Passive Lightspeed Attenuator Pre-Amplifier DIY



I have been working on building a light-speed attenuator pre-amp in a nice case matching to match the chassis used in the Nelson Pass F5 and F3 power amplifier builds. This is a scratch build rather than a kit. I needed to buy the case from Italy and 4 quad matched Silonex NSL-32SR2 from the states. Matched LDR's can be difficult to get hold of and to match these yourself requires buying a large quantity and constructing a matching circuit to test them all across the working range. Quite costly and time consuming.

The circuit is fairly simple so I built it on proto-board with hard-wire links on the copper side with the shortest routing possible esp on the audio links. I used some silver wire I had on the links that carry audio signals and these are situated at the back of the unit very close to the RCA sockets. I also avoided fitting a selector switch so it is 1 pair RCA input and 1 pair RCA output for the most direct audio signal route. This way we do not lose the advantage of using the LDR's to eliminate the usual audio carrying volume pot and its contacts and spoil that by routing audio through a selector switch or relay. We do not want any volume pot or switch contacts at all in the signal path.

 

Balancing

I used the original circuit by George Stantscheff as I know this works very well from a previous build by a friend but I made a slight modification. In this build I added 2 more 1K trimmer pots in series with the 100R resistors on the shunt LDR's LED's exactly as it is wired for the series LDR's. This allowed for much finer adjustment and better balancing. I balanced all 4 LDR's with the volume control at the optimum listening position which was half way in my setup and to get the best balance across the range of typical use.

 

Sound

As I remember from borrowing a friends build sometime ago, the LDR lightspeed is very unusual and surprising for what is still a 'passive' pre-amp. It seems to do things that you would think not possible with a passive, well not with the ones I have used in the past and that includes a AVC/TVC. It has a wide rich sound-stage that you can get from a good active pre-amp, esp tubes. Most 'pot in a box' step attenuator passives are detailed but always sound small, lacking energy and scale were the LDR lightspeed has the passive detail but with the width and organics of an active. Really unexpected. It maybe does not have the fine etched cutting sound of some of the best active pre-amps but if you want a smoother rich organic sound with ultra detail this is really hard to better.

 

Lightspeed Attenuator Pre-Amplifier
Lightspeed Attenuator Pre-Amplifier
Lightspeed Attenuator Pre-Amplifier
Lightspeed Attenuator Pre-Amplifier
Lightspeed Attenuator Pre-Amplifier
Lightspeed Attenuator Pre-Amplifier
Lightspeed Attenuator Pre-Amplifier