I'm currently putting together a 12v sound system and I need a volume control for the setup, but I'm unsure how to calculate the impedance of the potentiometer I would need.

The system consists of 4 JBL GTO 2000 amps each running in bridged mode powering 2 x 12" woofers and 2 x midrange and treble speakers. The signal input will mostly be coming from a sound card with an output impedance of 150 Ohms, but there will be instances where a mobile phone or other portable device will be plugged in as the source (for which I have no idea of output impedance).

I've not found any specific info on the input impedance for the JBL amps, but looking at similar amps, it seems to be in the region of 20k.

The signal will be split into 4 stereo signals using Y RCA connectors. How does that affect the the overall impedance of the amp inputs? 1/4?

As I understand it I should be aiming to have 1/10 of the input impedance of the amps as the impedance for the pot? That would mean I need a 2k pot for one amp and 500 Ohm for all 4 amps? To me this seems very low? Typical volume control pots seem to be more in the 10k or 50k range?

Can anyone shed some light on this as it's really the last thing I need before the project is complete.

Many thanks

Lavend