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I recently got a Marshal JCM800 2203 reissue and it sound unbelievable! But I seem to be having issues with the FX loop. When the FX loop is on the -10db setting (which is recommended for floor pedals) The amp seems to be noisy... but if I set it to +4db (chich is recommended for rack units) the noise seems to go away.

The problem is that I am only using this FX loop for floor pedals. (A Boss DD7 and a ISP Decimator G-String II)

Even with the noise gate, the hum seems to come through. What could cause this to happen? Is it typical for an amp to be noisy when the FX loop is set to -10db?

The effects seem to sound okay when it is set to +4db, but I'm wondering if this is typical or if I should bring my amp in to a technician and have it checked out.

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  • Have you tried looping directly (without any pedals in the signal path) the send to the return? If the noise goes way then most probably it's not a problem with the head, but some kind of mismatch between the pedals you're using and the input circuit for the -10bD return input.
    – José David
    Commented Apr 18, 2016 at 11:23
  • If I connect the send directly to the return it still produces some noise on the -10db setting. Not as much, but I would think with absolutely no pedals it should be silent.
    – tjwrona
    Commented Apr 18, 2016 at 11:43
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    In that case only someone who knows this particular amp model (which I don't) may tell if that's normal or not, but my expectation would also be not to have significant noise.
    – José David
    Commented Apr 18, 2016 at 17:00
  • A couple of things to try just be on the safe side, if you haven't done so yet. 1) It shouldn't be ground loop noise either, since with a direct effects loop the only active component you have is the amp it self, but it won't hurt checking the grouding of your electrical connection, if you can. That's one of the first things the shop or technician are going to ask anyway. 2) It could however be electrical hum picked by your guitar that's being more amplified by the -10db circuit. Is the noise the same with or without the guitar plugged in? Are the pickups single coil or humbuckers?
    – José David
    Commented Apr 18, 2016 at 17:01
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    @joseem, I contacted Marshall and they said my amp may have a bad MOSFET in the FX loop circuit. I won't know for sure until I take it to a tech though.
    – tjwrona
    Commented Apr 22, 2016 at 13:52

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It turns out that the issue was a ground loop between my FX loop and the front of the amp! I found out that it was actually only happening when I had my Noise Gate in the loop. The Decimator G String II takes an input from the front of the amp along with an input from the FX Loop which ends up connecting the two grounds. Apparently the two grounds were at slightly different voltages and it induced a ground loop and caused a humming sound.

I ended up getting an Ebtech Hum Eliminator and used it to lift the ground of the FX loop and the problem was solved!

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Really the issue is that the Decimator G String II should come with a ground lift switch... But unfortunately it doesn't.

Also on top of the hum there was a slight hiss on the -10db setting even without the noise gate. I ended up taking the amp to a tech and he explained that it has one potentiometer to adjust the bias, but it also has another to adjust noise and the "noise" adjustment wasn't quite set right. Tweaking the second potentiometer helped eliminate the last little bit of hissing.

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The most common cause for effects loop noise is placing a pedal or electronic device to close to the amp head. Make sure your pedals or effects devices are at least two feet away from the amp head.

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