Hi Alan,
Love the Quadraphonic story. All I did yesterday is measure the resistance of the LDR as I varied the voltage coming out of the transistor. Today I will hook it up to the HiFi and see how much the volume varies. I just need enough to lower the volume if I'm interrupted by a phone call or something.
The other use is to fine tune the volume when I'm doing an A/B test of different components. Years back A/B tests were not very rigorous, you would switch out components and then return to your listening position and rely on audio memory. The amusing part, I always heard a difference. Now, with instant switching between components and just sitting in my chair, the difference is extremely subtle and when there is a difference I can't honestly judge which is the better sounding. For the ultimate test, I compared the difference between an all valve (tube) preamplifier and a solid state one and the difference was minuscule, all the talk about tube warmth and better pacing and on and on is hyperbole. Sadly, before I made the A/B test I had zero doubt tubes made for a better sound.
My next step in my audio adventures and I'm sure I'll be asking for help is to have a remote controlled switching of components in the loudspeaker crossover circuit. My home made speakers have very good Danish made drivers, it took me months to tune the crossover to my liking, crossover makes a big difference to the sound. The crossover is outside the speaker box and i have terminals for each driver, I made it that way so I can try active crossovers in the future and so as to be able to change crossover components. I'm thinking of using relays to switch out different components, for instance changing the inductor between two different values.
To my ears there is little to be gained by spending money on amplifiers and other electronic components in the end it's the speakers that vary the most. Of course, the best place to spend money for music enjoyment is on opera and symphony tickets, unfortunately due to the pandemic the season was cut short. At least I got to see Ms. Anne-Sophie Mutter before the lock down, I was so looking forward to Andras Schiff but it had to be canceled.