Adding Input Gain Control to ACA Mini?

That's quite high when talking of solid state equipment.

If the preamp or source components you are connecting to the ACA all have a low output impedance (such as modern solid state gear) then you could usefully go to something a lot lower like a 10k or a 22k but your 100k will work just fine.
 
Well, I used the 100K potentiometer I had in my junk drawer....Works perfectly. Quick question, on the circuit board I left the RCA jacks and just soldered the pot leads onto the lands. If I connect the output of the preamplifier directly to the circuit board mounted RCAs, will the pot, not being used, still have some influence on the sound?
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If I connect the output of the preamplifier directly to the circuit board mounted RCAs, will the pot, not being used, still have some influence on the sound?

If I'm visualising what you have done correctly then yes, the pot will have an effect when the pot is turned down. I'm assuming you have fed the wiper of the pot onto what would be centre pin of the RCA socket. If that's not what you mean then its best to draw it out 🙂

At minimum volume it will shunt the ACA input (the centre pin of the RCA socket) to ground. A solid state preamp would work normally until the pot got down to around a few 10's or hundred or so ohms. So in practice nothing is noticed until the volume control is on almost minimum. Anything higher than that and no effect.