potentiometer code clarification

[phishman13]

I'm trying to replace a few broken pots in my console.  They are pan pots on each of the 8 bus modules.  The code in the service manual and on the pot is 20kAC.  I get the 20k is the resistance, and it is a dual gang pot, but what does the AC part of the code mean?  A stands for linear taper correct?  But the C? 

Anybody got any ideas?

Thanks

got schematics if ya need em.

 

[Conductor]

A = Audio (?)
C = Center detent (?)

 

[phishman13]

ooooh good one on the C

it is center detent.  didn't think about that.

Here is a quick draw up of the panpot config.  Hopefully this will clarify what the taper is.  Both those pots are ganged together in one dual gang pot.  One wired backwards from the other i'd say.  Info I have found says that A used to mean audio, now it means linear.  Maybe one of you can make it easier for me to figure out.

Thanks

 

[abbey road d enfer]

That's a tricky one. Depending on origin (US, Jap, Europe) same marking can have different meaning.
In Europe, A means Lin, B log, C revLog, S log-lin-revlog composite with B5, B10, B15, B25 variations check http://www.radiohm.com/admin/pdf/internet%20potentiometre/1er%20groupe/Resistance%20Laws%20&%20Application%20Notes.pdf
In Japan, A means Log, B lin, D revlog, W log-lin-revlog composite, and yes, C indicates center detent (sometimes)
In US, A means audio (which may be different of Log? )

 

[phishman13]

It says ALPS Japan on it.

And the aux send master level pots (theres one on each card) are 20kA.  I'd assume that those controls would be audio taper, therefore I would assume that A stands for audio.  I'm gonna buy some and see what happens.

 

[abbey road d enfer]

Yes, definitely, Alps conforms to the jp standard. A is log audio.

 

[jp-apb]

Phishman,
For Alps: the default  "A" taper is 15% forward log (often referred to as audio-taper).
What that means is that when the pot is at the center of its rotation, 15% of the resistance is between Term-1 (CCW)
and the wiper (Term-2). The remaining 85% of the resistance is between the wiper and Term-3.
"C" taper is 15% reverse-log; use my above statement but reverse the usage of Term-1 and Term-3.

What you have shown in your drawing for a pan-pot would typically use an "A" taper combined with a "C" taper.
The pot is fed so that at the center of rotation, each wiper is equally down approx 15%. With bus loading, etc, that usually gets you
close enough to 3dB (approx 30% down). As the pot is rotated toward either end, one side gets louder, one side gets softer.

Other standard tapers these days:
B is linear (there are variations 1B, 2B, 4B) that usually modify the last 10% or so of the end rotation.
D is 10% forward log (RD is 10% rev-log)
If the pot has a center-detent, you may see the suffix "CC" after the value. That stands for Center-Click.

Here's a link for tapers and specs for Panasonic rotary pots; Alps would use similar terminology:
http://www.panasonic.com/industrial/components/pdf/AOK0000PE1.pdf
DigiKey carries a series of Panasonic pots.

For replacement pots for a console:
In addition to the value and taper of the pot, you also have the shaft length & type to consider as well as
the mounting method (soldered directly into the circuit board, bushing mounted to the panel, etc.)
Who makes the console, did you try contacting them directly for repair parts?

JP

 

[JohnRoberts]

Wouldn't CC be "center crick"...? ;D

JR

PS: I've actually used reverse D in mic preamps when I had to order from the menu.... Got spoiled at Peavey with custom tapers.

 

[phishman13]

For Alps: the default  "A" taper is 15% forward log.

Alright, I got one of these suckers out.  This is written on the top of the can.  134N 20KAC. Across both decks is about 18k.  From 1 to wiper on one deck about 3k, from pin 1 to wiper of 2nd deck, 15k.  This is pretty much what you described, A taper on one, C on the other.

Its a Ramsa WR-T820B.  They still sell SOME parts through Panasonic, but the website is seriously misleading.  They show you the part and the price, you can add it to your cart, but when you check out, it THEN tells you they don't stock it.  And they pretty much don't stock any important parts anymore.  Oh, and they are bloody over priced.

I checked out the Panasonics on Digikey, and I guess I'm either not looking in the right place, or just can't find any dual gang pots.  Now that I know what I need, I just need to find it.....

Any ideas?  ;D

 

[jp-apb]

Phishman:
Here's a page from DigiKey for dual 12mm pots.
They show an "AC" taper pot is available with center-click,
both horiz and vert mount available with & without bushings.
http://dkc3.digikey.com/PDF/B092/P2010.pdf

If no luck there, I may have some 10K A/C pots available.
Depending on Ramsa's circuit implementation, they may work OK,
but it's going to be the physical aspect of the pot
that will be harder to match, not the electrical spec.
Can you post a picture of the actual pot for a better evaluation?


Careful, JR. It's a different world now 
For mic pre, best I was able to do as "standard" was a 05C (5% rev-log).
But for best results, custom taper is the way to go.
Did you spec that 25K custom rev-log that PV used?

JP

 

[JohnRoberts]

Careful, JR. It's a different world now   
For mic pre, best I was able to do as "standard" was a 05C (5% rev-log).
But for best results, custom taper is the way to go.
Did you spec that 25K custom rev-log that PV used?

JP

Best I did was 10% way back when...

There were several different custom parts for that application (mic pre) in different technologies for different product families and over time there was also an evolution to newer, smaller rotaries.  I usually only got involved when there were problems with oddball parts, but the engineer (Jack A) who brought in most of the parts was the senior engineer in my mixer group. I don't think you ever worked with him, he left either before or around the time we hooked up with Crest. IIRC the 25k you are talking about had something like 3 or 4 screen overlays on the resistive element to get good adjustability at the low resistance end. The 25k was from the high bulk resistance ink, with a couple lower resistance inks used down in the hop off region.

The custom gain control tapers were not only about good adjustability at hop off end. We had to re-tweak a gain control taper in the very popular top box powered mixers (XR600 et al) when a minor circuit change between generations to improve gain pot kill and crosstalk ended up changing the gain at 12 O'clock a couple dB lower. The market was convinced that the new version had less power than the old one because standard operating procedure for those products was to just turn everything up to 12 0'clock.  No amount of proof would convince the market otherwise, so in the spirit of the customer always being right we tooled up a new custom taper that made the gain identical to the old circuit at 12 o'clock and everybody was happy.

JR

 

[phishman13]

http://www.alps.com/WebObjects/catalog.woa/E/image/Potentiometer/MetalShaft/RK16/RK16_D_511.GIF

I couldn't take a pic of the exact unit, because I JUST put it back in.  I found a workaround until I find a replacement.  The shafts are broken off which is the reason for the replacement, so I just turned it around so the shaft was pointing INTO the console, which leaves me a flat slot on the back side (now pointing to the hole in the console panel) to put a flat head screwdriver in there to turn it!  It works for now! 

Those pots at digikey don't look like they'd physically fit.  the pins are all in a line as opposed to 2 rows of 3. Do you know of anything that might fit into that footprint?

 

[jp-apb]

Phishman,
Mouser carries that series of Alps pots, but does not stock an A/C version.

Here's the ALPS spec (linked from Mouser's site):
http://www.alps.com/products/WebObjects/catalog.woa/E/PDF/Potentiometer/MetalShaft/RK16/RK16.PDF

Here's the Mouser catalog page with that ALPS pot:
http://www.mouser.com/catalog/catalogUSD/639/693.pdf

Taiwan-Alpha makes something similar (also carried by Mouser), but again, no A/C versions.
Here's the Mouser catalog page with that TA pot:
http://www.mouser.com/catalog/catalogusd/639/690.pdf

You could "slug" a dual-linear (B-taper) to make an A/C version.
With a linear (B) taper, the signal is 6dB down (1/2 the level) at the mid-rotation point (with no loading);
with the A/C taper pots, it's only down by 3dB (when designed into the pan circuit).
Adding resistors from the driven terminals to the wipers will pull-up the signal from that -6dB level.
The circuitry loading the wiper has to be figured into the equation to get the right value for the pull-up resistor.
Many consoles were built this way because a dual-linear pot is usually standard, off the shelf.
The main drawback is the extra loading put on the driving circuitry by the pull-up resistors.
Not so bad when the pot is at the mid-point, but max loading happens when the pot is at either end (panned hard left or right).

Take a look at what is in-stock at Mouser, see if there's anything that is physically close in a dual-linear version that will fit.
If you want to go that route, send me the full schematic and I could calculate the pull-up resistors for you.

JP

 

[phishman13]

Here's the schematic on 2 pages.

http://www.twin-x.com/groupdiy/displayimage.php?album=random&cat=10406&pos=-3549
http://www.twin-x.com/groupdiy/displayimage.php?album=random&cat=10406&pos=-3548

I didn't exactly find anything at mouser that would fill the void.  Didn't seem to have any 20K values.  Nor could I tell if any could be center detented.

Is there a source to have these made cheaply and not have to buy 1000000000?  They seem like they'd be super easy to make custom.  Normal parts laying around.  On mine, it seems like they just used one type of wafer for both and instead of having the resistive element facing the same way, they just flipped one, which flipped the law as well.  Handy. 

 

[cuelist]

Here's the schematic on 2 pages.

This is clearly an A (audio) and C (reverse audio) taper law dual track pot. For a panpot in this configuration it can't be anything else. If you have had pull up resistors from the wipers to the top of the pots then it would have been a dual linear pot.

 

[phishman13]

This is clearly an A (audio) and C (reverse audio) taper law dual track pot.

Yeah after further measuring, you got it!

I contacted omeg and they quoted 5.06 GBP for an order of 10.  That is worth it to me!  Has anyone dealt with them in small quantity situations like this before?  I just want to know how it went, and if this might be THE solution!

 

[abbey road d enfer]

Omeg are known for being helpful with small orders. You could say it's one of their competitive advantages.

 

[phishman13]

Awesome!  Well I might just get some detailed measurements and put in an order!  ;D

 

[jp-apb]

Phishman,
OK, just came from the Omeg site (www.omeg.co.uk)
Their P16 series seems the closest to the Alps pot you showed us before.

The mounting heights (shaft center-line) match at 12.5mm

Don't know what the Ramsa used for bushing size/length and shaft type/length, but you can measure that (in mm).

Make sure you specify the correct section (front or rear) for the A-Taper vs the C-Taper
(or else your pan pot will be 20dB down at the center instead of 3dB) 

Don't forget to ask for a center-detent in the "Special Requests" box.

However, you may have a problem with the PC footprint:
Because of how Alps built their pot, the 2 wafer sections are spaced at 5mm (based on the datasheet I have).
The closest section-spacing Omeg offers is 7.5mm.
Double-check the Alps part, I have seen that series with wider spacing than 5mm.

Good luck with the retrofit, let us know how it turns out!

JP

 

[phishman13]

Thanks for that research!  I didn't even think about the spacing between the wafers.  I will measure my boards to see what it should be. 

The bushing length is pretty much the length of the threads right? 

All that was good advice.  I will look a little deeper and find out what I need.