Battery Powered Pedal board - ground loop

[yairisan]

I recently added a rechargeable battery bank to my pedal board, consisting of 10 x 1.2v batts, running through a 7809 voltage reg, with filtering. Works a treat and a single charge is more than enough to do a gig. Problem is a slight ground hum, which goes away when I touch any of the pedals footswitches or metalwork.

The pedals are as follows - Korg tuner -- TRex Dual Boost -- VOX delay -- Hardwire R7 reverb -- Session Award DI

Have tried taking out the tuner, and the the DI, but still get the same issue. It is usable as is, and I am very happy with the sound of the rig, but the hum is definitely noticable now I am tuned into it, and is probably robbing a bit of clarity.

I have heard of people disconnecting shields at one end of patch cables as one solution, but I can only imagine the signal would be cut off doing this, and besides, I wouldnt know where to start with this.

Incidentally, this is an acoustic rig; hence the desire to get the noise down to an inaudible level. For electric guitar, I wouldnt be half as bothered by it.

Thanks for any input.

Y

 

[dgatwood]

What sorts of filter caps are you using, and are you filtering on both sides of the regulator? Batteries are kind of noisy.

Oh, and you might also try adding some resistance in there (just a few ohms, probably wire wound to be safe) followed by a filter cap right at the power input on each device so that any noise that one device generates on its power/ground bus doesn't pollute the V+/ground for the other devices so badly.

+9V source ---->resistor-->Cap+ ---->input
ground---------->resistor-->Cap- ---->input

 

[yairisan]

What sorts of filter caps are you using, and are you filtering on both sides of the regulator? Batteries are kind of noisy.

Oh, and you might also try adding some resistance in there (just a few ohms, probably wire wound to be safe) followed by a filter cap right at the power input on each device so that any noise that one device generates on its power/ground bus doesn't pollute the V+/ground for the other devices so badly.

+9V source ---->resistor-->Cap+ ---->input
ground---------->resistor-->Cap- ---->input

Thanks for the response...

I have 1000uf before the reg and 100, 10 and 0.1uf after the reg output. It seems that all manner of values can be used, although 1000uf before the reg apparently helped someone who had a V out problem, which went away when he installed the 1000uf.

I had no idea that batts would be noisy.

I thought going to psuedo-isolated outs using individual regs for each pedal supply might solve the problem, but still the same. Haven't put it to the test of course, but I imagine indiv. regs at least offer some form of short circuit protection to other pedals in the event that one pedal's power feed gets shorted. I dunno, not going to find out, and neither is it relevant to the hum issue.

I will give your resistor, cap filter a go.

Thanks very much.

 

[dintymoore]

... I have heard of people disconnecting shields at one end of patch cables as one solution, but I can only imagine the signal would be cut off doing this...

That's what I thought when I first heard of that but that is a misconception.

Here's how I made mine:
https://homerecording.com/bbs/showthread.php?t=294090

You use 3 conductor wire (balanced or stereo) and the ground is attached at both ends. But the second ground (the shield) is attached at only one end. A bit freaky to understand at first but it gives you much better protection from ground hum.

But it's not like you were thinking, it would be if you used 2 conductor wire but you use 3.

Adding resistors and caps will screw up the sound to some degree (only makes sense since you're adding stuff) so if the cables worked it would seem preferable.

It's hard to say if the problem is in the cables though.

 

[dgatwood]

Adding resistors and caps will screw up the sound to some degree (only makes sense since you're adding stuff) so if the cables worked it would seem preferable.

Just to be clear, I was talking about adding resistors and caps to the power supply, not the signal lines. It should have no impact on the sound as long as they don't result in a significant voltage drop.

 

[yairisan]

Thanks again for the input, chaps. I have heard of that cable topology, and another interesting route to try.

And I agree that there can't be any harm in adding filtering to the power line, providing there isn't a significant voltage drop that it affects the performance of the pedal.

I have noticed a difference in sound between having a single and separate regulators though! The latter gives a more noticeably more defined sound, more robust and articulate. The single reg seems to place a slight veil over the sound, very interesting.

Haven't had a go at the resistor cap filter yet, but will. After that, I might try the cable idea, but then i have to chuck my nice George L's, but then again, the George L's are designed for electric guitar, with a midrange lift to them, so something more neutral would be better suited to this (acoustic) application.

Still hapy with the battery bank, which drives three hungry pedals and two others for 5 hours straight on a single charge, so the main purpose of the exersise has been accomplished.

Thanks again.

 

[yairisan]

Just to be clear, I was talking about adding resistors and caps to the power supply, not the signal lines. It should have no impact on the sound as long as they don't result in a significant voltage drop.

Put in 1 Ohm resistor and 47uF on each 9V feed as prescribed (though no values were suggested). No significant volt drop, which is good. I'm not sure how much difference it has made though though, as the hum was pretty minimal before. Right now it is VERY minimal, and can live with it quite happily. If I detect an increase when I take out your filtering, they will go back in for good.

Cheers!

 

[dgatwood]

Worst comes to worst, isolation transformers or optocouplers inline would probably... nah, probably not worth it.

 

[commanda]

Put in 1 Ohm resistor and 47uF on each 9V feed as prescribed (though no values were suggested). No significant volt drop, which is good. I'm not sure how much difference it has made though though, as the hum was pretty minimal before. Right now it is VERY minimal, and can live with it quite happily. If I detect an increase when I take out your filtering, they will go back in for good.

Cheers!

On a "off the top of the head, rule of thumb" value, I would have gone an order of magnitude bigger than 1 ohm. 10 ohm, 47 ohm, even 100 ohms.

 

[Warnerbros]

It's possible that you have a bum power supply. If your power supply is a salvaged wall wart that puts out DC, it may or may not have an internal filter. It may or may not have an internal regulator to make the power smooth.