removing a lot of components but I did try to break possible shorts it was making by carefully scoring and chipping at it with small screwdriver.
I also suspect that the old probe I was using is quite noisy due to this startup measurement where the old probe is on ch1 below and the new one on ch2 above.
I confirmed that this difference is actually in the probes and not the amp channels by switching them and confirming that the ch1 (old probe) was still noisier than ch2 (new probe).
Anyways, with the startup/shutoff noise miraculously out of the way (hopefully for good), I thought I'd try aligning the DC offset and idling current using a multimeter to see if that helps with the intermittent noise. The left channel DC offset was <10 mV already so I moved on to the right channel where I hit a bit of a roadblock. When trying to adjust VR402 to get the right channel DC offset down from 150 mV I noticed that the variable resistors have all been fixed in place with some kind of white glue:
Despite this, before realizing I somehow made it jump to -230 mV while trying to turn it without really managing to turn it at all.
Measuring the speaker output at startup again I now see and hear a lot of the same kind of intermittent noise right away on the right channel (lower side cut off a little bit in the measurement):
Interestingly, the DC offset looks more like -70-80 mV on the scope but in any case, the sudden increase in noise coinciding with the unfortunate DC offset adjustment (more of which I'm afraid to attempt with the VR's glued up) leads me to believe they're connected.
So, that leaves me wondering: Do I try to manually remove the glue on the VR's enough to be able to turn them and perform the alignment procedure or should I look for replacements instead? Am I wrong in thinking the DC offset and noise are connected and Is there some other probable cause for the noise that I should be looking into instead?
OK, so it's established the noise is on the right channel, good.
It would be worthwhile replacing those old adjustment pots on both channels and then doing the DC setup again. A new pot won't create bias hazards during adjustment.
It would be worthwhile replacing those old adjustment pots on both channels and then doing the DC setup again. A new pot won't create bias hazards during adjustment.
