NOTES & ERRATA Playmaster 300W Subwoofer amp (April/May 1995): While the BC556 transistors used for Q3 and Q4 have a `nominal' rating of 80V they can suffer from voltage breakdown in some amps, resulting in an audible 100Hz hum or even `motor-boating' at the output. The BC556 transistors can be replaced by BC640s (which have a higher voltage rating) as indicated in the associated diagram. - SEE sub3_pic.exe: a self displaying diagram for VGA screens.