Hi all,
I have a quick question about the potentiometers in the LM1036 datasheet schematic (attached below).
Volume, balance and tone are all controlled using 47k pots. As far as I can tell, this is a relatively rare value these days. I already have a bunch of 10k pots lying around that I'd like to use instead.
As I understand the circuit, the pots are just dividing down the regulated voltage from pin 17 so that the various control pins see some voltage between 0 and 5. So the value of the pots shouldn't matter at all for that purpose, right?
I'm more curious about the RC networks immediately after the pots, which use 47k resistors. I figure these are just low-pass filters to remove pot noise or avoid really sudden changes in control settings. Is there any particular reason they've chosen 47k resistors? Is it somehow important for this to match the potentiometer value? If I use 10k pots, should I replace the 47k resistors with 10k resistors and make the capacitors 4.7 times larger to keep the same filter properties? Or is it fine to just stick with 47k resistors after 10k pots?
I have a quick question about the potentiometers in the LM1036 datasheet schematic (attached below).
Volume, balance and tone are all controlled using 47k pots. As far as I can tell, this is a relatively rare value these days. I already have a bunch of 10k pots lying around that I'd like to use instead.
As I understand the circuit, the pots are just dividing down the regulated voltage from pin 17 so that the various control pins see some voltage between 0 and 5. So the value of the pots shouldn't matter at all for that purpose, right?
I'm more curious about the RC networks immediately after the pots, which use 47k resistors. I figure these are just low-pass filters to remove pot noise or avoid really sudden changes in control settings. Is there any particular reason they've chosen 47k resistors? Is it somehow important for this to match the potentiometer value? If I use 10k pots, should I replace the 47k resistors with 10k resistors and make the capacitors 4.7 times larger to keep the same filter properties? Or is it fine to just stick with 47k resistors after 10k pots?
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Hmm, yeah, that's probably the most sensible thing to do, since 50k pots are easy enough to source. Don't know why I didn't think of that myself, guess I got caught up on using parts I already had on hand. Thanks!
I'd still be curious to know if there's a good engineering reason for the R in the filters to closely match the pot values, if anybody knows any.,
I'd still be curious to know if there's a good engineering reason for the R in the filters to closely match the pot values, if anybody knows any.,
After a quick look at the datasheet... the potentiometer wipers connect through the 47k resistors to internal transistor bases. So they look to me to limit the current into those bases to a low value, which max's out when the wiper is at its upper end (when max voltage is across the pot). If using 10k pots, the wiper resistors would need to be similarly reduced to avoid starving the transistors of current. The caps may be just for bypassing noise. Someone else could probably expound/correct this.
47K is a standard 50%, 20%, 10% value from "back in the day".
Resistor Values E6 E12 E24 E48 E96 E192
Resistor Values E6 E12 E24 E48 E96 E192
47K pots load the internal reference...don't want too low of a value.
Control pins...max current is -2.5uA
This suggests to me that the current is coming out of the pins.
Stay close to 47K (50K is OK).
Otherwise you might want to start calculating voltage drop vs position vs design to see if there is a major shift in results.
Lots o'fun.
Control pins...max current is -2.5uA
This suggests to me that the current is coming out of the pins.
Stay close to 47K (50K is OK).
Otherwise you might want to start calculating voltage drop vs position vs design to see if there is a major shift in results.
Lots o'fun.
Unfortunately, it looks like the "back in the day" standard doesn't count for much these days - Mouser only have 9 kinds of 47k pots in stock! And most of them are expensive/unusual.
Thanks for the more thorough discussion of what's going on. I think I'll go with the 50K pots, seems like the easiest solution. No doubt I'll find plenty of other chances to use my 10Ks in future.
Thanks for the more thorough discussion of what's going on. I think I'll go with the 50K pots, seems like the easiest solution. No doubt I'll find plenty of other chances to use my 10Ks in future.
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