Audio Parts - Log pots - Page 1
Forum section: electronics / repair

author: RobSyd
author_rank: Newbie
timestamp: June 22, 2020, 01:44:52 pm
content: Hi EEVBloggersI'm wondering if you can help me - I'm repairing an audio desk and need to replace a few pots that have been damaged by thoughtless handling by others.Its a 10yr old Soundcraft mixer (GB4) and well worth fixing - but I cant seem to locate the log pots for the gain/tone controls that have been damaged.The usual places - Mouser, Digikey, Element14, RS Australia have the right size pots but they are linear rather than log.The pots are either Alps or Bourns 11mm versions -10k and 20k.Any thoughts where I might pick these up?Thanks!!!

author: greenpossum
author_rank: Frequent Contributor
timestamp: June 22, 2020, 02:51:24 pm
content: A search showed that element14 has some.https://au.element14.com/w/c/passive-components/potentiometers-trimmers-accessories/rotary-potentiometers?track-taper=log-audio-&st=log%20pots

author: TimFox
author_rank: Super Contributor
timestamp: June 22, 2020, 03:06:57 pm
content: Gain controls are normally logarithmic, but tone controls are usually linear (sometimes center-tapped). Are you sure your tone controls are log?

author: fcb
author_rank: Super Contributor
timestamp: June 22, 2020, 03:16:13 pm
content: Are these per-channel or are they on a buss. Basically, can you map out the response of a known 'good' pot in that posistion.Alps are now "Alps Alpine"www.alpsalpine.com, although it is likely to be a special variant of an existing design.

author: themadhippy
author_rank: Super Contributor
timestamp: June 22, 2020, 05:07:24 pm
content: QuoteThe GB4 has 2 types of tone control,the top and bottom are a fixed frequency boost/cut , the 2 mids are sweepable frequency with boost/cut.The boost/cut circuit is just a gain control with another name,and the sweep frequency follows a sort of octave step,so the first half of the pot goes from say 100HZ to 500HZ whilst the other half will cover 500HZ to 2.5k HZ

author: Audiorepair
author_rank: Frequent Contributor
timestamp: June 22, 2020, 06:32:33 pm
content: You will probably find the channel input gain pots are actually Reverse Log.Good luck finding those!

author: james_s
author_rank: Super Contributor
timestamp: June 22, 2020, 07:12:45 pm
content: You can make a reasonable approximation of a log pot by placing a fixed resistor across a linear pot. It's not exact but neither are real log pots, in fact if you plot the resistance vs rotation of an actual log pot most of them are pretty terrible.

author: TimFox
author_rank: Super Contributor
timestamp: June 22, 2020, 08:05:40 pm
content: One good thing about loading a linear pot to achieve quasi-logarithmic behavior is that the linear tapers are more accurate, therefore if you have a large set of them they will track each other better than will cheap log pots. This is useful if you don't care too much about accurate dB values, but want them to match so that equal settings give equal results.A bad thing, however, is that the load impedance that the pot presents to its driver is not constant; this may not be fatal if you are careful about the values. It can be a problem if the pot is driven from another variable resistor.

author: SpecialK
author_rank: Regular Contributor
timestamp: June 22, 2020, 11:52:59 pm
content: Something like this?https://www.fullcompass.com/prod/026777-soundcraft-dm1105-200b-master-aux-pot

author: RobSyd
author_rank: Newbie
timestamp: June 23, 2020, 01:14:43 am
content: Thanks for all the great info and replies!!!!I was hoping (beyond reasonable hope apparently!) that there would be a direct simple part replacement - these are only pots after all -The pots are PCB mounted and the PBC mounts to the frame - so like for like is pretty important.https://www.digikey.com.au/product-detail/en/PTV111-1415A-B1203/PTV111-1415A-B1203-ND/3828844/?itemSeq=330091858I'll get a better idea when I pull the matrix section out which is where the reall issues are - a couple of wonky pots in the channels may have to be lived with!

author: GLouie
author_rank: Regular Contributor
timestamp: June 23, 2020, 04:17:05 am
content: Have you tried contacting your Soundcraft distributor? These parts can have so many permutations that is often much easier to pay 100% more and just get them from the manufacturer.That said, in the USA I've found that guitar amp parts vendors often have something I can use. A few times, I've used parts from both bad & good to get the right combination of taper, shaft configuration, etc.

author: RobSyd
author_rank: Newbie
timestamp: June 23, 2020, 04:51:44 am
content: Yeah I did try Soundcraft but their line is 'No user servicable parts' - and if they did they sell boards not components, sadly.

author: GLouie
author_rank: Regular Contributor
timestamp: June 23, 2020, 06:23:37 am
content: Not being able to buy pots from a pro audio mixer company like Soundcraft is a travesty. No user serviceable parts indeed. I wonder if their sale to Samsung has anything to do with it?I have not had cause to look for Soundcraft parts myself, but find it amazing if they won't help find pots, or sell them at a good profit. Pots should be among the first items needed for repair and maintenance. Do you have the service manual with part numbers?I'll ask my former Soundcraft dealer in the states if we also have this problem.In the states, a 3rd party AV vendor sells some Soundcraft parts, although this probably won't help you downunder:https://www.fullcompass.com/searchresults.php?search_simple=true&txtAll=soundcraft+rotary+potalthough the odds of these being the ones you need are slim, and they are not usually stocked. I would think they could order parts for me if I had the part number.I mentioned guitar amp repair shops. I have obtained similar 11mm pots at this US vendor, although in one case the rotation stops were on the opposite side and I ended up frankensteining a pot. But they had a good selection of values and tapers, after I gave up on Mouser/Digikey.http://amprepairparts.com/pots.htm#marshall11You do need details on the tapers and shaft specs. Sorry if much of this seems useless to you in Australia.

author: Fred Basset
author_rank: Regular Contributor
timestamp: June 23, 2020, 06:49:05 am
content: Gottit!!At least I think so...https://www.ebay.com/itm/Korg-Marshall-11mm-Square-Potentiometer-10K-Audio/293507218158?hash=item445664baee:g:L1YAAOSwHy5eaPwlIt is a Korg Marshall part (makes sense in "SoundCraft" equipment), but they seem to be sold as "Marshall" pots. Contact the sellers and see if they have exactly what you want - At least one of them has a reverse log taper in stock, which I never expected to see. Ask them if they will ship to Australia, or try local suppliers of electronics parts for musical things like guitars/amps, etc.

author: RobSyd
author_rank: Newbie
timestamp: June 23, 2020, 07:31:25 am
content: LEGEND!I'll give them a shot!Thanks.

author: RobSyd
author_rank: Newbie
timestamp: June 23, 2020, 07:37:17 am
content: Thought I might see if I can track down an 'authorised repairer'Here the only one listed is the importer - might be worth a try though I suspect they have a reduced motivation to repair and not just sell an upgrade.

author: HalFoster
author_rank: Frequent Contributor
timestamp: June 23, 2020, 05:03:28 pm
content: I have a lot of that type of thing - please post a picture of them if you can and any other information and I will be glad to look through what I have. By 11 mm, I take it you mean the shaft length? If they are like the blue one you posted, probably not but if they are the older style chances are I have one.Hal

author: Audiorepair
author_rank: Frequent Contributor
timestamp: June 23, 2020, 07:11:48 pm
content: Manufacturers don't have to use "off the shelf parts" any more, as it is so cheap to have them custom made in China.I once seriously considered ordering 1000 unobtanium slider potentiometers that would fit a particular vintage drum machine.Typically, the minimum order of 1000 units would cost less than 50c a piece, I was looking at £300 - £400.Even charging £10 a piece, I knew I would not cover my costs before I died, so gave up on the idea.

author: Bassman59
author_rank: Super Contributor
timestamp: June 23, 2020, 09:08:58 pm
content: Quote from: RobSyd on June 23, 2020, 04:51:44 amReally? I've found the US-based Soundcraft operation to be responsive. (Well, ok, one of their top sales reps is an old friend who's come to several of my live shows and he even arranged for me to bring a Vi1 into a small club.)

author: RobSyd
author_rank: Newbie
timestamp: June 23, 2020, 10:48:54 pm
content: Quote from: Bassman59 on June 23, 2020, 09:08:58 pmIts not what you know....its who!In Aus Soundcraft have a local distributor (https://cmi.com.au/brands/soundcraft/) so the main Harman office is Singapore - we are not a big enough market to have a real local presense and so CMIs support is not focused on old analog mixers - more warranty repairs.

author: RobSyd
author_rank: Newbie
timestamp: June 23, 2020, 10:56:59 pm
content: Heres what they look like:

author: RobSyd
author_rank: Newbie
timestamp: June 23, 2020, 11:02:27 pm
content: Quote from: Audiorepair on June 23, 2020, 07:11:48 pmBeen there - thought that! Even found an (un)suitable vendor on Aliexpress - but I think the shipping cost on these ones tips it over the balance of being a good idea (!WTF).BTW I know these are not the exact pot but man that shipping will kill you.

author: themadhippy
author_rank: Super Contributor
timestamp: June 23, 2020, 11:56:31 pm
content: QuoteThe gb4 might be old and it might be analog,but there still a current model,.Who'd spend more than an x32 for one is another question.

author: wizard69
author_rank: Super Contributor
timestamp: June 24, 2020, 04:13:55 am
content: Quote from: RobSyd on June 23, 2020, 01:14:43 amThat is your first mistake, think that pots are as common as tea kettles these days. I had to go looking for linear pots that wouldn't fall apart when somebody looked at them for motor controllers. Come to find out my favorite supplier of military spec pots decided that there wasn't a reason to make them anymore. So that put me on an adventure in searching for rugged reliable pots fortunately I found a vendor with some old stock.I honestly think high quality pots at a reasonable cost are a thing of the past. They have become a specialist item with a price to match.QuoteI wouldn't live with ti. Bad pots will drive you crazy on audio equipment. However finding replacements might be harder than you may imagine. The first thing to do is to trace down the manufacture and hope that they are in business. Then you have to hope that they have something compatible in stock. If not you have to look for a third party solution which may or may not exist.It sucks that we have to go to extremes just to find parts these days.

author: Fred Basset
author_rank: Regular Contributor
timestamp: June 24, 2020, 06:25:22 am
content: If noisy pots are troubling you, have you heard how the BBC cured the problem in the 1950s? It is still one of the most inventive bits of EE I have ever heard of and they got called "Light Pots".Basically they used germanium transistors as opto-amplifiers and used normal pots just to control the intensity of light shining on them. The bulbs were under-run so they lasted a great length of time, and the set-ups were shielded against stray light, just a little being allowed to leak out to give a visual indication that all was well inside.Another nice little bit of lateral thinking involving light was a friend of mine many years ago. He wanted my last 8m of fiber optic cable from me. Knowing he wasn't really into that sort of thing and this cable being very old and thick so was not rated for modern comms, I had to ask what he was after it for? Turns out he had just got his car licence and had already got stopped for a tail light not working. So instead of designing some complex sensor, he was just drilling a small hole and poking the end of the FOC inside. The other end he was running to the dashboard. Maybe not a perfect solution, but quick, cheap and not a lot to go wrong really compared to home brew sensors.