April 1958 Radio-Electronics
Wax nostalgic about and learn from the history of early electronics. See articles
published 1929 - 1948. All copyrights hereby acknowledged.
Hmmmm.... at the time I marked this stereo preamp project for posting
I must have had a really good reason for it, but now I can't recall
what that really good reason was. Maybe it was simply to mark the
point in time when stereophonic electronic equipment was just beginning
to be mainstream. Oh well, somebody somewhere will do a Google search
on the topic someday and will be elated to find this. Thanks for
Playback Preamp for Stereo Tapes
By A. C. Moller. Jr.
Inexpensive unit lowers cost of stereo tape playback
Careful construction makes for professional appearance.
Plenty of room inside case. Twisted wires are heater
Now that stereophonic tapes and equipment have become plentiful
and so many fans are purchasing tape decks, it seems apparent that
a good two-channel tape preamp is the answer to inexpensive stereophonic
sound. This two-channel preamp is relatively easy to build, even
for the amateur audiophile. Its total cost is less than $30.
In complicated hi-fi systems the two-channel preamp eliminates
the constant changing of input and output leads. I leave the unit
connected permanently to the stacked stereophonic heads on my tape
unit. All recording is done with a separate head, thus enabling
me to monitor freshly recorded sound with one channel of the preamp
or use the two-channel preamp for all playback of tapes.
The unit pictured has been designed to the proper response curve
for NARTB recorded tapes. This is accomplished by the feedback loop
consisting of a 680-ohm resistor (R12) and the 51,000-ohm resistor
(R9) feeding through the .001-μf capacitor (C4). The resulting
curve provides approximately 15-db boost at 20 cycles and 15-db
cut at 15,000 to 20,000 cycles, which will equalize to a substantially
flat response output when using NARTB recorded tapes.
The unit has been tested with a scope and provided an almost
perfect waveform from 20 cycles to beyond 30,000. There is a slight
amount of deformity at approximately 500 cycles, but not enough
to distort the signal excessively.
The preamp is built into an aluminum chassis box 5 x 7 x 3 inches
which encloses all components, thus minimizing hum as well as providing
a neat appearance. Two 12AX7's are used, one for each channel, working
from a single power supply. It uses a full-wave transformer with
a 6.3-volt heater supply. Two selenium rectifiers are employed.
Three 40-μf capacitors do an excellent job of filtering the B-plus.
It is important to keep the power supply as far from the input
jacks as possible and take other precautions to keep the hum level
at a minimum. Placement and orientation of the power transformer
are equally important.
Heater leads are tightly twisted and dressed as close to the
chassis as possible. A 100-ohm hum balance potentiometer reduces
any remaining hum to a minimum.
A 1-megohm potentiometer in the output of each channel controls
volume and balance. The controls also provide some tone-control
compensation by reducing bass boost as the control is retarded.
If the unit is used with leads in excess of 6 or 8 feet, a 6C4 cathode-follower
output stage can be added if desired. It is not necessary except
in extreme cases. All ground connections are made to a No. 14 copper
bus and grounded at the input jacks.
All parts for the preamp are easily obtained at most radio parts
supply houses. Construction time, including drilling the chassis,
is approximately 6 hours.
If hum is present, check all connections and parts placement.
Also check each channel separately as a bad 12AX7 is sometimes encountered.
Circuit Schematic of the 2-Channel
R1, 2 - 100 ohms, 1 watt
R3 - pot, 100 ohms
R4, 5 - 15,000 ohms, 1 watt
*R6 - pot, 1 megohm, audio taper
*R7 - 220,000 ohms
*R8 - 3,300 ohms
*R9 - 51,000 ohms, 5%
*R10 - 1 megohm
*R11 - 470,000 ohms
*R12 - 680 ohms
*RI3 - 1 megohm, 1 watt
All resistors 1/2 - watt 10% unless noted
C1 - 40-40 μf, 150 volts, electrolytic, can
*C2 - 470 μμf, tubular ceramic
*C3 - .06 μf
*C4 - .001 μf
*C5 - .01 μf
C6 - 40 μf, 150 volts, tubular electrolytic
All capacitors 600 volts unless noted
*J1, 2 - phono jacks
RECT 1, 2 - selenium rectifiers, 130 volts,
T - power transformer: primary, 117 volts; secondary,
250 volts ct, 25 ma; 6.3 volts, 1 amp (Stancor PS-8416 or equivalent)
*V - 12AX7
Case, (chassis) 5 x 7 x 3 inches
Socket, s-pin miniature, with shield
*For two-channel unit 2 of each are needed
Posted October 3, 2014