Modifying potentiometer values

[RandomGlitch]

Hi, I'm just about to start building a zvex  fuzz factory clone. 

I want to use 9mm Alpha pots in order to get them all in the enclosure.  I like using these pots because I can solder them directly to the board, and it gives a nice way of securing the board, and it's neater, I feel.

The schematic I have uses a couple of 5k pots, one for the voltage starve (stab), wired as a variable resistor and one for the volume control.  Unfortunately I can only get 10k ones from Jaycar.  I know I can get them online, but they are more expensive that way.

What I was thinking was to use 10k pots.  I'd parallel the "stab" with a 10k resistor and just have a changed taper, hopefully it would be OK.  How much of a difference would using a 10k on the volume pot be?

Thanks!!

[midwayfair]

http://geofex.com/Article_Folders/potsecrets/potscret.htm

and

http://gaussmarkov.net/wordpress/parts/resistors/resistors-6-mods/

But you can just use the 10K pot on the volume control with no problems.

However, the stab pot might pose a problem. If you use a parallel resistor, you'll retaper the pot, and the Fuzz Factory is ridiculously touchy. You might have some problems dialing in settings on the far left and right of that control. If you use a 10K, it might again be hard to dial in some settings.

You can also use 12mm pots if you can get them. They'll still fit. Actually, if you're using a 1590B, you can almost certainly fit five 16mm pots with proper planning. I've fit four with some extra room.

[RandomGlitch]

That's what I was afraid of, the taper making it hard to use... so yes maybe a rethink is in order.

I'm going to use a 1590BB because I'm not that clever at fitting it all in to a 1590B. 

I tried putting a 16mm pot i had on some perfboard (which I plan to try on this build) but found the holes too small, maybe I need to ream them out a bit first.

I may even place the pots in two rows, 3 on top, 2 under that, in a conventional "portrait" rather than the zvex sideways arrangement, which I don't really like.

That and trying to avoid using wires to the pots really!

Thanks midwayfair for taking the time to reply.  I dig your videos btw!

[midwayfair]

That's what I was afraid of, the taper making it hard to use... so yes maybe a rethink is in order.

I'm going to use a 1590BB because I'm not that clever at fitting it all in to a 1590B. 

I tried putting a 16mm pot i had on some perfboard (which I plan to try on this build) but found the holes too small, maybe I need to ream them out a bit first.

I may even place the pots in two rows, 3 on top, 2 under that, in a conventional "portrait" rather than the zvex sideways arrangement, which I don't really like.

That and trying to avoid using wires to the pots really!

Thanks midwayfair for taking the time to reply.  I dig your videos btw!

Thanks

I didn't realize you were set on using board-mounted pots. In that case, your options are indeed somewhat limited.

I guess you could always swap out the Stab pot if a 10K poses a real problem. Who knows, you might get some more sounds out of it with the extra 5K to play with!

[LucifersTrip]

http://www.diystompboxes.com/analogalchemy/emh/emh.html

Linear Pot

r1/r2 = 5

[RandomGlitch]

http://www.diystompboxes.com/analogalchemy/emh/emh.html

Linear Pot

r1/r2 = 5

Oh that's very interesting!     Is the taper linear you say?  I'm going to put that into a spreadsheet to figure it out.

I'll never go hunting for weird pot values again!!! 

thanks!

[Pyr0]

Harald from Sabrotone has a great vero layout for a fuzz factory with all 9mm pots board mounted to fit in a 1590B, you might even find room for the extra resistors.

http://www.sabrotone.com/wp-content/uploads/2011/11/FuzzFactoryLarger.gif

[GGBB]

http://www.diystompboxes.com/analogalchemy/emh/emh.html

Linear Pot

r1/r2 = 5

Oh that's very interesting!     Is the taper linear you say?  I'm going to put that into a spreadsheet to figure it out.

I'll never go hunting for weird pot values again!!! 

thanks!

As the page says: "the resulting value can fluctuate as the knob is turned."  I take that to mean it's an uneven curve.  I've never understood how that is supposed to be used as a substitute for a linear pot of the value required.  Take for example your set up requiring a 5K pot from a 10K.  If you move the wiper to the one end, resistance from that end to wiper is zero and from the other end to wiper is 1/(1/5K + 1/10K) = 3.3K.  With the wiper in the middle, resistance is 1/(1/5K+1/5K) = 2.5K.  That's not linear and it's not really a 5K pot (only the overall resistance is 5K and only when the wiper is directly in the middle).  It might work fine for the volume control, but it doesn't really apply to use as a variable resistor because you'd be shorting one of the 5K resistors and you'd end up with a 3.3k pot, and 10K should be fine for the volume anyway.

If you go with your first idea, the taper won't be too far off linear.  It would be about two thirds flatter than a typical log taper (closer to linear than to log), so it may work just fine.

[RandomGlitch]

Well, after all that I've ended up buying the 16mm pots, as I'm putting it in a bigger enclosure anyway.

I can live with the off-board pots!  It seems less hassle than fiddling about as we discussed.

Thanks for all your help everyone.  I've learned a lot from it! 

[LucifersTrip]

As the page says: "the resulting value can fluctuate as the knob is turned."  I take that to mean it's an uneven curve.  I've never understood how that is supposed to be used as a substitute for a linear pot of the value required.  Take for example your set up requiring a 5K pot from a 10K.  If you move the wiper to the one end, resistance from that end to wiper is zero and from the other end to wiper is 1/(1/5K + 1/10K) = 3.3K. 

I'm happy you checked that out....I have to admit, I bookmarked that a long time ago, never had a need for it and never looked at it in detail.

I just did a quick test using two 10K pots and two 5K pots at 25% intervals. (Note: pots can be 10% off, so this isn't totally accurate, but close enough to get the idea)

5K linear:
0, 1.25K, 2.5K, 3.75K, 5K

10K with 10K across outer lug:
0, 2.2K, 3.75K, 4.66K, 5K

10K with two 5K's, each from wiper to outer lug:
0, 1.66K, 2.4K, 3K, 3.33K

[bancika]

I tried lowering pot value but it doesn't really work. Maybe I'm an idiot, but here's what I've got.
500K pot, trying to get it down to 1K pot with two 510ohm resistors across lugs, as described in the article. What I get is a 1K pot, but taper is just useless. At only 1% of the rotation I have 5K (1% of the 500k) on the pot in parallel with 510ohm and the remaining 495K in parallel with the other 510ohm. That gives pretty much 510ohm:510ohm and it remains like that for the remaining 99% of the travel. So the whole action happens in the first 1% of the pot, or even less, effectively making it an ultra reversed log pot
Am I missing something?

[Mike Burgundy]

Lucifers measurement match a quick spreadsheet calculation. Try R1=R2=Rpot=10k, a little straighter, close enough for R&R, pretty close to 5k. (well, 6)

[RandomGlitch]

I tried lowering pot value but it doesn't really work. Maybe I'm an idiot, but here's what I've got.
500K pot, trying to get it down to 1K pot with two 510ohm resistors across lugs, as described in the article. What I get is a 1K pot, but taper is just useless. At only 1% of the rotation I have 5K (1% of the 500k) on the pot in parallel with 510ohm and the remaining 495K in parallel with the other 510ohm. That gives pretty much 510ohm:510ohm and it remains like that for the remaining 99% of the travel. So the whole action happens in the first 1% of the pot, or even less, effectively making it an ultra reversed log pot
Am I missing something?

I was amazed this could work, so I put the math into a spreadsheet to check it and found that the more you change the value, the further away from linear the taper becomes. 

You're trying to change 500k into 1k, that's a big change, the math will show you (if you care to do it!).  That's the trade-off.

Try using a value closer to the 1k your after.

[bancika]

then it defeats the purpose, a value close to 1K pot will perform close to 1K pot on its own. If I had one I'd just stick it in the circuit
That means that article is not really correct, or at least deceiving http://www.diystompboxes.com/analogalchemy/emh/emh.html. Anything you do will change the taper. The only way to get a smaller lin pot is to start with a slightly larger log pot and bump the curve up to somewhat linear using parallel resistors.

[Baran Ismen]

Can someone tell me if its possible to make a C taper pot from a linear one ?

[antonis]

post #37