The Univox Uni-Comp is a compressor pedal made by Univox in the 1970s as part of the company's "foot" series1. It is an optical compressor: where many later designs use a separate LED and light-dependent resistor, the original used a Vactrol, an integrated light-and-resistor element labeled MCD5271. The pedal has been discontinued for decades, but it is remembered well, described by the technical site Tagboardeffects as seeming "to be loved by many and was recently requested"1, and it remains a popular subject for DIY builders.
What it is and where it came from
Univox built the Uni-Comp as part of its "foot" series of pedals in the 1970s1. The unit is identified as the Uni-Comp Model CO-1051. Its gain-reduction element was a Vactrol labeled MCD527, not the LED and LDR pairing common in later optical compressor designs1. By around 1979 the pedal had been discontinued; one owner of a gray example reported loaning it out around that time and never getting it back, only to discover it was no longer made1.
For a player today, that means original units are secondhand items only. The circuit itself is documented, which is where the DIY interest comes in1.
Circuit and DIY layouts
In March 2017, Tagboardeffects published two stripboard (vero) layouts for the Uni-Comp1:
- Layout 1: based on the schematic in the request section of that post1
- Layout 2: based on the trace posted at FSB (Freeschematics/Forum) of a Uni-Comp Model CO-1051
The layouts differ in origin, so a builder choosing between them is choosing between two independently traced versions of the circuit1.
The Vactrol question
The original did not use an LED and LDR, but a Vactrol labeled MCD5271. For builders substituting parts, the layout notes give two paths:
- If using an LED and LDR, cover them in shrink wrap to block out any light, which increases their efficiency and effect1
- If trying a Vactrol instead, the suggestions are an NSL32 or VTL5C3, which have been used on many different optical compressors1
The clipping diodes
There is a genuine uncertainty about the diodes in the circuit. Both people who traced their Uni-Comp pedals reported the diodes were germanium, but the MD234 datasheet shows silicon, so the diodes could have changed during production1. A builder reproducing the circuit may therefore encounter either type depending on which version of the original they are modeling.
Because the two traced examples disagree on the diode type, a Uni-Comp build may not match every original unit. The diodes could have changed during production1.
Controls
The published layout describes a single control:
| Control | Wiring detail |
|---|---|
| Comp pot | Lugs 2 and 3 joined with a single wire from the board1 |
That single Comp pot is the pedal's only described control, wired with lugs 2 and 3 joined by one wire running from the board1.
Questions
Is the Univox Uni-Comp still made?
No. The Uni-Comp was discontinued around 1979. One owner of a gray example reported loaning it out around that time and never getting it back, only to discover the pedal had been discontinued. Original units now circulate secondhand, and the circuit lives on mainly through DIY stripboard layouts published for builders.
Who made the Univox Uni-Comp?
Univox made the Uni-Comp in the 1970s as part of its foot series of pedals. The company's service department was still supporting it years later: one owner who contacted Univox's service department was sent the wiring diagram for the pedal on request.
Can a Vactrol replace the LED and LDR in a Uni-Comp build?
The original circuit used a Vactrol labeled MCD527 rather than a separate LED and LDR. If you build with an LED and LDR instead, the published layout notes advise covering them in shrink wrap to block out any light, which increases their efficiency and effect. Trying a Vactrol such as an NSL32 or VTL5C3 is another option, both used on many different optical compressors.
Does the Uni-Comp use germanium diodes?
Possibly, though the record is mixed. Both people who traced their Uni-Comp pedals reported the diodes were germanium, but the MD234 datasheet shows silicon, so the diodes could have changed during production. Builders should be aware the two versions may not be identical.
Sources (1)
- Guitar FX Layouts: Univox Uni-Comp, tagboardeffects.blogspot.com, 2017-03-13