Connect with us

Audio amp earth loop?

Discussion in 'Electronic Repair' started by Gareth Magennis, Nov 2, 2004.

Scroll to continue with content
  1. 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.
     
  2. Asimov

    Asimov Guest

    "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.
     
  3. jakdedert

    jakdedert Guest

    Disconnect the signal ground at *one* end.

    jak
     
  4. bg

    bg Guest

    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
     
  5. 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.
     
  6. Gareth

    Gareth Guest

    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.
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-