AF input selection
Use is made of a single-pole, 12position rotary switch, S1, with whichand 12 pull-down resistors the inputsource is selected. Since only one resistoris linked to the positive supply lineat any one time, the current drawn bythe circuit is only 15 μA, which, in thecase of a battery operated preamplifier,is an important advantage.
The 12 outputs of S1 are linkedto parity checker IC3. The output ofthis device is high only if an oddnumber of inputs is high. When S1 isturned, all inputs go low briefly andso the output of S3 also becomes lowfor an instant. The output of IC3 thentriggers monostable multivibrator(MMV) IC4a. Since this is retriggerable,its output will be a single pulseeven with contact bounce of S1. Aslong as trigger pulses arrive during theperiod the output is active, the outputpulse is stretched. To makeabsolutely sure, the time can be setwith P1 between 0.1 s and 1 s.The outputs of S1 are also linkedto D-type bistables IC1 and IC2,which ensure a stable change-over ofthe output levels. The bistables havethe advantage that they can be reset.This facility is made use of by resettingall relays before a change ofinput, so ensuring that only oneinput is linked to the circuit at anyone time. This arrangement providesa dead time between the releasing ofone relay and the tripping of another.This dead time corresponds to thesum of the mono times of IC4a andIC4b. MMV IC4b serves to clock theinputs of all the D-type bistable.Since this requires a pulse of only10 μs, the dead time is determinedprimarily by IC4a.