This one uses a diode bridge.
The circuit is quite simple: the input is fed into a 100:1 attenuator (47K and 470R) to the non-inverting op-amp input. The feedback is via another 47K with the diode bridge forming the gain defining resistor from inverting input to 0v. The op-amp is fed from a split supply (10 – 0 – 10v or thereabouts).
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The maximum range of the control is determined by the performance of the op-amp. If you are trying to get 60dB control range you need a good, low noise op-amp: the discrete one in the circuit is good enough to give 60dB range. 60dB range resulted by varying the diode bridge current over the range 1microamp to 5 milliamps. 40dB was given over a 35microamp to 5milliamp control range.
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