From: znpt01@trc.amoco.com (Norman P. Tracy)
Subject: Re: Capacitors, etc... CALMLY
Originator: znpt01@gpss43
Organization: Amoco Production Company, Tulsa Research
Date: Tue, 17 Nov 1992 16:14:41 GMT


In article <1992Nov15.223232.12901@eng.ufl.edu>, brian@crow.eel.ufl.edu (Brian Gentry) writes:
> I seem to have started this by asking why Andrew wanted to use a 10uF
> tantalum in parallel with the big electrolytics used for "stiffening caps".
> 
> This brings me back to the original point of using small caps with good 
> high frequency characteristics in parallel with large caps with poor high
> frequency performance.  If you put a 10uF tantalum capacitor in parallel 
> with a .5F electrolytic (or any size electrolytic) you will only add:
> E = .5 * C * V^2 = 0.5 * .00001 * 144 =  0.00072 Joules of energy storage
> capacity to the system.  In TTL circuits where the transistor's switching
> causes glitches in the power supply, this tiny increase in energy storage
> would be enough to control the high frequency noise in the power supply.
> However, we are talking about a high power car audio system that may demand
> hundreds of amps from the car's power system.
> 
> So, I agree that high frequency components will be "injected" into the power
> system by high energy transients, but I don't think that a small capacitor
> will have much of an effect on the problem because of it's VERY small 
> contribution to the overall energy storage capacity of the system.
> 
> Think of it this way:  A large amplitude bass drum note begins.  The current
> demanded by the amplifier increases rapidly.  The big electrolytic cap 
> can't change it's current that fast, so it adds almost nothing to the total
> current going to the amp.  The little cap can change that fast and does.
> The little cap gives up it's entire energy store.  This energy store is
> so small that it will make almost no difference in the total current demanded
> by the amplifier(s).  Remember, with a 10uF cap we're only talking about 
> 720 MicroJoules of energy.  However, this condition is only true when the
> base note first begins.  Once the bass note is acting as a sine wave (after
> the voltage changes direction for the first time) then the current demand
> is at a low frequency and the large cap can begin to deliver current to 
> the amplifier(s). 
>
> 
> 		   	  Brian L. Gentry
> 		     (brian@sioux.eel.ufl.edu)
> 
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.  The impedance of the 12v DC wiring then comes
into play.  When a musical transient causes the amps power supply to make a
heavy demand on the 12v line the impedance of the wiring prevents the battery
from instantaneously supplying the necessary current.  When the voltage sags
due to the wiring impedance the aux cap located next to the hungry amp steps
in and by discharging some stored charge fills in the gap until the voltage
potential at the batteries terminals overcomes the wiring impedance.  The
realm of theory is replaced with 'how' questions when you are contemplating
a real installation vs. sitting at our terminals (CRT not battery!) running
thought experments.  'How' far away it too far for the battery?  'How' big
a wire guage to use?  And if you decide an aux cap is called for 'how' close
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.  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.

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

Norman Tracy znpt01@trc.amoco.com
Newsgroups: rec.audio.car