It is a stereo amp, indeed. The schematic only shows one channel. Everything (including mods) is identical across channels. There are three 9622/ECC88s in it, one for each channel, and the shared input triode.
The problem is in both channels. It appears to be identical: loud hum, hiss, and now that "lawnmower" sound which is quite loud.
The feedback capacitor is missing from the board entirely, and has been replaced with another one of unknown value (again - 312 doesn't make sense to me. 3100pF? Strange value). I've currently cut off the wire connecting the speaker output cable to the feedback.
Oops, the power supply... I guess I've failed to mention some possibly important details about it. I highlighted the values of those capacitors because I've changed them. The original capacitor was a single unit containing four sections of 200/200/75/25 µF. The designer of this amp has repeatedly stated that this is a bad thing, and needs to be replaced. And I have.
I used four discrete capacitors of the values 220/220/100/33 µF (Epcos 400V capacitors, they seem reputable, owned by TDK / Siemens I believe), and I mounted them on their own little dedicated PCB that I made out of stripboard. This is mounted fairly far away from where the original capacitor was, so I used wires to connect them. Four 0.75mm² wires including a single ground wire of the same diameter. These are all connected with crimp connections on both ends. The wires are about 15 cm (6 in) long. Could this installation be the cause of problems? I guess all this stuff adds quite a bit of resistance to the connections.
The problem is in both channels. It appears to be identical: loud hum, hiss, and now that "lawnmower" sound which is quite loud.
The feedback capacitor is missing from the board entirely, and has been replaced with another one of unknown value (again - 312 doesn't make sense to me. 3100pF? Strange value). I've currently cut off the wire connecting the speaker output cable to the feedback.
Oops, the power supply... I guess I've failed to mention some possibly important details about it. I highlighted the values of those capacitors because I've changed them. The original capacitor was a single unit containing four sections of 200/200/75/25 µF. The designer of this amp has repeatedly stated that this is a bad thing, and needs to be replaced. And I have.
I used four discrete capacitors of the values 220/220/100/33 µF (Epcos 400V capacitors, they seem reputable, owned by TDK / Siemens I believe), and I mounted them on their own little dedicated PCB that I made out of stripboard. This is mounted fairly far away from where the original capacitor was, so I used wires to connect them. Four 0.75mm² wires including a single ground wire of the same diameter. These are all connected with crimp connections on both ends. The wires are about 15 cm (6 in) long. Could this installation be the cause of problems? I guess all this stuff adds quite a bit of resistance to the connections.
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It is on both channels, yes. It looks like it might be stock, though. I do not see the traces from -1 go to the 15K resistor, although inconveniently, the traces in that particular area were very, very scorched, and I had to repair them with resistor leads. They sit right next to -2 and the 22K resistor, so I joined them together. I am not 100% sure that is how it was originally, but it certainly looks like it.
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Yes. The original PCB differs slightly from the schematic.
No, it was used with a passive pre (just a volume control) and very sensitive Lowther horn speakers.
Sorry, only just spotted this (I had guests here for the weekend). The common ground wire will guarantee hum/buzz. Each cap must have its own ground wire, even if they all connect to the same point at the other end. Alternatively, rewire the PSU ground as a bus with the dirty end connecting to transformer and rectifiers and the clean end connecting to signal ground.Welcome said:
The aim is to keep the charging pulses for the first (reservoir) cap well away from the ground connections for the later (smoothing and/or decoupling) caps.
Hi guys, sorry about the delay in updates. I got kinda discouraged and forgot about this for a while.
Today I saw DF96's last post, and I decided to give it another try.
I removed the capacitor PCB that I had made, it was useless. There was actually room for all the caps directly on the amplifier's own PCB, so I drilled some holes and installed them there. The hum's totally gone now, and I also noticed that I had installed one of the miller caps wrong after trying to remove them, so the weird "lawnmower" sound that I mentioned earlier is also gone.
So now it works better than it ever has - in my hands at least. But all is still not well.
There is a strange and very loud noise (and constant - not dependent on volume) at all times, in the background. It sounds kinda like a 50Hz square wave. This noise changes if I cut the feedback wires, and becomes much stranger then, it's like doing that adds ocean sounds to the square wave.
EDIT: It's on both channels.
With no source connected, it gets even louder - and bassier.
Today I saw DF96's last post, and I decided to give it another try.
I removed the capacitor PCB that I had made, it was useless. There was actually room for all the caps directly on the amplifier's own PCB, so I drilled some holes and installed them there. The hum's totally gone now, and I also noticed that I had installed one of the miller caps wrong after trying to remove them, so the weird "lawnmower" sound that I mentioned earlier is also gone.
So now it works better than it ever has - in my hands at least. But all is still not well.
There is a strange and very loud noise (and constant - not dependent on volume) at all times, in the background. It sounds kinda like a 50Hz square wave. This noise changes if I cut the feedback wires, and becomes much stranger then, it's like doing that adds ocean sounds to the square wave.
EDIT: It's on both channels.
With no source connected, it gets even louder - and bassier.
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Yessss!
It's now sitting on top of my desk, playing beautiful music.
The fix was quite easy... In fact, I had read about it online earlier, but hadn't gotten around to testing it. The solution was that I had to move the input signal directly to the driver valves, bypassing the (shared) input valve and some circuitry altogether.
That simple procedure fixed it. The amp now works fine. I also cut off the feedback completely, as it had been tampered with, and I don't have the components to restore it right now.
As there were some mods between the input valve and the driver valves, I guess bypassing all that, I cheated and just ignored the problem instead of fixing it. ;-)
There's still a rather minor issue. There's still some buzz - which is dependent on volume. Practically inaudible - but there - with the volume totally down (and this is the case for all amps, in my experience) and increases as you turn it up. I have no idea what this is. I don't totally care, though. Not a big problem. Quick thought: could it be a resistor? It's in both channels, though (not as much in one).
For the curious, here's now the circuit - changes (by me and others) are in blue. Everything else is stock.
It's now sitting on top of my desk, playing beautiful music.
The fix was quite easy... In fact, I had read about it online earlier, but hadn't gotten around to testing it. The solution was that I had to move the input signal directly to the driver valves, bypassing the (shared) input valve and some circuitry altogether.
That simple procedure fixed it. The amp now works fine. I also cut off the feedback completely, as it had been tampered with, and I don't have the components to restore it right now.
As there were some mods between the input valve and the driver valves, I guess bypassing all that, I cheated and just ignored the problem instead of fixing it. ;-)
There's still a rather minor issue. There's still some buzz - which is dependent on volume. Practically inaudible - but there - with the volume totally down (and this is the case for all amps, in my experience) and increases as you turn it up. I have no idea what this is. I don't totally care, though. Not a big problem. Quick thought: could it be a resistor? It's in both channels, though (not as much in one).
For the curious, here's now the circuit - changes (by me and others) are in blue. Everything else is stock.
An externally hosted image should be here but it was not working when we last tested it.
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