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hi-fi audio signal booster- RCA input connections - how to wire the "shield" ??

Discussion in 'Audio' started by Skidood, Sep 4, 2015.

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  1. Skidood

    Skidood

    35
    5
    Aug 24, 2015
    Hi

    I recently built a small 4-channel audio amplifier for my vehicle. It is merely an audio signal voltage booster (2.5 X voltage boost) which goes between the head unit and the Bose factory speaker amp in the trunk area. One channel per speaker.
    I used audio hi-fi op-amps. (set up in the circuit for single-supply operation)
    I had to do this project because I replaced the Nissan head unit with a new Pioneer deck and used the 4 RCA outputs from the head unit (as opposed to the speaker outputs). They are not the 4 Volt high-voltage outputs (deck doesn't have that option) ...I believe the outputs are 2 volts RMS rated. Turned out, the Bose amp actually requires a signal more up around 4-6 V RMS
    The amplifier circuit I built is housed in a small metal enclosure (hereafter referred to as "box") and I installed the metal RCA jacks directly on the walls of the box so the shields (outer ring) of the RCA cables are electrically connected to the box wall, and are tied to DC ground (the circuit ground inside is connected to the box wall and the box wall is externally connected to DC ground for good noise shielding)
    Upon testing using my home stereo amplifier and external sound card on my laptop, it sounded amazing....no detectable degradation in sound quality, no noise.. (laptop plugged into an un-grounded AC outlet)
    Upon testing in the vehicle, I heard a nasty humming, lower than expected volume, and lousy sound quality.
    I then realized the shields should be "floating" or something (not tied directly to DC ground...after a little research, (and I mean a little) it seems that some manufacturers, instead of just grounding the shield, put a 200 ohm resistor between circuit ground and the shield..others also use a capacitor in addition to a resistor. I don't know if anyone uses just a cap.
    What should be my next move? (Take note, its a pain to remove and re-install the deck, and the amp gets tucked in behind it..)
    Should I:
    -just try a 200-400 ohm resistor between the shield and the circuit ground? (on both the inputs and the outputs)
    -use a resistor and cap?
    -use my meter to ohm out the deck (to see what the R is between the outer ring and DC ground on the deck, and also try to see if I can get a capacitance reading, and then emulate that in my circuit.

    I want to keep the box itself grounded for noise shielding but I can isolate the circuit ground inside...just not sure how to tie the circuit ground to the RCA cables shield ...and now instead of going out and buying special jacks (with the outer ring electrically isolated from the mounting surface ) I think I will just run RCA cables out through little holes in the box wall...
    (El Cheapo)
    I have not (so far) had to scratch my head realizing there can be a difference between a signal ground and DC ground/return in audio circuits so this is kinda new territory..
    Thanks in advance.
     
  2. Old Steve

    Old Steve

    734
    169
    Jul 23, 2015
    I have little or no audio experience, but...
    Have you tried with the shield disconnected altogether?
    Sounds like a ground loop problem. Duh.
    I'd go with the good ol' trial-by-error method. Try each method in turn and see what works best.
    Can you do that 'in-vehicle' with longer leads, without having to re-install everything back into the dash or console for each test?
    (Someone with more experience in this area will hopefully get on board, but since you haven't had any replies yet, I thought I'd add my 2c worth.)

    This might help. (I haven't read it myself yet.):-
    http://www.rane.com/note151.html
     
  3. Skidood

    Skidood

    35
    5
    Aug 24, 2015
    Hey OldSteve, thanks, yes I have read that article before. That was almost the limit of my research...
    I guess the crux of my question is, how do you build an audio amp with both a signal ground and a DC ground...
     
  4. Old Steve

    Old Steve

    734
    169
    Jul 23, 2015
    About all I can think of is keeping the supply and signal tracks separate all the way from the power supply to the output, decoupling the supply well with caps as close as possible to the components, then connecting the shield to the supply negative and not the signal negative.

    Still hoping an automotive audio expert will weigh in at some point, because that's all I've got.
     
  5. BobK

    BobK

    7,682
    1,685
    Jan 5, 2010
    It sounds like the Bose unit is designed to connect to the speaker outputs. If that is so, why not connect it that way?

    Bob
     
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