substituting potentiometer values

[brendandwyer]

i know this schematic has come up before. http://www.forsselltech.com/downloads/schematics/8chsum_1.pdf

i'm having a lot of trouble finding 25k lin dual gang pots. so i was wondering how to go about the calculations to figure out what needs to change to use say, 20k, or 50k dual gang pots.

i would imagine the 22k resistors to each buss would have to be increased?

anyway, thanks

 

[zebra50]

Hi!

From a quick look at the schem, these are left-right pots. 20K will work fine. 50k would not be a disaster either. It would still work, although the left-right profile may not be quite the same.

Most standard pots have fairly loose specifications. Many builders here would throw in the nearest thing they have in the spares box.

I don't ever want to discourage creativity and experimentation, but I'm curious about why you're building this? There are plenty of super-cheap mini mixers on the market, and dual pots can be expensive.

z50

 

[JohnRoberts]

Simple proportional scale up or down as needed.

For example to substitute 20k for 25, multiply other values by 20/25 or .8x

JR

 

[brendandwyer]

thanks guys i'll get the 20k and calculate the other changes.

 

[brendandwyer]

out of curiosity, what is the overall effect on the circuit performance when you substitute 20k pan pots for 25k?

 

[JohnRoberts]

I wouldn't expect any significant difference.

JR

 

[brendandwyer]

thanks man

 

[brendandwyer]

so last dumb question for today:

at what value would you start to see a change in circuit performance? 10k? 100k?

and maybe why too, again i'm a beginner

thanks

 

[JohnRoberts]

Everything makes a difference, whether that difference matters depends on your design goal.

In general higher impedances will generate higher self (Johnson thermal noise) noise, and higher noise from active device noise currents, and higher offset voltages from device bias and offset currents.

Lower impedances can load down pot tapers, cause LF loss when cap coupled, and perhaps higher distortion from inadequate drivers.

Note, since the OPA604 is a JFET input opamps the noise current is femto Amps so not significant. Offset current is pico Amps so still not a big deal, so remaining concern from high impedance is resistor noise in the context of the opamp's 10 nV/rt Hz.

I'm too lazy to do the calculations, but you can probably find the equations via google. I found http://www.sengpielaudio.com/calculator-noise.htm a noise calculator, but I've never seen it before so I can't vouch for it's accuracy.

JR

 

[brendandwyer]

thanks! i spent the last week on vacation reading a beginners electricity/electronics book. I'm now brushing up on my algebra and calculus