Mine goes up to eleven
January 2, 2007 5:02 PM   Subscribe

Do-it-yourself electronics: Building a volume knob.

Inspired by this, I'm trying to make a volume knob with female 1/8" input and output jacks.

I think I understand things for the most part, but my jacks have three prongs and I don't know which to connect to my potentiometer.

If I did this with no further advice, I would connect the centermost pole of one jack to prong 1 of the pot, and I would connect prong 2 of the pot to the centermost pole of the other jack.

I assume there also need to be grounds connected to various places, but I'm not sure where exactly, and I can't seem to find an online tutorial that really helps me.

posted by etc. to Technology (17 answers total) 2 users marked this as a favorite
I think your 100k pot might have too great of a range for this. like 1k ohms might be enough to turn the volume down to zero. You may need to switch to something with a much lower maximum.

Not sure offhand about the specific wiring; I'm sure someone else will have an idea though.

You could also just build a CMoy Pocket Amp. More features.
posted by autojack at 5:09 PM on January 2, 2007

You have the wrong kind of pot. That one will only handle mono (1 channel). You need a stereo potentiometer.
posted by mr_roboto at 5:14 PM on January 2, 2007

hi etc...

I agree with autojack... the pot may be too large. Audio taper is good. 1K is good, too. Generally, lower value pots = lower contact noise. I also agree you'll need two pots or a dual pot.

You want to wire it in a voltage divider manner.

That means that the outer leads of the pot get the two wires from the input jack, and the center lead and either one of the outer leads gets the other jack.

That should make it 'work', though you might not like how it does.

Feel free to email and good luck.
posted by FauxScot at 5:28 PM on January 2, 2007

Hold on; now that I'm looking at your jacks, those look mono, too (2 conductor). Is this for a stereo signal, or are you just doing mono (a guitar or something)?

If you want a mono connection, one of those terminals on your jacks is a shunt terminal; you can avoid wiring it completely. You just need to figure out which terminal connects with the tip of your plug and which one connects with the sleeve.
posted by mr_roboto at 5:31 PM on January 2, 2007

The normal setup is like this:

The first pin connects to the one part of the plug, the second pin is connected to the other part of the plug, and the third is connected to the second pin when no plug is present, and connects to nothing when a plug is present. In a cheap radio it would be connected to the speaker, so it disconnects it when you plug in headphones. You can ignore it, and just use pins 1 and 2.

Which pin is which you need to work out for yourself.
posted by cillit bang at 5:54 PM on January 2, 2007

A minature dual ganged pot like this might be what you want, although even a 1K pot is going to seriously cramp any load that draws appreciable current. In other words, if you just need to create a voltage divider network for use in padding the inputs of powered speakers, a 50K pot might be fine, but if you want to plug in headphones, you need a much closer impedance match in your divider to the headphones' nominal impedance rating, in order to deliver enough current to the headphones, in order to operate their drivers.

But the kind of minature pot I've linked contains two seperate potentiometers in the same physical package, mechanically operated ("ganged") to the same shaft. You take a ground lead from the barrell connector of your input jack, to one end of each pot, take another common lead from the barrel of your output jack to your common, create the voltage divider for each channel by taking the input lead from each channel to the opposite end of each pot to that you've nailed as common, and then connect the barrel of each channel of your output jack to the divider (wiper) contact of each of your pots. Voila! variable output signal level.

Or, for a few bucks, or maybe more, buy a headphone amp.
posted by paulsc at 6:11 PM on January 2, 2007

The big question is a volume control for what? A proper circuit matches the impedance of the pot with that of the signal. Typically headphones are around 8Ω while line signal inputs can be near 1 MegΩ.

As a voltage divider you want the total pot impedance to be near the highest i/o impedance, though you might be able to use a log taper 100K pot with 8Ω headphones and get maybe 5° worth of useful volume control.
posted by MonkeySaltedNuts at 6:15 PM on January 2, 2007

Response by poster: Heck, I was just gonna screw around and see if I got lucky, but here's the specifics for those of you who clearly know this shit:

I have a ukulele with a passive piezo pickup and a Smokey amp. Neither has a volume control, so I wanted to insert a volume control between them.

I just bought a bunch of crap at Radio Shack, but I'm getting the impression it isn't necessarily the right crap. A passive pickup obviously doesn't put out much signal.

I also bought this volume control, but it has SPST which as near as I can tell means it has three extra prongs sticking out of the bottom and I don't know what they do. But it's 10K (I think). Is that better?

I'd be happy enough if it works at all. I was working under the assumption this would be a rough draft, but any help you folks offer is much appreciated.

PS: Even though it's for an electric uke, I decided to go with 1/8" jacks for whatever reason. That's not the problem.
posted by etc. at 7:40 PM on January 2, 2007

Best answer: Piezo pickups have a really high output impedance, so your original 100K audio-taper pot is probably the right one to use.

The pot consists of a part-circular carbon track, and a sliding contact called the wiper. The two outer lugs on the pot connect to the ends of the track, and the centre one connects to the wiper.

When you turn the pot all the way down, the wiper will stop at one end of the track; when it Goes To Eleven, the wiper will be at the other end. You should be able to figure out which end is Down and which end is Eleven by looking at the pot and working out where the wiper has to go.

Your panel sockets have three connectors. Two of these will connect to the tip and sleeve of the 3.5mm audio plug. If your sockets are installed on a metal panel, you'll probably find that the sleeve connector also ends up connected to the panel.

The third connection will be a switch contact that connects to the tip connector if no plug is in the socket, or to nothing if a plug is inserted. You should be able to figure out which is which by close inspection.

On the input socket (from the pickup), wire the tip connection to the Eleven end of the pot.

Wire the tip connection on the output socket to the wiper connection of the pot.

Wire the sleeve connections on both sockets to the Down end of the pot.

That's enough to get you a working volume control.

If you want, as a nicety, wire the input socket's switch connection to its sleeve connection. This will cause the input socket to short-circuit the input when the pickup isn't plugged in, which should stop the input of your amp picking up hum and noise.
posted by flabdablet at 9:20 PM on January 2, 2007

None of this will make your volume any louder, by the way; it can only make it softer. If you want your volume control to go to Eleven, then your pickup will need to be capable of driving your amp to Twelve if you connect it directly without the volume control in between.

If your pickup's not currently putting out enough signal to make your Smokey smoke, you'll need some kind of pre-amplifier to go between the two, which will likely have its own volume control.
posted by flabdablet at 9:27 PM on January 2, 2007

Response by poster: Flabdablet, the uke and the Smokey setup with no volume control is scaring my cat right now, so the point of this is that I don't want to ALWAYS be playing at eleven, so this is perfect.

Thanks to everyone who helped. I think I'll give it a shot.
posted by etc. at 4:33 AM on January 3, 2007

It doesn't sound like you've got much electronics construction experience, so here's Soldering 101 - please ignore if redundant.

Fit your wire to the connector and crimp it on with a pair of fine point pliers so that the joint will stay assembled without you needing to hold the wire in place yourself while you solder it.

Use a fine-gauge multi-core electronics solder.

Wait for the iron to come to full operating temperature before you start, then wipe the tip on a damp sponge or cloth to get rid of the oxide film and expose the shiny solder that plates the tip.

Touch the end of the solder briefly to the tip of the iron, to melt a tiny drop of solder that will act as a heat transfer cushion. Press the tip of the iron firmly onto the joint so that it's touching both the connector and the wire at the same time, and feed the end of the solder into the joint, not to the iron. You want the flux that forms the solder's core to wick into the joint where it can do some good, not smoke off uselessly on the iron.

When the joint has got hot enough to melt the solder and suck in enough of it to wet both the connector and the wire without making a great big blob, withdraw the solder, then the iron. Don't touch or jiggle the joint while the solder solidifies; moving solidifying solder will stuff the joint up. A good solder joint looks shiny, not dull and grey.

Hope this works for you. The idea of a ukelele at eleven is scaring me at least as much as it scares your cat :-)
posted by flabdablet at 7:23 AM on January 3, 2007

Oh, yeah, one last thing: because this is a high-impedance circuit operating at low signal levels, it's going to be prone to hum and noise pickup unless you mount the whole thing in a decent metal box to keep the innards shielded (a tobacco tin would be a nice match for the Smokey). The mounting nuts for the 3.5mm sockets should make good electrical contact with the metal they're mounted in; scrape off paint if necessary to achieve this.
posted by flabdablet at 7:28 AM on January 3, 2007

Response by poster: Thanks again. And wish me luck. I'll post back here when I'm done (good or bad).
posted by etc. at 1:29 PM on January 3, 2007

Response by poster: Followed all of flabdablet's instructions in this thread and my little project worked perfectly. I've got exactly what I wanted.

Thanks to all.

Now I just need to decide what my next project is going to be...
posted by etc. at 10:43 PM on January 6, 2007

Glad you had success!

As for what's next: it's perfectly clear to me that a ukelele needs wah.
posted by flabdablet at 2:21 AM on January 7, 2007

#flabdablet: As for what's next: it's perfectly clear to me that a ukelele needs wah.

Actually building a 'Wah' is rather hard mainly because it really needs a precision pedal that turns a pot rather than just twisting some pot shaft.

As a next project, I recommend building a fuzz box. Years ago I built one that was basically one transistor, a pot, and a battery. I may have included a resistor (for impedance matching) and a capacitor (to decouple DC) but it was damn simple and worked to overdrive and distort the input signal.

A general Uke on fuzz might sound bizarre, however if the player realizes that in fuzz only solos and "power chords" (root + fifth) sound good then it is possible to make listenable music.
posted by MonkeySaltedNuts at 7:41 PM on January 7, 2007

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