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[John Roberts {JR}]

ssltech wrote on Wed, 30 June 2010 15:43

You could try 'steering' the law slightly, since it's a simple potential divider application... Resistor from wiper to top of track will bend the center 'upwards', resistor from wiper to bottom will bend the law 'downwards'. You could just try it and see whether getting unity to match the top panel makes the rest of the law better or worse... or more or less to your preference, whichever is applicable. Keith

I already suggested that,

He already tried it and found it wanting... (too slow below 0dB and poor kill at full off).

Sounds like time for some different faders, or relabeling the law of those.

JR

PS: Do people really look at the numbers on the fader plates? I never knew.  

[ssltech]

Ah, I forgot that, thanks.

-Keith

[Nid]

Talked with TKD Japan and they informed me that their faders use "ladder circuit" principle. I guess this has something to do with the failure of the parallel/passive resistor mod, no?

[ssltech]

Well, it has an influence on HOW the law is bent. It's done for law shaping and is also used for trimming on laser-trimmed variants.

It tend to 'anchor' certain spots more strongly than others; (think of the 'rungs' in the ladder versus the 'gaps' between them).

[Geoff Emerick de Fake]

Nid wrote on Fri, 02 July 2010 07:16

Talked with TKD Japan and they informed me that their faders use "ladder circuit" principle. I guess this has something to do with the failure of the parallel/passive resistor mod, no?

The output impedance of this type of fader is variable in an unpredictable way. As a result, none of the proposed methods will work satisfactorily. Still, the strange behaviour is unexplainable to me.

[John Roberts {JR}]

Nid wrote on Fri, 02 July 2010 07:16

Talked with TKD Japan and they informed me that their faders use "ladder circuit" principle. I guess this has something to do with the failure of the parallel/passive resistor mod, no?

It shouldn't. You can answer your own question easily enough by measuring the resistance from each end to wiper with an ohm meter for different amounts of wiper travel.

This surely seems to be more work than it is worth... but I remain curious about why it appears so difficult to bend the curve.

I am no longer in the trenches but I hear from folks who are, that Panasonic or whomever was the popular source of value faders is getting out of the business, so a replacement for a lot of mixers/consoles, needs to be found.

Good luck

JR

[Nid]

http://www.tkd-corp.com/02_products/pdf/p01_cpa-7101.pdf

If you look on the datasheet there is the CPA-7201 version that is 104.2mm and the taper looks like what we are after. But can a 104.2mm fader be used in a 100mm slot?

update: the slot seems to have 2mm extra space at top and bottom so I guess a 104.2mm fader will fit perfectly... We'll probably get one for testing and see how it goes.

[Nid]

OK, we got one 104.2mm fader as a sample and it fits exactly in the slot. Also the taper is very close to the old Alps taper. The Unity point is just slightly higher than the original. We could now try the resistor mod to "bring it down" a bit but I don't think it's worth it. Perhaps it's more harm than good in this case.

The 00 and +10dB markings (lowest, highest) are a bit off now because of the longer travel of the 104.2mm fader (because of the additional 2.1mm at each end) which means the knob's center reaches just a little beyond the markings but that's not really an issue.

As for the quality, it's superb! Super smooth, super quiet.

So I guess this solution ends our troubles. Hope this thread becomes helpful to other people thinking of replacing their faders. Any questions are welcome.

Thanks to everyone here! Also a big thanks goes to Mr. Franz Kobler (www.fk-industrie.de), the German distributor of TKD, for his supportive stance.