Triple Spring Reverb

Posted on Jan 18, 2013

This spring reverberation unit comprises three identical main sections each of which contains an Accutronics type 9 spring reverb tank with short, medium and long decay times. Controls are provided to adjust the amount of signal arriving at the output from the input directly and from the spring tank. Conventionally these signals are called `dry` and `wet.` A feedback control allows the tank output to be returned to the tank input to produce oscillation effects. The amplified output from the spring tank is available on a socket to send to other effects units and an insert input socket is provided for the return from that effect. The insert socket overrides the signal from the tank output. This return is prior to the feedback control so that the effects unit is within the tank feedback loop. In this way a frequency shifter could be used in the send-return loop to provide rising or falling reverb effects. The Accutronics type 9 is the best unit made by that manufacturer and is considered to be a high quality spring reverb used in many respected products.

Triple Spring Reverb
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This sheet shows the three tank drive circuits. These are simple class B drivers driven from the TL074 outputs. The ground side of the tank input transducers returns to ground via an 8.2Ohm resistor. This provides the first stage of frequency response shaping by making the mainly inductive tank input transducer look more resistive. The tank has an impedance of approximately 10Ohms at 1KHz which is mostly inductive. With the 8.2Ohm resistors the current in the tank driver will be about -3dB at 1KHz and reducing at approximately 6dB per octave above that frequency.Taking the feedback for the op-amp/driver combination from the top of this resistor makes the driver a constant current circuit, so the output swing will increase as the impedance of the tank transducer rises at higher frequencies. This continues up to a certain frequency defined by the capacitor in the feedback circuit. The current drive is rolled off above about 7kHz as the tank mechanical response rolls off rapidly much above 6kHz. Extending the drive bandwidth higher would be useless and result in lots of unnecessary current circulating around the circuit at 15 or more kHz. Stability of the circuit is very good despite the phase shift created by the inductive load above 1kHz. This circuit requires the use of the tank version with isolated input transducers. Using this circuit with the sense resistor grounded also allows for easy single-ended measurement of the...

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