Flanger and Echo circuit

Posted on Oct 9, 2012

This is a 3U rack unit with standard mains power supply. It provides audio routing and speed control for two domestic three-head cassette tape machines such that they can be used to create authentic 1960s style tape flanging effects. The tape machines link to the rear of the unit with a total of four dual phono leads. The speed control of one machine is achieved by voltage control of the capstan motor connected by a three wire connection. The flange effect can be adjusted manually on a front panel control or externally by voltage control. There are sufficient jack patch sockets and input-output flexibility such that the tape machines can also be used for tape echo or tape feedback effects. Inverting gain controls enable different tape decks to be used which may have an overall inverting signal path between the record inputs and the downstream tape monitor outputs.

Flanger and Echo circuit
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This sheet has standard regulators to provide regulated +/-5V reference rails and the motor speed control circuitry. Tape machine A in this prototype setup has a 12Vd.c. motor which has an internal electronic speed control, adjustable on a screwdriver control through a small hole in the rear. The motor has been set to its maximum speed on this control. The circuit here is provided with the 12V supply which would normally drive the motor with its internal regulation. This is reduced to a nominal *V which gives approximately the correct normal running speed. The speed regulator internal to the motor is thus running in drop-out, and speed regulation will not be particularly good. However this is not a major problem for a flanging effect where the speed is usually changing constantly. It should be considerably more controllable than the historic method of applying finger pressure to rotating tape spools. The external regulator is a traditional LM317, set to provide *V if the lower 1K resistor were to be grounded. The op-amp arrangement lifts this node to a higher voltage which is seen directly impressed on the voltage output. The op-amp input side sums with appropriate scaling the manual control voltage from the front panel pot with the external control voltage. An adjustment preset voltage is also summed in and sets the normal mid-point running speed. This sheet shows the main audio sections. The first four op-amps sum each...

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