ZX had been in for cap upgrades a year back, then changed hands. New owner wants SQ tweaks which includes high bias. Previously we added individual heatsinks to the predrivers for this but it’s a fiddly job.
Skills learned in 4 years of Metalwork at High school have come in handy. Piece of ally bar cut and tapped to allow fastening to end drivers. Green goo is thermal gap filler which transfers heat and holds the bar in place.
Ally bar soaks up heat from predrivers and it’s then transferred to the cooling tunnel by thermal gap filler. Middle predrivers bonded to the bar and have thermal compound to ensure good heat transfer. This keeps them cool and the bias stable.
Usual underbiased distortion waveform that we see with ZX amps as they arrive with us. If the bias is turned up they can go into thermal runaway as the predrivers get warm, so the factory sets it low.
After optimisation, and the bias is now completely stable. That’s less than a tenth of the previous figure.