A good way to mount "swappable" potentiometers in a pcb?  - Page 1
Forum section: electronics / projects

author: autotel
author_rank: Contributor
timestamp: August 21, 2019, 02:10:36 pm
content: As a hobby project I want to do a passive audio mixing board. I would like to make this board more eco-friednly, and one problem is that potentiometers have the tendency to become noisy with time. I decided to think of ideas how these can be changed later without wearing off the board.I'd like to find an elegant and scalable solution because I use 15 stereo potentiometers in this mixer; and who knows, maybe I do more of these to sell if it turns out to be cheap.The two ideas I thought are:To use a 5-pins modular pin header, where the tree pot legs are hostedTo solder it nevertheless, expecting that the swaps won't be too oftenThe problem with the first option, is that those pin headers also wear out and the link usually becomes loose, which is even a worse problem than noisy potentiometers. A solution would be a connector intended for permanent but serviceable connections, such as screws. I know no solutions for this that are applicable to a typical potentiometer pin.The problem with the second option, is that the potentiometers are three-legged, so they can be very difficult to de-solder: when you melted one connection, there are two other connections that are still holding it in place. I thought that maybe the board could have cutouts where the potentiomers go, and two of its legs make contact as if it was an SMD potentiometer, and the third leg doesn't make contact until you make a bridge with the solder, which would be easier to break without needing to move the potentiometer first.Apart from these ideas, what else could I consider as a solution? Have you tried any of these ideas?

author: Kasper
author_rank: Frequent Contributor
timestamp: August 21, 2019, 02:25:46 pm
content: Here are some ideas; I would go with 1 or 3b.1: Your idea of plugging trimpots into header seems fine. Make sure the PCB and the trimpots are fastened securely and the header shouldnt wear out.3: solder them in and get better at removing them.3a: Get a very large tip for your soldering iron so it can heat all 3 pins at once.3b: Cut the pins before desoldering.3c: Bend the legs and solder them smt not thru hole.4: Use terminal blocks to hold the pins.5: Make you own 'terminal blocks' aka use screws to attach the pins to the pcb.6: Make an extra PCB just to hold trimpots.Solder the trimpots to the PCB and make the PCB easy to swap. For example with header strip(s) connecting it to main PCB

author: grbk
author_rank: Contributor
timestamp: August 21, 2019, 02:40:59 pm
content: If it were me, I would solder wires to the pot legs and use crimped wire-to-board connectors on the ends of the wires. Heatshrink for strain relief on the wire-to-pot soldered connection.The downside is that you have to do the crimping, which is a hassle if you don't have a crimping tool suitable for the connector system. For many (most?) connector systems you can get pre-crimped wires, however.The upside is that if the crimps are properly done it will be very reliable, with no loosening over time like you'd get with pot legs inserted directly into a header. It's also easy to assemble if you have the crimping tool already and very easy to swap out pots if you use a decent connector system.If you don't want to use crimped connectors, you could still solder wires onto the pot legs and insert the wires into screw terminals, which may or may not be better than putting the pot legs into screw terminals directly. At least you would get more strain relief and placement flexibility that way. You could also use IDC connectors instead of crimps.

author: james_s
author_rank: Super Contributor
timestamp: August 21, 2019, 02:55:18 pm
content: I doubt you'll be changing them often enough to wear out headers. You could also design the pads on the pcb to have plenty of meat and make the holes big enough that you can easily desolder pots without tearing up the board. Most of the problems I see with lifted traces are old single sided boards and fine pitch stuff.

author: OM222O
author_rank: Frequent Contributor
timestamp: August 21, 2019, 06:30:45 pm
content: easiest fix would be not using a pot at alluse an encoder on the front end, then use that to control a digital pot. when used in the voltage divider mode, they are very well matched and stable since the manufacturer targets matched resistances, rather than an absolute tolerance or tempco (digital pots are actually quite bad in terms of absolute tolerance and tempco).If you want to avoid using an MCU, use the pots that have an increment and direction pin, which you can drive directly from the encoder, something like this:it's easy, cheap and reliable. also allows you to simulate a multi turn pot for better accuracy for much cheaper than a multi turn pot. just keep in mind most digital pots are linear, not log. if you want a log taper, then you should pre compute a list of values or use special digital pots meant for audio applications.

author: MosherIV
author_rank: Super Contributor
timestamp: August 21, 2019, 06:45:16 pm
content: I would suggest something similar to om2220.Instead of routing the actual audio signal through the pot, use a voltage controlled amplifier and the pot controls the voltage to the VCA. This will remove the noise as the pot waers, especially if you add a filtering cap to the potI spoke to someone who works for a company making commercial mixing desks and he told me that is how they use to do it.Now everything is digital.The pots are just read as a voltage and that is used in a DSP or micro to scale the digital audio for a channel before being summed.

author: themadhippy
author_rank: Super Contributor
timestamp: August 21, 2019, 07:08:31 pm
content: Dsp,vca's, encoders and mcu's ? Dont sound much like a passive mixing desk to me.Instead of pcb mounting, point wire them. Infact if you want ole school/boutique do away with the pcbs and just use tag strip

author: MosherIV
author_rank: Super Contributor
timestamp: August 21, 2019, 08:07:08 pm
content: QuoteMaybe not.QuoteAnd this the problem with a pure passive mixer.Either continually change expensive potentiometersORFor a bit more complexity, build it once and minimise the amount of repairs.

author: David Hess
author_rank: Super Contributor
timestamp: August 21, 2019, 11:07:58 pm
content: I would just solder them and plan on cutting the leads if necessary to make desoldering and replacement easy. Select potentiometers which provide access for cutting the leads.Oversized collet sockets are available which would fit potentiometer leads. Collet pins solder directly into oversized holes in the printed circuit board. They are expensive but no more than a good potentiometer.Quote from: autotel on August 21, 2019, 02:10:36 pmMake sure to block DC bias currents which would otherwise go through the wiper. In some cases, this means using alternative circuit topologies.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 22, 2019, 04:19:18 am
content: You may want something like this:https://www.digikey.com/product-detail/en/mill-max-manufacturing-corp/0327-0-15-15-34-27-10-0/ED90081-ND/947089as mentioned here,Quote from: David Hess on August 21, 2019, 11:07:58 pmMind that you will find pots with tin plated PC pins; use tin-plated contacts as well. This particular one is gold plated, which will lead to fretting corrosion against tin.Plating works roughly like this, in order of preference, best to worst:- Gold on gold- Tin on tin- Tin on gold- ...- None (just bare copper or whatever)That is to say, dissimilar plating is still much better than nothing, but it's not the best.Connectors are notoriously unreliable, so you may just be adding another unreliable component into the mix. The sum is less reliable than just the pots alone.So you want to make sure the contacts are good, and that you give them the best chance of success.Strain relief and secure mounting are key. Every sliding or rocking motion on the connector wears away some of the plating, or allows some dust or dirt, or condensed goo, into the contact area. The gunk can be washed away with contact cleaner and a few cycles of "wiping" movement, but when the plating eventually wears out, it's over, the surface oxidizes and the contacts get bad really quickly. So, you want to make sure that takes decades to happen.If you'll be using vertical mount potentiometers, then consider mounting them on the panel directly (threaded collar, panel mount type pots), and having the pins stick into the PCB. The PCB then is mounted to the panel with standoffs (which can be PEM(R) style press-in hardware, or screws and standoffs or spacers, or sheetmetal tabs even), so that even as the panel flexes and vibrates in use, the PCB follows it, and minimal strain is applied to the pins and connectors.There are many other possibilities for layout, of course. You could make a cheap sacrificial PCB and mate it to the main board with a ribbon cable or whatever.Quote from: OM222O on August 21, 2019, 06:30:45 pmThis may be a better solution than OP realizes -- if an optical or magnetic encoder is used, it will never wear out (mechanical ones are as bad as pots!). The semiconductors involved, if protected from transients, can last basically forever.The major downside, in regards to general circuitry, is that digipots and multiplying DACs have awful bandwidth (100s kHz). More than sufficient for audio, just not so good for other things.Tim

author: autotel
author_rank: Contributor
timestamp: August 22, 2019, 07:10:32 am
content: thanks for the answers! It is funny, I was most enthusiastic about the idea of a solder bridge. I guess these are in general considered a bad practice, and why do it systematically, rightI have done a couple of passive mixers using the boutique approach; soldering wires by hand. It is a +3 hours work and really not worth doing, if I were going to do it for someone else. Then they turn out to be a bit hard to repair, because of the double gang!Cutting the leads: yes, definitively a sensible idea. Why didn't I think of that! I usually try to recover the parts wholly, but in this case it would be pointless to try.I laughed about the topic of digital potentiometers in a passive mixer. I have actually considered this possibility. My mixer will have one tiny active output in case the user wants to use it with headphones. I didn't follow thinking about that because I expected that you'd also need a microcontroller for that, which in my mind defeats the purpose. I liked that digital pot that you can couple with an encoder directly! However active, it is a bit in the spirit of the project, I feel. The bad thing about that approach is the reliance on having power in order to be able to mix. Perhaps a meaningful compromise. Besides, it might be more eco-friendly to have an indestructible potentiometer than having to change these systematically.Thanks for all the ideas!