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From: pmr@drutx.ATT.COM (RastocnyPM)
Newsgroups: rec.audio.high-end
Subject: Re: Mods for Hafler 100 Power Amp
Message-ID: <8227@uwm.edu>
Date: 10 Dec 90 14:13:35 GMT
Sender: news@uwm.edu
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Approved: tjk@csd4.csd.uwm.edu
Originator: tjk@csd4.csd.uwm.edu


In article <8094@uwm.edu>, jdhill@BBN.COM (Jack D. Hill) writes:
> 
> In article <7696@uwm.edu> you write:
> >
> >About 8 years ago I built a Hafler DH100 power amp from the kit.
> > ... I am getting
> >increasing hum and hiss in both channels, and I assume it's due to aging
> >of important components and/or solder joints.
> ...
> A year ago I purchased an upgrade kit for the DH100 direct from Hafler. It
> costs under $20 and includes better op-amps and a few resistors and capacitors.
> The upgrade involves replacing the socketed op-amps and soldering in the rest.
> It turned out to be a nice upgrade. The biggest difference I heard was that the
> high end wasn't nearly as grainy and harsh like it used to be. It sounds like
> you might have problems in your power supply. Try repacing the diodes and
> electrolytics.

The latter part of this last sentence is probably the culprit.  Electrolytic
caps age and have a life (brand dependent) of from 5 to 10 years.  Audiblility
of this aging process usually comes in the form of hummmmmmm, graininess,
edginess, no bass punch, loss of inner detailing, poor imaging, and loss of
transient response.  There are two uses for electrolytics:  power supply
filtering and DC-blocking.  Failures in the latter cause severe signal
degredation and failure in the former causes hum/noise and transient-related
signal degredation.

If you can find replacement values and matching or greater voltage ratings of
non-electrolytic type capacitors (such one of the mylar varieties), a simple
one-for-one replacement will greatly improve the sound of the unit.  When
replacements are not possible because of large values, better quality
(high frequency) electrolytics should be used.

Another thing to check are the screw connections on the power supply
capacitors themselves.  On my amp, I have to go in about twice a year and
crank down on the screws (they back out from normal thermal changes).

		Yours for higher fidelity,
		Phil Rastocny