Phwoar, volume knobs. Arguably, the best kind of knob. And certainly the most enjoyable part of any decent audio gear - you can tell a lot about an amplifier or DAC by the texture and rotation quality of it’s primary enloudener.
But there’s no reason why a big, awesome, solid metal volume knob should remain the sole domain of expensive audio gear - I’ve got a design for an external control that’s dead simple, cheap to build and looks good. It’s also entirely passive, so it won't make your music any louder or sound any better. So... why would you want one?
For the audio geeks:
Frustrated with the stepped volume levels on your iPhone, computer or music player? Just leave that sucker on maximum and let your external control do all the heavy lifting, with a precision potentiometer of your choosing for smooth, gradual changes in volume.
Alternatively, use both - set the device to a volume you’d consider your maximum comfortable level, and then use the external pot to attenuate it from there.
If your desktop audio device is hard to reach and doesn’t have a remote, there’s nothing cooler (and handier) than a bigass volume knob within easy reach.
For the electronics geeks:
Having a robust, clean, external potentiometer is useful when you want to pre-adjust any sort of signal, or want greater measurement control than your scope or equipment can provide. Choose any input and output connectors you like - audio jacks, banana plugs, hookup wires for a breadboard, BNCs for a scope... the possibilities are endless. I built mine specifically to sit between my scope and amplifiers under test.
Step 1: Design
For my design, I chose BNCs as the main input/output connectors but added a couple of 3.5mm stereo jacks for the sake of convenience.
The BNCs make piping the signal to various pieces of equipment dead-easy. Although only single channel, BNC "T" adapters, BNC-to-RCA plugs and plenty of coaxial cable can be used to split the signal and route it to any combination of scopes, analysers, dummy loads or amplifiers.
Keeping the unit switchable between the input/output methods will also let me have a quick "hardware" method of flipping between (say) playing the signal back through some speakers and running it to an oscilloscope.
Your design might be more or less complex, as your needs require:
1. If all you need is a volume control for listening to music, just use a pair of 3.5mm jacks and get rid of the switch. Music In -> Volume Pot -> Music Out.
2. If you're only interested in using the device as a bridge between a signal and measuring/test equipment, consider using two sets of BNC connectors on the input and output sides, allowing you to adjust two sources simultaneously. Or use a single set of BNCs and swap the dual gang ("stereo") potentiometer for a single gang to keep it simple.
3. Banana jacks are also extremely useful, particularly the kind that can accept both banana plugs and can be unscrewed to receive bare wire.
Step 2: Parts!
Here are the parts I used for my build:
1 x Aluminium enclosure (Hammond 1590LB)
1 x Dual gang audio-taper 10KΩpotentiometer (ALPS RK09 series - RK09712200MC)
1 x CHUNKY METAL VOLUME KNOB
1 x DPDT switch
2 x 3.5mm stereo jack - panel mount
2 x Female BNC connectors - panel mount
...and of course hookup wire, audio wire, soldering tools and workshop tools!
Selecting a potentiometer:
The potentiometer is obviously the heart of this little project, so it's important to choose a high quality pot that will serve your needs. There are three considerations to make:
1. The resistance
Potentiometers are just variable resistors, and as you'd expect they have different resistance ratings. You want something with a high resistance so you can reduce the level of the signal enough to be useful, but not so high that the slightest turn of the knob crushes it completely. For audio, stick to ratings of 10kΩ, 20kΩ or 50kΩ... perhaps up to 100kΩ if you expect a very loud incoming signal.
2. The taper
When you turn the knob, some potentiometers adjust their resistance linearly - that is, each degree of rotation corresponds to the same amount of increase/decrease in resistance. These are referred to as linear taper pots.
Another common type is the logarithmic pot, where the resistance change is less at the start and end of the knob's rotational area than in the middle. Manufacturers state which kind of taper is used on the relevant datasheet.
Interestingly, human hearing is logarithmic - meaning for volume pots, logarithmic is the way to go. If you're using the external control for non-audio related signals, linear would offer more overall gradual control.
More reading: this question on Stackexchange.
3. Unfortunately, the brand :(
A high quality pot will be housed in a rigid metal case, have a shaft that requires a decent amount of force to rotate (no flapping around in the breeze) and - most importantly - produces a clean, smooth variance in resistance free of noise and audible "scratching". When in doubt, stick to major brands, and try buy legitimate parts from Digikey/Mouser/RS etc. I'm a huge fan of Japanese-made ALPS pots, who make a range of potentiometers specifically with audio applications in mind.
Step 3: Prep the Enclosure
Before getting your hands on your knob, you'd better whip out your tools! Measure the diameters of the holes you need in the enclosure for the various jacks and components you'll be using. Then mark out the holes and punch them before drilling. (I couldn't find my centre punch, so just used a large sharp screw and a hammer.)
Ideally drill the holes using a drill-press, but a cordless drill works too if you're handy with one. Finally, use some needle files to deburr the box and sand if needed!
Step 4: Dry Fit!
Before proceeding, double check not only that your components fit the enclosure, but that there's still enough room for hookup wires and space to plunge a soldering iron.
Step 5: (Optional) Spray It!
I used a raw aluminium box, which is useful if you want to apply colours or finishes beyond what the manufacturer supplies. In my case, a high gloss black was in order. So I sanded the enclosure with 220 grit sandpaper and an orbital sander until it was dull and matte. Then my "go to" combination of spray paint: 4 light coats of combo paint-primer (with 30 minutes drying time between each coat) followed by 2 light coats of clear lacquer (allowing an hour between each coat). Then leave it to dry for around 8 hours. Nice!
Step 6: Solder It Up!
Wiring up the components was a bit tricky - not because the circuit is particularly complex, but because I was using such a small enclosure. Which meant that the parts were all extremely close together.
The best strategy I found was to assemble the circuit outside of the enclosure, and then carefully use tweezers to nudge each component in place inside the box afterwards.
The enclosure itself was used as ground - meaning each component needed contact with raw aluminium on the inside of the box. I used one of those tiny, hourglass shaped washers to tie the two ground poles of the pot together, putting them directly in contact with the metal case. From there, each of the other connectors either bridged to this point using short wires or made direct contact to the case themselves.
Total time was about an hour, with liberal use of helping hands to make the job easier.
Step 7: Assemble!
Fortunately, assembly was a breeze, mainly owing to the old adage of "measure twice, cut once, fix your mistakes, measure again, cut again, learn from your mistakes, make promises you can't keep".
I ended up swapping out the smaller grey knob for a huge black aluminium one, which matches the aesthetics of the case and makes finer adjustments a little easier.
Step 8: Admire
Step 9: Using It for Music?
Dead simple - connect your music source to your input, and feed the output to your headphones or speaker amplifier. I'd generally recommend leaving your device volume on maximum (and if you can, disabling that annoying European volume limiter if you were cursed with it) and then just using your new knob to do the full range of adjustment.
Step 10: Using It for Electronics Nerdery?
As mentioned, the possibilities here really are endless, but this comes in particularly handy when you're attenuating a signal that doesn't have a readily accessible way to adjust amplitude.
In my case, while all bench style waveform generators have quick and easy hardware access to adjust the amplitude of the signals they produce - via knobs or buttons - many USB generators and scopes don't. Such as my beloved Analog Discovery 2.
So, I output a waveform at a higher peak voltage than I need, pipe it through my "volume" controller and then on towards whatever device I need it to go to - typically amplifiers. That lets me find the sweet spot in terms of an amplifier's maximum power output before the signal starts to clip, without fiddling with little on-screen adjustment controls or drop-down boxes. I'll make an Instructable about how I test and measure audio equipment at some point, all of which revolves around the brilliant (and comparatively cheap) Analog Discovery.