[sdiy] LDRs and distortion (was: Volume control with LDR´s)

[Michael Buchstaller]

Thank you all for your answers and tips.

In the meantime i built several test circuits to see if it works well.
In an acoustical sense, on the first try it sounded well. But then i took
the circuit back to the work bench and wanted to look with the
distortion analyzer if it really behaves well.

I realized that everything is fine (THD < 0.14 %, THD of my signal
generator is 0.12%) for high volumes (low ohmic value on the upper leg
of the voltage divider), but when i turn the volume down, the distortion
rises.
I tried several LDR03´s, LDR05´s and LDR07´s, but the behaviour was
similar. If the ohmic value of the upper LDR goes above approx.
1.5Kohms, the distortion rises dramatically.

So, my conclusion for this experiment is: the LDR approach might be
OK for switching an audio signal (lamp full on, therefore low on-resistance of
50...300 Ohms), but not for volume control.

Or does anybody know of other photo-resistors that are more useful for
audio (do not exhibit distortion when the resistance rises) ?
Have you tried the "FW..." parts that Conrad sells, for example ?



-Michael Buchstaller