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 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.

Gareth Magennis wrote:
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.

Disconnect the signal ground at *one* end.

jak



Perplexed,


Gareth.

Try connecting the two together using 1 to 1 isolation transformers, this
will negate any possibility of an earth loop. If the noise persists, then
the problem is not due to a loop.
bg

"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

Thanks for all the replies. I shorted the networks and took the
amp back to the guys bedroom studio, and it cured the loop problem
totally, but the Boss's audio is still a lot noisier than I would
expect. I was going to contact Roland Service today - bad PSU?.
However, I talked to the guy this morning and he has moved the
equipment to a different room in the house and now it's really noisy
again, so I think the problem is actually his house wiring! Back
again tonight to undo the short and experiment I think.

Alesis did send me a generic pdf this morning about earth loops but
nothing specific to their amps.

I really hate earth problems.


Thanks to all,

Gareth.