Xref: utzoo rec.audio:7522 sci.electronics:3470
Path: utzoo!utgpu!attcan!uunet!lll-winken!lll-tis!helios.ee.lbl.gov!pasteur!ucbvax!hplabs!amdcad!rpw3
From: rpw3@amdcad.AMD.COM (Rob Warnock)
Newsgroups: rec.audio,sci.electronics
Subject: Re: Homebrew audio equipment
Message-ID: <22476@amdcad.AMD.COM>
Date: 28 Jul 88 13:03:59 GMT
References: <1075@gethen.UUCP> <6315@bloom-beacon.MIT.EDU> <118@luna.UUCP> <5741@pogo.GPID.TEK.COM>
Reply-To: rpw3@amdcad.UUCP (Rob Warnock)
Organization: [Consultant] San Mateo, CA
Lines: 45

Previous poster observed that big power-supply caps mean less ripple, but
also mean higher peak currents needed through diodes (and transformer, don't
forget). There are two solutions to this, one old, one new:

1. Old -- use an inductor (a.k.a. "choke") between the diodes and the caps.
   This smooths out the current peaks, causing current to continue to flow
   even though the voltage across the inductor is sometimes negative. This
   technique is best when you have a large more-or-less-constant load, but
   can be useful even with light/variable loads, particularly when combined
   with "parallel" regulation (a transistor which turns on under very light
   load to keep the voltage from skyrocketing). Computing the right series
   inductor value can be tricky, though.

2. New -- use a much higher voltage rectifier system, with poor regulation
   (smaller caps, cheaper diodes), then run a switching voltage converter
   (which also uses a series inductor, see "Old" above) to step the voltage
   down to your target. Also known simply as a "switching power supply".
   Used to be thought bad for audio, but coming back since switching
   frequencies have climbed *way* above the audio range (would you believe
   up to megahertz???). (Most modern "switchers" actually rectify the 110v
   input line, and step the voltage down with the switching inductor, which
   becomes the isolation transformer.)

Hmmm... just thought of another, more suited to home construction:

3. "Regulate under ripple" -- Use a slightly higher transformer voltage,
   slightly smaller capacitors, get more ripple, less peak diode current.
   Then add series regulation (using a cheap regulator chip with a "booster"
   transistor) just to regulate out the ripple. The series pass transistor
   doesn't have to dissipate the whole power supply capacity, as it's
   only dropping the ripple (and anyway, the input voltage drops as the
   current rises).

Of the three, I think the best for home construction might be #1, the series
inductor, but you'd need to provide some kind of over-voltage limit, since
under very light load (quiet passages) the output voltage can climb to
near infinity (or high enough to kill your system).


Rob Warnock
Systems Architecture Consultant

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