Recently I’ve been fortunate to be in possession of two excellent passive preamps that won’t break the bank, in fact far from it. The following comments are based on some observations I made when listening to and comparing the Slagle Autoformer Modules to the Lightspeed Attenuator. While both are passive preamps, they use different designs. Both were built with separate volume controls per channel. For the Slagle Autoformer Modules this is standard and for the Lightspeed it was a custom feature. The cost for each is approximately $450.
My intent is not to make this so much a review, but to provide some comparative notes for others to use. I have professed my preference for passive preamps many times in these forums and feel in the right system, obviously one that is passive friendly, they are the best choice if one is looking for a simple design that offers a high dose of purity, dynamics, transparency, and neutrality.
The Slagle Autoformer Modules are a joint design between John Chapman of Bent Audio and Dave Slagle of Intact Audio. The design uses autoformers to attenuate the signal, but with a custom board that eliminates the need for excessive wiring. In addition, each module (two are required for stereo operation) uses a unique dual volume switch. The first switch offers 11 steps with 3.75db increments. The second switch provides 3 positions with -1.25db, 0 db, and +1.25db settings. Overall using a combination of the switches attenuation is from +1.25dB to -41.25 dB in 32 steps. For more information on the design go here:http://intactaudio.com/module.html
The Lightspeed Attenuator is designed by George Stantscheff and is basically a resistive passive design with a twist. The uniqueness of the Lightspeed's design is that there are no contact points in the form of a "wiper", as is the case with other passive designs (resistive and transformer) that use a potentiometer or discrete stepped attenuator. The key components in the Lightspeed Attenuator are Silonex NSL-32SR2 Optocouplers. The optocoupler device is a sealed unit that consists of a high performance LED that shines on a light dependent resistor (LDR) thereby achieving proper attenuation. Since the optocouplers require a power supply the Lightspeed does require the use of a power plug. For more information on the design and how it works go here:http://www.diyaudio.com/forums/showthread.php?threadid=80194http://diyaudioprojects.com/Solid/DIY-Lightspeed-Passive-Attenuator/
Optocouplers are not new to preamp designs. If I recall correctly Melos was using them long ago in one of their active preamps and Dartzeel uses them too. Nelson Pass has also published a schematic that uses optocouplers in an active preamp design.
Ever since John Chapman reintroduced the TAP-X passive preamplifier using Dave Slagle's autoformers I've been hoping to try it out. Unfortunately I never got the opportunity, but John did build a couple of Slagle Autofomer Module prototypes (single input, dual output, single ended RCA) to play around with, stuffing them into a Bent Flex chassis, and I was able to get my hands on one of these via John and Dave.
I had already had the Lightspeed Attenuator (single input, single output, single ended RCA) for some time now and compared it to my JRGD Capri active preamp as well as an Audio Consulting Silver Rock TVC. The former comparison was done in my system while the latter was done in a friends system. I have already commented elsewhere in the forums as to how the Lightspeed Attenuator fared against the Audio Consulting Silver Rock, but to reiterate it certainly held its own and in my opinion was slightly more transparent and neutral than the Silver Rock. It just seemed to get out of the way of the music a bit more so than the Silver Rock. Otherwise, price aside, all things were pretty well equal. In comparison to the Capri, the Lightspeed showed that the Capri had a tendency to err on the side of warmth. The Capri provided slightly more punch in the lower frequencies, but all told it wasn't missed much when I swapped the Lightspeed into the system. Both units had excellent top end extension, but I would give a slight edge to the Lightspeed here.
I received the Bent/Slagle Autoformer Modules a few weeks ago. I have been rotating the three preamps in my system since I received it. Over the last week the Capri was removed entirely and the focus was on comparing the Bent/Slagle to the Lightspeed Attenuator. I was hoping that the comparison would be similar to the one I did previously with the Audio Consulting TVC in that it would be pretty evident as to the differences between the components. However, this would not be the case. The Bent/Slagle and Lightspeed Attenuator are very difficult for me to differentiate. They are very close on nearly every level: transparency, neutrality, frequency extension, and noise. Overall, I would have to give the Lightspeed a slight edge over the Bent/Slagle, but there is something seductive about the Bent/Slagle that also makes it an enjoyable piece of equipment and I feel fortunate to have the both.
So there you have it, a couple of passive preamps that are fairly inexpensive and whose performance in my opinion is high end. They’ll be alternating in and out of my system for the near future. Now if I can get a balanced version of each that might make for some more interesting comparisons.