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:
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:
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.
I started using my DIY Lightspeed in 2007. Have loved it from the beginning.
I am now using a DAC with a 6np2 tube output stage the passive and 110w 6c33c otl mono's. Speakers are also minimalist in terms of no resistors in the signal path.
The Lightspeed remains a really great sound. However, I have recently built the Slagle AVC using his basic modules. To my ear, there is more micro detail in the AVC and it is the preferable device. Very Natural. Very detailed. very 3d. You could love either, and I expect its very system dependent.
I have learned that any form of resistor removes some of the micro level detail and deadens the sound. I think the LDR is one of the best resistors, but it is still a resistor.
I think Dave's AVC (AFTER 100 Hrs) takes it up a notch. Some may prefer the LS. It has a lot of air and the Pratt that everyone references. But to me it cuts out one layer of detail like any other resistor albeit a good one.
After I use the Slagle AVC for an extended period, I'll drop the LS back in and rereport. You can't go wrong with either if your system is up to using a passive.
I think there are a couple LDR passive preamps out there with remote and multiple inputs. The names escape me at the moment though.
As for DIYing one perhaps the remote John Chapman sells could be put to use with a Lightspeed type design. Actually this is probably better discussed over in the Lightspeed thread at DIYAudio. Lots of people building them and discussing their projects. I am sure multiple inputs has been addressed but not sure about remote.
It seems to me that someone with good DIY chops could build around a LSA to get two inputs in the highest quality way possible (i.e., without having to add another interconnect in the chain) and add some sort of motorized rim drive remote for the volume. Can live without remote for analog but not for music server playback.
Yes he is missed around here. I introduced Paul to the Lightspeed and that was the common denominator that in essence started a very nice friendship between us.
Please be advised that Pubul57 passed away this earlier year. You may want to go visit the thread he started on the Lightspeed attenuator. I have been using one for a few years now in my single ended system. I pair it with a Music Reference RM-10 amp. I also still have the Slagle and another unit I made with passive parts that produces voltage. So I am still on the passive band wagon for 2012 and Most likely beyond.
The simple way is to just get a source switch box, then you can have your cake and it too. So when you want to get the best out of your system you can still go direct, as I've have proved with my prototype, which does have two inputs with the best source selector switch available, the single input production Lightspeed Attenuators still sound a bit better every time I put them into my system to do listening tests before shipping out.
I was a bit skeptical that it would better the TVC and AVCs I previously owned, but it certainly as good, and they were quite a bit more money. This is a no brainer to audtion with any single source system.
Got a Lightspeed Attenuator for myself. George said that I could wait 5 minutes before playing. Wasn't too sure after the 5 minutes, but as it stabilized, it came into its own and I'm convinced. I'm leaving it on 24/7 with the volume at 12 o'clock like George says.
No more edge on vocals or instruments unless it's meant to be there. Wasn't sure it could better my transformer based passive pre, but it's more liquid.
Wish there were two inputs, but I can live with one if it sounds like this.
That is worthy competition indeed! My impression is the Lightspeed Attenuator is the best preamp I have ever owned or heard in other systems. More folks should try it against there 10-20K preamps and listen for themselves. I don't know what more I can say about it - it cost less than the NOS tubes in my other preamps:)
That's not a pity, that's why it is so pure and so good. This coming from someone who has to keep swapping cables rather than use an input selector switch, as well as gets most of his exercise adjusting the volume manually;)
Batalok, what were using before (for comparison and context). I feel the same way after having used CAT, Lamm, Joule, Atma-sphere, and Placette - pretty good stuff for this LS to go up against.
I got the Lightspeed right now. All I can say, Wow. Stereo image is improved signifantly, brushes are more clear(strikingly good), piano becomes more "resonant", low end goes deeper, it all happens down deep there. Maybe the speakers (Master Contempory C, you will not regret it, on the contrary!) are the main reason for that, but for sure the Lightspeed adds another dimensionality. Good works George!
Well, I have spending a lot of time listening over the past few days, if for no other reason than to get over "new thing euphoria". I have also had two audiophile friends with good "credentials" come over for some listening sessions. All I can say is the Lightspeed Attenuator is the best sounding preamp I have used in my system with my Music Reference RM9 and RM10MKII amps, and my friends loved it too. Lack of dynamics, pace, or bass simply not issues in my set-up. It is as simple one input/output device and single-ended and, oh my god, no remote, but if it fits your ergonomic needs, and don't know if there is a better sounding preamp, and if there is, it would be so much more expensive as to make that pursuit meaningless IMHO. Not for everyone and all systems, but if it works for you, you are in for a real treat. At the price I would especially recommend it to those that have had success with TVC/AVCs and understand a little bit about the use of passives. Soon I will try it with the Atma M60 amps, though I think there may be a gain issue there, we'll see.
Pubul57 There is something about the idea of a passive linestage that makes me think it has to be better of doing nothing destructive to the signal, and that in some way an active, any active simply does too much to a signal be as pure to the source. Assuming appropriate gain and impedance match.Pubul57
Hi Paul, there is a way you can see how little the Lightspeed Attenuator embeds it's own character onto the sound of the source. I don't tell many people to do this but if your careful it can show what colorations/distortions preamps passive or active are doing. First plug your source directly into your poweramp and the put on a known very quite low level CD (have your finger on the stop button just in case) then get a cd/track that you think will be at your normal listening level and see what it sounds like, this is the purest sound you will get from your system. Note the level you were listening at, insert the Lightspeed and listen again at the same volume, I've done this many times to members of our audiophile society and they cannot pick if the Lightspeed was in or out of the system, yet when tried with other active/passive pre's they could distinguish the difference, this listening test shows how transparent the pre is. Saying that you will get people that like the colorations that some pres can give if the system is flawed and needs that coloration embeded on it. Cheers George
It would be nice to get Terry's views on the LA as he has owned the Placette and Bent, and his current tube preamp, based on the review is very good indeed. There is something about the idea of a passive linestage that makes me think it has to be better of doing nothing destructive to the signal, and that in some way an active, any active simply does too much to a signal be as pure to the source. Assuming appropriate gain and impedance match. If, and that is a big if, Terry finds the LA competitive with the Concertr Fidelity I think alot folks have to start reconsidering their approach, and expense, of their preamps. To me, a preamp should be nuetral to the source signal (as Ken Stevens says, the color of water), and flavoring should come from the amp/speakers because we know all speakers will impart their flavor.
I've got the Bent Tap-X which I really enjoy. The design and execution of the circuitry is excellent (utilizing fibre optics links to control the autoformer modules) and implementation of the optically encoded remote control is great. Now, how cool would it be to have John Chapman design LDR modules that could be interfaced with the Bent unit remote control circuitry. Personally, the only drawback to the Lightspeed Attenuator is the lack of remote control (volume, balance, mute etc.). I think the Bent remote functionality with LDR attenuator modules would be awesome. Maybe some of us Bent owners (or previous owners) should contact John and do some arm twisting!!!
I'm in agreement with that and I think George stated as much above. Also, on the 6 input version of the TAP-X John Chapman included a remote controlled buffered output, so depending on source, one could select buffered or non-buffered mode. Now that's flexibility.
Can we agree that with the right combination between source amp an ICs, that no buffering is needed, and if not needed for impedance matching no buffer is better than having one. In other words, no buffer is optimal, but a buffer is much more flexible.
I think Steve might just have the right combination with the VRE-1. He has always been a big proponent of a passive with active buffer. His preamp designs have always been well regarded and his older designs remain in high demand.
It would be great to hear a comparsion of the LA up against the likes of the Concert Fidelity. Congrats on the XA.5s, I regret having sold my 30.5s, but 4 amps was getting a little ridiculous:)
I just received the other two components in the Silicon Arts Si2 microline today. Masa-san sent me the preamp and amp to evaluate with the DAC that I already had. The preamp in particular is supposed to have trickle down technology from the Concert Fidelity line.
It will be an interesting few weeks coming up to hear all this stuff.
Paul, yes I did a review on the XA-60.5's here on the GON. I was surprized how much of a sonic difference, for the better, the 60.5's made in my system compared to the XA-100's. Yup, it would be fun to hear the L.A. in my system. Don't know if you read my review of my new reference, the Concert Fidelity CF-080, which replaced my Audio Valve Eklipse. You know there is no "BEST", however this is the best preamp I have ever heard. I recently described it as a passive with balls to an audiophile friend. Clio09 has heard the CF-080 at the CES show and has high regard for it also. This preamp is more not there then any passive I have ever heard, yet offers natural timbres, great image density, and the best macrodynamics I have ever had in my system. That's way the L.A. would have a very up hill battle in my system. The CF-080 gives me everthing a great passive offers, plus dynamics/prat which I have found missing in my system with passives in the past. If the L.A. would even come close for what it costs it trully would be one of the greatest bargains in all of audiophile land.
Terry, have you reviewed and compared the XA60.5 on Gon? I loved the XA30.5, would be good to here your view of the L.A. if you get a chance to hear Anthony's.
Hi Paul, I'm not using the XA-100's anymore in my system, they were replaced with XA-60.5's. Remember I bi-amp with a Bryston X-over, so I have never had an impedence matching problems with any active or passive linestage in my system.
Clio09, thank you for your very kind offer to lend me your Lightspeed piece, I'll send you an E-mail when I have some time to audition it.
Anthony, it will be interesting to here your observations. My only concern with the M60s was their Input voltage for full output of 2.83V, but not sure if the S30 are the same. I'll be trying it soon. Really curious to hear what a minimalist passive like the LS will sound like with a minimalist circuit like the Atma-OTL. One would expect a very low level of distortion - pretty much like George describes it - connecting your source directly to the amp - with some means of controlling volume. Interesting article on Arthur Salvatore's sight as to why the passive preamp is the best approach and why if any active, at any price sounds better in ANY way, it just means you have an impadance/gain mismatch somewhere in your system - pretty much what Roger says. Of course, this does take a bit more vigilance in paying attention to the rest of the chain, but you know, it is well worth it when you get it right.
Pass XA100's are very low input impedance, I think around 12k or 20k this is why Nelson designed a very simple 2 Fet buffer for the Lightspeed Attenuator, you can see it on my Lightspeed Attenuator forum on Diy Audio, it's for memory it's around page 135, or if you like tube buffers, I have posted the circuit for a SLCF (Super Linear Cathode Follower) page 320, it's about 100ohms output impedance, and probably the best buffer I have heard, but still the best buffer is no buffer if you get the impedances right as I've posted above. Cheers George
Teajay, if you are still using the Pass XA100s it would seem that the LS will be unsuitable, I'm not sure that an unbuffered passive would work with those amps. Tomorrow, I will try the Lightspeed with the Atma-sphere amps. They are not very sensitive, but I can swith the EMM Labs to 3.7v output so it should be able to drive them with no gain issues.
It looks as though your server or fire wall is doing the self editing, as if I try to insert the more than or less than signs (the V's on their side) it does the self edit thing, you guys need to fix this.
OK lets see if this one gets through unscathed. If I could just chime in here about the PRAT factor with the Lightspeed Attenuator. If you have a source (cdp or phono stage) that is less than 100ohms output impedance and power amps that are more than 50kohm (100kohm or more input even better) your PRAT factor if anything will be better than anything you've heard, it gives the impression that your listening to something like the old DBX units (Dynamic Range Enhancer) without all the pitfalls of those things. Cheers George (Lightspeed Attenuator Manufacturer)
Sorry again, you moderators are nutz your going to blow some ones system up if you keep editing my figures like that, you have know idea what your editing. I'll try a fresh post please leave it alone. Cheers George
George, you should just add on your signature that you are Lightspeed, or whatever the company name is, folks like to see that, even if it is obvious on this particular thread.
If I could just chime in here about the PRAT factor with the Lightspeed Attenuator. If you have a source that is 50kohm (100kohm or more input even better) your PRAT factor if anything will be better than ever, it give the impression that your listening to something like a DBX unit (Dynamic Range Enhancer) without all the pittfalls of those things.
There was not a lot of difference in the Lightspeed and Slagle AVC in my system as I noted above. However, the ease at which the music flows with the Lightspeed in the system is quite impressive. It was very obvious versus the Audio Consulting, not so much versus the Slagle, but I believe this contributes to the PRAT that I hear with it, especially with digital music. Analog is off the charts with the Lightspeed. As Pubul57 mentioned, the soundstage holds up quite well versus actives.
Terry, I think for $500 its worth a try. I would be happy to lend you mine as well if you thought it made sense for you to try it. I would be interested in your impressions versus the Concert Fidelity. BTW - Masa-san is sending me the Si2 amp and preamp next week to try out. Should be interesting.
Thanks Paul, your feedback helps me understand why your enjoying the Lightspeed so much. Yes, when I use the term image density that's my way of describing palapablty or 3D imaging. Sounds like there is no problem in that area with the Lightspeed.
Would my assumption that you have never lost PRAT or "aliveness" with this piece be correct compared to your active linestages? As Im mentioned before this was something that happened in my system and really got in the way of the music.
I do agree totally that for $500.00 this piece is one hell of a deal.
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