Hi,

I'm trying to find the answer to something I just don't get.

In connecting a Boss digital multitrack to an Alesis RA500 power amp, the
amp outputs a fair amount of power supply and disk drive activity noise
from the recorder. The Boss has an external switch mode PSU wall wart. I
found that the amplifiers signal input earths are connected to earth via a
4.7 ohm resistor in parallel with a cap.
To me, that looks like Alesis have built in an earth loop. Why is this RC
network there? (The system exhibits the same noises if you just connect the
signal earths together, hence my earth loop diagnosis).

I am not yet convinced the Boss is as quiet as it should be but this
apparently designed in earth loop just looks all wrong. This is a budget
amp and the Power amp circuit and preamp circuit are built on the same PCB.
Could this RC circuit be a bodge to stop oscillations?

Alesis have not responded to my emails about this.



Perplexed,


Gareth.