Hello, I'm liking everything in your LDR implementation but I would like to see the distortion figures at various input voltages (1V, 2V or 2,5V and 5V).
Thanks and happy new year!
The best info I have on the applicaton of LDRs along with their distortion characteristics was published by Silonex several years ago. I've included a link at the end of this post to a PDF version of this info. Silonex was the leading manufacturer of audio LDRs and have since gone through multiple cycles of acquisition by other entities. As far as I know this info is no longer published by the current manufacturer although the product really hasn't changed.
I refer you to Figure 13 in the file which is most closely representative of the series/shunt configuration used in our LDR based preamps. Figure 13 illustrates how LDR distortion depends greatly on the amount of attenuation. At low attenuation levels (loud music) the distortion is quite low and then climbs as attenuation increases (music gets quieter). While not explicitly shown in this info, increasing the voltage drop across the LDRs does increase their distortion all other things being equal. I've corroborated this behavior through some testing several years ago. In practical applications I found LDR distortion to typically be a few tenths of one percent or less.
You can find endless debate around the topic of audio distortion. If you believe distortion to be the ultimate measure of audio quality then you may well conclude you can do better than using LDRs. Most pots and stepped attenuators have better distortion specs. You can also find very credible information online that shows distortion levels of 1% (THD) or less are effectively inaudible. Simply put, you can't hear the level of distortion at which LDRs perform. If you spend some time listenting to LDR based preamps/attenuators you quickly come to this conclusion.
The best comment on the topic of audio distortion comes from none other than Nelson Pass. I've included it below. “The ear is not a microphone, the brain is not a tape recorder, and measurements are limited in describing subjective quality. I like to have low distortion and so on, but these things take a back seat to what I experience when I listen. There are plenty of products which have great specs – I will not be offended if you buy those.” – Nelson Passhttps://www.tortugaaudio.com/documents/docs/Silonex%20Optocouplers%20Application%20Notes.pdf
Happy New Year!