Grounding issues using spring reverb with interface.

[BigDeano]

Hey guys, back with another post about my favourite topic, grounding

So what I am experiencing is some ground hum buzzing which occurs when I use my Vermona VSR3.2 spring reverb as a hardware send/return. What is interesting is if I touch the chassis of the reverb with one hand, and anything else that is going to ground (interface case, connected jacks etc) the hum goes away.

To be honest, I have always been plagued with this issue one various setups, but I’m sure there must be a way to solve this, other than employing a person to hold two bits of gear whilst I record!

To give some context, the setup is -
MacBook Pro -
Motu ultralite MkV -
Line out - verb line in
Verb line out - Motu line in.

The outputs on my interface are balanced, but the reverb is unbalanced in/out.

I have tried putting an orchid line isolator on either side, but it doesn’t make a difference.

If anyone has any suggestions on how to solve this I would be very grateful!!

Cheers guys
Dean

[Wonks]

This sounds like one of those situations where noise is due to not having a solid connection to earth. I expect all the devices have Class 2 power supplies with no (or a high resistance) mains ground/earth connection.

[BigDeano]

Yeh all of the devices have wall wart type power supplies with no ground.

So in this situation, would there be a solution?

[Hugh Robjohns]

^ Wot Wonks sez!

The reverb is powered by a low-voltage external supply which is almost certainly Class-II and has no Protective Earth connection from via a mains cable.

Your system is based around a laptop which will also have no mains Earth connection, and the same goes for the interface.

So, nothing in the system is Earthed and, as a result, the metal case of the reverb is picking up whatever electromagnetic fields are floating around and injecting them into the audio ground over the unbalanced connections, where they get added to the wanted audio signals.

You've proved this by semi-grounding the metal case with your hand.

So, all you need to do is provide the system with a decent Earth connection.

There are lots of ways you can do that, but the easiest is to connect something to the interface that is a Class-I device with a three-prong mains connection so it picks up the mains supply Protective Earth. A guitar amp, for example, or powered speakers (they don't have top be turned on, just plugged into the mains wall socket and interface!). The mains earth will then be extended to the interface via the audio cable screen.

Alternatively, connect a long wire to the sleeve of a jackplug, stick the plug into a spare socket of the interface, and connect the other end of the wire to something you know to be solidly Earthed.

[BigDeano]

Aha, thanks Hugh! I knew you would come to the rescue! I’ll give that a go later a report back!