I bought a used Marshall MG10CD... yeah, I know.
I guess it's one of those pieces of gear, like a DS1 or Metal Zone, that you need to have owned to understand all the hate. <3)
I have found the "10" stands for: needs 10x less gain, 10x less treble, 10x less output and 10x less hiss. :duh
I'm planning to mod it and turn it into a head with a Celestion fitted cab but I need to tackle some issues first.
Problem 1: hisssssssssssss
I can probably fix the gain and treble/voicing issues but I'm really struggling to figure out where the hiss (white noise, not hum) comes from. I see other MG owners complain about the exact same issue.
I understand that with lots of gain, there will be hiss but the problem is that even with nothing plugged in, there's quite a lot of hiss. With no gain other than coming from the power amp I don't understand where this hiss comes from.
At the levels I'm playing, turning up the gain to 10 only adds hum, and pretty much no extra hiss.
There's a pretty good analysis and nice schematic of this amp here: https://www.electrosmash.com/marshall-mg10
But unfortunately he made some mistakes when tracing, especially in the tone stack and Sallen Key sections, so the frequency plots are wrong.
R18 and R19 are 10k instead of the 100k in the schematic and he's missing R12 47k to ground right before R13
Link to the schematic in case the horizontal scroll bothers you:
https://www.electrosmash.com/images/tech/mg10/marshall-mg10-schematic-parts-big.jpg
Here's the only correct schematic I have found:
Link to the schematic in case the horizontal scroll bothers you:
http://oi63.tinypic.com/eqrqm1.jpg
With nothing plugged in, the preamp section is muted right before the tone stack, so turning gain and volume controls do not affect it.
If I ground the circuit right before the power amp (R22) the hiss is completely gone.
So, it must come from either the tone stack or those Sallen key filters.
If I bypass/jumper R15 and thereby disable that passive LP filter, the hiss is brighter but not louder so I think I can narrow it down to the tone stack or that first Sallen key filter.
Any suggestions on what to try next? Or any ideas on what could cause the hiss in that part of the circuit?
I have a Vox Pathfinder10 and you can honestly not tell if this amp is on or not when nothing is plugged in. Even with my ear against the grill cloth, one inch away from the speaker I still cannot tell if it's on. So it must be possible to have a dead quiet amp with nothing plugged in. And this amp is pretty bright as well and mutes/grounds the circuit in a similar spot.
Problem 2: OD channel volume knob mystery:
Another issue I can't seem to figure out is the following: on the OD channel, with the volume knob on 0 there is still some sound coming through. It's all treble frequencies, no bass.
When I ever so slightly turn the volume knob, like 1/10th of a mm, the bass frequencies kick in and the sound seems normal but it's actually already too loud by then.
I measured the resistance between wiper and ground to make sure there's no 'residual' resistance left when on '0' and it measures to about 1 Ohm.
One Ohm would give me 1/15000 of the maximum output (load of 10k + 5k pot). Maximum output of that opamp is about 2V because of the clipping leds in the NFB so output from the volume voltage divider would be 0.00013 volts. Can I hear that after the power amp amplified it? How would it explain the missing bass frequencies?
I guess it's one of those pieces of gear, like a DS1 or Metal Zone, that you need to have owned to understand all the hate. <3)
I have found the "10" stands for: needs 10x less gain, 10x less treble, 10x less output and 10x less hiss. :duh
I'm planning to mod it and turn it into a head with a Celestion fitted cab but I need to tackle some issues first.
Problem 1: hisssssssssssss
I can probably fix the gain and treble/voicing issues but I'm really struggling to figure out where the hiss (white noise, not hum) comes from. I see other MG owners complain about the exact same issue.
I understand that with lots of gain, there will be hiss but the problem is that even with nothing plugged in, there's quite a lot of hiss. With no gain other than coming from the power amp I don't understand where this hiss comes from.
At the levels I'm playing, turning up the gain to 10 only adds hum, and pretty much no extra hiss.
There's a pretty good analysis and nice schematic of this amp here: https://www.electrosmash.com/marshall-mg10
But unfortunately he made some mistakes when tracing, especially in the tone stack and Sallen Key sections, so the frequency plots are wrong.
R18 and R19 are 10k instead of the 100k in the schematic and he's missing R12 47k to ground right before R13
Link to the schematic in case the horizontal scroll bothers you:
https://www.electrosmash.com/images/tech/mg10/marshall-mg10-schematic-parts-big.jpg
Here's the only correct schematic I have found:
Link to the schematic in case the horizontal scroll bothers you:
http://oi63.tinypic.com/eqrqm1.jpg
With nothing plugged in, the preamp section is muted right before the tone stack, so turning gain and volume controls do not affect it.
If I ground the circuit right before the power amp (R22) the hiss is completely gone.
So, it must come from either the tone stack or those Sallen key filters.
If I bypass/jumper R15 and thereby disable that passive LP filter, the hiss is brighter but not louder so I think I can narrow it down to the tone stack or that first Sallen key filter.
Any suggestions on what to try next? Or any ideas on what could cause the hiss in that part of the circuit?
I have a Vox Pathfinder10 and you can honestly not tell if this amp is on or not when nothing is plugged in. Even with my ear against the grill cloth, one inch away from the speaker I still cannot tell if it's on. So it must be possible to have a dead quiet amp with nothing plugged in. And this amp is pretty bright as well and mutes/grounds the circuit in a similar spot.
Problem 2: OD channel volume knob mystery:
Another issue I can't seem to figure out is the following: on the OD channel, with the volume knob on 0 there is still some sound coming through. It's all treble frequencies, no bass.
When I ever so slightly turn the volume knob, like 1/10th of a mm, the bass frequencies kick in and the sound seems normal but it's actually already too loud by then.
I measured the resistance between wiper and ground to make sure there's no 'residual' resistance left when on '0' and it measures to about 1 Ohm.
One Ohm would give me 1/15000 of the maximum output (load of 10k + 5k pot). Maximum output of that opamp is about 2V because of the clipping leds in the NFB so output from the volume voltage divider would be 0.00013 volts. Can I hear that after the power amp amplified it? How would it explain the missing bass frequencies?