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I don't have a scope (can't afford one yet), so the logic process is all I have to work with. I believe I've learned basically what I could learn with a scope though -- just with a lot more effort and soldering -- that the area around "MUTE"/OPT6/OPT7 is where this is happening. Specifically, the spot where the three channels join together after each master volume, before the FX send and PI. I still believe the voltage divider formed by the 470k series resistor (R12 + OPT01/clean PCB) and the load at the 3-way join is somehow responsible.
After the seemingly backward result of putting faster optos in the lead PCB and having the fade-in worsen, I started thinking of the process as a kind of crossfade. Drive channel fades out, clean fades in. By putting the faster optos in only the lead PCB, I was cutting half of that crossfade and leaving the other mostly intact. So I decided to reverse my approach.
So I put the old NSL-32s back in, ONLY at OPT6 and OPT7. The other SR3s in LDR3, OPT2/3/4/5 all stayed. Sound went back to stock. So it really is the stuff at the join. Next, I put an SR3 in the OPT01 spot on the clean PCB, and also halved the value of R1 on lead PCB (current limiting resistor for the clean channel optos), effectively doubling the current through OPT01 and OPT02. This FINALLY caused a desirable change -- though with some qualifiers.
In the current state, with the lead or crunch master vols all the way down, the clean fade in is as stock. As I turn up the volume on either of the drive channels, the transition from that channel back to clean becomes more seamless, until at about 3, there is no audible fade-in. This is with the clean volume at about 2.5. This confirms to me that loading is part of the issue. However, interestingly enough: now for the first time, with the clean volume all the way down, switching to either of the drive channels produces a fade-in! Not nearly as pronounced and not something I'm going to worry too much about, but it's there.
I'm thinking at this point that just replacing OPT01 on the clean board with an SR3, and increasing the current through it, will mitigate the problem to most people's satisfaction (since in a live performance situation, the volumes aren't going to be that close to zero, where the effect is most pronounced).
It's possible that ONLY increasing the current through the stock clean optos would produce the same effect... something to try. This makes me wonder about the relationship between current and rise/decay time. I understand that final values of resistance/conductance will be affected by current according to the curve in the datasheet... but I'm wondering about the logic that less current makes the transition faster. Maybe it makes the decay faster, but not the rise, or vice versa. Maybe it makes both slower. The assumption is that since the resistance has farther to rise with more current, it will take more time. I'm not sure about that. At least OTOH with conductance, it seems that applying higher current causes the resistance to drop farther, but in the same amount of time (based on my experience here, not on any hard data), thus mitigating OPT01's addition to the 470k series component of the voltage divider issue.
EDIT: I tried increasing current through the crunch channel but not the lead channel (doubled it). It actually slowed things down. I'd guess that increased current shortens rise time, but lengthens decay time. So it might be possible to address the issue without replacing optos, by increasing current to the clean channel optos to about 16mA, but decreasing it to the lead/crunch channel optos (maybe to around 5mA? less?). Though, my approach has not been very disciplined or scientific -- so current may have little to do with it and it may just be a case of changing OPT01 for an SR3.
EDIT: Forgot to point out from before that there's something else that may or may not matter to some people (matters to me): independent of the fade-in issue, there is a split-second delay on the actual mechanical RELAY engagement when switching from either drive channel back to clean. That is, RL1 and RL4 are instantaneous when switching TO the drive channels, but delayed when switching away. This has no bearing on the fade in, it just means that the whole channel-switching reaction doesn't even start until some time after you press the button. You can demonstrate this by powering the amp up in standby and pushing the channel switching buttons on the front. The relay click is instant EXCEPT when going from Lead/Crunch back to clean.
The reason is C40 on the lead PCB. It controls the muting time for FET1 and FET2 on the clean PCB. Since I had removed FET1 and FET2, I didn't need C40 anymore, so I took it out (some boards have a C39 instead -- same function). The cap has 0V across it while RL1 and RL4 are engaged, but when TR1 pulls up from ground, the cap starts to charge. Thus current continues flowing through RL1's and RL4's coils for a split second after the channel change. Removing C40 solved this issue. Again, does NOT affect the fade-in, but in a performance situation, when changing channels on a specific downbeat, it does matter. I can't say whether removing it would add noise if FET1 and FET2 were still in... but my amp has no muting circuit, so taking this out was helpful for me. Just FYI.
After the seemingly backward result of putting faster optos in the lead PCB and having the fade-in worsen, I started thinking of the process as a kind of crossfade. Drive channel fades out, clean fades in. By putting the faster optos in only the lead PCB, I was cutting half of that crossfade and leaving the other mostly intact. So I decided to reverse my approach.
So I put the old NSL-32s back in, ONLY at OPT6 and OPT7. The other SR3s in LDR3, OPT2/3/4/5 all stayed. Sound went back to stock. So it really is the stuff at the join. Next, I put an SR3 in the OPT01 spot on the clean PCB, and also halved the value of R1 on lead PCB (current limiting resistor for the clean channel optos), effectively doubling the current through OPT01 and OPT02. This FINALLY caused a desirable change -- though with some qualifiers.
In the current state, with the lead or crunch master vols all the way down, the clean fade in is as stock. As I turn up the volume on either of the drive channels, the transition from that channel back to clean becomes more seamless, until at about 3, there is no audible fade-in. This is with the clean volume at about 2.5. This confirms to me that loading is part of the issue. However, interestingly enough: now for the first time, with the clean volume all the way down, switching to either of the drive channels produces a fade-in! Not nearly as pronounced and not something I'm going to worry too much about, but it's there.
I'm thinking at this point that just replacing OPT01 on the clean board with an SR3, and increasing the current through it, will mitigate the problem to most people's satisfaction (since in a live performance situation, the volumes aren't going to be that close to zero, where the effect is most pronounced).
It's possible that ONLY increasing the current through the stock clean optos would produce the same effect... something to try. This makes me wonder about the relationship between current and rise/decay time. I understand that final values of resistance/conductance will be affected by current according to the curve in the datasheet... but I'm wondering about the logic that less current makes the transition faster. Maybe it makes the decay faster, but not the rise, or vice versa. Maybe it makes both slower. The assumption is that since the resistance has farther to rise with more current, it will take more time. I'm not sure about that. At least OTOH with conductance, it seems that applying higher current causes the resistance to drop farther, but in the same amount of time (based on my experience here, not on any hard data), thus mitigating OPT01's addition to the 470k series component of the voltage divider issue.
EDIT: I tried increasing current through the crunch channel but not the lead channel (doubled it). It actually slowed things down. I'd guess that increased current shortens rise time, but lengthens decay time. So it might be possible to address the issue without replacing optos, by increasing current to the clean channel optos to about 16mA, but decreasing it to the lead/crunch channel optos (maybe to around 5mA? less?). Though, my approach has not been very disciplined or scientific -- so current may have little to do with it and it may just be a case of changing OPT01 for an SR3.
EDIT: Forgot to point out from before that there's something else that may or may not matter to some people (matters to me): independent of the fade-in issue, there is a split-second delay on the actual mechanical RELAY engagement when switching from either drive channel back to clean. That is, RL1 and RL4 are instantaneous when switching TO the drive channels, but delayed when switching away. This has no bearing on the fade in, it just means that the whole channel-switching reaction doesn't even start until some time after you press the button. You can demonstrate this by powering the amp up in standby and pushing the channel switching buttons on the front. The relay click is instant EXCEPT when going from Lead/Crunch back to clean.
The reason is C40 on the lead PCB. It controls the muting time for FET1 and FET2 on the clean PCB. Since I had removed FET1 and FET2, I didn't need C40 anymore, so I took it out (some boards have a C39 instead -- same function). The cap has 0V across it while RL1 and RL4 are engaged, but when TR1 pulls up from ground, the cap starts to charge. Thus current continues flowing through RL1's and RL4's coils for a split second after the channel change. Removing C40 solved this issue. Again, does NOT affect the fade-in, but in a performance situation, when changing channels on a specific downbeat, it does matter. I can't say whether removing it would add noise if FET1 and FET2 were still in... but my amp has no muting circuit, so taking this out was helpful for me. Just FYI.
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