Loud hum with pedalboard in effects loop only? Ground loop?

Grumpyrocker

I built a pedal board last week and I'm really happy with it in front of the amp. 

However when I tried it in the amp's effects loop there was a massive hum. 

I assume this is the cheap "one spot" style PSU I'm using. But why would there be a bad hum with the pedalboard in the loop, but not when in front of the amp?

Will a decent power supply stop this or do I need to chase a different problem? Recommendations please folks.

Drew_TNBD

Are you powering pedals out-front as well as in the loop? Classic case of ground hum that one. You could also try lifting (IE: severing) the ground connections in the cables that connect to your send and return on the amp. I've seen people have success with that.

Grumpyrocker

No this was just pedals in the loop.

I did try my little isolator box and that helped quite a bit. But not completely. So perhaps there's some interference as well.

I don't really have the money for a fancy effects power supply - and I'm not gigging right now so it's hardly a priority. Was mainly posting because I was surprised the hum was so bad. given there was none with the same effects in front of the amp.

Given that I don't need a very reliable power supply - in terms of gig length. Did consider one of those eneloop power packs.

Drew_TNBD

These issues are notoriously difficult to solve. I really wanted to use our Macbook as my effects rig at one time - the range of free effects you can get out there just piss all over most cheapish stompboxes - but no matter what I tried, I could not get it to place nicely with a real guitar amp.

And so it goes.

ICBM

It does sound like a ground loop problem of some sort. In theory there should also be one if the board is in front of the amp, but some amps have deliberately partially-ground-lifted inputs to prevent this. It may be that the FX loop is directly grounded.

I did think the One Spot power supplies weren't grounded though... does it have a plastic earth pin on the plug?

spacecadet

http://www.thegigrig.com/acatalog/HumDinger.html

Done.

Bidley

ICBM said:

I did think the One Spot power supplies weren't grounded though... does it have a plastic earth pin on the plug?

Yes, they do.

I had this problem, and it was solved by using two One Spots - one powering the pedals in the front, one for the pedals in the loop. Doesn't sound like yours is quite the same. Does it do the same with just one pedal in the loop, or a certain combination? Sounds like your best course is to systematically take pedals in and out of the chain until the hum disappears, if it does at all. Either way, it doesn't sound like it's the One Spot.

viz

Try one pedal in the loop, with a battery. If you still get a hum, spray the send/ return jacks and sockets with contact cleaner.

Grumpyrocker

@Bidley

I think I will have to go through the whole chain and try them one by one. I didn't have the problem when I was using several of the effects before putting the board together. So it could be one particular combination causing the problem that I need to track down.

Just checked the PSU - the ground plug is plastic.

Grumpyrocker

Right then...

Started the task of finding out what was the cause of this very loud hum. First thing I tried was swapping over the PSU. I have two very similar power supplies, both one spot type, both made by the same manufacturer. They aren't identical models though. I changed the one that came with the pedal case to the other one I have.

And that fixed it. Hurrah.

There was still a slight hum though, which would disappear when I switched the effects loop off. I suspected this wasn't necessarily the fault of my pedal board. I put my POD HD in the loop instead and got the same small hum.

So I assume this low hum that remains is either intrinsic to my amplifier or an issue with the power supply in my house. But I'm glad I got rid of the very loud hum very easily.

ICBM

Sounds like a faulty power supply. I had a switch-mode supply that came with a mains-powered pedal which hummed, swapped it for another apparently identical one which I'm fairly sure came from the same Chinese factory, and the hum went away. Oddly the 'faulty' power supply didn't hum with other pedals though!

Sometimes these things just are, there doesn't seem to be much logic behind it, although I'd still like to know why!

Grumpyrocker

Just tried a cable across the effects loop - no hum. So it's not the amp.

So this small hum is likely to be the better PSU. But it's pretty low and the same as my POD. If I use my G3 and MS70CDR with batteries in there's no hum - which I expected. 

Drew_TNBD

Try snipping the ground connection in the cables like I said before.

Grumpyrocker

I've got a little hum eliminator that does that. Didn't make a difference to this low hum. It could be one of the pedals - or perhaps that I have them daisy-chained.

viz

Try a battery operated pedal like I suggested lol

Grumpyrocker

Requires quite a few batteries to do that. I'd need six AA and six 9v. And three of those pedals would burn through a 9v in minutes.

viz

Fair point

Grumpyrocker

Fuck it, none of this crap with a modelling amp with built in FX, might go try a new Blackstar.

Grumpyrocker

Was fiddling around with all this again this morning. Discovered the hummy PSU added hum even if it wasn't connected to anything in my signal chain. Just plugging it into the same  4-way as my amp/pedalboard enabled it to emit a field of annoying hum around itself. Piece of crap.

I'm going to buy one of these.