Car audio, line noise and pre-amplification

Discussion in 'General Electronics Chat' started by Litch, Aug 15, 2013.

  1. Litch

    Thread Starter Member

    Jan 25, 2013
    I can't fit a head unit in my dash so I've just run some audio cable under the carpet from the boot and popped it out in the center console with a 3.5mm plug (Music via smart phone).

    Anyway, I've got some serious gear from about a decade ago; couple of 2.5KW Class D Monoblock amps, 4 x 6.5" component speakers, smaller 2CH amp for splits, etc...

    Currently I've just got a couple of 12" subs running off 1 amp and some 6x9s running of the 2chan - I'm getting a lot of noise though, even when nothings plugged in to the 3.5mm plug.

    Now I've encountered this before and it's usually an earthing issue, but my earth is zero-gauge cable bolted to the chassis, which has been sand papered to reveal the bare metal (prep in case I wanted to run both amps with some better subs, which I plan to).

    Also, the noise is worse when my iPhone is plugged in to the charger (that plugs in to the 12v cig lighter socket) - that's an expected result.

    Now I plan to throw 2 x 1500W 15" subs in, and utilise both amps - before which I'd like to get this noise under control.

    The circuit, as far as that goes is 0-gauge from that battery +ve all the way to the boot, where a solid brass distribution block breaks that out to 2-gauge to the amps, then another distro block is grounded (bolted) to the chassis for ground with the ground from the amps connecting to that.

    Finally, the questions:

    * Since I can't throw inline inductors in the power feed (We're talking >= 200 Amps at peaks when up loud) I'll need alternate ways of filtering out nose in the power - what type of circuit could I utilise? A 0.47μF cap across the power as close to the amp as possible, but that's only going to be effecting to a degree.

    Also, inline inductors are a big no-no anyway since at 12 volts, any impedance will severely degrade performance.

    * Rather than have my iPhone feed 6 channels at near max-volume (3 x Amps, Stereo channels from iPhone wired in parallel over each amp) - it's not designed for that environment and the SQ degrades at high volume. Instead, I'd like a high impedance input to a preamp, which can then break out 6 or 8 channels (3 or 4 sets of duplicated stereo outputs from the 1 stereo input), amplification to make the outputs with a decent gain so the amps don't need to do all the work from a small signal (thus lowering distortion) - happy to go out and purchase something but if there's a simple way of achieving this I'd like to build it.

    Thanks in advance for the advice.
    Last edited: Aug 15, 2013
  2. bountyhunter

    Well-Known Member

    Sep 7, 2009
    1) make sure noise is not caused by ground loops. This is a big problem in auto electronics.

    2) You probably need some monster sized caps at the power terminals of the units. They are readily available.
  3. Litch

    Thread Starter Member

    Jan 25, 2013
    From seminar.pdf

    Page 1:

    "Ground noise, usually heard as hum, buzz, clicks or pops in audio signals or seen as hum bars or specks in video signals, is generally much more noticeable and irritating."

    Well I think you may be on to something there bountyhunter, as that describes the effect to every point.

    Seems that further research is warranted - thanks for the hint.