# What potentiometer should I use for this phono pre-amp?

Please consider this DIY phono pre-amp.

The text states, that resistors R9 and R22 may be exchanged for potentiometers for gain control.

Since the circuit uses 1.2kΩ resistors for R9 and R22, I was wondering what kind of potentiometer I'd need. I was thinking of a logarithmic 10kΩ pot. However I'm not sure. In case there's a formula for computing this kind of resistance, please let me know.

The arrow "-VA" at IC1 (pin 4) should connect to the "-VA" point above C2, right? Likewise on the right channel.

... I was thinking of a logarithmic 10kΩ pot ...

The gain of IC2 & IC4 are (R9+R10)/R10 and (R22 + R23)/R23 respectively.

IF gain with R9 = 1k2 is "about right" then 10k may be too large - ie you will get about 8x the gain using a 10k pot. A 2k2 or 4k7 may be more appropriate. Linear would be OK enough. Log would better match ear response but either is OK.

Extra: You could either replace R9 with a pot with wiper tied to one end OR could replace both R9 and R10 with a pot, one end of track to IC2-6, other end of track to ground and wiper to IC2-2. The disadvantage of this is that when wiper goes to ground Gain is "infinite" (or open loop.)You could avoid this by placing a resistor between ground and the lower end of the track - but may as well then just use the pot in place of R9.

The arrow "-VA" at IC1 (pin 4) should connect to the "-VA" point above C2, right?

I think you meant to write "above C7" - in which case, yes. This is op amp negative power supply , and R7, C13, C9 form a ripple filter. The resistor should almost be unnecessary as long as the capacitors are used and are close to IC1, IC2 physically - but should do no harm.

• Another quick thing though: Should 0V be also connected to mains earth as well? – polemon Jun 7 '12 at 5:50
• 0V to mains earth. -> Usually a good idea. – Russell McMahon Jun 7 '12 at 6:05