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The higher impedance, the more noise. Modern circuits with OP-AMPs often work best with impedances in the order 5-10KOhm. But, the capacitors need to be bigger.
50KOhm is often used and not "hard to drive".
Just to give you an idea about what I mean with an OP-AMP (stage) board to put before the tone control board:
Carte d'amplification cc de preampli de carte d'amplification de double OP pour NE5532 OPA2134 OPA2604 AD826-in Operationnel Amplificateur Puces from Electronique on AliExpress
Mini preampli Op amplificateur module double canal NE5532 preamplificateur carte de tonalite 5 fois large tension 12 30V G10 004 on AliExpress
NE5532 OP AMP HIFI Audio preamplificateur double carte de preampli Bluetooth pre ampli-in Operationnel Amplificateur Puces from Electronique on AliExpress
NB: SMD components are less easy to replace if you need a different gain (amplification).
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If you are using such an pre amp board, you will need a power supply. It should look like this one and use 317/337 regulators which are adjustable, as these are better than the usual single voltage ones. +12V 0V -12V should be a usual voltage you ha
ve to adjust for the OP amps.
LM317t LM337t alimentation lineaire reglable 5 40 V DC bricolage Kits Module pieces composant F7 007-in Amplificateur from Electronique on AliExpress

A complete pre amp could look like this: An optional 10uf film capacitor to block DC, in front and /or after the first/ last stage.

A NE5532 module to buffer the input signal, then your passive pre and finaly an output buffer NE5532.

If you wire anything up in the right order, such a combination should do nothing to the sound but make the volume adjustable and work with any source and amp, as in- and output are well buffered.

Be prepared to change the resistor that sets the gain of the OP amps, most boards are x10 or x100 which is too much if you connect a line source like a CD player.

Last, if anything works, change the OP amps for 100% original NE5532, as all the modules from China have fake ones (I have bought a hand full, even some of the ones shown, not one was delivered with a real TI OP amp). This will cost you a maximum of 5$ for two.

If you like the "sound", which means the unchanged signal, you can experiment with "better" OP amps.
Maybe you hear any kind of change, if you go for something more expensiver than the NE5532. Be warned, most tests showed no audible improvement, even with Op-amps more than 20x the price. The NE5532 is used in countless high end products.
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Thanks for that information I will look into getting a op-amp .
Why does this passive tone board have 50k pots ? Surely 100k pots would not be as hard to drive

Higher input impedance means more hum pickup as well (when the source is unplugged or off), 6dB worse for 100k compared to 50k.

Passive preamps/volume/tone circuits are always a compromise and affected by the impedances of the source and the amp they drive.
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