C4 Opto + Tube compressor

A 4-channel optical compressor with high voltage vacuum tube make-up gain. The response of the opto circuit is switchable between three different speeds, and pairs of channels can be stereo linked. The C4 also makes a very good tube line amp if little or no compression is needed. The tube stage overdrives smoothly and subtly, and is very musically "friendly".


A unique feature of the C4 is that it comes standard with two different tubes installed, effectively doubling the sonic capabilities of the unit. Channels 1&2 use the short plate Electro-Harmonix 12AX7EH, and channels 3&4 use the long plate JJ ECC803S. Both are excellent sounding tubes that have slightly different tonal characteristics. The solid state circuitry in the C4 is exceptionally clean and clear, and is very revealing of differences in tube tonality.


An 8-segment LED gain reduction meter displays the amount of compression, from -1 to -15dB. Three colors of LED’s make it easy to see the amount of compression at a glance.

The comp knob sets the amount of compression. Turning the knob clockwise increases compression. The ratio is fixed at 4:1.

The gain knob controls the amount of make-up gain provided by the tube circuit. The "0" setting corresponds to unity gain through the unit with no compression.

The fast/med/slow switch selects the response speed of the opto compressor circuit. Attack times are 15, 40 and 70 mS, and release times are proportionally longer and largely program dependant. The three settings add great versatility to the unit, and are useful on a wide range of sources as well as subgroup or stereo bus compression.

The bypass switch activates a hardwired bypass of the unit. Switching is done through sealed, gold-plated contact relays.

The link switch allows pairs of channels, 1&2 and 3&4, to be linked for stereo operation. During link operation the sidechains are linked, but the unit still operates as dual mono, there is no master channel. Both channels should have their controls set to identical settings to ensure balanced stereo operation.


Inputs are actively balanced, and can accept either balanced or unbalanced line level signals. Outputs are impedance balanced, capable of driving either balanced or unbalanced lines. Connections are made via the rear panel ¼” TRS jacks.

All specifications subject to change without notice.
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