"Gareth Magennis" wrote in message
...
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.

Gareth
The RC combination is known as an earth lift, it used to be very common in
valve amps where, because of the high voltages present, you have to have a
low impedance to ground on the signal ground in case of a fault condition; I
have used this combination with 4.7 Ohms in parallel with 0.22uF in my own
amps. Of course, valve amps have linear power supplies, the HF noise from a
switcher may still manage to form an earth loop through this circuit.
Martin.

--
martindot herewhybrowat herentlworlddot herecom