File:Replacement of 4053 switch with optorelays.png

English: Replacement of 4053 CMOS switch in Revox B215 with low-resistance optorelays. The Soviet/Russian 293 series conveniently has model 293КП9А in which one channel is normally-closed and another normally open. The datasheet says max Rds is 5 Ohm, in practice it's well under 3 Ohm. The minimum LED drive current is 0.5 mA, but then the off-on transient is too high. Here, LED current is limited to around 3 mA so that the transient is under 1 ms. The input transistor is configured in such a manner that the normally-closed MOSFETs pass the tape signal (which is the default state, when there's no LED current), and the normally-open MOSFETs pass the source signal. The driving logic remains the same. Due to very high capacitance of the normally-open MOSFET (corresponding to SOURCE state and thus relevant only in recording mode), some source treble will inevitably bleed through into the line out path (but not the recording path). This may be reduced by decreasing resistors in the output buffers (R32-R35) from their stock 11K and 18K values to, say, 2.4K and 3.9K. The other problem is fitting the implant board into the very cramped corner of the motherboard, just next to the Dolby board connector.