home *** CD-ROM | disk | FTP | other *** search
- Newsgroups: rec.audio.car
- Path: sparky!uunet!usc!zaphod.mps.ohio-state.edu!saimiri.primate.wisc.edu!eng.ufl.edu!hopi.eel.ufl.edu!brian
- From: brian@hopi.eel.ufl.edu (Brian Gentry)
- Subject: Re: Capacitors, etc... CALMLY
- Message-ID: <1992Nov20.214616.333@eng.ufl.edu>
- Sender: news@eng.ufl.edu (Usenet Diskhog System)
- Organization: UF EE Department (student)
- References: <gilchr.721386805@ee.ualberta.ca> <1992Nov10.193217.852@cmkrnl.com> <gilchr.721639884@ee.ualberta.ca> <1992Nov15.223232.12901@eng.ufl.edu> <1992Nov17.101441@trc.amoco.com>
- Date: Fri, 20 Nov 92 21:46:16 GMT
- Lines: 69
-
- 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)
-