Lightspeed Passive

If you search the diyaudio.com forum you will come across a passive attenuator design by a guy called George Stantscheff. What makes his design unique is that the audio does not pass through any contact points in the form of a "wiper" as with a pot or a stepped attenuator. The main component in the Light-speed Attenuator are Silonex NSL-32SR2 Opto-couplers.

Sound

Sound is very surprising for a passive and not what I expected when I borrowed a DIY one off Dave H down the road from me. Most passives sound detailed but thin, weak, small and lacking dynamics. This passive is quite the opposite having a very wide sound stage, really deep bass and a natural textured sound. This is by far the best passive pre-amp I have ever heard even compared to TVC / AVC units. In a system with sensitive speakers it works amazingly well. However with difficult loads such as a power amp needing an active pre-amp with voltage gain like my Nelson Pass F5 or the F3, or using speakers under 90+ db then it is not as great. It still has the same wide sound, deep bass, detail and naturalness, but it sounds 2D with no projection. It just lacks that edge and bite of an active stage.

Other Use

This circuit can also be used to replace the pot inside any active pre-amp with good results and you can take the 5V supply off the tube heaters in some tube pre-amps as long as the heaters are not floated. Ideally the Opto-couplers needs to be matched pairs otherwise you will get balance problems as the volume is adjusted. If your power amp is a nice easy load with a high input impedance then this should work fine as a pure passive.