From: gilchr@ee.ualberta.ca (Andrew Gilchrist)
Subject: Re: Capacitors, etc... CALMLY
Organization: University Of Alberta, Edmonton Canada
Date: Fri, 13 Nov 1992 07:31:24 GMT

jeh@cmkrnl.com writes:
>However, I am dubious about the application to car power amps, especially 
>high-power ones.  

>The reason I'm dubious is that these power amps do not run directly from the
>12V (oh, all right, 13.6V) input.  Instead they have a DC-to-DC converter.

>So these add-on capacitors are not really filtering (or removing HF noise from)
>the voltage that's supplying the amp's output devices.  They're separated from

Usually not directly, true.
>the output devices by the amp's power supply.  Also, it is worth mentioning
>that the amp's power supply has its own filter caps.  These will tend to be

On the load side of the power supply, yes.  Since this is not an off-line
switcher, caps on the supply side are not really necessary.  Anything put
there is to keep noise from getting onto the power system.  Any noise that
goes back onto the system will affect eveything else put there.
The point in this case, is to isolate the amp from the rest of the power
system.
>smaller capacitances (hence lower inductance) than we'd typically see on the
>output of a linear power supply, since the amp's DC upverter typically runs at
>supersonic frequencies rather than 60 Hz. 

Oh yeah?  Do you think the power company picked 60 Hz by flipping a coin?
It is true that a switcher can run at supersonic frequencies, but you will find
that this is not usually desireable.  Still, your point about filter caps
is valid, since the frequencies used are generally higher than 60Hz.

>Has anyone actually measured the output of a high-power car audio amp, with and
>without various additional capacitors on the 12V supply, to see if they make a
>difference?  How about putting a scope on the amp's internal supply rails?  Or
>even the 12 V supply to the amp?  The "high frequency noise" that we're looking
>for is limited to audio bandwidths, so it shouldn't be THAT hard to see...

No. As you point out, the noise will be the switcher frequency modulated
by the audio power draw.  If it's a PWM stabilized switcher, it gets even
more messy.  On the other hand, it could be an open-loop (saturation
stabilized) switcher.  In which case, large current spikes will appear at
the switch points.
If you measure with a scope, you should see the voltage ripple.  It has
several sources. (contact impedance and reactance of power lines primarily.)
Assuming this ripple is slower than the RC constant of the capacitor,
the cap will help to reduce it.


In any case, you are quite correct in pointing out the ability of a well
designed switcher to improve immunity to power rail fluctuations.

However, the current has to come from somewhere, and it comes from
where everything else gets power.  If you play a loud transient (bass
drum, eg) the switcher will suck power out of the charging circuit
much faster than a linear regulator could hope to.  The load voltage will
drop, and while this may not affect the amplifier's power rails, since it is
supplied by the switcher, your head unit will feel it. And so will your
CD-changer or whatever.

If everything is supplied by a good quality switcher, you should be fine,
and using stiffening caps probably won't help much.

As I said, your mileage may vary.

Andrew
Newsgroups: rec.audio.car