update on alternator noise over speakers

[Huw Powell]

Lawrence C Leung wrote:
> 
> Most of these filters are choke coils (inductors). You want as much coil
> as possible. You can connect more than one in parallel (i.e. + to + , -
> to - , and yes there really isn't a + and  - in a coil, but it's more
> descriptive for those whom aren't as into electronics) to each other, but
> in series to the circuit, AS NEAR AS POSSIBLE TO THE AMP/HEAD (i.e. coils
> all connected together, in-line to the amp) to gain inductance.

wiring coils in parallel may reduce th series resistance - but it lowers
the inductance.

> 
> In ADDITION, you can also add a sizable capacitor into the circuit,
> connected directly across the power terminals of the amp. Here + and -
> matters, match + to +, etc. This type of filter is not commonly sold, and
> due to the range of possible engine electric noise frequencies (most vary
> by RPM) I don't know a way to calculate the appropriate value. I used a
> 2000 micro-farad MONSTER capacitor 

Farad-value range (1 farad = 1000000 microfarads...) caps are commonly
sold these days for the car stereo market, though usually to the "Boom!"
market rather than for noise reduction.

Though with fat power wires I see little point in just a cap, since the
cars battery is one heck of a capacitor.

If, however, you have a choke coil in your amps power wire, a capacitor
in parallel between it and the amp is useful, since they combine to make
a second order filter (rolls off higher frequencies twice as fast).

> Generally, filters will only be very effective over a certain frequency
> band (or in a car's case, RPM range). 

In the case of a ("low pass") filter as described above, all frequencies
above some cutoff point are attenuated, and more so as the frequencies
become higher.  Which means if there is a square wave or something close
to it leaking into your power wiring (or being picked up as RFI) most of
it will be filtered out.

But a properly wired car stereo hooked up to a properly functioning
vehicle electrical system can be made noise free without filters.

One-point grounding (watch that antenna wire!), "directional" line level
wires, and proper care with all connections and wire placement will do
it.

Everything on my coupe is grounded to one of the shifter mount bolts
(except the antenna...) - I ran 8 ga ground wires forward from my amps,
pulled the line level cables sheilds together and ran a wire from there,
and the ehad unit itself.  No noise.

-- 
Huw Powell

http://www.humanspeakers.com/audi/

http://www.humanthoughts.org