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RE/P Files: Producing A Controllable Phasing Effect

An interesting look back at techniques for recording electronic instruments, circa 1970...

By Jerry Ferree February 17, 2015

Take the PSW Photo Gallery Tour of audio equipment ads appearing in RE/P magazine, circa 1970

An Easy Variable Speed Oscillator (VSO)

by Brian Ingoldsby

The VSO (variable speed oscillator), sometimes called VFO (variable frequency oscillator) has become a widely used studio tool. Phasing, vocal enhancing, and adjustable echo (tape delay) are but a few of the many effects available with this simple device.

Using equipment normally found in most recording studios, set up as is shown (below), a basic VSO can be constructed.

The output from a standard oscillator (at least +4 dBm output) is connected to the input of an audio amplifier (minimum 75 watts mono). The amplifier must have 70-volt output terminals (standard on most PA amps), which are connected directly to the capstan motor.

By bridging the amplifier output with a VOM, as shown in the accompanying diagram, voltage to the capstand motor can be maintained at line voltage (110—120 volts).

Now, varying the oscillator frequency in the neighborhood of 60 Hz will alter in capstan velocity, which is the desired effect.

VSO diagram.

The degree of flexibility required in the oscillator controls depends on the VSO application. Usable tape delay effects can be achieved with oscillators equipped with step-type frequency controls. Phasing and other more complicated effects require more fine tuning capabilities.

Editor’s Note: This is one in a series of articles we’re featuring from Recording Engineer/Producer (RE/P) magazine, which began publishing in 1970 under the direction of Publisher/Editor Martin Gallay. After a great run, RE/P ceased publishing in the early 1990s, yet its content is still much revered in the professional audio community. RE/P also published the first issues of Live Sound International magazine as a quarterly supplement, beginning in the late 1980s, and LSI has grown to a monthly publication that continues to thrive to this day.

Take the PSW Photo Gallery Tour of audio equipment ads appearing in RE/P magazine, circa 1970

Our sincere thanks to Mark Gander of JBL Professional for his considerable support on this archive project.

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