Potentiometer help needed!

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Potentiometer help needed!

I'm working on an audio project and I'm having problems with the potentiometer. My intent is to take a stereo headphone-level signal and run it through the potentiometer as a volume control. I picked up a stereo 100KAX2 pot at Radio Shack and, while it works, I only get volume control at the last 10% of rotation...meaning, I can turn the knob for 90% of its rotation and get no sound, then the sound increases in volume rapidly. I want a nice, smooth taper. The part I picked up at Rat Shack is an Alps model, and the markings on it read "411G 100KAX2." It's the only stereo pot that Radio Shack sells. I have the input signal going into pin 3 and the output on pin 2. There's also a fourth pin on the other side of the pot (there's three in a row on one side, and a single one on the other). I'm not running ground through the pot...should I be? I have a feeling I'm just wiring this thing wrong, so how should it be done properly?

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Taper? Value = 100k-ohm?

Change the taper or change the value. Linear or log taper? Maximum resistance is 100k (based on the part number)? Save for a "shoot from the hip, no basis in reality" answer, I'm useless!

tony b.

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Re: Taper? Value = 100k-ohm?

yotonyb wrote:
Linear or log taper?

I'm starting to think it's a log taper (though that's pretty stupid for something marked as an audio taper). I wired and rewired the pot every which way but Sunday (though since it's 11:30 Saturday night as I write this, I'm not too far off from Sunday), including grounding the hell out of it, and I'm still just getting the same thing.

I'm going to have to look for a new pot...I'd hate to have to order one, since it's only a $2 part. I don't suppose anyone out there has a nice, small, stereo *linear-taper* pot they could send me?

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[size=3][/size][font=Courier]

<br />
               100K Ohm<br />
  O------\/\/\/\/\/\/\/\/\----<br />
Input           /|\           |<br />
                 |            |<br />
                 |     Gnd  -----<br />
                 |           ---<br />
                 |            -<br />
                 O  Output</p>
<p>

You must have ground connected. That will fix your problem.

Best,

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Well, here's the part I'm hav

Well, here's the part I'm having problems with:

http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&category%5Fname=CTLG%5F011%5F002%5F011%5F000&product%5Fid=271%2D1732

...and I wired it according to the diagram here:

http://sound.westhost.com/pots.htm#using

I did notice that when I soldered the ground wires to that weird 4th pin, I got a little more useful of a taper (though still not anywhere near linear). But it would only work with input soldered to pin 3, which means that to increase the volume I had to turn the pot counterclockwise...no good. Input on pin 1 gave me nothing.

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It's stereo, right. Diagram

It's stereo, right. Diagram #4 on that http://sound.westhost.com/pots.htm#using you sent over is correct, although the output labels are reversed (R- and L- should be swapped).

Unless I'm mistaken, a logarithmic pot for audio is correct. Decibels are nonlinear units - something like an order of ten magnitude increase, or the inverse thereof. Nonlinear at any rate.

You have the right part, most likely. It sounds like, from the symptom of only getting sound at the last 10% of the turn and it increasing rapidly, you've got the input and output reversed. The single pin you mention is most likely a common ground.

There should be five connection points, not four: R-In, R-Out, L-In, L-Out, and GND. It is conceivable, though not likely, that the mounting point iself is the ground, but I strongly suspect it's that pin off by itself. Try to just get one channel working properly first to minimize the number of combinations. Then once you figure it out tackle the second channel. Hope that helps.

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Okay, here's the pot I have:

Okay, here's the pot I have:

Pot

Those green wires are, obviously, ground. I've tried putting input on pin 3 (the farthest to the right) and output on pin 2 (the middle one), just like the diagram says. I've also tried switching the two. No dice; I still get the same thing.

Further down the page I linked to above, it talks about adding a 22k resistor to the output (between signal and ground) to give a better taper. I'll try adding one to each channel and see what I get--but somehow I doubt it'll help.

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ISTR from long ago . . .

tying the center and one side together, with ground only connected to the outer case and not to the actual circuit.

{snipped misinformation}

EDIT: Ooops, I just read the page you referenced and see that my idea is not-so-good. Maybe that's why all those volume controls I made as a kid worked so poorly. Blum 3

How about? http://sound.westhost.com/pots-f8.gif

That didn't work? How about using a trimmer pot in the R position to allow you to futz the best value?

dan k

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Is that audio taper for power

Is that audio taper for power amps outputs or passive?

I would order a number of different vales and try each one. Heck, each potentiometer is only 50p or so.

Alternativly, you could try wiring it up as a potential divider. Its more complicated but it would give you more control.

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The pot is for a headphone-le

The pot is for a headphone-level signal. Basically, the project is to make a switch with volume control for use with a laptop. My dad bought a laptop for desktop use, but uses either headphones or a pair of speakers. He hates to use the software volume control, so I'm building a box that has a pot for volume control and then a switch to select one of the two outputs. The entire box is passive. Everything works fine without the pot. I picked up some 22k resistors yesterday, and I'll try adding one tonight and see what that does.

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I do not profess to know what

I do not profess to know what is wrong however just to give you another idea of something to try if you want to build a little device like that again.

I bought from the thrift store a pair of those little crappy computer speakers (self powered) that had volume controls and headphone jacks on them.

Took the electronics out. Cut off the speakers. I then went to radio shack and bought a little project box that just fit the amp board into it.

Drilled holes for the volume control and the headphone jack and the power and signal wires. Mounted the board into it and closed it all up.

I know have a little tiny box that is self powered (my laptop has horrible amplifcation for headphones) and my little box has a volume control on it. I guess you could use it for speakers as well but i don't bother.

Just another idea to toss around if you want to make something again for use with a laptop.

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I dug into that Radio Shack w

I dug into that Radio Shack web page and there is some documentation for that part. I found some cursory instructions here:

http://support.radioshack.com/support_supplies/doc31/31139.htm

I haven't tried to interpret whether or not this is the convention you followed, or if this convention has been suggested by others. I wasn't sure if you saw this, so I just wanted to be sure you didn't overlook this. It also says that pin 4 is a "40% loudness" pin connection. Presumably that's what gets wired to the loudness attenuator on your home stereo.

Good luck and good skill!

tony b.

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That RadioShack document spec

That RadioShack document specifically refers to input from a receiver and output to a speaker...so I'm thinking that I simply have the wrong part. It also mentions a completely different wiring scheme that eliminates ground, so I'll give that a shot too. I took a look at some diagrams for headphone amplifiers and they all specify using 1k pots, not 100k ones. I added a couple 22k resistors, but they didn't really help. I think I just need to find a better part.

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POS pot

Turns out that the pot is simply a piece of s--t. When I wire it the way Radio Shack says to, it works fine--I get a nice, gradual taper. The problem? I have to turn the knob *counter*-clockwise to increase the volume, and clockwise to decrease it. My father barely knows his head from his ass nowadays, so having a backwards pot won't do any good. I've tried reversing the wiring scheme, but I get the problem I was having earlier--taper at only the last 10%. Seems to me like a piece of horrible engineering.

Looks like I get to try to find a new pot now...

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Victory is mine!

I at last have achieved success!

Turns out that Rat Shack's wiring instructions are ridiculously wrong. While surfing at work today, I Googled for "Radio Shack 271-1732" and found this page:

http://www.bcae1.com/potentio.htm

...and there's a diagram towards the bottom of the page with the pinout. Turns out that these pots aren't *complete* pieces of crap, either.

So I wired the pot according to the diagram and whaddaya know--it works!

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FYI: Audio taper is a log sca

FYI: Audio taper is a log scale as that's how the decibel system scales power. Also their full range is about 300 degrees of rotation.

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Woo-hoo!

Congratulations on completing the project! Sorry to have steered you to the manufacturer's INCORRECT wiring instructions.

tony b.

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what does it mean?

hey ppl! i have one question that bothers me for a long time. hm well i have problems with pot in my guitar wah pedal (ibanez wh10). actually i replaced the pot cuz the last one was scratchy and i bought the new 50k. looking for it for a long long time and once when i found it and install it it sounds weird. well, it sounds like there is no much of wah effect if you know what i mean. and on the old one it was written: 500kohms + 50 kohms a . so its a linear dual pot. but i dont know if its 500kohms or 50 kohms? so what does it mean? please help. thanks

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good pot

Well first of all you should always replace parts with the exact same vlaue if possible. If the old pot said 500K + 50K, it might mean the pot had a 50K resistor on the wiper to add 50K to whatever value you had it set at. The best way to find out is to hook up an ohm meter and measure the center connector to one of the edge connectors at both end of rotation. if it measures 500K and 0, then it's an ordinary 500K pot. If it measures 550K and 50K then use a ordinary 500K pot and put a 50K inline without output. A thrid possibility is measureing 500K and 50K, in which case you'll never find a replacement part, but you can do the same as option 2, a pot with an external resistor, and you'll lose a few degrees of volume control, but will likely work just fine.

Don't use radio shack pots. They are complete crap. If the audio signal actually passes through your pot, I'd suggest getting an ALPS part. If the pot controls a variable gain amplifier then you can use a much lesser part. Given you said the old part was scratchy, the audio signal probably was going through the pot, and you want to use the highest quality pots you can find. You can buy high quality parts from Erno Borbely at:

http://www.borbelyaudio.com/

I've personally done business with Erno and he's a great guy. He does ship to the USA, but there's a $50 minimum order for international orders.

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I still need help

Hi Dr. Webster,

I followed your postings and have tried every trick you described. I have experienced the same problems you have experienced and guess what? The rat shack pot I am wiring still does the same behavior that you originally posted. I looked at the diagram at http://www.bcae1.com/potentio.htm and am still experiencing the same behavior. Was the page you referenced entitled "The diagram below shows how to wire a volume control to be inserted into the preamp line. If noise is a problem when you touch the metal shaft, ground the metal part of the pot. A potentiometer specifically designed for use as a volume control (a pot with a 'logarithmic' taper) will give the most linear volume control. The Radio Shack pt# 271-1732 is a good choice."?

Thanks for your help!

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Re: I still need help

MarkJ wrote:
I looked at the diagram at http://www.bcae1.com/potentio.htm and am still experiencing the same behavior. Was the page you referenced entitled "The diagram below shows how to wire a volume control to be inserted into the preamp line....

I've long since forgotten about that evil pot, but if memory serves that section of the page was the one that fixed my problem.

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same problem

I followed the directions noted in the posts above and have the same problem. I'm using the pot between a reciever and speakers to control volume but it only changes the volume in the last 10% of turning the knob. Has anyone figured this out?
There are great instructions at http://spystyle.arcadecontrols.com/01/index10.htm
and http://www.bcae1.com/potentio.htm but it comes with the same problem. Even tried a pot I tore out of an old computer speaker -same thing.

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solution

I was having the same problem, but i figured out what it is:
your positive and negatives (to the RCAs or whatever) are reversed. Switch them back and everything should work.

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