Audio research st 70 owners manual

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Modification Manual
Audio Research Corporation

ST-70-C3 Super Modification Kit

for the

Dynaco Stereo 70
Power Amplifier

PRICE $400

Page 2

W.

.57.

,l/.

IO? «1on

NOISE

LEFT CHANNEL

IOmV P N) P
320 Hz

RIGHT CHANNEL
/'\ /'\ /'\ 5mV onF
|20 Hz
Noise patterns in the two

channels. Input sensitivity is :
830ml] for 22V RMS/lél?

-.~-

IoKHz

Total Harmonic distortion in a modified ST-70-C3, both channels driven.

POWER BANDWIDTH

H

0

25V RMS/lsfl 1 39W

Ion mom

FREQ. RESPONSE AT IWATT

IOKHZ

IOHz loon

TABLE IV
Performance Specifications

Power: 16:2: 30W per channel
(Both channels operating)
THD < .12 @ lkHz.
< 5% @ 30Hz
< 1.0% @ lSkHZ

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IOKHz

VOLTAGE CHART

Tube Pin Any 6L6GT Vl Either Either V6/V7
V2/V3 Vii/V5

1 +l75VDC +9ovoc +hlovnc +270voc _
2 i=6. 3VAC +90VDC +100voc
3 +420VDC +105v0c +110qu
1: +420VDC lovnc 6:6 . 3VAC *6. 3vAC *6. 3VAc
5 -hzvoc IOVDC
6 l75VDC 90voc +260voc +MOVDC
7 *6 . 3VAC Hoovuc +9ovoc
8 +IlOVDC +l05VDC
9

These are approximate voltages taken at ll8VAC line level and may
vary 2202; from unit [0 unit.

7'=Appro>

Printed m the
U,S,A.13Copyrighl x978 by
Edward T Dell, Jr, All rigMS
are reserved

Audio Research Remakes
Dynacos Stereo 70

by Edward T. Dell, Jr.

HE IMPULSE to make the good

better, runs deep in the crafts
man. The simplicity and elegance
of the Dyna circuitry in the Mark
III and the more modest Stereo 70
exerts a strong pull on the mind of
anyone who regards an amplifier
circuit with the same sort of
pleasure rising in him that accom-
panies the first taste of a good
wine.

That impulse lay behind the edi-
tor's project of a craftsmans ver-
sion of the Stereo 70 back in 1966.
(Stereophile, No. 4,1976, reprint
available for $1.50 from TAA). The
Dyna amplifier circuit was left
virtual 1y unchanged with only an AC
balance and a metering circuit
added, but military grade compon-
ents were used throughout. The
power supply was doubled, and modi-
fied to provide more filtering and
much more capacitance.

What moved William Z. Johnson of
Audio Research Corporation to un-
dertake modification of Dyna's
Stereo 70 only he knows. I would
guess it grew out of Johnson's
habit of making the good product
even better. The result is a lower
powered cousin of The Audio Re-
search D-76-A whose amplifier con-
figuration is shifted from the
spare economic simplicity of Dyna's
three tube circuit (Fig.1) to the
balanced completely push-pull ele-
gance of the D76A (Fig.2). The
result of the changes are evident
in Fig.3. The modified ST70 is a
lower powered unit, of course: 30
Watts versus 75 Watts per channel
in the D76A.

In amplifier configuration, the
ST-70-CS lacks only the constant
current source stage of the D76A.
The other differences are size of
output, tubes and transformer, and
a much larger, partially regulated
power supply. The modified unit
has a zener regulated supply for
its input tube and a D.C. filament
supply for it as well. The power
supply uses the Dyna transformer,
fuse, line cord,and power switch,
but adds silicon diode rectifiers
and 33011]: total filtering of the
high voltages. The experimental
prototype used a thermistor in the
ac line but proved to have little
usefulness.

But, strictly speaking, the S'l«

Photo A. The completed Audio Research ST-70-C3 stereo amplifier.

70-C3 is not a modification at all.
It's a transformation. The result
isn't a modified Dynaco product
its an Audio Research power amp-
lifier using Dyna transformers and
chassis.

Is the transformation worth the
effort and the cost?

Dynaco Stereo 70s are bringing
about $50 to $70 second hand these
days and the modification kit
price raises that to well above
$200. For this quality of ampli-

fier, however, the price tag is
not out of line. The parts in the
kit are first quality and were
chosen to give long service.

Those who want to build the de-
sign from scratch can probably do
so for slightly less money although
output transformers of the quality
of the Dyna product are relatively
rare. The Dynaco line began around
the outstanding features of their
output transformers. Dyna does not
any longer sell output transformers

FIG.I EL34
6
G)
OUTPUT
TIBS '
J PI (9 6 - ems
INPUT PHASE
@ AMP INVERTER ° '
9
OUTPUT
@ l
FEEDBACK I

Fig. 1. Block diagram of the Donaco Stereo 70, one channel. The + and -
signs on this and Figs. 2 and 3 denote instantaneous signal polarity with
respect to input JP-I. Power supply is unregulated, sopF total capacitance.

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