Potentiometer help


I'm new here and pretty new to home audio/electronics projects so please bear with me here.

I created my first clock amp a while ago using a Minty Amp kit, an Ikea alarm clock and some other bits and bobs. Here are some images:




I failed to take progress pics so couldn't create an instructable for the project but I plan to do that with my next one (I've had a lot of interest with people actually asking to have them made to order - which Mr Minty Amps, Bob Hickman, assures me is cool).

Anyway, so I've ordered the parts to make another clock amp but decided to get some extra bits from Maplin.co.uk this time. In teh first amp I used a 250k guitar potentiometer for the volume but it's not sensitive enough and really only has three settings (full, quiet and silent). So, this time I ordered this 1k pot from Maplin.

When the pot arrived it has a 50mm shaft which is too long for my project. I assume I can just trim the shaft down so long as I'm careful since it's only plastic. Is that right?

Also, because it's not a guitar pot, the shaft is smooth and so the standard control knobs I have will not fit it. Does anyone know of a (cheap) type of control knob that will fit such a pot? I really don't want to have to glue a knob on there.

Thanks for your help.

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NachoMahma9 years ago
. It will still work, but it will react differently than what you expect (most of the adjustment will be toward one of the extremes of rotation). Won't hurt anything, but you'll want to get log (sometimes called audio) taper pots for any units you sell.
clockamps (author)  NachoMahma9 years ago
Right, I see. The funny thing is, what you've described (with most of the adjustment being at one of the extremes of rotation) is pretty much exactly the problem I had with my last pot. Would a guitar pot be linear and not logarithmic then? (excuse my ignorance, like I said I'm pretty new to all this). Thanks again for all your help.
. Not sure; I'm an acoustic kind of guy. You can check it with an ohmmeter. If at 25, 50 and 75% of rotation you get 25, 50, and 75% of the resistance, it's linear. If it's not linear, it's probably log.
clockamps (author)  NachoMahma9 years ago
Ah right, I now understand the difference between log and lin pots (thanks to your good self andthis page). I think the pot i used previously IS log but it's 250k so maybe that's the reason for its unresponsiveness.

gmoon clockamps9 years ago
That's a good page. Yep, most audio pots are log (also called audio taper, as NachoM wrote) since loudness is a log function. So guitar and amplifier pots are too, generally.

How do you have the pot wired? 250K volume pots are common in guitar (as are 500K) so that's not the issue.

Generally, a volume control is a "voltage divider." You'd need one side of the pot wired to the incoming signal, the other side wired to ground, and the pot "wiper" is then fed to the amp input. Just placing varying resistance "in line" with the signal doesn't really work.

The total resistance of the pot would have an effect on the "input impedance" of the amp, but 250K is usable (if not ideal.)
clockamps (author)  gmoon9 years ago
I wired the pot up according to the Minty Amp instructions ( PDF here).

Bob Hickman at Minty Amps did tell me in an email that "The volume pot is one of the trickiest things to work out. Basically, there's no perfect solution (which is why I don't carry them in the store)". He also suggested that I might get better results by adding resistors in parallel with the pot.

For now though, I think I'm just going to see how this new pot works out.
gmoon clockamps9 years ago
Sorry, guys, I didn't know anything about the amp internals, 'till I saw the PDF. Volume control on the output is more unusual, and is usually termed an "attenuator."

A large resistance value POT won't work well here--you're attenuating the "current side" of the amp, which driving only an 8 ohm load. Once you exceed a relatively small amount of ohms, there's not enough current left to drive the speaker. And a 386 driven to the max might put out enough power to damage a normal pot, too (if it were adjusted down to 1 or 2 ohms.)

Another 386 amp--see the MAKE cracker box amp, which uses a 25ohm rheostat as an attenuator. Note the low resistance value. Accordingly, rheostats are designed to handle more current than your typical pot.

The reason for attenuating the power side of the amp? Same as an attenuator on a full-size amp--push the amp hard into distortion all the time. But attenuating after both the preamp and power amp stages lets you play at reasonable volumes, and keep the overdrive tone (allegedly--not everyone agrees.)
. Looking at page 6 of the PDF clockamps links to, the pot is wired as a voltage divider, but it is on the output of the amp (legs to amp output and gnd, wiper to speaker). I too have been assuming the pot was on the input or feedback.
You could turn the 250K pot to just beyond its useful range, and then measure its resistance using a multimeter (you may need to cut/desolder one end, to make sure you only measure the pot, and not the rest of the circuit it's attached to). That'll give you a better idea of what value pot you should replace it with. Chances are, if you jump straight from a 250K pot to a 1K pot it won't have the range you'd like...
clockamps (author)  Patrik9 years ago
Patrik, I don't actually have a multimeter at the moment (that's how much of a noob to all this I am) and I'd be reluctant to take the pot apart in any case, just because I'm not all that confident with what I'm doing right now. I have just today purchased a new 4K logarithmic potentiometer (with a standard sized, grooved metal shaft) that I hope will work well but we'll have to wait and see. The original 250K pot did it's job but I just wanted a bit more sensitivity and range. I'll let you guys know how it works out (all I need to do now is take a trip to Ikea for another clock...)
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