LDR as a digital pot

[JohnRoberts]

The inherent attack/release characteristics of the original technology was accidentally responsible for the success of some simple early designs. We like to think that all popular legacy designs are the result of masterful engineering. Sometimes it was just chance.

or not...  :

JR

[edit]  not to get too invested in this, but in theory using a pair of LDRs in opposition could speed up gain recovery, but this adds another dimension to the design (impedance vs time). [/edit]

 

[ruffrecords]

Really? As far as I can tell, distortion increases continuously with level, so the "limited range" is set by how much is tolerable. I don't see any reason why it would be different in a push-pull configuration.
Similarly to a tube amp, once the push-pull arrangement has decreased 2nd-order distortion, the remaining products increase continuously. That's why NFB is necessary; unfortunately it doesn't apply to voltage-controlled resistors.

Agreed, but the FET square law produces predominately even numbered harmonics which are cancelled by push pull. The remaining odd numbered harmonics should be significantly lower.

Cheers

Ian

 

[Matador]

I had two reasons for looking in to this:  first, my beloved Triaxis is going on 30 years old, and I wanted to be able to repair it as the LDR's are the most replaced component.

Second, I've wanted to make a generic 'digitally controlled pot' board that I could sub into existing tube designs (like microphone preamps) to give either voltage control over various functions like gain knobs, volume controls, and EQ controls, or by extension, microcontroller based-controls that use voltages to control these things.  Mesa's approach seemed like a good starting point because their circuit was designed to sub-in to any existing circuit since it's fully floating.

https://lunainc.com/wp-content/uploads/2016/08/Audio-level-Control.pdf

Luna Electronics describes John's approach which is to use an op-amp to force a set voltage across an unknown LDR resistance (that is configured as a voltage divider with a known resistance, hence one can program any resistance just by setting a control voltage).  Silonex bins a sorted version of their single LDR (the NSL-32SR2S) which are matches together within 5% for only about 70c more.  So one could get a potentiometer with a center tap with a quad op-amp and 4 LDR's, or a 'rheostat' control with a dual op-amp and 2 LDR's.

These should be easily fittable on a square inch of dual-sided PCB.

 

[rackmonkey]

UPDATE 2:

Actually it looks like VTL5C4 is much slower (1.5s vs 35ms turn off time)...

VTL5C3    30kΩ @ 1mA 5Ω @ 10mA 1.5Ω @ 40mA 10MΩ 20 (typ) 75db (typ) 2.5ms 35ms
*VTL5C4    1.2kΩ @ 1mA 125Ω @ 10mA 75Ω @ 40mA 400MΩ 18.7 (typ) 72db (typ) 6.0ms 1.5sec

There is an Xvive VTL5C4/2 but not on Ebay.

FWIW, Small Bear Electronics carries the Xvive clones of the Vactec parts. They have the VTL5C4/2 in stock. Not cheap, but none of these old optos are now. $11 and change per.

I’ve used and experimented with the Xvive and Macron parts quite a bit and the Xvive stuff is indistinguishable from the corresponding Vactec original, performance wise.

http://smallbear-electronics.mybigcommerce.com/photocoupler-dual-xvive-vtl5c4-2/

 

[benb]

As far as I know, the only thing "special" about the Vactrol brand was they were selected (or maybe manufactured) to tighter specs than other optocouplers. If you know what you want and how to measure them, you can get a bunch of surplus no-name brand parts and likely find several of what you want. You then have a bunch of others for general LDR/opto use.

I got a bag or two from Goldmine when they were on sale at 50 for $10 or so. This was a few years ago, dunno if they'll still have a sale like that as no doubt the demand for surplus has gone up since Vactrol stopped making them.

Here's the usual price 99 cents each:
https://www.goldmine-elec-products.com/prodinfo.asp?number=G15396

 

[abbey road d enfer]

Agreed, but the FET square law produces predominately even numbered harmonics which are cancelled by push pull.

  That is true in a gain stage, but in an attenuator the secong harmonic is already cancelled by the symmetrical structure of the FET, that sees an equal dose of positive and negative signals. In a gain stage, the voltages are all in the same quadrant.