I recommend a modification to the v4 to avoid a potential failure mode that has been exposed in bench testing of two newly constructed units.
The amplifier stages were tested with bench +/-15v supplies with c. 50ms turn on time power supplies.
This caused, in one case, a catastrophic failure of a transistor in the unexpected power removal protection circuitry.
For existing units that employ the custom power supply I recommend the addition of a single diode, D1, a 1N4003 or similar, as shown below, the same type as used elsewhere in the circuit.
This does not seem to be a problem in existing builds as the power supply turn on time is very slow, but I
nevertheless strongly recommend the addition as the transistor Q39 can fail in unexpected ways and in one case the collector base shorted putting a near short between ground and the -15v supply. This would cause the -15v regulator to go into shut down and likely enter thermal protect, but it is in no way a good thing.
For those that use other +/-15v supplies I recommend a slightly more complicated change.
I will request
@hirscwi to update the v4 layout with this.
I have updated the Kicad files with the schematic change.
