Stereo linking two 1176 units has traditionally been done in a rather crude but effective way: by linking the control voltage outputs of two units. Two random 1176 limiters are unlikely to have gain reduction FETs that even vaguely match, so the 1176 SA Stereo Adapter was created. It contained a small battery and a pot to introduce a DC voltage to correct the discrepancy between the point in which the onset of gain reduction occurs between two unmatched units.
If you use this basic method of linking two 1176s at point 7 on the schematic, you are probably much better off sorting through a bag of 100 or so FETs to find two that closely match each other and forgo the Stereo Adapter circuit. With two matched gain reduction FETs (Q1 on the schematic) you can simply link point 7 on each of the boards using a shielded cable and achieve practical stereo limiting. Be prepared to do a fair amount of knob tweaking to get satisfactory results.
A Better Way
There's always more than one way to do almost anything and Purple Audio came up with a clever new way to link two 1176 units together. Rather than play with the Gain Reduction Control Amplifier outputs, they devised a scheme to combine the signals coming to the audio inputs of the gain reduction control amplifiers. In this configuration, the output of the first signal amplifier stage is buffered in each unit, then combined with a simple audio mixer. Each unit's sidechain sees exactly the same input signal while the rest of the circuit performs as usual without any further interconnection. No "stereo interconnect" (point 7) connection at all!
Circuit Description
The schematic shows the complete circuit. A302 provides a virtual ground so the opamps can be powered from a single 30 volt supply. Its output sits at half of Vcc or 15 volts. A201a, half the TLO72, is configured as a buffer stage which receives an input from the Signal Amplifier output of the main 1176 board. This signal is split - one line goes to the local mixer while the other goes to the second unit's mixer. A201b mixes the local buffer output and, when connected to another unit, combines it with the buffered Signal Amplfier output of the second unit. The output of the mixer goes back to the local ratio switch matrix, then into the Gain Reduction Control Amplifier as usual.
Since each unit sees the same inputs into the local mixer, each will have the same output as well.
Hookup
The component and connection designations are from the Purple schematic. Have a look at their MC77 manual if you want to cross-reference. "Loop" (Relay Wiper) is the input to the buffer. On your regular garden variety 1176 this will connect to the output of the Signal Preamp between the output capacitor and the output potentiometer.
Hairball Audio has started a support thread for the circuit.
