From: brian@hopi.eel.ufl.edu (Brian Gentry)
Subject: Re: Capacitors, etc... CALMLY
Organization: UF EE Department (student)
Date: Fri, 20 Nov 92 21:46:16 GMT

In article <1992Nov17.101441@trc.amoco.com>, znpt01@trc.amoco.com (Norman P. Tracy) writes:

|> Brian,
|> It seems to me your analysis is leaving out one component of this hypothetical
|> mega power car audio system, the battery.  The only reason for placing these
|> auxilary caps in the power supply circuit is that the battery(s) have to be some
|> distance from the amp rack.  

I thought that the major reason for using these caps was because of the
poor transient capability of the car battery.  I know that a battery 
can put out a lot of current for a short time (i.e. starting the car) but
I thought it had bad performance for very fast transients.

|>The impedance of the 12v DC wiring then comes
|> into play.

Do you have any references for the inductance of wire by guage?  I thought
that it was on the order of microHenries for large guage wire, but I could
be mistaken.  In any case, the highest frequency we should be worried about
is 22.05kHz as this is the highest frequency reproducable by a CD player.

[...]
|> to theoretically perfect does it have to be?  Bypassing a huge electrolytic
|> (as favored by you car audio guys) with smaller, higher quality, caps is an
|> accepted method in High-end home audio, and other forms of electronics, of
|> trying to make the big mama electrolytic behave more like a perfect cap.  

My point about the small caps was that they couldn't provide much of a boost
at high frequencies anyway.  They can't provide much because they have so
little energy stored in them.  I have seen a comparison of capacitance vs.
frequency for a big electrolytic, before and after being bypassed with a
smaller poly cap (this cap was about 10% of the value of the larger one).
The original curve (unbypassed) was very nonlinear in the high frequency 
range.  The bypassed curve showed about 1 dB of improvment over the nonlinear
range.  In short, it helped, but not much.

I think that especially for this application (stiffening capacitors) the use
of small "bypass" caps is really overkill.  I think that you would not be
able to tell the difference audibly and would probably be hard pressed to
measure it.

|>This
|> is done because the big can electrolytics have significant ESR (equivalent
|> series resistance) and rather poor DF (diaelectric factor) both of which tend
|> to degrade its high frequency performance.  In home audio when we want to really
|> get tweekie a set of staggered values is used, for example 0.1, 1.0, 10.0, 100,
|> 1000 uF, etc. with the smaller caps being NPO cermanic or plastic types known
|> for excellent high frequency performance.  So how good is good enough?  Only
|> an oscilloscope or your ears can tell you for sure.

I can see the need for some high frequency bypass caps on the INTERNAL rails
of an amplifier or on the power rails of a device that uses IC circuits (because
of their high frequency switching).  However, we are talking about primary
power supply filtering before it gets to the amplifer's transformer.  What you
describe above seems like overkill to me.  I guess I'd have to measure the
performance difference myself.

|> Remember the old hot rod saying "Speed costs money, now how fast can you afford
|> to go?"

True, but stereos reach a point of diminishing returns much faster than a
land vehicle.  Acceleration increases linearly as the power to weight ratio
goes up--in a stereo system, the "performance" increase is more like logarithmic
with the amount of money and time you spend.
 
|> Norman Tracy znpt01@trc.amoco.com

			   Brian L. Gentry
		      (brian@sioux.eel.ufl.edu)