Binson Echorec Schematic

by Phil Taylor

The Echorec is a superb piece of engineering! In their quest for perfection Binson got many important aspects of the design right, maintaining a balance between simplicity and adding genuinely useful features to the device. The engineers developed their own unique magnetic drum storage medium that offered superior reliability and stability over 1/4″ tape used in all other tape echo units. No expense was spared in the design of the mixing and amplifier circuitry – there are seven miniature B9A tubes inside the unit. All the tubes are still readily available 12AX7 and 12AU7 triodes. The level indicator is an EM81 tube which are also now being manufactured.

This webpage contains a description of each functional part of the circuit, a flowchart clearly showing the signal flow, schematics for the Echorec IA and II.

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Input Buffer

The input buffer is an inverting gain stage based on section 4A of a 12AX7 tube. To optimize this to work with electric guitar pickups the the 47K resistor before the input coupling capacitor can be removed. Virtually all guitar amp input stages are direct coupled i.e. there is no capacitor. This is okay as a guitar pickup only generates an AC component with no DC offsets. Note: there is no cathode bias resistor. Typically triode amplifier configurations have a 1K8 resistor to maintain the grid voltage below the cathode.

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Input Mixer

Mixer stage with common anode resistor. This is where the ‘dry’ signal and delayed signal are mixed to create multiple repeats. Trimmer, T2 is used to set the range of the ‘LENGTH OF SWELL’ knob.

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Record Amplifier and Bias Oscillator

The record amplifier is based on tube section 6A which provides some amplification. Additionally passive 1st order filtering before the grid provide -6dB/octave roll-off of low and high frequencies.

The bias oscillator circuit is based around tube section 6B and generates a 50KHz alternating current waveform. This is mixed with the instrument signal and applied to the record head.

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Playback Amplifier

The playback amplifiers are based on tube sections 1A, 1B, 2A and 2B, boosting the small signal from the playback heads. My interpretation of this circuit is that the 220pF would lower the resonant peak of the playback head, perhaps to improve the subjuective sound quality. Trimmer, T5 is a tone control and T4 output control for each of the four playback amplifiers.

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Switching Circuitry

The ‘SELECTOR’ and ‘SWITCH’ knobs form the switching circuitry. The SELECTOR knob allows selection between ECHO, REPEAT and SWELL settings. ECHO is just one distinct repeat; REPEAT is multiple repetitions, depending on the ‘LENGTH OF SWELL’; SWELL introduces a little signal from all four playback heads into the final mix, to produce reverb type effects. The REPEAT setting is the ‘one’ to use to create those ‘Pink Floyd Live at Pompeii’ delay effects.

The ‘SWITCH’ knob enables selection of different heads and combinations of heads for single and multi-tap delay. All the schematics I have seen so far show a simplified interpretation of the action of this switch. A full schematic for this is on my ‘todo’ list.

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Tone and Volume Controls

Basic passive tone and volume controls operating on the delayed signal.

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Output Mixer and Buffer

The output mixer buffer is a cathode follower configuration. Suggested modifications to extend the H.F. response a little are to increase the 47K resistor on the output to 100K and increase the 270nF output coupling capacitor to a polyester 680nF

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Level Indicator

The level indicator is based on an EM81 ‘magic-eye’ tube which displays the level of signal repeats. The ‘fan’ can be adjusted using trimmer, T3.

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Power Supply

Low tension 6.3V supplies the tube heaters. High tenision power supply with various levels of smoothing for different parts of the audio circuitry.