LDR control

[mikeyB]

Would like to try replacing pots/switches with LDR's with a view to making preset(able) unit control. Are LDR's ok on the noise front- or is the idea a non starter - particularly with the valve/high impeadance gear?

 

[Johan]

used as switches, they are probably allright..vactrol's are used in many guitaramps to handle channelswitching duties, but used as a pot is probably harder to do.
As I understand it, they have a "memmory" of what the just where..so going from one set value to another and then back again, might not result in the same actual value, even if everything else is identical. Thats ok for compressors and limmiters, where everything is dynamic anyway, but for a "pot" that is to be left in a sertain position..?..probably not..
I'm just guessing, but that is how I understand it..I'm sure others will fill in the gap's...

johan

 

[bcarso]

I would avoid.

It is true that photoconductor-based optocouplers have many benefits, such as no offset voltage and isolation of control from signal. As well the inherent dynamics of the response are often quite useful for compression/limiting applications.

But Johan is correct: they suffer from a host of non-ideal issues, including as mentioned a severe dependence on history. They are also hard to obtain in well-matched multiples.

You could use a well-matched dual with a servo loop and get better results than an open-loop arrangement, but you sacrifice one half of each dual for the servo.