Marshall SV20H FX Loop – Issue with digital reverb (Immerse MkII)

[Lichdraik]

Hi everyone,

I own a Marshall SV20H, and I’m facing a persistent issue when using a digital reverb pedal (Neunaber Immerse MkII) in the FX loop.
As soon as I plug the Immerse into the FX loop of the SV20H, even with the Mix set at 9 o’clock, the reverb becomes excessively present, too strong, almost as if it were overamplified.

On another amp (ENGL Savage 120 with a buffered FX loop), the Immerse works perfectly, even with higher settings.

So the problem clearly doesn’t come from the pedal or the signal chain — it seems to come from the FX Send level of the SV20H.

What I’ve already tried:

• All reverb parameters set to minimum (Mix, Depth, Pre-delay, etc.)
• Tried with and without buffer, delay placed before or after
• Tested various configurations (full FX loop vs Immerse alone)

Have other SV20H users experienced a FX Send level that’s too hot for modern digital pedals like reverb or delay?
Is there a reliable solution to attenuate only the Send level, without affecting the rest of the loop?
Has anyone successfully used a passive FX loop attenuator (like the Westminster Loop Attenuator) with this amp?

I’d be really grateful for any serious feedback or experiences, especially with this kind of pedal or this specific Marshall model.

Thanks in advance!

 

[chumbucket]

Hi everyone,

I own a Marshall SV20H, and I’m facing a persistent issue when using a digital reverb pedal (Neunaber Immerse MkII) in the FX loop.
As soon as I plug the Immerse into the FX loop of the SV20H, even with the Mix set at 9 o’clock, the reverb becomes excessively present, too strong, almost as if it were overamplified.

On another amp (ENGL Savage 120 with a buffered FX loop), the Immerse works perfectly, even with higher settings.

So the problem clearly doesn’t come from the pedal or the signal chain — it seems to come from the FX Send level of the SV20H.

What I’ve already tried:

• All reverb parameters set to minimum (Mix, Depth, Pre-delay, etc.)
• Tried with and without buffer, delay placed before or after
• Tested various configurations (full FX loop vs Immerse alone)

Have other SV20H users experienced a FX Send level that’s too hot for modern digital pedals like reverb or delay?
Is there a reliable solution to attenuate only the Send level, without affecting the rest of the loop?
Has anyone successfully used a passive FX loop attenuator (like the Westminster Loop Attenuator) with this amp?

I’d be really grateful for any serious feedback or experiences, especially with this kind of pedal or this specific Marshall model.

Thanks in advance!

This may work for you - https://www.morleyproducts.com/line-level-shifter/

 

[311boogieman]

Seems odd for sure. What @chumbucket posted may work but it sucks that you'd have to use something external like that to fix the problem. Maybe the mix pot on the amp is borked. Also, when I had the serial loop mod done to my Recto I remember him saying something about how he replaced the send or receive jack for some kind of similar reason? Also, it wasn't clear by your OP, but do other effects pedals sound overly jacked or is it just Reverb?

 

[griff10672]

I think it's more to do with the phase inverter section on a Plexi circuit ......

from what I understand ..... and have experienced with my Granger Plexi that I had .... and SV20H that I had for like a week ...

Loops don't work well in Plexi circuits ..... that's why Suhr won't put them in his ...

maybe some of the amp guys will chime in ....

 

[Lichdraik]

Hi everyone,

I just wanted to sincerely thank all of you for your help, your suggestions, and the time you took to answer my questions.

Thanks to your advice, I’ve finally solved the issue with my SV20H’s effects loop. The Westminster FX Loop Attenuator turned out to be the perfect solution — it tames the signal just enough to make my digital reverb behave properly, without compromising tone or dynamics.

Everything’s working great now.

Thanks again for your support!

All the best,
Lich

 

[311boogieman]

Hi everyone,

I just wanted to sincerely thank all of you for your help, your suggestions, and the time you took to answer my questions.

Thanks to your advice, I’ve finally solved the issue with my SV20H’s effects loop. The Westminster FX Loop Attenuator turned out to be the perfect solution — it tames the signal just enough to make my digital reverb behave properly, without compromising tone or dynamics.

Everything’s working great now.

Thanks again for your support!

All the best,
Lich

That's great to hear. Do you know if it is similar to the Morely box @chumbucket suggested?

 

[Lichdraik]

That's great to hear. Do you know if it is similar to the Morely box @chumbucket suggested?

Thanks!

From what I understand, the Morley Line Level Shifter is an active device that uses a transformer-based circuit to convert signal levels (e.g., from instrument level to line level or vice versa), and it may also provide ground isolation.

The Westminster FX Loop Attenuator, on the other hand, is completely passive — no power, no circuitry, just a simple attenuator to lower the Send level before it hits the pedals in the loop.

So while both aim to solve similar problems related to FX loop signal levels, they do it very differently. The Westminster doesn’t convert levels per se, it just trims down the amplitude — which was exactly what I needed in this case.

 

[chumbucket]

Thanks!

From what I understand, the Morley Line Level Shifter is an active device that uses a transformer-based circuit to convert signal levels (e.g., from instrument level to line level or vice versa), and it may also provide ground isolation.

The Westminster FX Loop Attenuator, on the other hand, is completely passive — no power, no circuitry, just a simple attenuator to lower the Send level before it hits the pedals in the loop.

So while both aim to solve similar problems related to FX loop signal levels, they do it very differently. The Westminster doesn’t convert levels per se, it just trims down the amplitude — which was exactly what I needed in this case.

The Morley is a passive unit. I've used one for this exact effects loop scenario although mine still has the Ebtech name before Morley bought them out. The tranformer impedance matching would affect the tone less than just a potentiometer.


From their PDFs on that webpage -

Completely passive design with audiophile quality
components assures the best possible noise and distortion performance

GUITAR AMP FX LOOPS - The signal levels in most guitar
amp effects loops are all over the place. Some may be too strong signal for floor effects , other
may be too weak . Unless your amp has adjustable levels on the send and return you're stuck with
what you got; unless you have a Line Level Shifter®! Use the Line Level Shifter® to increase or
decrease the Send and Return as needed for your particular situation. A more efficient matched
signal level means better tone and in the world of guitar, tone is EVERYTHING!

 

[311boogieman]

Thanks guys. I learned something today so cheers