Playback Preamp for Stereo Tapes
April 1958 Radio-Electronics Article

April 1958 Radio-Electronics [Table of Contents] Wax nostalgic about and learn from the history of early electronics. See articles from Radio-Electronics, published 1930-1988. 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 your indulgence.

Playback Preamp for Stereo Tapes

By A. C. Moller. Jr.

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 Preamp

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 type

*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, 20 ma

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

Knobs

Miscellaneous hardware

*For two-channel unit 2 of each are needed

Posted  October 3, 2014