The Lightspeed passive pre has gotten a lot of buzz in the last couple years and George was gracious enough to share his circuit with the DIY crowd. As a result there are several boards/designs available accompanied by threads on diyaudio. Since the Lightspeed refers to a finished product offered only by George I will only use that term in reference to that product.
I chose the board now known as the Optical Volume Control offered by Oliver at diyaudio. Tiny board, only needs a few parts. In fact I couldn't help but feel a little skeptical about the whole thing because there are so few parts needed. I got the board itself done in less than 30min. before heading to work and let's just say I'm not sharpest knife in the drawer at that time of the day. So
anyone can build this thing.
What is it? It's a passive that functions as a volume pot but without the need of physical contacts of any kind as is unavoidable with other passives. The premise being that all such pots compromise the sound quality to some degree. In place of physical contacts this design uses optocouplers which contain a combination of LED and a Light Dependent Resistor or LDR. Eyes glazing? Yeah, me too. Cause that's where my knowledge ends. From the diyaudioprojects.com article:
...in the case of the Lightspeed Attenuator, we change the volume (resistance) by changing the intensity of the LED. By using a series and shunt combination of optocouplers the attenuator can be configured to produces a constant input and output impedance, regardless of where the voltage control setting is.So. Yeah. Kind of crazy. Also, although it comes with a potentiometer, that pot is not operating within signal path. All you need for power is a wall-wart or linear power supply or batteries. Depending on which design you choose the requirements vary. The OVC for instance comes with a 7805 regulator on board for the needed 5v, so I just need to supply anywhere from 5 to 20v.
The easiest thing to do of course would be to just buy George's finished Lightspeeds. Otherwise, I recommend Uriah Dailey's kits at buildanamp.com. They come with very good instructions and he supplies all parts needed. He also has a more refined version called the Lighter Note which is a little more expensive but still much cheaper than George's finished product.
First off there are some caveats. It needs a high input impedance amp. It needs a low output impedance source [edit: there is a post from George on this forum last year stating <1kohms for source and >47k for amp]. So ideal scenario would be CD/DAC w/ 2v output w/ low impedance and probably a tube amp, etc. The SDS boards from classdaudio.com are 47k, and therefore just make the cut. CDA, no. You probably know where this is all going for some of you: buffer. If the impedance matching is an issue it's recommended you use a buffer. I can definitely see the B1/DCB1 as a perfect fit w/ this design. The potential of that duo makes me giddy.
Okay, so how does it sound? It sounds really, really, really good. I can't really describe it. Honestly. Not because it's the second coming or anything. Just not really sure how to translate the difference. I have listened to it coming straight from my DAC (470R ouput imp and low-ish voltage) and also after my Aikido. The only word that comes to mind is 'pure'. Like spring water. Neutral, nothing is really emphasized. Low-level detail. No glare. Kind of relaxed, warm-ish even which was a surprise. I don't know. You just have to hear it. I I do think for some people it will be an eye-opener. Here's another word: high-end. That's what I kept thinking while listening. It makes my speakers sound really expensive. Weird thought, but that kept popping up. I felt like I owned really expensive gear and was hearing pure, unsullied, no colouration music. Even with the Aikido, you hear the Aikido contribution but it's cleaner and clearer. I know none of that sounds exciting, but I guess that's because in a way it doesn't sound like anything. It's not what it's doing that's impressive. It's what it's not doing. Anyway, you just have to hear it.
Basically, I intend to use this circuit to replace a potentiometer in any configuration I use from now on. Period. Not bad for $50
(if you're going from scratch I'd say $100 including enclosure, etc). I love DIY.
links:
Official Lightspeed
http://members.optusnet.com.au/~georgehifi/Ground up DIY with great explanation of design.
http://diyaudioprojects.com/Solid/DIY-Lightspeed-Passive-Attenuator/Kits and matched LDRs
http://www.buildanamp.com/http://www.diyaudio.com/forums/group-buys/153388-optical-volume-control-professional-pcb.html
Here's a nice pic of the OVC board from the forum:
