[GregS]

In article FXBih.5658$od6.1148@trnddc04, wrote:
In a recent thread here and many times in the past, I've seen discussion
about adding a BIG cap to the power supply of a car stereo.

Can someone explain to me how that works?


There are many ways to analyze this. I chose a time-domain approximation
because it makes the "back of the envelope" math easier.
Assumptions:

I used very round numbers to make the math easier. It's linear math, so
it should scale.

1500W = 100A at 15V (peak power)

What happens if we put a 100A transient load on our battery?
My battery is 8 years old, so measuring it won't help.
But for every 10 milliohms of resistance, internal resistance of the
battery plus all the wire and connection resistances,
we get a volt of transient drop at 100A.

I've also read that there's an electrochemical lag inside the battery,
but never seen any actual measured numbers.

So, the voltage at the amp is bouncing around with the load. What are some
typical numbers for this?

when we add the cap, we add additional series resistance in the form
of more wire and connections to the cap. That makes things worse,
even if we do know what we're doing and put the cap in the optimum wiring
configuration.

You don't have to add more series resistance. If the cap is in parallel with
the supply it simply provides a kind of node at that point. When the cap draws
current, it does cut down on the available current out of the alternator,
but if the alt, starts to crap out, then the current comes back out of the
alternator. You could add 4 caps, all with their own wiring. In star
configuration, all the wires from each cap come to the same summing point.
The cap should be right at the power amp. Sometimes you see the bussing
of multiple caps. This is not the best method, but with heavy straps, the
resistance is minimal.


But the cap and the connections have internal resistance too. Every 10
mOhm gives us a drop
of 1V at 100A, then there's the decay of 1V every 10mS for a 1F cap.

Let's not forget that there's also a hundred amps (average) of AC coming
out of
the alternator into all these resistances.

Only if the RPM's are up. At 600 RPM the alt can only furnish a few
amps.

The battery voltage is bouncing all around with engine speed and load.
The amplifier has a "common mode rejection ratio" that describes its
immunity to these variations. That same immunity mitigates the effect
of the cap.
One possible way of describing it is, "How much more undistorted power
output
can I get with the cap installed?"

it really about peak power. The cap will discharge and the average power
is not going to be any higher than with just the alternator.

Given a well-designed amp with good CMRR, the output should be
independent of
power supply voltage until the amp saturates at the lower power supply
voltage.

What's a typical speaker impedance? If it's one ohm, and you have a bridged
amp off of 12V, you can't get much more than 10A into a 1Ohm load.
That's 100W peak power. To get more than that, you have to up-convert
the 12V
to something higher. That up-conversion should make the system pretty
much immune
to what's going on on the 12V side???

To a certain extent.

Then, layered on top of all this is the liklihood that my stereo gets
installed
by a high-school dropout who's never heard of a volt or an amp. And the
installation
is more dependent on dealer profit, aesthetics and fixed postioning of
the automobile components
than it is on performance.

My questions are these:

1) In a well-installed car stereo, what are the typical resistances in the
wiring, connections, batteries etc.?

2) What is a typical speaker impedance for a high power system?

3) If I add a big cap, What improvement would I HEAR under what conditions?

4) If I hooked up an oscilloscope to the power supply at the amp and to
it's audio output,
what improvement would I SEE under what conditions if I added a cap?

5) Why would lights dim? IF the lights connect to the battery.
And the stereo connects to the battery.
And the sense point for the alternator/regulator connects to the battery.
And the alternator is big enough.
??

6) What percentage of people actually care about the technical merits of
their
car stero...as long as their ride is "fly" and gets 'em laid? ;-)

The caps provides better peak power, and also helps control lamp flicker.
That about it. How about the FAQ. There should be a good
explanation in there.

greg