The ST-150-BJ-1
A Boak-Jung Modification
of the Dynaco Stereo 150 Amplifier
I
first discovered the pleasures and
satisfactions of “hands-on” audio in
1976, when I purchased and assembled
from kits the Dynaco Stereo 150 power
amplifier, as well as a PAT-5 and an
FM-5. The Stereo 150 power amplifier,
introduced by Dynaco in 1975, is a
neutral-sounding, good, 75 watts-per-
channel amplifier. It was overshadowed
however, by its big brother, the Dyna
400, and has generally been little
regarded, both in the audiophile press
and among audio amateurs and
modifiers.
My first “modification” audio pro
ject was the ST-70-C3, the Audio
Research Corporation modification of
the Dynaco Stereo 70 {TAA , 4/77). The
ST-70-C3, a clean and warm-sounding
tube amplifier, had a cleaner, truer and
more natural sound than the Stereo 150.
It became my amplifier of choice, and
the Stereo 150 went on the shelf.
In 1978 I began, and have since con
tinued, a correspondence with Walt
Jung, arising out of his modifications of
the Dynaco PAT-5. In May of 1980, my
ST-70-C3 was down for repairs. I wrote
to Walt mentioning that the Stereo 150
was back in use and really sounded in
ferior. He wrote back with some sugges
tions for improving its sound.
Through Walt, I met Jim Boak, a
TAA Contributing Editor, whose most
recent published article is “A Family of
Regulated Power Amplifier Power Sup
plies,” ( TAA 1/80), and who has moved
recently into the Cleveland area. Jim
listened to and looked at my Stereo 150,
and also had a number of suggestions.
With guidance from Walt and Jim, I
have completed the series of Stereo 150
modifications outlined in this article.
The Stereo 150 sound is considerably
cleaned up, its transient response
tightened, and its power increased. It is
now definitely my amplifier of choice.
The ideas and electronic details of this
modification have come from Walt Jung
Reported by Patrick j. amer
or Jim Boak, both of whom have review
ed and corrected this article. The
“hands-on” assembling and installation
work, and thus the construction sugges
tions, are mine, and I volunteered to act
as reporter for the project.
The modification as I executed it fell
into four stages, each of which in
dependently results in a clearly audible
sonic improvement. Stages One, Two
and/or Four can be implemented
without doing Stage Three. A major at
tractive feature of the modification is
that Stages One and Four are cheap and
Stage Two almost free. The changes
were implemented in the 1975 version of
the Stereo 150, but may be adapted for
implementation in the later post-1976
versions. I assume that the modifier will
have at hand not only a Stereo 150 but
Dynaco’s Instructions for Assembly and
Operation manual as well.
STAGE ONE: Capacitor Changes
In their landmark article, “Picking
Capacitors,” in the February and
March, 1980 Audio, Walt Jung and Dick
Marsh demonstrated that the material
or composition of a capacitor in an
audio circuit has a significant effect on
distortion in the audio circuit. The rele
vant factors were further illustrated in
Dick Marsh’s “Dielectric Absorption in
Capacitors,” TAA 4/80. Applying these
principles, Walt Jung identified four key
locations in the Stereo 150 most apt for
capacitor changes, and these changes
constitute Stage One of this modifica
tion.
1. Cioi, the input coupling capacitor: if
your preamplifier has (or all your pre
amplifiers have) an output coupling
capacitor, the input coupling capacitor
on the Stereo 150 is redundant, and you
can jumper Cioi. If you wish to retain
the DC blocking function of the input
coupling capacitor, remove Cioi (a 33/iF
25V tantalum) and substitute a 5/xF
100V (or more) polypropylene capacitor
and a 0.47/tF 250V polypropylene
capacitor in parallel. The TRW types
are recommended throughout. Use
hookup wire and a terminal strip to out
board these capacitor pairs.
2. Cio6» the capacitor on the feedback
circuit: Remove Cioe (a 33/tF 25V tan
talum) and substitute the following ca
pacitors in parallel:
A. a 220fiF 50V aluminum elec
trolytic (exact value and voltage not
critical; use a high quality unit, as
recommended in Jung/Hollander, “St.
Pooge and the Driaagon,” TAA 1/81);
B. a 5/tF 100 + V polypropylene; and
C. a 0.47/xF 250V polypropylene.
I suggest you mount these caps on a
four-lug terminal strip for each channel
and mount the terminal strips on the
PC-36 mounting brackets (see Photo 2).
This keeps these capacitors up and out
of the way, and leaves chassis room for
the regulators described in Stage Three.
If you have doubts about the practical
effect of a capacitor change on the sound
from your audio system, listen carefully
before and after making this single
change. I think you will be astonished at
the increase in clarity and detail.
3. Ciis, the output RC phase compensa
tion capacitor: Remove Cns, a 0.1/iF
100V disc, and substitute a 0.1/xF
200 +V polypropylene.
4. C301 and C302, the power supply elec
trolytic can capacitors: Install in parallel
with each 10,000/xF can a 5fiF 100 + V
polypropylene and a 0.47j*F 250V poly
propylene. This step will be reversed
and these capacitors used elsewhere if
you go on to Stage Three. It will facili
tate installation if you install a terminal
lug under the mounting screw on the
positive terminal of each can cap, and
use the existing lugs on the negative ter
minals.
You should hear a significant
improvement in clarity and spatial
placement of sound as .a result of these
capacitor changes; I did.
32 THE AUDIO AMATEUR 2/1981
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