Optonica SM-4100 Input Selector Board

The input selector circuit is very simple and based on LF353N Dual Operational Amplifier. Capacitor-less FET inputs is as good as it gets, and sometimes a simple circuit like this provides better audio quality, than having signals go through digital signal processors, digital switches, and decoupling capacitors, which you will find in modern amplifiers. This simple FET input design outperforms all of that modern DSP wizardry with input decoupling capacitors. In addition, a simple rotary input selector switch is as good as it gets, and I prefer these over digital switches.

Here is the LF353N operational amplifier IC in a PDIP package. This integrated circuit is still in use today because FET inputs are useful in medical instrumentation etc...

I just love the look of these mylar and polyester film capacitors. You do not see these much now a days.

A thick shielded connecting cable brings the selected signal to the front PCB. This is more advantageous over PCB tracks that are prone to crosstalk noise.

This is the input selector switch with a connecting rod going to the front of the amplifier.

Here is the metal connecting rod.

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