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lmaurits 21st January 2013 01:17 AM

Pot values in LM1036 preamp circuit
 
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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?

sofaspud 21st January 2013 01:23 AM

I would just use 50k pots in place of the 47k.

lmaurits 21st January 2013 01:37 AM

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.,

sofaspud 21st January 2013 02:30 AM

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.

DUG 21st January 2013 02:30 AM

47K is a standard 50%, 20%, 10% value from "back in the day".

Resistor Values E6 E12 E24 E48 E96 E192

:)

DUG 21st January 2013 02:42 AM

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.

sofaspud 21st January 2013 03:06 AM

Quote:

47K pots load the internal reference...don't want too low of a value.
Yes, that's the first thing I thought when reading the OP.
Quote:

Control pins...max current is -2.5uA
I missed that spec.
The datasheet does state that an external reference can be used in place of the internal zener reference. This, or the modified response circuits, would seem the best option for 10k pots.

lmaurits 21st January 2013 04:43 AM

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.

mickeymoose 21st January 2013 08:53 PM

Pin 17 on the LM1036 is a 5.4 volt zener regulated voltage. In the datasheet it states 5 mA drive capacity. 4x1k pots would draw 22 mA. Use an emitter follower to boost current capacity or an external DC reference with the 1k pots. E

DUG 22nd January 2013 04:33 AM

4 10K pots is only 2.16mA

Looks OK to me


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