additional circuitry

Supporting the audio circuitry are two other subsystems, the power supply and the relay system.

The power supply is designed with intentional "sag", and most of the voltages and currents in the amp are relatively low, because these factors lead to desirable sonic characteristics.  The sound and responsiveness of this amp shares something in common with Eddie Van Halen's "brown sound", which he obtained by dialing back the AC input power to his Marshall amp, with a variac.  Of course, in EVH's case, he was also reducing the heater temperatures, whereas the McGrath Mini runs the heaters at full power: the "sag" is only for the high-voltage supply.

The power supply circuit has morphed a bit since I made the schematic.  E.g., the resistor between B+2 and the B+3 nodes is now 10k instead of 1k.  The various changes have just been things I've tried, in seeking to improve performance or address perceived problems.

However, so far, nothing has made any important difference, for the better or for the worse.  The power supply worked fine as originally drawn, so basically I have no worthwhile changes to suggest, as of yet.

As noted, the relay contol circuitry is based on orange LEDs.  If you wanted to use LEDs with a bigger voltage drop, such as blue LEDs, then changes would be needed.  Probably, just running the circuit on 12vdc instead of 5vdc would do the trick -- of course, different relays would then have to be used.

Also, I've spoken about wanting to make other functions controllable by the footswitch.  These might include the effects loop bypass, and some of the gain-boosting controls such as the "NFB" switch or the "coldness" switch.  I would probably *not* add more buttons to the footswitch, as I like it being a simple passive unit which connects with a regular stereo patch cable.  Instead, each of the multiple functions would have a 3-way switch, giving choices of on, off, or "ftsw".  In the "ftsw" position, each function would be assigned to a pre-determined footswitch button, either 1 or 2.  The "additional gain" functions would share button 1 with the linear preamp, and the effects loop would share button 2 with the M-section.

Probably the additional switching complexity would be handled on the input side of the comparators, so there would just be more comparators and transistors to drive more relays.  Another LM339 may be needed.  If, however, it turns out to be desirable to drive multiple relays from the same transistor, then these transistors should be 2N2222, which have more current capacity than 2N3904 (otherwise, they are interchangeable, so if you only purchase one kind, I suggest you make it 2N2222 for versatility in multiple projects).