Slagle AVC Modules & Lightspeed Attenuator

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'm surprised that no-one has commented upon your review, as I feel these are the two best passive volume controls in existence.

Thank you for taking the time for posting this info. It reflects the same conclusion I have come to in regard to transformer volume controls vs optocouplers.

In the end, I decided upon optocouplers for volume control at the 'grid stopper' position of the input tube. Removing that last resistor in the signal path has allowed the optocoupler volume control to surpass the TVC without reservation.
I missed this review the first time around. I have been thinking about both approaches for an interstage volume control in my tube preamp. I will probably try the LDR approach first as it is so simple & inexpensive.
Huh, I missed this review too. I've been using the Slagle autoformers for some time now, and find them wonderful - transparent and unfailingly musical. One nice feature of an autoformer is that you can overcome the inherent problems of transformer and resistive based passives in regard to frequency anomalies introduced through impedance and capacitance mismatches. One thing I recommend (though this may be moot with the introduction of Dave's circuit board-wired modules which eliminate over 100 point to point solder connections) is that you should utilize the best switch you can afford when wiring the autoformers - I use a 24-position Shallco switch with John Chapman's remote stepper motor interface. It makes an appealing retro "ca-chunk" sound that catches everyone's ear.

Now, I'm also thinking of trying one of Dave Slagle's custom matched phono SUTs. Anyone with any experience?

Well better late than never. I appreciate the comments and information. To update everyone I'm using both in my system simultaneously now. The Lightspeed handles the phono and Slagle the digital. At some point I may send the Slagle out to be rewired as two input/one output. That will make things a bit easier for me.

I think my Slagle modules have the Elma switch. I might investigate the Shallco. With the Lightspeed the design eliminates the volume switch from the equation so a cheap switch can be used with no detriment to the sound.
Anthony, any word from George on expected lifespan of the LED and LDRs?
Here is some information from the manual:

A couple of do’s and don’ts, the Lightspeed can be left on continuously. When not being used leave the volume control at half volume (12 o’clock), this let’s the LED/LDR packages inside operate at half level, this means instead of them lasting 25 years they will last 50 years.

The Lightspeed uses a double regulated power supply, the wall wart that is supplied is linear 12dc regulated, 300mA centre pin positive, please only use the same and not switch mode wall warts. At 300mA this is enough to drive 6 x Lightspeeds. There is a secondary 5vdc regulated supply within the Lightspeed itself.

Allow the unit at least half an hour to stabilize at half volume before any serious listening when first powered up.

Hope this helps.
That's long enough:)
The more important issue with LDRs is matching. Out of 100, you might be lucky to get half that are closely matched pairs. That's a major reason George does not do balanced designs.
I know George is reading this and will send me the most tightly matched LDRs in the history of LDRs:). Think I will get the Bent AVC again.
I am actually rewiring a Bent/Slagle AVC right now. It came with single input/two outputs. Switching it around. The new circuit board John and Dave designed makes this real easy. Check out the new models Dave is rolling out:
The two make quite a pair with their respective techncial skills.
Anthony, got the LS LDR, I can see why you love it. It, with a Music Reference RM10 is what I would recommend to anyone starting out looking to spend $2000 or so for a preamp/amp combo, in fact it is what I would recommend to someone wanting a great system with no desire to upgrade in the future. You reviewed the product, and I have nothing to add other than to say I agree.

Glad you like it. Now do me a favor and try it with the M-60's for laughs. I've been speaking to Ralph of late and he insists the input sensitivity won't be a problem for a passive preamp. You won't be able to hook it up balanced, but nonetheless give it a shot.
Will do. Also, my CD player can go up to 3.7v output at the flip of a switch, so I don't think gain will be a problem with the M60s.
Should be perfect (heaven actually)with your 3.7v output from the CDP and the 100kohm unbalanced input of the M60 MkIII monoblocks.

Cheers George
George, there is always theory and practice, and theory seems to be worth very little compared to how something actually sounds in the real world. Your Lightspeed Attenuator, is extremely good by almost all the "audiophile" criteria, but it just plane sounds wonderful.

There much debate about the resitor based and transformer based approach to passive preamps, with most of the Audiogon consensus favoring transformers or autoformers. Roger Modjeski (Music Reference / RAM Labs) one of the best designers on the planet swears by passives (he simply believes no active can be as good as a good passive - obviously, sometimes your system needs an active to deal with cabling, gain and impedance issues and a passive will simply not work)but is also a proponent of the resistor approach - he explained to me why, but is was over my head.

Obviously, you have chosen the path of least resistance, what is your view of the theoretical advantages and disadvantages of resistors/transformers? Regardless, your product is exceptional compared with all the passives of every ilk I have tried, and some of the finest active tube linestages made.

I'll say this, every audiophile with a single source should try your preamp (under $500 delivered!!) and compare it to whatever they own. Some may not want to believe it, but you really can have world class sound from a preamp considerably cheaper than the interconnect to the amp.
So in practice did you try it with the M-60's?
Not yet. Just with the MR RM10 and 9 so far.
Of all the customers about 25 that I have that have both, they all said they prefer the Lightspeed Attenuator because it has better dynamics and is more transparent than the TVC's.

Cheers George
George, it just might be possible the Lightspeed Attenuator is the best preamplifier I have had in my system (and I have had many passives and even more actives). I need to do some more comparisons, but I'm not sure that trumpet, bass, drums, and piano have ever sounded more real. Anthony thanks for finding this thing. The LS with the Music Reference RM10 is one hell of an incredible combo, and for $2,500 for both at list, it is stupidgood. Combine this with some Merlin VSMs or some other great speaker that does not need much power and has smooth impedance and I think you can put this up against most anything out there, no matter how much more you can or are willing to spend. George, the next thing is going to be demand for a multiple input option.
I understand George's reluctance to do more than a single input. You could always try to get a transparent switcher, or just swap cables which is what I do. Yes it is inconvenient, but worth it. I would much rather see George come up with a balanced design.

Also, I did not discover the Lightspeed. It came up on a thread here and I just followed the link to and perused through the many many pages on the thread. When I got to the part where Nelson Pass chimed in I knew this was something worth pursuing.
Thanks for the wrap guys, just a note, my prototype that has been powered on 24/7 now for more than 5 years has in fact got dual inputs with a very good gold plated 2amp rated switch the best I could find.
Still when I listen test every production Lightspeed Attenuator for a few hours they always sound just that little bit better, this is why I will not do a dual or multiple input, and would rather the customer get a source switching box, this way when you really want to hear your system at it's best to show off to friends, you just go direct and bypass the input switch box, this way you can have your cake and eat it too.
Cheers George
Also Paul, wait till you hear the Lightspeed Attenuator with your M60 OTL's.
I heard OTL monoblocks 8 x 509 tubes in each with a Lightspeed up their skirts last weekend, I must say it was the most crystalline sound I have ever heard, it reminded me of when I heard Magnat Ionic tweeters on top of double stacked 57 quads for the first time before getting gassed to death. AMAZING!!!
Cheers George

I am considering getting the Atma-sphere amps as well. The one sticking point is that they work best in balanced mode, although they do provide a single ended input as well. So while I understand the reasons very clearly and have read some of the experiences of others who have tried to DIY a balanced design, I still wish a balanced version were available.

BTW - do you remember the name of the OTL amp you heard the Lightspeed with?
Ah, that's what the weekends for:) You may have to come out with a audio jewlery version (you can call it Signature Edition, Special Edition, or Deluxe I Really Mean It MKII). of this for $3,000 so people can believe how good it is. No difference inside, just more bling and weight (20lbs should be convincing) to justify the cost:). Like my Merlin speakers, this is one piece of gear I will not be selling, unless you come up with a better way, but in 35 years, I imagined you have sorted this out pretty well already.
I have found this thread very interesting based on my own experience with passive preamps in the past. As Pubul57 knows, since he purchased from me the Placette buffered, I went from this great resitor based passive to John Chapman's TVC based Tap unit. Loved how the TVC sounded in my system and went on to write a review here on the GON to share what I thought were its sonic virtues.

As Paul mentioned already on this thread, factors such as cabling, gain, and impedance can have a dramatic effect on the performance of a passive linestage in any system. So, my following comments of course our in the context of my system. However, at least five other audiophile friends have come to the same conclusions in their systems which are very different then mine.

The clarity/transparency, the microdetails, natural timbres, if feed by the right source, rank with the best in the world regardless of cost. But I always, regardless of which passive I had in my system, noticed two very important shortcomings that become apparent over time and in comparsion with great active linestages.

1) The depth of my sound stage was shortened to a great degree and overall their was less image density without as much air around individual players.

2) Paul, had mentioned some were either on this thread or another on passives that people claimed that passives lack macrodynamics and he questioned what did they really mean by this statement. When I first got interested in passives I two found this negative comment often stated and yet felt that there was extension on the bottom end with good kick and power. What I'm now aware of is a deficiency I would not label as macrodynamics, but what I would call the PRAT factor. Very subjectively stated it's as if the "aliveness" of my system dramatically decreases and it makes it harder to get into the music regarding pace and rhythm. It's pretty dramatic in my system compared with an active linestage, the overall musical perspective sounds like my system as gone to "sleep".

So, here's my questions regarding the Lightspeed piece, is their any reason to believe that George's passive would not have the same short comings I have found in other passives? If it would address these concerns, what would be the explanation how/why it would do this?

A final question for Paul, it's very apparent your a great fan of the Lightspeed, would you share details regarding why you think it is much better then the Placette or TVC's that you have experienced before. Thanks in advance.
Hi Terry. It is really hard to analyze the differences since it has been so long since I have had the RVC and the BENT AVC. I guess what I love about the Lightspeed is that the soundstage is huge in width and depth, localization is specific and does not drift, and the timbre of trumpet and sax seems extraordinairily accurate, and the body and weight, and snap of acoustic bass is so real. I am not sure what image density means, is it the same a palapable? The sense that the sound is coming from a 3 dimensional instrument? In that sense the Lightspeed has image dimensionality. I wish I had the RVC here for comparisons, but I can tell you that when I got your Placette Active, in that case I immediately preferred it to the passive (that is one heck of a good preamp). I think what the LS brings that perhaps a TVC and AVC cannot do, is provide this sense of a transparent window - much like OTLs versus Transformer tube amps - (less haze, noise, gunk - real harisplitting here)into the musical space. Even without gain and all that, a TVC and AVC surely adds alot more wire between source and amp. I am going to be comparing the LS against my Joule and Atma-sphere preamps for the next month or so - truth is I do like them all. What blows me away is that for $450 delivered, it competes with anything I've heard, and if you are on a budget and $5,000-$10,000 preamps are not on the agenda, you could, and should, live happily with a system based on the LS with the right amp and speakers, ofcourse.
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.
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.
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.

Cheers George
I would like to see a Teajay review, he does it a lot better than me.
Geroge, do you mean the source or the amp?
Sorry, moderators have edited and stuffed up the figures in my last post.
Source output impedance (cdp or phono stage) should be less than 50kohm

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.
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

Cheers George
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)
I may have jumped the gun and have to appologise.

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.

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.
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

I purchased an S-30 so I'm going to give it a go too. Ralph said 2V would be enough, although I may need to use more of the volume control.
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.
BTW - Roger will be addressing the next LA/Orange County Audio Society meeting at Brooks Berdan May 22. I can't wait to hear what he has to say.
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.
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.
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.

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.
Those who are considering the $18,000 Concert Fidelity CF-080 might also consider the US made SMc Audio VRE-1, which is also a "passive with balls" ($14,950).
...addendum to the above.

I owned Pass Labs XA-60.5 amplifiers and mated them with the SMc Audio VRE-1. Exceptional results, IMO.
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 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.