50K Reverse audio taper, what else can I use?

[svstee]

Building a hotcakes clone and I don't have the 50K RA pot for gain. Tried using a regular audio pot, but it only works for about the last 1/10 of a turn. Is there anything else I can use that would get me closer?

[mdh]

Did you try wiring it backwards, i.e., swapping the wires to the CW and CCW lugs?  If you do that, you would turn CCW for more gain, but the taper should be right.

[CynicalMan]

Just label it "Clean".

[svstee]

I'll try that mdh. Thanks.

[Ripthorn]

Look at "the secret life of pots" I think there is a way to get something roughly like C taper using like a 100kB or something.

[John Lyons]

A 100k lin pot with a 100K resistor soldered across the outside lugs will get you a RA 50K pot.
It's not a perfect audio taper pot but it will get pretty close and turning clockwise will = more
gain.

john

[svstee]

By outside lugs, you mean lugs 2 and 3, right? I was reading that article, good, stuff. I just need to digest it and reread.

[tednet]

So basically to get lets say a 100k rev log pot, you need a 200k pot and 200k resistor?

Is there a formula or something or examples? I am very interested in this, because there are almost no log pots, and no rev log pots in Bulgaria.

Also, how do you get a normal log pot from a linear pot?

[GREEN FUZ]

I had a similar question recently and in the course of my research came across this thread.

http://www.diystompboxes.com/smfforum/index.php?topic=70732.0


Lots of useful information on emulating various tapers.

[skiraly017]

Why not just order a C50K from Small Bear?

http://www.smallbearelec.com/Detail.bok?no=116

[jefe]

By outside lugs, you mean lugs 2 and 3, right? I was reading that article, good, stuff. I just need to digest it and reread.

No, I believe the outside lugs are 1 and 3, 2 is the center.

[svstee]

That's what I was thinking at first, but this
http://www.geofex.com/article_folders/potsecrets/potscret.htm
made me think otherwise.

[mdh]

Tapering resistors don't do any tapering if placed across the CW and CCW (lugs 1 and 3), because both resistances in question are constant with rotation of the pot (unless of course one of those lugs is tied to the wiper).  You want to put the tapering resistor between the wiper (lug 2) and one of the outside lugs.  I always have to think about the details when I do this (particularly if it's not entirely clear whether the resistance in question should increase or decrease with CW rotation of the pot).  You could always take the empirical approach if you're not sure, but my guess (note that this is just a guess) would be that you would want the resistor across the CCW lug and the wiper, i.e., between lugs 1 & 2.

Also, FWIW, IIRC according to R.G.'s article, a tapering resistor around 1/4th to 1/5th the value of the pot gets you the closest to a reverse log taper.  So a 68k fixed resistor with a 250k pot would get you pretty close (~53k, approximately reverse log).  And don't worry too much about trying to hit 50k exactly, because pot tolerances are terrible anyway.

[jefe]

That's what I was thinking at first, but this
http://www.geofex.com/article_folders/potsecrets/potscret.htm
made me think otherwise.

.... just perused the article again... I'm pretty sure 1 & 3 are the "outside" lugs, 2 is the center, wiper, whatever you want to call it. I've been known to be wrong though! lol

[mdh]

That's what I was thinking at first, but this
http://www.geofex.com/article_folders/potsecrets/potscret.htm
made me think otherwise.

.... just perused the article again... I'm pretty sure 1 & 3 are the "outside" lugs, 2 is the center, wiper, whatever you want to call it. I've been known to be wrong though! lol

It's true that 1 & 3 are the outside lugs and that wiper is lug 2, but it's equally true that putting a resistor across those lugs will not accomplish any tapering unless one of them is tied to the wiper.  If you look at the article again, there is absolutely no instance in which a tapering resistor is placed across the outside lugs, because that would be nonsense, and that's not R.G.'s usual MO.  In this case, it is almost certain that what is desired is a reverse log tapered resistance increasing with CW rotation, so putting the tapering resistor between lugs 1 & 2, and using those as the terminals of the variable resistance is the right thing to do.

Sorry if I sound snippy, I just want to make sure that the OP gets the most unambiguous, accurate information available.

[MikeH]

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

[jefe]

It's true that 1 & 3 are the outside lugs and that wiper is lug 2,

That's alls I'm sayin hoss... just trying to make sure we use proper lingo.

[svstee]

OK, just to be sure, I'm going to solder a 100K resistor across lugs 1 and 2 (or CW and wiper) of a 100K log pot in order to get a 50K RA pot (or close to it).
Am I ok?

[GREEN FUZ]

My understanding is that to get the value you want from your 100k linear pot, you need to first bridge lugs 1 and 2 with plain wire then bridge lugs 1 and 3 with a 100k resistor.

[MikeH]

OK, just to be sure, I'm going to solder a 100K resistor across lugs 1 and 2 (or CW and wiper) of a 100K log pot in order to get a 50K RA pot (or close to it).
Am I ok?

That wont work, that will give you a 50k log pot, I'm pretty sure.