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From: rutgers!sales.stern.nyu.edu!sbhattac@cmcl2.NYU.EDU (Shankar Bhattacharyya)
Newsgroups: rec.audio.high-end
Subject: Re: Modications to Hafler preamp DH-101 Wanted.
Message-ID: <12104@uwm.edu>
Date: 13 May 91 12:59:07 GMT
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In article <11819@uwm.edu> johnt@sdd.hp.com (John Thomas) writes:
>Does anyone know any modications for a Halfer DH-101 preamp.  Please post
>responses.  Thank you. 

In addition to the modifications provided by commercial establishments,
there was a pair of articles in 1980 in Audio Amateur. One dealt with
rewiring parts of the DH-101. That was mostly about eliminating a ground
loop, and shielding the phono leads, reducing the length of wiring involved
in the two-phono-source setup (which sacrifices one phono input), and
other such things. These reduced hum rather substantially, and essentially
eliminated cross-talk between high-level sources and the phono signal,
which is pretty bad in a stock DH-101.

The other article was about the guts of the preamp, and covered alteration
of the RIAA equalization, bypassing of coupling capacitors at various points,
and increasing the bias current in the phono section. This increased bias
was standard in the DH-101 from about 1980 on. The article also recommended
an improved power supply, and suggested some options.

I have done this to a couple of DH-101's, and the owners were rather
pleased with the results. My own preamp is a scratch-built implementation,
with on-board regulators, good capacitors, and most of the other obsessive
doo-dads. It is not the state of the art, but I'm pretty happy with it.

I'm not inclined to worry too much about the equalization. It makes a
difference, but I think that is generally pretty trivial compared to what a
pair of loudspeakers or a room can do.

If you are comfortable with a soldering iron, you can do all the
modifications in about one long afternoon. Collecting supplies is not
included.

Incidentally, if you do reach into the DH-101, you will find that some of
the transistors run distinctly hot. I suggest making yourself familiar
with that before you start messing around. They run hot enough that if you
don't know that it is normal, it can perturb you . It perturbed me, anyway.

- Shankar