Noise issues, ground lifting and transformers

[vtguedes]

Hi everyone, this is my first post here, but I've been stalking through the forum learning a lot of stuff for my DIY projects for a while now. So first I'd like to thank the community for such a rich source of material!

Lately I have tried to get rid of my pedalboard noise issues and I have realised that:

- With digital pedals on the board, an isolated output power supply is a must (problem solved);
- Even when using a single pedal with a 9V battery, I still get some noise and I think it has something to do with my ground connections.

My amp is set to a clean tone, and if I connect the guitar direct to it I get rid of some of the noise, but as soon as I touch the floor or some other "connected to ground" object I get some annoying hum. I can replicate this to both of my amps but I can get rid of it if connecting DI to both of my audio interfaces available.

Now the questions:

1. Is having multiple connections to ground creating the noise?
2. How can I get rid of it? Should I lift the ground?
3. If so, where is the right spot to lift the ground, at the begining or the end of the pedalboard, or both?
4. How is it achieved? Do I need an audio transformer?

Thanks so much for the help!

[amptramp]

Welcome to the forum!

Now the questions:

1. Is having multiple connections to ground creating the noise?

In most cases, we recommend a star ground with all ground connections made at a single point.  If you have a distributed ground in several places, you may have current running between these "ground" points and the current times the resistance of the connection between them creates a voltage difference between the grounds.  If this is an AC current, you will have the "ground" voltage modulated by the signal and noise voltages.

2. How can I get rid of it? Should I lift the ground?

Many designs use a shielded twisted pair connection with the signal current going between the two wires of the twisted pair and the shield only being grounded at one end, usually the amplifier end.  Others use coax cable and put up with the shield being the return path for signal.

3. If so, where is the right spot to lift the ground, at the begining or the end of the pedalboard, or both?

Shields should be grounded only at one end for audio (both ends for RF) to avoid carrying currents on a shield.  Obviously, this is not possible with coax cable.

4. How is it achieved? Do I need an audio transformer?

You will find designs within this forum that use a transformer for isolation but the cost and limited frequency response of a transformer mean that most designers will try anything else before resorting to transformers for isolation.  Some people use differential amplifiers or instrumentation amplifiers at the input to cancel out any common mode noise where the input ground and signal appear to be moving up and down at the same amplitude and time.

Thanks so much for the help!

No doubt other people will weigh in with their ideas and some of them may be different from mine.  Use whatever works.

[PRR]

> as soon as I touch the floor or some other "connected to ground" object I get some annoying hum.

What is your electrical supply? Properly grounded to utility company and to dirt and all inside wiring?

With all groundy-things properly bonded, "multiple grounds" is usually a minor issue. If they are all floating at various potentials, hum happens.

BE CAREFUL! Un-bonded electric supply is UN-safe. We occasionally find mis-wired services where electric "ground" is high-voltage to dirt/concrete and contact can be fatal.

[Rob Strand]

Does it happen when you just have the amp by itself?   (even a little bit that you can hear when you crank the volume.)

It's pretty weird if it only happens with pedals and even pedals running off battery.
Debugging that simple configuration would be a place to start.

(There's a difference in knowing why it's happening and just fixing it.  Although knowing why often helps fix it.)