Hey there,
Can you please critique my basic analysis of the master volume design in these amps. My amp has developed a hum problem, and during the course of tracking it down (still haven't, more on that later), some questions have arisen in my mind, funny that.
My questions center around the implementation of the 1Meg pot in what appears to me as the feedback loop of each OpAmp. Actually I'm not even sure if it's in the feedback loop, as it's different to the Tubescreamer gain feedback loop, for example.
I have not decided which extreme of the pot dictates full volume or no volume. Logic says to me that when 'shorted' there is an easy path for the guitar signal around the OpAmp, but then there would be no amplification factor and I'd have to assume that the OpAmp is being used purely as a switch.
The reason I'm analysing this, is I'm trying to narrow down where my hum is emanating from. My hum is occurring with no guitar inserted, and only when I turn up the master. This happens with both channels. Am I correct in assuming that the hum must be emanating from further upstream in the preamp? The main dual-section caps were replaced not long ago just so you know.
My shortlist of usual culprits:
1. C3 preamp filter (changed, no improvement).
2. Low voltage supply caps and Zeners (changed Zeners as there was a slight voltage mis-match, no improvement, will entertain replacing caps C21, 22, 31, 32).
3. A noisy OpAmp.
4. A leaky coupling cap passing some DC between stages?
I have substituted all valves.
Obviously I have not invested in a scope yet...it's on my sizeable want list and is juxtaposed with paying bills, feeding that cat and self, etc.
Any insights appreciated.
View attachment JCM900 Dual Rev Pre.pdf
Can you please critique my basic analysis of the master volume design in these amps. My amp has developed a hum problem, and during the course of tracking it down (still haven't, more on that later), some questions have arisen in my mind, funny that.
My questions center around the implementation of the 1Meg pot in what appears to me as the feedback loop of each OpAmp. Actually I'm not even sure if it's in the feedback loop, as it's different to the Tubescreamer gain feedback loop, for example.
I have not decided which extreme of the pot dictates full volume or no volume. Logic says to me that when 'shorted' there is an easy path for the guitar signal around the OpAmp, but then there would be no amplification factor and I'd have to assume that the OpAmp is being used purely as a switch.
The reason I'm analysing this, is I'm trying to narrow down where my hum is emanating from. My hum is occurring with no guitar inserted, and only when I turn up the master. This happens with both channels. Am I correct in assuming that the hum must be emanating from further upstream in the preamp? The main dual-section caps were replaced not long ago just so you know.
My shortlist of usual culprits:
1. C3 preamp filter (changed, no improvement).
2. Low voltage supply caps and Zeners (changed Zeners as there was a slight voltage mis-match, no improvement, will entertain replacing caps C21, 22, 31, 32).
3. A noisy OpAmp.
4. A leaky coupling cap passing some DC between stages?
I have substituted all valves.
Obviously I have not invested in a scope yet...it's on my sizeable want list and is juxtaposed with paying bills, feeding that cat and self, etc.
Any insights appreciated.
View attachment JCM900 Dual Rev Pre.pdf
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The volume control is a classic 'Baxandall' active type and does indeed work by altering the feedback factor. You get minimum volume when the pot is at minimum resistance.
Having the two opamps in parallel is done to increase drive ability, however at face value (as drawn) it is very poor design indeed because even the slightest gain imbalance will cause one or other opamp to effectively work into a 'virtual' short circuit as it tries to force current into/out of the other opamp. Hard to believe it is actually like that in reality, however...
Opamps in themselves don't hum so they can effectively be discounted.
Try removing both R20 and R23 and seeing if the hum disappears. If it does then the problem is most likely in the part of the circuit to the left of these parts.
Ideally you need to use an oscilloscope to look at what is happening. The problem could easily be in the power supply, perhaps the HV supply to the valves.
Having the two opamps in parallel is done to increase drive ability, however at face value (as drawn) it is very poor design indeed because even the slightest gain imbalance will cause one or other opamp to effectively work into a 'virtual' short circuit as it tries to force current into/out of the other opamp. Hard to believe it is actually like that in reality, however...
Opamps in themselves don't hum so they can effectively be discounted.
Try removing both R20 and R23 and seeing if the hum disappears. If it does then the problem is most likely in the part of the circuit to the left of these parts.
Ideally you need to use an oscilloscope to look at what is happening. The problem could easily be in the power supply, perhaps the HV supply to the valves.
Ok thanks for the insights Mooly. I'll start pulling parts to try and pinpoint the source of the noise.
I initially planned to sell this amp to fund the purchase of something like an OWON SDS1102. I might just have to pull the trigger on that anyway.
Will post an update in that it may help others in future.
I initially planned to sell this amp to fund the purchase of something like an OWON SDS1102. I might just have to pull the trigger on that anyway.
Will post an update in that it may help others in future.
Sorry Mooly again you are not in the Guitar World.
Those Op Amps are NOT in parallel.
M5201 is a *channel switching* dual Op Amp, so a blessing in a channel switching Guitar Amplifier.
Basically a TL072 but both halves, which have a common output pin: #5, are never ever "in parallel" because either one or the other is ON or enabled while the other is OFF, depending on control voltage on pin 1
Hard to believe but that´s reality
Ok well thanks for that. You appear to have some ongoing beef with Mooly's level of expertise, which may or may not be justified. Either way I don't care. Can you offer anything on a positive note? I didn't create this thread for people to use as a vehicle for denigrating others.
Can you verify any of Mooly's post as being accurate? Do you wish to posit anything? Or do you just wish to be surly to get whatever garbage you have on your chest off it?
For anybody who may be interested... I replaced the low voltage supply caps with no difference in hum. Pulled one leg each of R20, 23, hum still present. Considering next course of action. Thanks.
Can you verify any of Mooly's post as being accurate? Do you wish to posit anything? Or do you just wish to be surly to get whatever garbage you have on your chest off it?
For anybody who may be interested... I replaced the low voltage supply caps with no difference in hum. Pulled one leg each of R20, 23, hum still present. Considering next course of action. Thanks.
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I'm sure it's nothing like that
I took the two opamps as drawn to be in parallel but didn't appreciate they were a unique switchable type. Ordinarily you can't parallel two active voltage sources (like an opamp) as they would fight each other. So my mistake there.
I'll stand by everything else I stated.
So I would say the next course of action is to determine if the hum is entering the final output stage (those two opamps) and given that they are 'switchable' (thank you Mr Fahey, I've pulled the data sheet on them
) i.e. only one device is active at one time then we have to first remove the audio input to both to prove that nothing is entering via those routes. Lifting C15 and R22 should achieve that although I have a question now... why does the diagram show C15 and R22 as joined together? That looks like it will apply the same 'mix' of audio to both opamps suggesting that whichever is active, the final output will be the same.
Mooly is right 99.9% of the time. Normally we have nothing to add.
This 0.1% time Mooly lacked a very obscure fact. That is an unusual chip. I have not seen it used anywhere except this series of Marshalls.
JM was informing Mooly about this chip. This is such a rare event that JM used his loud voice.
Rudeness aside, Mooly's further comments are on-track.
Thanks for that PRR. Regarding that guy's response, there's a fine line, in my opinion, between being 'cantankerous' and being a donkey's hind quarters.
Regarding this amp, I have:
1. Lifted points indicated by red crosses in this new attachment, hum still present, still changes with master volume.
2. Disappears with V2 pulled.
3. Lifted C15, R22 noise gone.
4. Lifted R21, 23, noise still present.
Could something within the green box actually introduce noise through the HT (A)?
I can't see how noise can be introduced any other way, as the only other connections are ground connections. Maybe a bad ground connection?
Regarding this amp, I have:
1. Lifted points indicated by red crosses in this new attachment, hum still present, still changes with master volume.
2. Disappears with V2 pulled.
3. Lifted C15, R22 noise gone.
4. Lifted R21, 23, noise still present.
Could something within the green box actually introduce noise through the HT (A)?
I can't see how noise can be introduced any other way, as the only other connections are ground connections. Maybe a bad ground connection?
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Lets try and work with these logically:
So you have R20 and R23 lifted and hum is still present. That shows hum is entering after this point.
Something bothers me here and that is the point I mentioned earlier in that C15 and R22 are drawn as being connected together. That doesn't compute to me... given that the final opamp is a switchable one and so either one or the other opamp is feeding the output, the diagram seems to show that both opamps receive the same signal.
Can you just do a quick check and confirm or otherwise that these two components are or are not not joined.
If they are joined then what is the difference to you as a user when you have Master A or Master B selected. Both opamps use identical circuitry and component values. I can't see any differences there, even the pot is marked as being wired in the same direction.
If there was a ripple component on the HT line (marked A) then logic suggests that would still couple to the opamp via C15 and whether or not V2 was present. Yet it is quiet with V2 removed.
Is V2 back in place now or not?
We also need to know what those opamps are doing (that common connection again) and which opamp is active.
At this point I would also suggest trying C15 and R22 individually to see if the hum is entering via just one or via both.
Lets concentrate on that before considering anything more.
So you have R20 and R23 lifted and hum is still present. That shows hum is entering after this point.
Something bothers me here and that is the point I mentioned earlier in that C15 and R22 are drawn as being connected together. That doesn't compute to me... given that the final opamp is a switchable one and so either one or the other opamp is feeding the output, the diagram seems to show that both opamps receive the same signal.
Can you just do a quick check and confirm or otherwise that these two components are or are not not joined.
If they are joined then what is the difference to you as a user when you have Master A or Master B selected. Both opamps use identical circuitry and component values. I can't see any differences there, even the pot is marked as being wired in the same direction.
If there was a ripple component on the HT line (marked A) then logic suggests that would still couple to the opamp via C15 and whether or not V2 was present. Yet it is quiet with V2 removed.
Is V2 back in place now or not?
We also need to know what those opamps are doing (that common connection again) and which opamp is active.
At this point I would also suggest trying C15 and R22 individually to see if the hum is entering via just one or via both.
Lets concentrate on that before considering anything more.
So with regards to your first 4-5 paragraphs, the rationale behind this design is that you have switchable master volumes. C15 and R22 are indeed connected. You can individually tailor the volume of each channel via IC8, as it switches over when ICs 1 and 6 switch over, so for example, you can go from a lower-gain, lower-volume channel A rhythm sound, to a louder, higher-gain B channel lead sound.
I always have to remind myself that they designed this amp at the tail-end of the eighties, when hair metal was all around, and that they probably didn't see Nirvana coming. While it's obviously not everyone's cup of tea sonically, I do admire the engineering that went into it. I mean, when it works, it works fine.
I'm going to methodically work my way through the points you discussed in your previous post, probably tomorrow now. I've burned myself out for today by continually going round in circles on this thing.
Thank you for your input and I'll revisit tomoz with a fresh mind.
P.S. I should reiterate that the noise is present in both channels, and is not influenced by the preamp volume of either channel.
I always have to remind myself that they designed this amp at the tail-end of the eighties, when hair metal was all around, and that they probably didn't see Nirvana coming. While it's obviously not everyone's cup of tea sonically, I do admire the engineering that went into it. I mean, when it works, it works fine.
I'm going to methodically work my way through the points you discussed in your previous post, probably tomorrow now. I've burned myself out for today by continually going round in circles on this thing.
Thank you for your input and I'll revisit tomoz with a fresh mind.
P.S. I should reiterate that the noise is present in both channels, and is not influenced by the preamp volume of either channel.
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Thanks
That makes perfect sense once it is explained and in view of that I would go back to C15/R22 and initially retest as follows:So we know that with C15 and R22 fitted (and V2 in place) there is hum and with C15 and R22 removed it is quiet.
1/ Now fit just C15 see if there is hum or not. Yes or no?
2/ Remove C15 and refit R22. Hum, yes or no?
Hopefully that will give a clue as to the route the hum is entering.
If it is via C15 then is it possible there could be some problem with V2 itself and that it is allowing the filament voltage/current to couple into the audio chain. Valves can suffer from heater/cathode leakage and shorts and I'm assuming the heater supply is conventional AC. You mention it was quiet when the valve was pulled.
Have you swapped the ECC83 around as a test. Try using V1 in its place and leave the V1 position vacant.
Of course.I'm sure it's nothing like that
I took the two opamps as drawn to be in parallel but didn't appreciate they were a unique switchable type. Ordinarily you can't parallel two active voltage sources (like an opamp) as they would fight each other. So my mistake there.
I'll stand by everything else I stated.
I read about 1500 answers by Moly and said nothing or agreed with them because .... well ... they were right!!
By the same token, I commented on 2 (two) where "common sense" , as defined in the "normal" Hi Fi World does NOT apply, because some *specialized/dedicated* ICs do things different in the Guitar World.
Which, by the way, is EXACTLY what I wrote.
Paranoia does not help much solving problems and in fact explaining how that seemingly "impossible" circuit works DOES help solve your problem.
Chimed in , interested because I work with high gain Guitar amps for a living, for decades now, but given your "attitude" I can only wish you good luck with your problem.
As to Mooly, sorry if what I wrote sounds rude to somebody else´s oversensitive ears, today´s custom is to be offended by everything, I was actually "justifying" your not knowing what PRR correctly calls "an obscure IC".
And yes, we old Techs often tend to become grumpy, and try to get straight to the point using the shortest path.
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It's not a matter of being "over-sensitive", it's a matter of having a learned listening of your contributions over the years as being abrasive, judgemental, LOUD, bold, as you have waded through your forum participation.
Thanks for all your contributions Mooly. I replaced R4, R18, and the noise is gone. I'm not sure which did it and at this point I don't have the energy to find out. Maybe one of the joints was dry.
Thanks for all your contributions Mooly. I replaced R4, R18, and the noise is gone. I'm not sure which did it and at this point I don't have the energy to find out. Maybe one of the joints was dry.
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