The 8 seg LED VU meter follows the output of the final stage. The Summing input uses the Tonelux DISCRETE op-amp for summing, then feeds the Tonelux DISCRETE output op-amp into a transformer for the summing (or ACA) output. The return from the ACA out comes back into a DISCRETE booster amp, the master fader and transformer for the final output stage.
This configuration allows you to insert a compressor after the summing amp and before the master fader (either the front panel pot or the external fader option), then returning it back into the fader and to the final Booster output. This modules works the same way any typical professional recording console would work.
The INS (insert) button allows the engineer to compare or insert the device such as a compressor for IN/OUT comparison or when required for a special CUE point.
The SM1 is also available with only a Summing/Booster/Fader output when the INS function is not required, as when it is used for a send master.
Both the ACA output and the BOOSTER output are transformer coupled.