ABC pedal causing hum

Dodge · July 2023

I've bought a new passive ABC switch which seems to be injecting a fair bit of hum into my signal chain.

The only power requirement is for the LEDs, it's completely passive otherwise. I've removed all the effects beforehand so I have guitar > ABC pedal > 3 x amps. I noticed that with amps B & C switched off, that amp A was making a fair bit of hum. I then realised, it was only humming when the ABC switch was connected.

What I'm experiencing is connecting an amp to output A with nothing else connected it's mostly hum free (I would say a normal level of background hum). Adding a cable (not connected to an amp) to output B adds a slight but obvious amount of hum. Adding another cable to output C (again, no amp) increases the hum the same amount. All of which with the switch set to A and the amp connected to output A.

Cables all check out, taking the switch out of the equation and the hum goes. Using a 30 cm patch cable instead of a 3-5 metre instrument cable causes less hum - it's like the non-connected cables are somehow adding hum, despite not being active on the switch.

The other thing I've noticed in my DAW with all amps connected via the switch to loadboxes and into the sound card that there's a slight amount of signal bleed through. E.g. amp A switched on, when I play guitar the meters are showing a slight amount of signal bleeding through outputs B and C. Same whichever amp I select, there's a bit of bleed through to the other channels.

Have I got a dodgy switch?

Cheers.

Gassage · July 2023

Sounds like there's no ground lift or buffer in it.

One side needs to be fully isolated and normally the other is buffered, which needs power.

Passive boxes are nowhere near as good. Lehle, Radial, Gigrig as a bare minimum.

Dodge · July 2023

Gassage said:

Sounds like there's no ground lift or buffer in it.

One side needs to be fully isolated and normally the other is buffered, which needs power.

Passive boxes are nowhere near as good. Lehle, Radial, Gigrig as a bare minimum.

I get that in an ABY switch, but why would I need a buffer in an ABC switch?

ecc83 · July 2023

Sounds like you have a "ground loop". Make up a guitar cable where the screen is not connected in one of the jacks. Also ensure that all the amps share a common power source i.e. same strip. There is a very good article about this in the current copy of Sound on Sound magazine.

Dave.

Dodge · July 2023

ecc83 said:

Sounds like you have a "ground loop". Make up a guitar cable where the screen is not connected in one of the jacks. Also ensure that all the amps share a common power source i.e. same strip. There is a very good article about this in the current copy of Sound on Sound magazine.

Dave.

Hi Dave,

All amps are powered from the same strip.

But, with one amp connected only (output A), the ABC switch not powered (it only wants power for the LEDs, it's passive) as I add a second and third cable to outputs B and C (still no other amps), the hum increases at each stage with output A selected.

Could that still be a ground loop?

Cheers mate.

Sporky · July 2023

A photo or nine of the insides might help.

ecc83 · July 2023

Dodge said:

ecc83 said:

Sounds like you have a "ground loop". Make up a guitar cable where the screen is not connected in one of the jacks. Also ensure that all the amps share a common power source i.e. same strip. There is a very good article about this in the current copy of Sound on Sound magazine.

Dave.

Hi Dave,

All amps are powered from the same strip.

But, with one amp connected only (output A), the ABC switch not powered (it only wants power for the LEDs, it's passive) as I add a second and third cable to outputs B and C (still no other amps), the hum increases at each stage with output A selected.

Could that still be a ground loop?

Cheers mate.

Yes, to form a ground loop you need two pieces of mains powered equipment that have an earth (i.e. almost all guitar amps) and then an audio path that also carries an earth shield. This is why breaking the shield one end usually fixes the problem N very B! I f you make up such a cable MARK IT UP as using it for say guitar to amp will result in the MOTHER of all hums! This does not depend on all the gear being powered up.

Also, despite what you might read or be told NEVER remove any plug earths!!!

Yes Sporks' Picture worth...

Dave.

Dodge · July 2023

ecc83 said:

Yes, to form a ground loop you need two pieces of mains powered equipment that have an earth (i.e. almost all guitar amps) and then an audio path that also carries an earth shield. This is why breaking the shield one end usually fixes the problem N very B! I f you make up such a cable MARK IT UP as using it for say guitar to amp will result in the MOTHER of all hums! This does not depend on all the gear being powered up.
Also, despite what you might read or be told NEVER remove any plug earths!!!

Yes Sporks' Picture worth...

Dave.

Image: https://www.dropbox.com/scl/fi/p8pk2l4x2vlaminujqwpg/PXL_20230728_082337904.MP.jpg?rlkey=961s4sgkfi3qo54hasaoylntt&raw=1

Forgive me for being a bit basic here, I should be detaching one end of the shield from one of the second or third ABC switch > amp cables?

Sporky · July 2023

Let's not worry about the shield thing yet (though ecc is right as usual).

My guess for the noise issue is that the enclosure doesn't appear to be grounded. The jacksa are all plastic bodied (which is fine) and I can't see any link from signal ground to the casing. That very often results in noise.

If you have a multimeter check for continuity between signal ground (the black wire linking the jacks) and the enclosure.

Assuming there's no link, if you can solder, strip a bit of wire, connect it to the ground on any jack at one end, and trap the other end between the jack body and the casing. That may resolve your noise issue, and we can then move on to dealing with ground loops if they're a problem. Which they probably will be, as the ABC box is linking all the amps' grounds together.

Dodge · July 2023

Sporky said:

Let's not worry about the shield thing yet (though ecc is right as usual).

My guess for the noise issue is that the enclosure doesn't appear to be grounded. The jacksa are all plastic bodied (which is fine) and I can't see any link from signal ground to the casing. That very often results in noise.

If you have a multimeter check for continuity between signal ground (the black wire linking the jacks) and the enclosure.

Assuming there's no link, if you can solder, strip a bit of wire, connect it to the ground on any jack at one end, and trap the other end between the jack body and the casing. That may resolve your noise issue, and we can then move on to dealing with ground loops if they're a problem. Which they probably will be, as the ABC box is linking all the amps' grounds together.

There is good chassis to ground line continuity which I'm pleased about, because I don't really want to be modifying a new pedal - I'd rather back it and buy something else.

So, lifting a sheild from one of the ABC switch to amp instrument cables?

Whilst I think I get the concept of a ground loop - I don't understand why breaking the shield from the ABC > amp cable wouldn't generate noise like breaking an instrument to amp cable would (as @ecc83 mentioned). They're all unbalanced cables after all.

Sorry for being stupid.

Sporky · July 2023

Because the amp and the pedal each have their own ground, whereas a guitar hasn't.

Danny1969 · July 2023

That design is always going to hum as all the grounds are connected together. Basically if there's more than than path to ground with a different impedance then there will be a voltage difference and that will cause hum at 50Hz

A better way to do it is to either mute the unused amp and switch grounds as well as hots or to provide some isolation but that would generally mean a buffer - then isolation transformer.

Dodge · July 2023

Sporky said:

Because the amp and the pedal each have their own ground, whereas a guitar hasn't.

I understand that - so I leave the shield connected to one of the three ABC switch > amp leads and break the other two, so the switch then has one ground reference via one amp and no more ground loop? Is that right?

Sporky · July 2023

I found that switching the grounds tended to pop.

ecc83 · July 2023

You are not being "stupid" in the least Dodge! Ground loops are not that easy to get one's noggin around and can drive you bloody nuts in complex setups*

Take the simplest setup, a mains powered guitar amp feeding a second amp via its FX send jack. That will usually result in a hum. Breaking ONE end screen breaks the loop but because BOTH amps are earthed there is no hum. As I said before, if you were to use such a modified cable with a guitar...hums like ****! Because guitar is not then earthed.

*You mention "balanced" setups? It is true that balanced working can all but eliminate ground loops but it is not by any means foolproof. Also, where you have low impedance device outputs, interface, mixer, etc you can use transformers to isolate grounds but 'gitkit' is too high impedance and transformers would insert too much loss.

Dave.

ecc83 · July 2023

Sorry for the duplication. This old, one eyed duffer is a bit slow on the ole qwerty these days!

Dave.

Dodge · July 2023

@Sporky @Danny1969 @ecc83 - thanks for all the input.

I don''t necessarily mind a pop wjhen switching the grounds too, however as that'll be modifying the pedal I'd rather leave it.

So, just for clarity - I disconnect the shields of two of the three cable from switch to amps meaning there's only one ground reference from one amp (so all the outputs sockets all share the same ground regardless as to which one is active.

Is that right?

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ABC pedal causing hum

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