Marshall SC20H Studio Amp Head - Effects Loop Useless



Recently purchased this amp head only to find out that the effects loop does not works as advertised

I have tried an number of buffered and unbuffered pedals and the result is very frustrating

 When the loop switch is engaged with the amp in 5 watt mode there is an audible pop a noticeable hum comes from the amp/speakers and any effects connected pop loudly when engage/disengaged.

 When the loop switch is engaged with the amp in 20 watt mode (full power) there is an  audible pop a louder more annoying  hum coming from the amp/speakers and any effects connected pop loudly when engage/disengaged.

 I have also used just a quality effects pedal cable between the send/receive only to see if it’s just the pedals and the circuit still click and hums when engaged.

 This is totally unacceptable in any way shape of form and I would expect that with a brand such as Marshall and its long history of quality equipment that these type of issues would not exist as pre user testing would have sort them out!

This appears not to be the case as a number of online amp forums have pointed out that this is a know issue with this amp and no know solution or repair fix has been forthcoming from Marshall.

 Please issue a fix for this to be carried out by authorised service techs at your expense as this type of marketing and selling of equipment that is faulty and "not fit for purpose" is illegal under Australian consumer law and is most likley the case in other countries


I look forward to receiving a thoughtful and considered response to resolving this issue!


asked 11 May 2021 at 01:30 AM

Ross Ferro (1)
Answers: 2


Hello Ross,

I am sorry to hear that you are not happy with your SC20H, thank you for bringing this to our attention, i will pass the details on and this will be looked into.  So we can help you further in the meantime please email your contact details to us at



Kind Regards



answered 11 May 2021 at 08:48 AM

The Australian retailer Musos Corner in Newcastle NSW, has requested I return the amp to them for their amp tech to review. Is this ok with you or should I await contact from the Australian distributor?? Regards, Ross - Ross Ferro 11 May 2021 at 11:35 PM
It it more then likely that the retailer has already been in contact with the Australian distributor, so it is fine to return it to them.

Kind Regards
- Marshall 12 May 2021 at 07:31 AM
Are you able to confirm who the Australian distributor is? - Ross Ferro 12 May 2021 at 11:35 PM
Marshall SC20 Amplifier Head
Are you able to confirm the signal path from preamp through the loop to speaker output?
I had a stab at trying to establish this and came up with the following signal flow pardon my lack or electrical knowledge and it goes something like this:

Input- Preamp Volume -Tone Stack- Master Volume - Loop Send -----Loop Return - Power Amp- Speaker

Tested this by plugging my guitar directly into effects loop return input and noting that the Amp Master Volume control does not increase/decrease the guitar output level to the speaker.
I found that only the guitar volume pot controls the amp output.
So, by definition does this mean that the effects loop is wired as if this amp is a No Master Volume type of amp like the SV20H??
Does this also explain the click when engaging the effects loop on/off switch as the amp voltage tries to normalize and plugging and effect into the loop further exacerbates the issue? Hum and volume drop?
Is this why non buffered effects pedals pop/click?
Is there a way this can be fixed with easily?
- Ross Ferro 13 May 2021 at 02:14 AM
Hello Ross,

As we have your contact details we will email you directly, so we can help you further

Kind Regards
Marshall Support
- Marshall 13 May 2021 at 08:42 AM


Have sent contact details as requested and will await contact from your Australian distributor 

I trust this will be in a reasonable time frame!


answered 11 May 2021 at 11:11 AM

Ross Ferro (1)
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