Lightspeed Attenuator - Best Preamp Ever?

The question is a bit rhetorical. No preamp is the best ever, and much depends on system context. I am starting this thread beacuase there is a lot of info on this preamp in a Music First Audio Passive...thread, an Slagle AVC Modules...thread and wanted to be sure that information on this amazing product did not get lost in those threads.

I suspect that many folks may give this preamp a try at $450, direct from Australia, so I thought it would be good for current owners and future owners to have a place to describe their experience with this preamp.

It is a passive preamp that uses light LEDs, rather than mechanical contacts, to alter resistance and thereby attenuation of the source signal. It has been extremely hot in the DIY community, since the maker of this preamp provided gernerously provided information on how to make one. The trick is that while there are few parts, getting it done right, the matching of the parts is time consuming and tricky, and to boot, most of use would solder our fingers together if we tried. At $450, don't bother. It is cased in a small chassis that is fully shielded alloy, it gets it's RF sink earth via the interconnects. Vibration doesn't come into it as there is nothing to get vibrated as it's passive, even the active led's are immune as they are gas element, no filaments. The feet I attach are soft silicon/sorbethane compound anyway just in case.

This is not audio jewelry with bling, but solidly made and there is little room (if any) for audionervosa or tweaking.

So is this the best preamp ever? It might be if you have a single source (though you could use a switch box), your source is 2v or higher, your IC from pre-amp to amp is less than 2m to keep capaitance low, your amp is 5kohm input or higher (most any tube amp), and your amp is relatively sensitive (1v input sensitivity or lower v would be just right). In other words, within a passive friendly system (you do have to give this some thought), this is the finest passive preamp I have ever heard, and I have has many ranging form resistor-based to TVCs and AVCs.

In my system, with my equipment, I think it is the best I have heard passive or active, but I lean towards prefering preamp neutrality and transparency, without loosing musicality, dynamics, or the handling of low bass and highs.

If you own one, what are your impressions versus anything you have heard?

Is it the best ever? I suspect for some it may be, and to say that for a $450 product makes it stupidgood.

Showing 50 responses by clio09

Regarding the question on the diode effect. For those that missed it or don't recall it George explains it as follows:

The (diode thingy) "diode effect" is bought around by very fast music transients from the source, CDP or phono, which can be in the order of 100's of volts per micro seconds (volts per uS) these happen as the name implies in micro seconds.

When we view a static (constant) 1k-10k square waves on an ocilloscope all looks fairly fine going through a good pot (Alps Bournes ect).But it's when you pulse the waves at micro seconds (transients) an then store it on a very good super fast digital storage ocilloscope play the micro second pulses back frozen in time, and magnify the corners of the waves it's then you see the ringing/occilations effect of the light wiper contact on the conductive track of the pot, (I have named it for want of a better word, "dynamic contact bounce")It's the wiper being ever so slightly bounced on the track.

When pressure is applied to this wiper (with in my case a wooden skewer) so the wiper presses down hard down on the conductive track, the ringing occilations stop when doing the same test. When a soldered resistor is in place of the pot it's not there either, when a Lightspeed Attenuator is in place of the pot it also is not there either.All pots no matter how good bounce to a certain degree this is why they all sound different, they are all fundamentially flawed.

The diode effect that George references and that the LSA eliminates is clearly something designers of some well known preamps understand and can hear. Here is a quote from one:

Having done some audition in this regard (we 'switched', if you will pardon the pun, to our custom Shallco part about 15 years ago as a result of some of these auditions), the difference in contacts that George is describing above is clearly audible.

Its my opinion that the volume control is what shoots many preamps down (especially preamps with remote control) before they can even get off the runway. One of the consequences is that the majority of line stage technologies are in a deplorable state- I don't fault anyone for thinking that a passive might be better.

FWIW - The custom Shallco switch referenced above is used in his top of the line preamp that uses gold contacts, with a double-spring-loaded wiper. Highly superior and much more costly compared to other switches.

The designer also had this to say about the LSA:

I had the opportunity to compare the Lightspeed against one of our own preamps and I have to say it was the most neutral passive I have heard.

Anybody want to take a guess at who it is? I doubt anyone will question his knowledge on the subject.
2549, two conductor with shield. The two conductors are soldered to the pin and the shield is the return.
Its quite possible that John Chapman's modules can not be implememted manually.
You know how I feel.

Even with the Atma-Sphere S-30 I own where the sensitivity rating is nearly 3V this unit performs exceptionally well. I have used it with VAC Auricle Musicblocs and a Music Reference RM-10 as well. In each case its performance is consistent. I don't even have my Slagle Autoformer box in the system anymore.

The sound from the Lightspeed flies in the face of all the negative attributes that you hear people comment on regarding passive preamps. I am convinced anyone predisposed to active preamps that hears the Lightspeed in their system (and assuming proper system matching) will experience a reality check. At its price point there is not much to lose in trying one. I've dumped $450 (and much more) on interconnect cables that never came close to the price to performance value the Lightspeed provides.

Okay, back to our regularly scheduled program.

This is the email I use to contact George. Between the Stereophile article and a few recent threads I'm sure his time has been a little more consumed than normal. Also, you might be going into a spam filter. Hopefully he'll see your note here.
La45, yes this was the one I started using a while ago, around Thanksgiving last year. A while back I started a thread called Slagle AVC Modules & Lightspeed Attenuator.

Music Reference RM-10 + Lightspeed Attenutor = Depends

Don't forget to get a box of those as well before listening to this combination. This is one of the most enjoyable amp/preamp combos I have ever experienced.

BTW - Roger has a few RM-10 MkII amps left at the $1450 price. A great deal only bettered by being able to find one used in near new condition.
In the Lightspeed design taking the switch out of the equation further simplifies an already simple design. The Pot in the Box is also a very simple design, but the Noble pots, as good as Roger feels they are, will impart a sonic signature. Transformers and autoformers are wound with lots of wire. I've been watching the transformer winding process and will be learning to do it myself. All that wire has to have some effect on the sound rendering the TVC or AVC less transparent and neutral (silver or copper windings would also have some effect). I can think of two reasons why TVCs and AVCs are popular. One they are more forgiving of impedance mismatches and two, in the case of transformers, they offer some isolation.

I'm considering building a PITB for the fun of it and to compare against the Lightspeed. Also, many people like the Luminous Axiom passive devices. I'd be curious as to any opinions on these from owners. Come to think of it I've got a pair of EVS attenuators lying around I should throw in the system for the fun of it.
Active preamps are for the most part tone controls (a good case could be made that all components are tone controls as well). A good example to reference is the Audio Horizons thread. A number of people chose to take the very well received stock unit further through modification (whether through the factory options or their own doing). The idea was that they wanted more or something different from the sound of the original unit. Fair game, in a way that is part of the hobby and can be very enjoyable, whether you're swapping tubes, caps, wiring, or whatever else can alter the sound.

What is somewhat amusing to me is that some people want tone controls (bass, treble, etc.) on the tone control itself. I guess a little is not enough in those cases.
With Ralph's assistance I'm tweaking the S-30 so that it will be even further optimized with the Lightspeed for single ended use and my Otari reel-to-reel running direct for balanced use.

Pubul57, sorry to see you sold the Joule, I know how much you liked it. If the Lightspeed is that good that you could let the Joule go, then that is saying something.
At full price the LSA isn't going to break the bank. It's a very low risk proposition. That's one reason I like it so much. Of course that it works optimally in my system is another.
I also feel the need to share this story, because it offers a bit of a different use case for the Lightspeed.

My speakers are Audiokinesis Jazz Modules. They are a 92db efficient 12 ohm design with a smooth impedance curve. Very easy speakers to drive. I've used a 225 rated TRL D-225 amp (tests out closer to 300 watts as you know Bill) with it, as well as some lower powered amps. The designer, Duke LeJeune, voiced the speakers to an extent with the Atma-Sphere S-30 amp. While the amps input impedance is 100k (single ended) or 200k (balanced) ohms, the input sensitivity is only 2.83V. This figure is not usually what one, including myself, would consider passive friendly. Since I wanted to hear this combo for its synergy I asked Ralph Karsten, the amps designer (who has heard the Lightspeed), if the single ended Lightspeed and another balanced passive unit I'm going to test out would work well with the amp.

Ralph responded that I was making too much out of the volume control issue. My CD players output was more than enough on its own to drive the amp, basically validating a comment George just made (I was able to run my Otari MX-5050 BII reel-to-reel into the S-30 amps using the decks volume control and there was plenty of drive). In fact Ralph feels the further around the volume control you can get on a passive (closest to wide open) and achieve your optimal listening SPL, the better. With my very sensitive amps I get anywhere from 10 o'clock to 12 o'clock. With the S-30, I can push it a bit further, 1 o'clock or so. I can say it sounded wonderful.

Ralph recommends using an active preamp with his amps but it is important to note that his designs support the 600 ohm balanced standard (pro audio). Not all balanced active preamps support this standard. Of course Atma-Sphere preamps do. The benefit of the 600 ohm standard is that it takes the balanced cable out of the equation. Otherwise, based on my experience, when using appropriate cable lengths, there should be no reason why a passive preamp wouldn't work with less sensitive amps, assuming the other spec criteria are met.
Pubul57, what is the active preamp "conditioning"? Are you referring to the preamp being some type of noise filter?
I heard a Redgum integrated recently, it sounded quite good and was well built.
And don't forget all interconnects and speaker cabling have some resistance this should be added to the input impedance of the load, this is why one should not use hair thin cabling as it has more resistance and adds the load resistance.

What type of cabling should one use (or do you use) in terms of gauge? Is there a preference over stranded versus solid core. Would cabling like Mogami or Belden suffice? These typically have low LCR specs.
If you buy this off the shelf, is it center pin positive by default?

If you buy what off the shelf?
Hopefully will have it in a few weeks. Ed Schilling at the Horn Shoppe builds them one by one and he has a bit of a back log right now. I'll probably start a new thread at some point in the future to provide some feedback and comparisons.

While the 3V output of the CDP is quite sufficient, the output impedance is 220 ohms single ended. George may need to weigh in here on how that might affect the matching between CDP and Lightspeed. It should work as typically you want a ratio greater thant 10:1 between the input impedance of the passive preamp and the output impedance of the source. The greater the ratio the better. Same is true for the ratio between the input impedance of the amp to the output impedance of the passive preamp. What complicates matters is that the output impedance of passive preamps vary by volume setting.

The 67k ohm input impedance of the Samson amps is lowish (maybe best to check with Paul on that spec, I'm going by memory here) but workable (under 50k ohm would definitely pose some issues). I prefer at least 100k ohm. However, I did use a passive preamp with my D-225 for quite some time with no problem whatsoever.

To me, if 2V plays loud enough then go with the lower of the two voltages. Why use a jackhammer to drive a nail. 2V was plenty for the RM-10, Auricle Music Blocs, and S-30, and the S-30 had nearly 3 times the sensitivity rating as the other two amps.

I was under the impression the output impedance did vary on the Lightspeed. If it doesn't, that is another significant benefit versus other passive designs.


He's been posting here of late so I doubt he is on vacation. I suspect your email may be getting caught in a spam filter. George usually responds to emails within a day or two.

I bought mine from George using PayPal. I don't think he takes credit cards. Let's hope he sees your posts here and can contact you through the Audiogon system.

Be patient, it's worth the wait.
Clipsal, what were the other components in the system in which you heard the shoot out?

The B1 is an excellent product. Nelson Pass also has published schematics for his own version of the Lightspeed. My take on his development of the B1 is that through the use of an active buffer stage the preamp matches up much better to the low input impedance First Watt amps he designs.
Why doesn't this link show up in the forum section any longer? I had to do a search to find it.
What I am most curious about is how well The Truth preamp will drive the S-30. The LSA does drive it well, but I'd like to hear if The Truth will improve upon dynamics. The bandwidth measurements are off the charts with the Truth and the output impedance is a constant 2 ohms. Apparently the input impedance is too high to measure.

No risk to try it so we'll see.
I just started running the Lightspeed off a battery power supply. It is the same batter power supply used by Galibier Designs to power their turntable motors (Sears Diehard Portable Power 750), 12V DC and 12aH. Thom Mackris made me the appropriate adapter cable (mine needed to be center pin negative). I've been listening for a bit now and so far I like what I hear.
Marqmike: The amp should have greater than 50k input impedance, more is better. Input sensitivity isn't as critical. Typically 1V or so would suffice, but I've run my LSA with an Atma-Sphere S-30 that is nearly 3V sensitive and it works fine. I even tried it by altering the amp to be even less sensitive (nearly 6V) and it was still fine, although I was running out of room to advance the volume control. It is also important that your source output impedance be low, 100 ohms or so, and have enough output voltage to drive the LSA, 2V or more.

As mentioned, you need to worry more about the amp/speaker combo when deciding what components to use with the LSA. Given that the above requirements are met, the LSA is benign in the scheme of things.
The battery is plug and play. The link to the connectors show several. The one I indicated is the one you want. It is center pin positive. Most are default that way, but you need to be certain as reversing the polarity on yours with a center pin negative connector will damage it. The information for the one I referenced says it is "inside positive" which means center pin positive. Buy this one with the battery and all is plug and play.
Yes, the sound stage and imaging manipulation that active components and even some cables add to the equation is very evident after listening to a passive preamp in the system. I would agree that the LSA does not have the spatial capabilities of some other active preamps, but I'm fine with that as it does reproduce the music in a truer form.

Hanging out with a recording engineer of late, I have learned that most of what we perceive to be the sound stage created by our systems comes straight from the recording. Room acoustics play another part, as does speaker placement. I myself dislike hearing a drummer whose arms appear to be 8 feet long, or a piano that appears to be 12 feet wide, or a vocalist whose mouth appears to be a 3 foot round oval. Too much for me. I'm not sure who it was that said this, it was a reviewer I think, but the point was that the sound stage should not extend beyond the speakers. I think I might agree with this thinking. Depth and height, as well as space between the performers are another matter though.

I have a couple of compilation discs from Ridge Street Audio that offer excellent recordings where the sound stage is well reproduced. I made some copies for a friend and he commented on the improved depth of the sound stage he heard with these recordings. He seldom hears the same level of depth with other recordings. Goes to show what can happen when the recording engineer is paying attention.

The Truth awaits us...
I have used the Lightspeed with both MC and MM cartridges. No issues whatsoever. With MC I use 68db gain on my phono stage and the Lightspeed is at about 2 o'clock or so on the dial. With MM I use 42db gain and the Lightspeed is at 11 o'clock or so on the dial.

I have even used the Ligtspeed with an amp whose input sensitivity is nearly 3V. I even doubled the sensitivity to nearly 6V and in both cases the Lightspeed worked fine. at 6V I was pretty far around the dial, but that is what the amp manufacturer recommended having tested a Lightspeed on his own.
Great sound does not have to cost a lot. Great looks are nice to have, but it doesn't necessarily equate to great sound. Convenience seems to be something many audiophiles want, but a remote or multiple inputs can do more harm than good. Tubes are fun to play with, but can inject noise or colorations.

We get to make our choices and we get to live with the results, for better or worse.
What was the output of your low MC's? (maybe I screwed something up, I'm a vinyl newbie)

Dynavector 20XL low output version rated 0.25mV. However, mine was going into amps whose sensitivity was rated 1V and 0.7V respectively.

As for being a vinyl newbie, maybe I'm mistaking you for someone else, or maybe the vinyl set-up listed in your system is a mirage. However, I've been to that pictured location in San Pedro listed on your system page (I think you've moved since then) and bought one of your two Scheu turntables at the time. Having two of those tables, me thinks you're probably not a vinyl newbie, but hey, when it comes to vinyl set-up we all make mistakes.

Try again and see.
Yes the idea of an active preamp, or in the case of The Truth preamp, a buffered passive, the idea is to control the ICs. However, in unbalanced designs its my understanding that while this might provide some benefit to using longer runs (ex. you can use 30 ft. ICs with the Truth with no sonic degradation), it doesn't mean that the coloration/artifacts of the cable itself can't be heard.

I've convinced my tech here to design a passive balanced preamp that supports the 600 ohm standard. If I use this with my Atma-Sphere S-30 I should be able to eliminate the cable coloration/artifact from the equation.
George, what don't you like about that 1uf coupling cap? Any suggestions for modifying the circuit?

I will try the 82k ohm resistor. Thanks for the suggestion.

BTW - the amps I have in house are full range only.
Maybe you have even better amp/speaker synergy. That's where the real magic in sound reproduction lies.
I like the idea of blind listening sessions, why are so many audiophiles and reviewers against it, or at least to believe in it as a effective method for assessment? That always seemed odd to me. Yes, it would be a good idea to do such a test absent the knowledge of when one is listening to a particular piece, especially when they have invested themselves in a piece financially and/or emotionally.

Most of these tests as conducted today are flawed to some extent. Unfortunately you can't create an accurate enough control environment. That being said I think they are fun and a method by which opinions can be shared and discussed. Not that we're going to get any definitive answers, more likely the usual subjective opinions.

One thing that would be nice in the LSA, and most modern preamps, though it might "ruinous" or a least "deleterious", is a scheme for balance control to account for recording mixes, room geometry, and human frailty as we age - not sure how many ears are balanced L/R- perhaps than we would like.

Mine technically has a balance control by virtue of it's dual volume controls. I like this feature for all the reasons mentioned above.
I did not realize that the Concert Fidelity pre was a "passive with balls" also.

Not sure what you mean. The CF-080 preamp has a gain stage so technically it can't be a passive. The Si2 preamp which I own and uses the same volume control as the CF-080 is a zero gain device using an active buffer. That could be classified as a passive with balls IMO. IIRC I believe the VRE-1 engages a gain stage when the volume control reaches a certain level. So it too has passive attributes. Quite a nice preamp to boot.

For some light reading on the Si2 lineup look here:

I was pleasantly surprised to receive an email from my friend Hajime Sato regarding RMAF and the use of Concert Fidelity gear by Dale Pitcher. It will be interesting to hear with his speakers.
I should know today, but I hope to be going to RMAF. If I attend I will be doing so as an assistant to Hajime Sato in the Concert Fidelity/Silicon Arts Design room. So I will definitely have time to stop by Dale's room as well. Last time I was at RMAF Dale canceled his visit at the last minute, so I never had a chance to meet him. It's something I hope to check off my to do list.

I agree the thread over at the DIY site is a bit winded, but there is a lot of great information about the history and uses of the opto coupler. Nelson Pass even partakes for a bit. In comparison, The Truth preamp I have uses photo cells, but not opto couplers. The idea is basically the same though as attenuation is controlled by an LED that shines light on the resistors. Looks pretty cool in action.
I wouldn't say the battery option improves the sound, but there is something different about the sound when using the battery power supply. I just can't put my finger on it. I go back and forth between the two.
Pubul57, I know what you mean. It's my preference to have dual volume controls and when I ordered my LSA I just asked George if he could do it that way for me. I don't think it is an official option as George feels his stock design is best as is.

On another note, I have found that moving your listening position laterally one direction or another can act as a "balance control" too. We're not talking much distance here, maybe less than an inch or two and small increments make a difference.
I have used the LSA with the VAC Auricle Musicblocs, Music Reference RM-10 MkII, and Atma-Sphere S-30. The Atma-Sphere/Audiokinesis combo was the last configuration it was used with. In all three it worked fine. In some weird sort of way I might like it with the S-30 best. It allows me the most use of the volume control. Something Ralph Karsten recommends with most passive designs.

All in all, given the proper matching criteria I think the LSA would feel at home with just about any amp/speaker combo. I don't often pay much mind to what Stereophile says but the fact the LSA even got on their radar is enough of a testimonial IMO. The fact they liked it, well its just hard to ignore the truth sometimes.

Speaking of The Truth, more to come...
Wonder what Arthur Salvatore would think of it and if it might make his Class A recommended list. He's known to favor a passive approach with the right system and doesn't give a hoot about what something costs when he associates it with a specific class.
Maybe it was previously mentioned, but can you make these with two volume knobs for left and right channels to adjust balance?

That's what mine is. Cost difference was negligible. Of course mine is not remote. Dual volume remote I'm not certain about.
I think it came from a Stereophile article. Not sure who it came from though.