The CMP-5 compressor
Here are some notes about this 80s compressor pedal, and also some tweaks I did to mine to solve some issues.
The CMP-5 compressor was sold under many different brands: Aria Pro II, Axtron, Loco Box and others. I have one that is branded “Vision” (http://www.effectsdatabase.com/model/vision/5/cmp5).
It’s a Ross/MXR Dynacomp style OTA compressor using the CA3080 chip, built into a cheap but quite durable plastic enclosure. I have rehoused mine in an aluminium enclosure.
It has the standard controls for attack (slow attack is CCW, fast attack CW, so at “0” you get the “default” Ross/Dynacomp setting), sustain and level. When both the sustain and attack are at the higher settings (CW) you can get a crackling noise especially when a note is dying out. See below how I tried to fix that in my pedal.
It has a Boss style soft bypass switching, which means it has a buffered bypass. I did some tweaks to that too, see below.
The transistors used in this pedal are 2SC1815 and 2SK30A (the two JFETs in the switching circuit)
Here’s the pcb with some of the components that I mention in the text marked:
Here are a few observations when I compared the circuit with the original Ross/DynaComp circuit.
Power regulation and filtering
The power is regulated with an emitter follower voltage regulator (Q1 in the picture). The regulated voltage is around 8V and the bias voltage is around 4.3V.
There is an emitter follwer buffer after the CA3080 (Q3 in the picture). That means that the transistor after the OTA doesn’t have to serve as phase inverter, diode driver, and signal output buffer (as in Ross/DynaComp). The buffered bypass switching adds an extra output buffer (Q10) after the level control.
Extra rectifier filter transistor pair
Unlike most other Dynacomp/Ross derivates, it has an extra transistor pair (Q5 and Q6) in the stage that provides the current feedback to the OTA amplifier (Q7 and Q8 are the other transistors in the output stage). The signal is split after the phase inverter transistor. I’m not sure about the advantages of an extra pair. The extra pair has a high pass filtering with a cut-off at 160 Hz.
I increased the bias resistors (R1 and R2 in the picture) on the input and output buffer stages to 1M (from 470k). This, together with increasing the input cap to 47n (C1) and the cap in the bypass circuit (C2) to 470n (from 47n), the frequency response was changed to allow more bass through (both in active and bypass). The original pedal cut bass from 150 Hz even in bypass. I haven’t really heard any difference, only verified it with frequency analysis, but it is more similar to the Boss compressors of the time.
Sustain control mod
I got a crackling noise especially when a note was is dying out when the sustain and attack was in the highest settings (attack in the “fastest” setting). The problem wasn’t reproducible with attack in “slower” settings. My fix was to increase the 15k resistor to reduce the max current that can be fed back to the OTA (I added a 56k in the feedback path). It’s still plenty of sustain.