Potentiometers

Yo!

I am looking for some nice pots for the Shruthi-1 digital board. I bought ALPS - RK09K11310KB (1191725 from Farnell), and I am very disappointed with the poor quality loose feel. The pots that come with the Shruthi-1 kits are very nice, so I am looking for something like these.

Any suggestions which ones I should get and where I should buy them?

Cheers!
flip

Huhhh… Alps parts are supposed to be of high quality! But it’s true that the first beta tester of the Shruthi did not like them and was looking for something with a sturdier response. He liked the Panasonic parts better but they did not fit mechanically (very long shaft).

I get mine straight out of the factory in HK, by batches of 1000 :smiley: Apparently, the guys who make them sell a lot to the hi-fi component market.

Maybe you can give a try at Bourns PTV09A-4025F-B103 ; they seem to be quite similar to the Alpha Taiwan parts used for the WTPA, but might suffer from the same “light” feel.

I am all ears about alternative PCB-mounted parts.

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yes, you are right, actually its the “light” feel I am complaining about, not necessarily the build quality :wink:

I bought some pots with longer shafts once. Can’t remember the brand, but I think I got them from Farnell. I ended up simply sawing off the top 1cm or so, and they where fine. Incidentally, I’ve found it necessary in the past to file the square plastic ‘nipple’ on the top of tact-switches in order to get the caps to fit. I think I got those particular ones from RS Components, as the Farnell part was a US order, hence an extra £16 delivery, and a long wait.

a|x

I bought 652-PTV09A-4025FB103 and I find them too wobbly. I could not find the specked pots from the BOM. I guess it’s good research as I plan to build a second Shruthi. Once I build my second Shruthi, maybe I’ll try to replace these pots.

I know I’m replying to an old post, but there a lot Shruthi PCBs being sold. I sourced my own parts. I really like the Shruthi, it’s a brilliant synth and an awesome DIY project!

I can recommend the Alpha potentiometers they sell at thonk.co.uk, e.g. these: Alpha 9mm Pots – Vertical D-Shaft – Thonk – DIY Synthesizer Kits & Components

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I actually have some of those green pots except they’re the round version. I put it side by side with the other Bourns potentiometer I used and they are the same height. Those Bourn pots are extra tall, the green pot has a slightly higher mount. I’m going to use those green pots for my next build. Thanks for that tip.
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Please note that there are hundreds of pot models from various manufacturers that look exactly like this but feel VERY differently. To my experience, Alpha has the best feel.

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Agreed, not all 9mm rotary pots are created equal.

I use Alpha Taiwan when replacing poor performing pots. They have acceptable End Resistance specs, and their audio taper pots have a reasonable approximation of logarithmic taper. Behringer are some of the worst by far - just to add a data point, not to start a row.

Sadly, they stamp the vendor logo on the bottom of the component where you can’t tell unless you remove it.

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Here’s a photo of a B* pot I took apart a while ago. The blue arrows indicate areas of poor performance:

  • The resistive element terminations are rough and poorly contoured for accurate wiper positioning near the end of track. This causes poor selectivity in the end regions, and excessive stair step from zero to the selected resistance.
  • this photo shows a 1meg audio taper pot. Unlike better pots, the taper is ‘modeled’, by splicing together two linear elements. So instead of a true logarithmic taper, you get two linear element spliced together.
  • another point is at the ~60% point where the taper splice occurs. Here you will encounter missing resistive values as it jumps from one taper to the other. In one such application - Moog 911 Envelope Generator clone for System 55 - attack setting in the range of 400ms to 1sec are entirely unaddressable.
  • additionally, note any rotary damping mechanisms are missing. There is no LF damping to help stabilize last value when releasing the knob - such as tuning knobs. Instead a HF damping ‘goop’ is injected in the mechanical stop cavity (where you should find a tensioner plate for LF damping).

So, while the component is poor, the rest of the cloning was pretty good. Swapping out the poor pot for an Alpha yields as excellent performance as you can get with a 9mm. ER specs are good, taper is accurate, damping is acceptable. I replace the pots in every one of their products, it’s like night and day.

Sadly, the brand logo is under the part, so you can’t ell what they are unless you pull them. Interestingly, the B* pot is almost identical to the Alpha, except the assembly pins are positioned on opposite corners.

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Follow up on the dampener.

Here’s a shot with three vendors; Bourne (this is included for reference, I haven’t looked at it yet) Alpha, whatever Behringer uses.

  • note the Alpha shaft. In the cavity directly below the top of the package, there is a small brass-looking disc. This is the LF dampener. Note it’s missing on B’s.

  • Bourne attaches the wiper directly to the end of the shaft. And the LF dampening mechanisms seem to be related to the plastic shaft and compression enclosure.

I need a better camera for close up work

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What excellent information - thank you so much for sharing!

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When I bought 911 envelope generator. I wanted to see why it didn’t match the front panel markings. The Moog 911 ADSR is literally the only module that ever recorded the intended envelope timing directly on the front panel, instead of 1 through 10, it’s in milliseconds. Naturally, the original module, with a true audio (log) taper pot would match the front panel to within a 10% or so, depending on the tolerance of the potentiometer.

Since the B version was made for a log pot as above; but isn’t. The markings were worthless, and the only timing that matched were at extreme CC and CCW settings.

I mapped the circuit response with a resistor decade box to ensure the circuitry wasn’t the issue; and it wasn’t. The circuit behavior matched the OG version close enough. So I looked at the pot and found all of the above (and more).

  • B uses a two-piece linear resistive element to model log response. Instead of a smoothly increasing log value wrt to rotation; you get a slight linear rise until the 60% point - which is where the splice is - then you get a much more steep linear increase in values. The 60% point represents the inflection point at which a log response (10% log or so) would transition to a steeper slope. Unfortunately, the splice is an abrupt transition, and values are usually missing in this area. On my modules, the entire 400ms to 1sec attack/decay/release timing is missing.

So, in addition to poor end resistance making timing hard to hit, it’s completely missing in other areas of rotation. Not a great device.

This is where the extreme cost reduction/clean sheet design of B’s do a disservice to their customers.

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I’ve swapped out over 150 potentiometers from their equipment I’ve bought. It worked much better now. You get what you pay for, and sometimes quite a bit less…

Still, it’s not all bad. emphasized text

The Buchla 281 also has the envelope timing on the front panel, in seconds:

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Excellent! Yes, there were a few when I looked back on my statement. A Buchla should still be using larger format panel mounted pots? How does it match up to the gradations?

That is clearly a log taper control :0)

Doepfer also use larger format pots with better characteristics.

The 9mm rotary SMT pot is too darn small for 4 orders of magnitude inside.

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Awesome potentiometer info!