"Gareth Magennis" bravely wrote to "All" (02 Nov 04 12:16:53)
--- on the heady topic of "Audio amp earth loop?"

GM From: "Gareth Magennis"

GM Hi,

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

GM In connecting a Boss digital multitrack to an Alesis RA500 power amp,
GM the amp outputs a fair amount of power supply and disk drive activity
GM noise from the recorder. The Boss has an external switch mode PSU
GM wall wart. I found that the amplifiers signal input earths are
GM connected to earth via a 4.7 ohm resistor in parallel with a cap.
GM To me, that looks like Alesis have built in an earth loop. Why is
GM this RC network there? (The system exhibits the same noises if you
GM just connect the signal earths together, hence my earth loop
GM diagnosis).
GM I am not yet convinced the Boss is as quiet as it should be but this
GM apparently designed in earth loop just looks all wrong. This is a
GM budget amp and the Power amp circuit and preamp circuit are built on
GM the same PCB. Could this RC circuit be a bodge to stop oscillations?

GM Alesis have not responded to my emails about this.

The 4.7 ohm value may have been misread when the amp was made. Perhaps
it was simply dropped into the wrong tray on the work station. In my
experience it is an obvious error. I'd see more likely something like
47K or rather 470K, even 4.7Meg in such a location. The purpose of this
type of network is usually for rfi bypass using a low value capacitor.

The resistor is there for static charge dissipation which may build up
on the bypass capacitor. It really should have a high value and
certainly never so low as you have found. It doesn't make sense to me
either and would indeed cause a bad ground loop noise.

Basically, the earth ground is not supposed to be used for circuit
operation but only for safety as per electrical code in most locales.
In my opinion, Alesis may be ignoring you to avoid a law suit. Threaten
them with a lawyer and you'll see them jump but fast!

A*s*i*m*o*v

.... A stereo system is the altar to the god of music.