To find eight closely matched LDRs as needed for a balanced passive, requires measuring & sorting through many dozens of pieces. The task is much simpler with a single-ended passive. Only two matched pairs of LDRs are necessary for a two channel SE passive, and variation in matching between L and R channel LDRs is noncritical. In contrast, in a balanced configuration, even small variations in LDR matching will produce poor common mode noise rejection. George understands these devices well and has probably been wise to steer clear of balanced configurations.
For those unfamiliar with the technology, an LDR is a photosensitive variable resistor, whose resistance is controlled by an adjacent LED. The volume pot on the passive preamp sends a variable control voltage to the LEDs, which determines the light intensity applied to the variable resistors. Two LDR elements per channel in a series/shunt arrangement, provide all the functionality of an expensive ladder-type attenuator based on conventional resistors. Apart from being simpler than a conventional ladder attenuator, the LDR attenuator is better insofar as there are no switch contacts in the signal path. Moreover, the sound of LDR resistors is reportedly as transparent as the best conventional resistors. Finally, as the control pot is not in the signal path, the chosen pot may be of the cheapest variety and have no impact on sound. So it's a win-win of simplicity, economy, & performance as seldom found in audio.
