[R-390] R-392 Audio

[Drew P.]

Hi All,

There must be an error in the schematic for the "official" R-392 solid state audio replacement module posted on Rovero's website (not the MOSFET version Perry is offering).

As shown, the posted version would have excessive crossover distortion because there is no forward bias on Q603 and Q604, the output transistors.  Sure enough, R647 & R648 appear to supply such bias to the output transistors' bases, but the bias would be shorted to ground via driver transformer T604's grounded center tap on the secondary winding.

In many circuits of this type, the driver transformer would have a voltage divider supplying the center tap of the driver transformer secondary, through which forward bias would then be supplied to the output transistors. Frequently the voltage divider would incorporate a temperature sensitive element (diode or thermistor) which would be thermally coupled to the output transistors to avoid thermal runaway.

Drew