Fixing sc20's badly behaving effects loop.

[Timo V]

My gray matter needed some action, so I dug out my scope and signal generator and into the amp I went.

Issue number 1. A loud pop when true bypass effects are activated. A cause is an effects loop's electrolytic coupling capacitor (C35) that is 100(!) times bigger than needed and is also floating. When a non buffered effect is activated, the capacitor discharges to effect's input stage causing a loud pop. A solution is to replace the cap with a 470n plastic capacitor. After replacement a pop is barely audible.

Issue number 2. A volume drop when effects loop is turned on. Like issue number 1, somebody at Marshall screwed up.
An effects loop's recovery stage's gain is set too low. When I feed send stage's opamp with 1 Vp-p signal I get 0.545 Vp-p output after a recovery stage, a signal is cut almost to half. An ideal solution would be to rescale opamp's gain setting resistors, but my quick and dirty fix is (at least for me) good enough. There is a voltage divider (R50,R51) that sets recovery stage's output voltage to a phase inverter. Bypass the R50 with a jumper wire and output voltage sits at 0.925 Vp-p against 1Vp-p when effects loop is turned off. A volume drop is virtually gone.

A couple of pics follow. I used 1uF polyester cap for C35, it's twice the size needed, but works just fine.




Timo

 

[PelliX]

Issue number 1. A loud pop when true bypass effects are activated. A cause is an effects loop's electrolytic coupling capacitor (C35) that is 100(!) times bigger than needed and is also floating. When a non buffered effect is activated, the capacitor discharges to effect's input stage causing a loud pop. A solution is to replace the cap with a 470n plastic capacitor. After replacement a pop is barely audible.

Interesting. I've never noticed that, but as you mentioned it would be (most) audible with unbuffered effects gear/pedals. I wonder what their logical was behind the original value of C35.

Issue number 2. A volume drop when effects loop is turned on. Like issue number 1, somebody at Marshall screwed up.
An effects loop's recovery stage's gain is set too low. When I feed send stage's opamp with 1 Vp-p signal I get 0.545 Vp-p output after a recovery stage, a signal is cut almost to half. An ideal solution would be to rescale opamp's gain setting resistors, but my quick and dirty fix is (at least for me) good enough. There is a voltage divider (R50,R51) that sets recovery stage's output voltage to a phase inverter. Bypass the R50 with a jumper wire and output voltage sits at 0.925 Vp-p against 1Vp-p when effects loop is turned off. A volume drop is virtually gone.

The volume drop is most noticeable at low MV settings and decreases as one turns it up. That said, it's not meant to be switched "live", hence the button being on the back and there being no footswitch/MIDI option. This also applies to point 1 ^.

This might be of some interest to you:





I haven't performed these adjustments on my SC as I hardly use the loop and don't find either issue problematic when I do (also only dealing with buffered gear).

 

[Timo V]

Interesting. I've never noticed that, but as you mentioned it would be (most) audible with unbuffered effects gear/pedals. I wonder what their logical was behind the original value of C35.



The volume drop is most noticeable at low MV settings and decreases as one turns it up. That said, it's not meant to be switched "live", hence the button being on the back and there being no footswitch/MIDI option. This also applies to point 1 ^.

This might be of some interest to you:

View attachment 135911

View attachment 135912

I haven't performed these adjustments on my SC as I hardly use the loop and don't find either issue problematic when I do (also only dealing with buffered gear).

Thanks for the Marshall advisory! However, my sc20 came with that fix already done at a Marshall factory, and it still fell badly short. A volume drop must have been REALLY bad with a original setup.

And concerning the infamous pop issue, it DOES NOT happen when the effects loop button at a back of the amp is pushed, it happens when the effects loop is already on, and THEN you stomp on a for example non buffered reverb pedal in the loop.

 

[PelliX]

Thanks for the Marshall advisory! However, my sc20 came with that fix already done at a Marshall factory, and it still fell badly short. A volume drop must have been REALLY bad with a original setup.

I haven't checked it with a dB meter, but I didn't really find it as harsh as some. That's news to me that Marshall are implementing this from the factory... you didn't by any chance get an open-box or returned model or something of that kind? What's the manufacturing date of yours?

 

[Timo V]

I haven't checked it with a dB meter, but I didn't really find it as harsh as some. That's news to me that Marshall are implementing this from the factory... you didn't by any chance get an open-box or returned model or something of that kind? What's the manufacturing date of yours?

Week 15, 2023. It arrived in an unopened box, and with no signs of soldering after original assembly.

 

[PelliX]

Right, brand spanking new then. Interesting. Any bump in the revision on the boards?

 

[Timo V]

Right, brand spanking new then. Interesting. Any bump in the revision on the boards?

Damn, I did not check that! Might take a while when I get back to that, my amp is back in one piece and my curiosity is satisfied.

 

[scozz]

I bought my Studio 800 in 2019 when they first came out, back then people were talking about these two supposed issues. I’ve never experienced a pop with mine, but mine does have the volume drop when engaging the loop.

But it’s only a slight volume drop and it goes away completely by the time you hit 4 on the master. For me though, I have the loop on all the time, (3 pedals in the loop + a looper), so it doesn’t matter.

In fact it works as an advantage for me when I’m occasionally playing at low volumes. Being an at home player it’s like a mini attenuation, haha.

@PelliX is right about the design of the loop on/off switch, it’s not designed to be turned on and off during songs. It designed to be on or off as needed,… but not in the middle of the song.

I know some people feel strongly about these things, even being a deal breaker for some. For me though, neither of these things are issues the way I use the amp.

 

[Timo V]

I bought my Studio 800 in 2019 when they first came out, back then people were talking about these two supposed issues. I’ve never experienced a pop with mine, but mine does have the volume drop when engaging the loop.

But it’s only a slight volume drop and it goes away completely by the time you hit 4 on the master. For me though, I have the loop on all the time, (3 pedals in the loop + a looper), so it doesn’t matter.

In fact it works as an advantage for me when I’m occasionally playing at low volumes. Being an at home player it’s like a mini attenuation, haha.

@PelliX is right about the design of the loop on/off switch, it’s not designed to be turned on and off during songs. It designed to be on or off as needed,… but not in the middle of the song.

I know some people feel strongly about these things, even being a deal breaker for some. For me though, neither of these things are issues the way I use the amp.

Sigh... Et tu Brute. So far no one who have replied to my OP on this or on other forum, have not noticed that in so many words I said that it's not activating the loop that causes the pop, it's activating THE EFFECT that's in the loop that's causing the pop. And that is a VERY REAL problem.

 

[Dogs of Doom]

to be fair...

Marshall might consider that a fault of using an unsuitable effect in the loop...

They used to put in the handbooks:

* pedals go in front of the amp...
* rackmounts go in the loop...

Impedance is one thing, but, also having a suitable pre-amp in the effects unit.

I use a micro-rack effects (reverb) in my loop & it's always on, so, I don't notice any problems. The micro-rack has no on/off button, or switch. I just set the input & output on the reverb to taste.

I understand that pedals have improved in a lot of ways, over the years, but, there's still some things like that to consider.

 

[Timo V]

to be fair...

Marshall might consider that a fault of using an unsuitable effect in the loop...

They used to put in the handbooks:

* pedals go in front of the amp...
* rackmounts go in the loop...

Impedance is one thing, but, also having a suitable pre-amp in the effects unit.

I use a micro-rack effects (reverb) in my loop & it's always on, so, I don't notice any problems. The micro-rack has no on/off button, or switch. I just set the input & output on the reverb to taste.

I understand that pedals have improved in a lot of ways, over the years, but, there's still some things like that to consider.

Yes, in this case one size does not fit for all, and those Marshall effect locations advices are familiar too, but i'm very well versed with these kind of circuits because I designed them for years, and truth to be told, sc20's effects loop is terrible. There are at least 3 wrong sized capacitors (and two of those are wrong types too, low quality electrolytics in signal path.) And then there is that damn volume drop issue too. It's like this whole effects loop circuit is some kind of poor copy-paste job.

But enough already, I've drained my batteries now. But stay tuned, there's more to come.

 

[LCW]

Just got a used SC20 off Reverb. Appears to be in great shape. But has massive volume drop when the loop switch is engaged. So much so it’s similar with it on in 20W mode to 5W mode with it off.

Dated 2019. Week 26 if that’s what the 2 digits following the year indicate on the sticker.

I don’t use the loop so it’s not critical but I’ll still bring to my local tech to fix this. I’ll eventually sell this I’m sure so I don’t want the next guy to bitch about it.

 

[PelliX]

Just got a used SC20 off Reverb. Appears to be in great shape.

HNAD!

But has massive volume drop when the loop switch is engaged. So much so it’s similar with it on in 20W mode to 5W mode with it off.

Dated 2019. Week 26 if that’s what the 2 digits following the year indicate on the sticker.

I think that's how it works, yeah. Would check for mine, too.

I’ll eventually sell this I’m sure so I don’t want the next guy to bitch about it.

If you're like me, this one can grow on you...

 

[LCW]

HNAD!



I think that's how it works, yeah. Would check for mine, too.



If you're like me, this one can grow on you...

Haha I may keep it lol

 

[PelliX]

With those two amps you can cover a lot of Marshall territory.

 

[stephan_l]

Yes, in this case one size does not fit for all, and those Marshall effect locations advices are familiar too, but i'm very well versed with these kind of circuits because I designed them for years, and truth to be told, sc20's effects loop is terrible. There are at least 3 wrong sized capacitors (and two of those are wrong types too, low quality electrolytics in signal path.) And then there is that damn volume drop issue too. It's like this whole effects loop circuit is some kind of poor copy-paste job.

But enough already, I've drained my batteries now. But stay tuned, there's more to come.

thx for your help fixing this. Today i changed the 2 resistors and made the changes from your 1st post. Before, the loop was nearly unusable on my SC20H.

If there is time, maybe you can share which other caps are good for a change to the better. thx again

 

[Timo V]

thx for your help fixing this. Today i changed the 2 resistors and made the changes from your 1st post. Before, the loop was nearly unusable on my SC20H.

If there is time, maybe you can share which other caps are good for a change to the better. thx again

I'm sorry it took this long, i somehow missed your reply! IIRC there is at least one one critical component swap that gives the FX-loop better high frequency response. This has been kind on a back burner for a while, but stay tuned, i'm about to open my amp to see what i did (I have a shitty memory). Also, glad to hear my fixes were usefull! It escapes me why they put so obviously bad loop in otherwise lovely amp.

Edit. And now i noticed he has not been here since that post. I failed him, f**k.

 

[stephan_l]

I'm sorry it took this long, i somehow missed your reply! IIRC there is at least one one critical component swap that gives the FX-loop better high frequency response. This has been kind on a back burner for a while, but stay tuned, i'm about to open my amp to see what i did (I have a shitty memory). Also, glad to hear my fixes were usefull! It escapes me why they put so obviously bad loop in otherwise lovely amp.

Edit. And now i noticed he has not been here since that post. I failed him, f**k.

no fail at all. I was waiting patiently

 

[Timo V]

no fail at all. I was waiting patiently

Dude!

 

[Pete Farrington]

Issue number 1. A loud pop when true bypass effects are activated. A cause is an effects loop's electrolytic coupling capacitor (C35) that is 100(!) times bigger than needed and is also floating. When a non buffered effect is activated, the capacitor discharges to effect's input stage causing a loud pop. A solution is to replace the cap with a 470n plastic capacitor. After replacement a pop is barely audible.

I think the root cause of the pop is that C35 is missing a pull down resistor, to ensure that the send signal sits at 0V DC. As it is, it can just float off.
Without that, even the miniscule DC leakage of a film cap will cause the V DC there to eventually settle at about 4.5V.
I agree that C35 needn't be such a high value as 47uF. But in the absence of a pull down resistor, reducing its value just reduces the energy in the pop, rather than eliminating it.
A resistor in the range 10k to 100k should be added across the send output.

Issue number 2. A volume drop when effects loop is turned on. Like issue number 1, somebody at Marshall screwed up.
An effects loop's recovery stage's gain is set too low

Yes, and then to compound the issue, enabling the loop also seems to actuate RL1, which reduces the signal level even further.
It's baffling, RL1 seems to be completely unnecessary.

The potential divider for the send signal reduces it to about 1/16 (0.064).
But the recovery stage only has a gain of 9.4.
RL1 brings in the R50&51 potential divider that reduces the level by a further 0.6
The net total is that engaging to loop reduces the signal to about 0.4 of the dry level (an 8dB reduction).

IIRC there is at least one one critical component swap that gives the FX-loop better high frequency response

I can't see an issue with that aspect of it?
The upper corner frequency formed at the recovery stage C37 // R46 shelf filter looks to be up above 100kHz.