The Build Your Own Clone 5-Knob Compressor is an expanded version of the classic Ross compressor that has been cloned by many boutique builders. It has some extra circuitry that serves to mix the direct signal with the compressed output. The mixer portion of the circuit is illustrated in the schematic above.
On one of the pedal forums, I noticed that some builders of the circuit were complaining that the signal was distorted so I took a quick look at the schematic and it was immediately apparent what was causing the problem. The added circuitry has two bipolar transistor amplifier circuits that are identical; one built around Q6 amplifies the direct signal and the other is the Q7 mixer amplifier which serves to restore the signal level after the passive mix control.
The transistor boosters are basically the same as the old LPB-1 circuit except the value of the emitter resistor has been increased to lower circuit gain. With the values shown in the schematic, the gain of each amplifying stage is slightly less than 10x.
The change in the emitter resistor has caused the transistor bias to shift. With the values shown in the compressor schematic, the transistor is biased nearly to cutoff, and signals of a modest size run out of swing against the voltage rail when amplified, and therefore are distorted.
I breadboarded one of the gain stages and measured the dc voltage at the collector of the bipolar transistor. Using the values from the above schematic, and with a new 9v alkaline battery powering the circuit, the collector was at 7.1v. This is way too high!
To fix the bias so that the collector voltage is near an optimum point, one resistor on each gain stage needs to be changed. The 47k resistor (R26) on the Q6 amplifier should be removed and replaced with a 62k resistor, and R33 on the Q7 amplifier should also be changed to 62k. This substitution will lower the collector bias to 4.95v which is an ideal value and will give more undistorted signal range. The distortion in the compressor will be significantly reduced.
Another mod that can be implemented is to connect the middle lug of the “Blend” control to ground instead of to a reference voltage. The mix control is dc isolated by C19, C20 and C22 and does not need to be biased to a reference level. It also eliminates the need for the VrefB network so C5, R6 and D2 are not needed either (at least I don’t see any other connection to them). This will slightly reduce the current draw of the pedal and allow longer battery life.
Try these mods if you want lower distortion levels in the 5-knob BYOC compressor!