FC526

FET Compressor Module

The FC526 is a somewhat unique compressor/limiter module based on commonly used circuit blocks. The audio path is comprised of three 2520 style opamps and one 2623-1 output transformer. The first 2520 is used as a balanced transformerless receiver amp. This input stage can be hit with a +28dBu signal before clipping occurs. The 2nd opamp is known as the signal preamp. This is the stage where the audio signal is detected, rectified and then used to reduce the audio signal level, aka gain reduction. The final opamp is the output amplifier, which is followed by a 1:2 balancing transformer. This part of the circuit is very much the same as the 2nd stage of a VP28 or ML2. The output stage also clips just beyond +28dBu. The red front panel peak LED indicates clipping of the output stage.

The FC526 uses a common FET gain reduction method. This FET hangs just after the input amp and is employed as a voltage variable shunt resistor aka VVR, very much similar to the legendary 1176. The control voltage for this FET is a product of the FET's bias voltage and a rectified DC signal of the output of the signal preamp. By definition, this is a "feedback" style peak limiter. The sidechain detection circuit is a mash-up of an 1176 and an 1178 using two amps from a quad IC opamp to rectify the audio's AC to DC. This DCV is then mixed with the FET's bias voltage, varying the resistance of the shunt FET in a manor that is proportional to the incoming audio signal.

The gain reduction meter uses a FET with similar characteristics to the VVR FET so you get a visual representation of the gain reduction that is occurring. The LED meter string is very fast and accurate which is accomplished with LM339 quad voltage comparators.

The rotary ratio switch has the common 1176 ratios available as well as 2:1 and ABI, which stands for All Buttons In. ABI takes the VVR FET out of its conductive range so it is normal for the green gain reduction LED to turn off when switched to this ratio.

Via a front panel switch, the sidechain has an optional HP filter that rolls off at 6dB per octave starting 3dB down at approximately 180Hz.

The upper right switch is a hard relay bypass. This simply couples the module's input directly to the module's output, completely bypassing the FC526's circuitry.

The lower right switch turns the gain reduction on or off. This switch turns off the audio going to the sidechain detector amp allowing the input signal to still travel thru the all discrete transformer coupled path.

This kit features high quality components throughout like C&K mini toggle switches, a Grayhill rotary switch and Bourns potentiometers/trimmers.

Attack Time: ~20 microseconds adjustable to 800 microseconds
Release Time: 50 milliseconds adjustable to 1.1 seconds
Current Draw: Less than 118mA per bipolar 16V rail

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