Hi Morten
I would expect for a 3 input design that there be 6 series LDR's and 2 shunts, otherwise we are back to compromises with switches, electronic or relays, which correct me if I am wrong why LDR's are used in the first place, ideally wired directly to the RCA socket.
It appears impossible to have anything less than 8 LDR devices if you want purity of signal path with 3 inputs
Perhaps some discussion or answer on this could be enlightening.
Cheers thingfish
I will sidestep the topic of "purity" since wading into that territory all too often becomes indistinguishable from a discussion about religion or politics.
As to using LDRs as analog switches in lieu of mechanical switches, I'm a big fan!
When LDRs are "off" they are well north of 1M of resistance - off enough! When they are full "on" they can be in the range of 40-60R which is de minimis relative to the downstream attenuator impedance. Again, on enough!
Thus, LDRs can indeed be used for selecting from multiple audio inputs. The argument for doing so is the avoidance of yet another mechanical interface point (the relay) in the signal path, gold plated contacts etc. notwithstanding. In fact, we use LDRs precisely for this purpose in our 6-input LDR6 Passive Preamp component. Including the LDR input "switch", we end up with 2 LDRs in series (1 fixed, 1 variable) and 1 in shunt (variable) per channel - or 6 total per stereo input.
Customers building their own preamps using our LDR3x DIY Passive Preamp Controller Board are left to their own devices as to how to implement input device selection. The LDR3x provides current sink switching to ground for up to 3 relays.....or LDR circuits. While we don't currently offer an input switching board, we are considering offering one in the future and it would indeed use LDRs in lieu of mechanical relays.
I would offer one caution in using LDRs as signal switches. For purposes of longevity, I don't recommend running them hard up against their LED current limit (20-25 ma in the case of the Advanced Photonix units we use) even though this yields the lowest "on" resistance. Better to back them off somewhat despite the slight rise in resistance. Still, commercial LEDs have a nominal lifetime expectancy of 50,000 hours. If you listened to music 5 hours a day, that's 10,000 days or 27 years....long enough I'd say!
As to why LDRs are used at all, my emphatic answer is because they are sonically superior in the signal path, i.e. they sound better! Better than pots, better than resistors, and by accounts from a growing number of our customers - better than transformer based volume controls.
Cheers!
Morten
