replacing 4.7k pots with 5k on negative feedback op-amp circuit

[Liam O'Rourke]

Hi, I'm doing a repair job on a friends PA mixer/ amp. i've done the essential fixes (dodgy NPN accompanied by a blown resistor) However, I'd quite like to replace all of the pots (24 of them) for sturdier ones as the ones currently on there are proper wobbly and it will make a HUGE difference to the build quality/ feel of the device at a very small price. Anyway, the pots are reading around 4.7k on a continuity test but i'm finding it hard to find 4.7k audio taper pots with the correct size shaft. I can however, find 5k ones. Now from what i understand of negative feedback op amp circuits the rating of the feedback resistor will partially depict the amount of gain. And if i increase the risistance I will increase the gain as the op-amp will need to output more from the mains connection to try to match it's two inputs. So i guess what im asking is...
A: is my theory right here?

and B: would using a 5K resistor make too much of a difference and possibly have adverse affects/ not work?
Thanks

also... If anyone knows of a supplier of 4.7k audio taper metal pots with 15mm shaft length could you please let me know as this would be much better than using 5k ones.
Thanks a lot.

 

[Gryd3]

Hi, I'm doing a repair job on a friends PA mixer/ amp. i've done the essential fixes (dodgy NPN accompanied by a blown resistor) However, I'd quite like to replace all of the pots (24 of them) for sturdier ones as the ones currently on there are proper wobbly and it will make a HUGE difference to the build quality/ feel of the device at a very small price. Anyway, the pots are reading around 4.7k on a continuity test but i'm finding it hard to find 4.7k audio taper pots with the correct size shaft. I can however, find 5k ones. Now from what i understand of negative feedback op amp circuits the rating of the feedback resistor will partially depict the amount of gain. And if i increase the risistance I will increase the gain as the op-amp will need to output more from the mains connection to try to match it's two inputs. So i guess what im asking is...
A: is my theory right here?

and B: would using a 5K resistor make too much of a difference and possibly have adverse affects/ not work?
Thanks

also... If anyone knows of a supplier of 4.7k audio taper metal pots with 15mm shaft length could you please let me know as this would be much better than using 5k ones.
Thanks a lot.

5.0 certainly sounds close enough... but how are they wired in?
Your continuity test would show the 'maximum' possible most likely by measuring pins 1 and 3... Using the wiper you would get anything and everything from 0 to 5.0 (which includes 4.7). If it were wired in with only 2 pins, we can guess immediately that it would be a perfect fit.
Now... funny enough. When you buy a standard/cheap 4.7Ω resistor, it may be ±10% of it's rating. Many parts have this tolerance, so swapping out with another component that falls in the design tolerance is perfectly acceptable.
Worst case is that the pot is wired in with more than 2 pins and the device is adjusted or balanced at a much finer degree to the original 4.7Ω rating. In this case, you may need to make another adjustment (most likely to a trim-pot) to balance out the change.

 

[BobK]

I would not worry at all about 5K vs 4.7K pots.

Bob

 

[Liam O'Rourke]

Thank you very much for your replies. By the way... does my theory on negative feedback on op-amps sound correct?

 

[AnalogKid]

The tolerance on low cost pots is 10% or worse, so they probably were intended to be 5K all along.

ak

 

[Liam O'Rourke]

Okay... Thanks a lot. So just out of interest... was I right in what i was saying about increasing the rating of the feeback resistor would subsequently increase the gain... as the output would have to drive a larger signal in order to attempt to match the inputs? Or does that sound like nonsense?
Thanks again

 

[AnalogKid]

That is correct as a general statement, but without a schematic we don't know that the pot is in the feedback loop.

ak

 

[Liam O'Rourke]

Thanks analogKid... here's a quick schematic. I havent put resistance ratings on there because I'm not bothered so much about that I just want to know whether or not increasing the FB resistor would increase the gain. Obviously the resistor going to ground would need to be changed accordingly as well.

 

[BobK]

Changing the pot from 4.7K to 5K would increase the gain by about 6%. Not significant at all. It would not go to 11, more like 10.6

Bob

 

[Liam O'Rourke]

Thank you very much.

 

[Liam O'Rourke]

So just out of interest... would replacing them with 10k pots possibly be pushing it a bit too far do you think?

 

[BobK]

Why would you want to? It would most likely result in only half the range of the pot being usable before you got distortion. Unless the inputs are unusually low, in which case it might be helpful.

Bob

 

[Liam O'Rourke]

Just because I've got loads of 10k pots lying around at home not being used whereas i would have to buy the 5k ones in especially. It doesnt matter tho. A bit of patience and a bit of money spent will result in a proper job so i'll just do that. Thank you very much for your help tho.

 

[AnalogKid]

So just out of interest... would replacing them with 10k pots possibly be pushing it a bit too far do you think?

Without a schematic, it's *impossible* to say. But almost certainly all of the pots do not do the same thing, so changing the value might affect some circuits more than others.

ak

 

[Liam O'Rourke]

Okay thanks again