Audio Circuit Power Supply Isolation

Discussion in 'General Electronics Chat' started by DanRilley, Mar 15, 2011.

  1. DanRilley

    Thread Starter Senior Member

    Jan 13, 2008
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    Hi,

    I am working on a circuit that involves an audio signal (microphone, preamp, mixing, output, etc.) as well as a motor (that drives a tape deck). My question is, what is the proper way to separate the motor circuitry and subsequent noise (it is driven by PWM) from the audio signal circuitry. As it is now, the motor noise is quite audible on the audio signal, which I assume is because they share the same ground.

    I am currently taking power from an AC transformer and running it through a basic rectifier/smoothing circuit (just 4 diodes and a capacitor). Would it be sufficient to use the same AC transformer, but use two separate rectifier/smoothing circuits to create isolated grounds for the motor and audio signal?

    Thanks for any help. I've been doing electronics for a few years, but am still quite new to it, and really know nothing of power supplies.

    Thanks,
    Dan
     
  2. Papabravo

    Expert

    Feb 24, 2006
    10,135
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    Can't give much help based on that recitation. How about a schematic and maybe some pictures so we can see whats going on.
     
  3. DanRilley

    Thread Starter Senior Member

    Jan 13, 2008
    107
    0
    OK, I have attached a quick layout of what I meant.

    The audio circuit here (shown to the right) is purely hypothetical for demonstration purposes (I don't think I'd use an LM386, etc.) The point is that I am confused on how to separate the motor circuit (shown left) from the audio circuit (shown right) while keeping them both running off a single transformer (shown above).


    Thanks,
    Dan
     
  4. Papabravo

    Expert

    Feb 24, 2006
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    The first problem I see is that you are running amplifiers with unregulated voltages and you do not appear to realize the need for bypass capacitors.

    You can eliminate many problems if you connect the motor ground and the analog ground at a single point near the filter capacitor. In your layout route the hot lead and the return lead to the motor without going near any of the other components or their power and ground connections.
     
  5. DanRilley

    Thread Starter Senior Member

    Jan 13, 2008
    107
    0
    OK Thanks. Yeah I plan on using regulators I just didn't include them, but in the past adding regulators has not helped isolating noise from the audio signals, so I didn't include them here. But yes, I will be using those.

    And yes I should be using bypass caps.

    So just to clarify, the ground connection from the motor should connect close to the ground of the 1000uF cap, and the 'hot lead' that is going into the motor's MOSFET should be as close to the positive side of the 1000uF cap?

    Also my audio power similarly should be going to that same point? Keeping in mind to keep the traces for the motor away from those for the audio except for that single point of connection?
     
  6. russ_hensel

    Well-Known Member

    Jan 11, 2009
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    Why not use a 3 term regulator for the audio circuit?
     
  7. DanRilley

    Thread Starter Senior Member

    Jan 13, 2008
    107
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    Yeah, I will use a regulator. Is a three terminal regulator any different from a normal regulator? I thought most regulators had three terminals, but again, I'm fairly green on everything.

    In fact I have a bunch of these lying around:

    http://www.national.com/mpf/LM/LM317.html#Overview

    and was planning on running the Audio Circuit and the LM358 of two of these at 12V.
     
  8. Papabravo

    Expert

    Feb 24, 2006
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    The key feature is to isolate the motor leads geometrically, not necessarily electrically. The idea is that noise has to travel from the motor back to the power supply capacitor and then if it wants to interfere with the analog stuff it has to travel from the power supply cap back out to the analog section.

    Read this:
    http://www.ese.upenn.edu/rca/instruments/misctutorials/Ground/grd.html
     
  9. DanRilley

    Thread Starter Senior Member

    Jan 13, 2008
    107
    0
    Thanks. I have also found that the PWM bursts can find their way into the Audio Signal at times, is this the same thing, in terms of isolating geometrically?
     
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