how to approximate centertaps in pots with external parts?

ok so centertap pots are getting nearly impossible to find and when found the prices are prohibitively high for normal use.

Lets discuss the possibility of using some kind of external stimulus to create a center tap on a pot.

Any ideas?

I don't see how a faking should be possible at all - other than to use a even rarer dual V/W curve (classic balance pot, each half is only active at half the movement) pot connected as a monopot.

But then again - it will off course depend on what you're using the centre tap for...

Jakob E.

modifying the law of the pot from center ala $$L..

[quote author="Svart"]modifying the law of the pot from center ala $$L..[/quote]
Neve modify the law of the pot from center.... I don't remember SSL doing that... they do however use the center tap to affect the 'Q' f the filter.

Neve make the cut/boost "tug" towards the center (greater resolution at center and then gradually accelerating boost towards the edges by adding a pair of resistors of approximately half the total cut/boost pot resistance value, one from each end of the track to the wiper center.

That might be what you're thinking, and they do it also because they don't have center taps. A side-"benefit" of increasing the resolution st the center of the travel is that any mechanical inaccuracy of the pot becomes less significant, without having to resort to a center tap.

Keith

I was looking mainly at the 4k EQ, I don't have the schemo on hand at work though. The lin boost/cut pots have equal Rs connected from CT to the other posts. it looked like they were attempting to modify the actual shape of the law. I could be and probably am wrong here.

Any snippets of the Neve setup? I don't even know where to begin to look for that.

Unless they conect to the wiper, they won't have the slightest affect on the law. You could put ten-ohm resistors in parallel with the pot, and no affect on the law.

SSL 82E02 EQ schematic:
http://www.tenmilecreek.net/images/SSL/ssl_82E02.pdf

SSL 82E242 EQ schematic:
http://www.tenmilecreek.net/images/SSL/ssl_82E242.pdf

Keith

Don't forget the RG Keen article about modifing pot response. Doesn't address the CT issue tho...

http://www.geofex.com/Article_Folders/potsecrets/potscret.htm

[quote author="mediatechnology"]I'm assuming linear pots here.

So would resistors of equal value from the wiper to each end not "bow" it to the middle? The ratios of 1/2 Rpot to Rbow would need playing with. Not an ideal solution but would seem to make the "middle" wider.

I think SSL on the 4K/6K Pan Pots used trimmer resistors to tweak middle.[/quote]
Yes essentially, though the 'trimming' resistors in the pans are -I'm almost certain- designed to achieve the center pan law of -4.5dB or so. (a compromise between the -3dB 'equal power in stereo' at center and the -6dB "equal level in mono". -DDL -like many other console manufacturers opted for a compromise.

Amek used to have a switchable -3dB/-6dB pan law switch, but SSL went with a compromise. -If you listen to a left-right pan sweep in stereo, there's an apparent 1.5dB dip in the middle. Switch to mono and there's an apparent 1.5dB gain in the middle. The resistors tugged the center position of linear pan pots to -4.5dB instead of the -6dB that you'd usually expect.

-That's how i remember it, at least! :wink:

Keith

You could put ten-ohm resistors in parallel with the pot, and no affect on the law.

Click to expand...

Damn, you are quite right.. I don't know why I was thinking that. :shock:

So how do we get around needing the center tap on something such as the 4k EQ?

heya Wayne, those are the schematics I am talking about..

If you look at www.omeg.com you should find that they do centre tapped pots to special order. I seem to remember noticing this the last time I ordered some pots from them

Thanks Rob but Omeg stopped doing centertapped pots last year. I called them last year and they are not going to do them at all anymore.

Why can;t you use the KM pots - used as balance pots
eg - this alps one here
http://cgi.ebay.co.uk/ALPS-RK27112-Das-Blaue-Audio-Potentiometer-upgr-T-amp_W0QQitemZ260036394792QQihZ016QQcategoryZ12078QQcmdZViewItem

10K MN - Alps for EUR14
"MN taper (" balance control") Special taper developed for home
stereo. Consists of two sections (one for each channel) operating
opposite each other. Exactly one-half of each section is a zero
resistance surface (i.e., solid-copper or equivalent), the next 50% of
travel is linear taper. Therefore for one channel rotating the slider
through the first 50% of travel does not change the level at all,
while the other channel is reduced from full to zero, and vice-versa,
with the middle position (usually featuring a center-detent) always
passing full signal to each channel."

On synth-diy there's a group order for center tapped Alpha pots going on.

Look here:
http://www.sdiy.org/xyzzy/parts/tappedpot.html

And here's the most recent message about this from the list:


To: [email protected],
From: Tom Arnold [email protected],
cc:
Subject: [sdiy] Alpha Tapped Pot Progress Report
Date: Thu, 7 Sep 2006 01:57:36 -0700


--------------------------------------------------------------------------------
Samples arrived. I've taken pictures :
http://www.sdiy.org/xyzzy/parts/tappedpot.htmlBR>
Note : The Center Detent is *not* easily removable. In previous Alpha pots
the detent worked on the back casing, in this one it is buried somewhere
inside the shaft where it wont be easily removable. The Detent is fairly
light and I think quite nice. The pots will be 80cents each but as I
mention on the webpage a month or so after the pots have arrived I'll go to
a quantity based pricing structure. Initially however 80cents each and I
hope I sell'em all so I don't sit on several hundred tapped pots forever...

If you are still interested, please contact me OFF LIST and let me know.
If you want a bunch of these and trust me to prepay that'd be much
appreciated also. Note that the lead time is quoted at 12 weeks, they were
able to get a half dozen samples here in 2 weeks so I bet the reality is
somewhere in between.

--
-----------------------------------------------------------------
- Tom Arnold - Free Synth DIY webspace http://www.sdiy.org- Free Synth DIY webspace http://www.sdiy.org

- SynthGeek -
- BBD Fanatic - "...is it a virus, a drug, or a religion?"
- echo evho wjxo - Juanita Shrugs. "What's the difference?"
--------------------

If you're interested, contact Tom, not me.

JH.

I need pots of various tapers, values and also ganged pots with centertaps.. and I spoke to Alpha some time ago about this, 1k is the minimum order.. I only need about 12..