Since it was decided to make use of rotary switches for the ratio settings, it was possible to fit two modules in a single 2RU enclosure.

A separate power supply was used, the components only needed to be sourced for the rest of the compressor circuit.

Here's a few of the more difficult to get components and the sources (Australian) from which they were purchased.

Most components can be purchased from Mouser, DigiKey and other electronics parts suppliers.


Description Ref Supplier
28K 1% resistor R90, 92, 93, 94 Rockby Electronics
182K 1% resistor R66 Rockby Electronics
44.2K 1% resistor R69 Rockby Electronics
38.3K 1% resistor R67 Rockby Electronics
7.68K 1% (I used a 7.5K and 180R in series) R71 Jaycar Electronics
25K log pot with switch R77 Rockby Electronics
2N5457 JFET (need a handful if you plan to match or select these)
Q1, Q10 Rockby Electronics
Flashstar VU Meter (requires additional rectifier to be added)
M1 Rockby Electronics or JLM Audio

The 25K pots with switches were only DPST switches and a SPDT switch was really needed to short the input of the GR amp during bypass. To solve this little problem a relay module with DPDT signal relays which are switched using the switch on the pot was designed. This PCB handles two channels independently so only one PCB is required for the dual unit.

For metering, Kyoritsu KM-66 VU meters were used in one unit. These have 6V lamps in them and look great. This did require an additional 5V supply rail to power both the VU meter lamps and the 5V relays on the GR disable module, and the PSU-8 module has a third rail for such use which could be setup to 5V.

On the second unit Flashstar meters were used, which have a 12V lamp. The GRDM-1 module was modified slightly to allow a series resistor with the relay to accommodate different lamp rail voltages. The PSU-8 third rail was set to 11.5V and used to power both the meter lamps and the GRDM-1 module relays. The Flashstar meters did not have internal rectifiers so a schottky diode bridge was added to each meter.