Pete Cornish OC-1 Optical Compressor

The Pete Cornish OC-1 is an optical compressor pedal introduced by the Pete Cornish Partnership in 2014, the maker's first optical compressor after what the company described as many years of development¹². It is a feed-forward design with a deliberately short two-stage audio path: an input buffer and an output make-up gain stage, with a light-controlled variable attenuator driven by a sidechain¹. Pete Cornish says the design goal was a compressor that produces no change to frequency response, no distortion and the absolute minimum of added noise².

The pedal is aimed at guitar players specifically, though the maker notes it is not limited to guitar¹. Aion FX, which traced the circuit in 2019, describes Cornish pedals as best known for being extremely expensive³.

An optical compressor with a clean blend

The OC-1's compression comes from an optical element called a vactrol, a package containing an LED and a light-dependent resistor. Aion FX's trace found the circuit begins with the classic Cornish buffer, followed by a three-way split into a compressed path, a clean blend path and a sidechain peak detector⁴.

The sidechain's job is to turn the audio signal into a control current that illuminates the LED in the vactrol. Aion FX noted that this sidechain topology was one they had not seen before in any other circuit⁴. The compression itself happens through a simple voltage-dividing attenuator, formed by a fixed resistor (R13) and the light-dependent resistor inside the vactrol. In effect, it is a dynamically operated volume reduction: the louder the input signal, the more the LED lights, the lower the LDR's resistance, and the more the signal is attenuated⁴.

So while the OC-1 is an original design, its core compression method has a long lineage, going back to the first optical compressors of the 1950s, such as the LA2A⁴.

The clean blend

What sets the OC-1 apart from a basic optical compressor is its clean blend path. Pete Cornish describes the BLEND control as allowing a small percentage of the original, uncompressed signal to be added to the output, which is useful at very high compression settings, where the original attack of each note can still be heard alongside the long sustain². The Deluxe OC-1's page makes the same point: a controlled amount of the original attack can still be heard together with the long sustain at very high compression settings⁵.

Aion FX's trace adds a detail players will want to know: the blend is not a balance control. It does not go from full clean at one end to full effect at the other. Instead, the clean blend path gradually combines the clean signal with the compressed signal as you turn it up⁴.

The controls and how they interact

The OC-1 has three controls: COMP, VOLUME and BLEND. The table below summarizes what each does, based on the maker's own descriptions.

The maker's suggested starting procedure is simple:

1. Set COMP, VOLUME and BLEND to minimum, at which point the pedal creates no compression or output boost¹.
2. Increase COMP to approximately 50% (12 o'clock) while playing, and note the decrease in output volume alongside the increase in sustain¹².
3. Use VOLUME at the end of the signal path to match the bypass level².
4. Add BLEND if you want more of the original attack back in the mix².

One interaction is worth knowing before you dial it in. Aion FX found that the volume and blend controls are somewhat interactive with each other, and the volume may need to be backed off when blended signal is added, to maintain a constant level⁴.

Bypass and buffering

The OC-1 is not true bypass. Aion FX's trace found the classic Cornish buffer in the audio path, meaning the pedal operates as a buffered bypass⁴. Pete Cornish states that with all controls at minimum, the pedal creates no compression or output boost, so the bypass mode sits at unity level¹. The pedal also includes an RFI rejecting input and bypass buffer, a feature shared with other Cornish pedals such as the CC-1, which the maker describes as having a high impedance RFI rejecting input and bypass buffer⁵.

Original design, hidden circuit

Pete Cornish describes the OC-1 as a completely original design, not copied from any previous or current device, and notes that it shares this original design approach with the CC-1, a compressor the company had recently introduced at the time¹². Aion FX's independent trace reached the same conclusion, stating that the OC-1 was in fact an original design, not quite like anything else they had seen before⁴.

The reason the circuit could not simply be inspected is that it is covered in thick epoxy. Aion FX described the goop as hard as a rock, so they traced the circuit using X-rays instead⁴. Those X-rays were enough to confirm the originality of the design⁴.

The OC-1 and the Deluxe OC-1

The Pete Cornish Deluxe OC-1 is a later version of the same optical compressor concept. It is housed in a rugged die-cast box with lined control knobs, and its power uses a coax connector of size 5.5/2.1mm⁵. Both pedals carry the same core control set (COMP, VOLUME and BLEND) and the same description as completely original designs, not copied from any previous or current device²¹. The Deluxe OC-1's page also describes it as the maker's first optical compressor, designed specifically for, but not limited to, guitar⁵.

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