Sorry plus the 21k added to the 9.3k, yes 30.3k.
Cheers George
Cheers George
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.
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.
1,866 responses Add your response
Devilboy, I have a pair of the 10db Rothwell's, and as measured with my not particularly accurate analog multimeter the series resistor has a value of about 21K, and the shunt resistor has a value of about 9.5K. As you realize and as George has confirmed, the 470K input impedance of your amplifier certainly presents no issues. (As you no doubt realize, the attenuators should be placed directly at the input connectors of the amp, so that there will be no interaction between their output impedance and cable capacitance). With the attenuators so located, and driven by the LSA via a short cable, the LSA would see a load impedance of about 30K. I suspect that is not an issue either, based on the assumption that the impedance "looking back" into the LSA is essentially resistive and does not vary significantly as a function of frequency, given that you will be driving it with a solid state DAC having low output impedance. Hopefully George will confirm that assumption. You will not be able to drive your amp to full power, though, even with the LSA turned up all the way, as 10db attenuation will reduce the DAC's 2V maximum output to about 0.63V. For most volume control settings of the LSA, btw, the attenuation resulting from insertion of the "10db" Rothwell's will be more like 11 or 12db, because the impedance "looking back" into the LSA will sum together with the Rothwell's series resistor. Concerning the sonic effects the Rothwell's may have under properly impedance matched conditions, I'm not using them with the VAC Renaissance 70/70 MkIII amplifier I currently have, but I noticed no adverse effects when I previously used them with a lesser quality EL34-based Paxthon amplifier. Regards, -- Al |
10-11-11: Devilboy Thanks, George. The input impedance of my amp is 470K. I can't imagine that being an issue with the Rothwell's.Devilboy No that is not a problem, what is the problem if you go passive or tube preamp, is that while the 470k input of your amp is fine for these preamps, when you plug in the Rothwells then this changes that 470k for something much smaller. What I don't know as I have never measured the shunt to ground resistor of the Rothwell, It maybe say "10k?" then this is seen by the passive or tube preamp not the 470k, and then it will not be a good impedance match for them. As Clio09 found out when he used them, it wasn't the Rothwells them selves that he didn't like it would have been the input impedance change of the poweramp he heard that then was not a good match with whatever pre he was using at the time. There is no free lunch, you have to do your homework, regarding impedance matching. Like I said before it goes on right through the whole system from source through to speakers, up to about 6 to 10 times, you can't do anything within the components themselves unless you are a good tech, hopefully the designers of individual components have done the right matching inside their units, but you can look at the output and inputs of each component to make sure you have a good match and give everything a fighting chance to sound it's best. Cheers George |
The Rothwell attenuators work just like the Lightspeed Attenuator. That is, they have a soldered fixed series resistor with another shunt fixed resistor to ground, this forms a voltage divider just like the Lightspeed Attenuator. Except the in the Lightspeed Attenuator these series and shunt resistors have variable resistance with the amount of light that shines on them. So what you have is a variable voltage divider in the Lightspeed Attenuator. The only problem with the Rothwell's is that they change the input impedance of the device they are attached to, so this needs to be taken into consideration when using them, with passives and tube preamps. Cheers George |
I never liked those Rothwell attenuators. You can accomplish the same thing by adding a resistor in series with your interconnects (there might be a little more to it, but this is the basic idea). I believe Luminous Audio makes cables like this upon request. Since you are cutting up some cables now might be a good time to consider doing it. |
My amp is 1.3V sensitive. After I get my 2V DAC, the only other change I will make will be to use 1/2 meter pair of solid core interconnects leaving the LSA to go to the amp (I'm in the process of cutting a one meter pair to half them). So, as George said, I can use RCA attenuators to cut the gain down a bit but I really don't want to add anything else in the path. I'm more of a "less is more" mindset. Anyone have experience with these devices degrading sound quality? I've come so far, I don't want to start going backwards... |
Different amps will have different gains, so will sources, and speakers also have different efficiencies. This will all govern where the volume control (on any passive preamp or any active preamp tube or transistor), position will be for normal listing. Now with the sources giving higher outputs than they ever have, and the recordings being higher in level also, too much gain is really becoming an issue with most systems. This is why they are starting to make rca attenuators like the Rothwell Attenuators in -10dB and -20dB to take care of this problem. http://www.decibelhifi.com.au/category42_1.htm Another reason why there is no need for active preamps with gain, we have too much gain coming out of nearly every component in a system. Cheers George |
I had close to that issue with my CD player. I used to max out at around 10, which gave me very loud, but not ear splitting levels. Now I'm at 12 with loud (so my wife says) but comfortable levels. I don't know why or when the change. I have changed amps sometime inbetween so that might be it. With vinyl, I'm now at 3 for pleasing but not head banging levels. I don't think I can get head banging levels with my amp which I'm now light loading. |
Ok...When I first received the LSA, it was placed in the system downstream of a Burson HA-160 DAC/pre. When the Burson's volume pot was fully open, I had a wide range of volume control on the LSA, with moderate volume achieved at around 10-11 o'clock on the LSA. The Burson is gone and I am awaiting arrival of the Metrum Octave DAC. Now....I took out an old, cheap-o dac I had in a closet (I have no idea what the specs are), and connected it to my Mac Mini via toslink. Doing this results in HIGH volume levels at 9 o'clock. I put a cd player into the LSA. I don't know the cdp's output impedance but at 2V it has the same output voltage as my incoming DAC, the Metrum. Same thing! 9 o'clock on the LSA is too loud. Anyone else have an issue with the LSA achieving very high volume levels at a low position on the volume pot? |
Here is what you are looking for: http://www.jensen-transformers.com/isokit1.html In the photo you will see two plugs in the lower left with orange barrels. These can be purchased separately and IIRC are less the $20. They are diagnostic only, but more accurate than shorting plugs at isolating ground loops. Only other reason I can think of for the noise is RFI. Phono stages and tubed ones especially are notorious for picking it up, cables too. |
There should be only one mains earth point in a system, (if you have more than one you will have a loop), all the other components will get their earth via the interconnects from the one that is earthed. Once you have two components earthed then it will become a loop via the house earth line. Even when I had two monoblock 805 tube SE that I built, I got a virtually no hum when only one was earthed and the other got it's earth via the interconnect to the Lightspeed and back down the other interconnect to the unearth monoblock. PS wonder if I'll get the magic 1,000 post with this as they moderate my posts for up to 6 hours before approval. Cheers George |
Clio09: what's a jensen plug? I looked up Jensen 'iso-max' products but there is a legion of them. Which one are you referring to? Is it for diagnosis or treatment? Regarding lifting the ground, the only other item I have with a 3 prong plug is my stage. I use the walwart that George pointed to and it is two pronged. My tt's power supply is also 2 pronged. I tried a cheater plug with the stage and it didn't help. I had entertained the thought that it was the LSA's walwart that might be responsible. But I swapped in a battery supply I have and the same noise persists. I once considered a switcher but I'm over it as well. I use almost exclusively my tt, and swapping IC's is easy because I have an open rack. I can see the appeal though if one had a cabinet and switched sources constantly. I'd be interested in hearing the difference a switcher makes though. Anyone use one? |
Hi guys, as I have found out input switching is almost as detrimental to the sound quality as a volume potentiometer, this is why I always recommend a separate good quality input switching unit so you can still go direct when you want to bypass all contacts and get the best out of your system or when you want to show it's transparency off. http://www.decware.com/newsite/rotary.htm Now the obsessed fiddlers on diy audio have found this out also, and to their credit they have now devised their own way of input switching unit using ldr's http://www.diyaudio.com/forums/analog-line-level/197894-ldr-based-input-selection.html I still think the one I posted was simpler and it auto shorted the unused inputs. http://www.diyaudio.com/forums/analog-line-level/80194-lightspeed-attenuator-new-passive-preamp-153.html#post1640716 Cheers George |
Banquo363, regarding the RM-10, perhaps some DC on your AC line if you can hear the hum at the amp. It doesn't take much. When I lived in a tri-plex in CA if my neighbor used her hair drier or turned on her window unit AC I would get similar symptoms to what you are describing. Unfortunately dedicated lines were not an option for me. To really isolate the problems after you begin plugging other components into the amp you should get a set of the Jensen plugs. One of the better investments I have made. Also, I won't advise you to do this, but in my system I float all but one of the grounds. Definitely on all my sources, then either ground the preamp, or ground the amp, but not both. The RM-10 manual discusses this too as I'm sure you know. The K&K phonostage, along with the Hagerman Trumpet were two of the best phonostages I have owned. The K&K was the quieter of the too. In general it is a very quiet phonostage due to the solid state PS and input. The newer versions are supposed to be better and I'm seriously thinking about revisiting it. |
Clio09: with shorting plugs, I had to put my ears to the speakers to hear the hum AND buzz on the rm10. However, I just tried it again and now I get dead silence. ??? There is a hum emanating from the transformer when I'm near the amp. It varies in its loudness depending on the fullness of the moon :). I thought until just now that that hum was making its way to the speakers--guess not, or not always. Yes, with shorting plugs in the LSA, I get roughly the same level of hum and buzz at all volume levels (well, up to 3 o'clock--I've never turned the volume past that). And it was the same level as the RM-10 with shorting plugs, i.e. very, very low. However, today, with the amp dead silent, I get the same buzz and hum with the LSA. Once I introduce the phonostage, then the objectionable noise appears. With IC's into the LSA but the stage off, the hum is loud. After I turn on the stage, the hum diminishes greatly and is overtaken by or competes with the tube noise in my stage. At ambient room levels, it is barely audible at my seat with the LSA at 3 o'clock. I very rarely listen at that level so it doesn't affect my music. But being neurotic, it still bugs me. My phono cables are grover huffman's latest. They are supposed to be well shielded. As for lead wires, I swapped out the stock wires on my AT LS-12 headshell with copper litz wires. With everything else going on, it's hard to say what effect that had. I should just sell my stage and get one more compatible with the LSA and quieter. To be honest though, the system sounds great, so I'm hesitant to mess with it. Do you have a K&K stage, Clio? How's that in terms of noise? |
Banquo363, have you tried the cheater plug on the RM-10? How loud is the hum? If you have to put your ear to the driver it is okay, but if you can hear it standing in front of your speaker or further away then maybe you should talk to Roger about it. In my system I do not get any hum at all from the RM-10 or EM-7 V12. When adding the LSA and shorting the inputs did you check for noise at various volume levels? The phono stage is more complicated. If a cheater plug did not work try running a ground wire from your phonostage to another ground point, maybe the center screw on the nearest outlet. Any possibility it could also be your phono cable or cartridge wires? |
I don't know what it is (thanks Almarg for the explanation above), but I've been switching my short (1.5 ft) and long (3ft) interconnects as described above and I always get better sound when the short IC is between LSA and rm-10. Perhaps it's as Almarg noted: that there's an attenuation of bass due to variations in the impedance of my tubed phono stage. At any rate, I now get more dynamic and better bass (articulate and rich). I get 15-20db dynamic swings (as measured by my radio shack meter) on symphonic music, from the average sound level to peaks. Not bad at alI, I think. What counts as a good dynamic swing? Joe Morello's drum whacks can now sound like a shot in the dark. I've made a myriad of changes to my system of late and I'm continually impressed by how the LSA allows me to note them all, good and bad. I guess that it's the LSA's neutrality 'at work'. Thanks, Clio09 for the shorting plugs idea above. I made some and went through the procedure. My rm-10 is not dead quiet, close but not quite. Transformer hum, I believe. But the LSA with shorting plugs adds no additionally noise. My phono stage however...There's a dreaded ground loop I think. I bought a cheater plug but that didn't help. Mercifully it's not too loud. I need to have the LSA at 3 o'clock to be bothered by it at my seat. On a side note for those using the rm-10: I've been using NOS GE El-84's and they're fantastic. Superior to the stock tubes and to the Genelex reissues. |
2: Never had any problems with the ldr side, as my prototype is still as it was when new and that's 7 years now powered 24/7. When I say in the instructions to leave the Lightspeed Attenuator at 1/2 volume when leaving it powered up and not listening, it's the led's I'm thinking of, even though when at full or min volume they are never at their max mA rating, it is in my opinion to share the use evenly so I say leave it at half way if not in use (listening). 5: The best is as it is now, it's simplicity in it's purest form, as the music signal only goes through one soldered resistor (ldr) from input to output, anything else would need to go through more devices. As for the power to drive the led's, it is known that battery is the purest form of dc power, and so many times we have A/B'ed between the wall wart and a Lithium Ion battery and nobody could detect the difference reliably some say they hear a difference but can't tell what it is. Even golden eared Sam Tellig, need to see with his own eyes if he has the wall wart or battery attached to his Lightspeed when he's listening. As for the Warpspeed not interested in it, too much other crap being put into the signal path. As for doing any other things to the Lightspeed, I would like to have more inputs, but the only way I would do them is to use Mercury Wetted relays for the input switching (which I believe are being made illegal) and that would be still not as good as only one input with no switching. So use one of these, and then you can still go direct when your really want to hear your system at it's best. http://www.decware.com/newsite/rotary.htm The only thing I may do in the future if I find a reliable set is a remote control volume and I would make sure it's retrofittable to all older Lightspeeds. But I have not yet seen a system I'm happy with that has long term reliable supply and quality. For a remote volume you need, a quality remote, receiver circuit, motorized pot and power supply to run it and the receiver Cheers George |
True, I can't see what more can be done to the circuit. I think George already addressed that going from MkI to MkII. I think we can forget about any upgrade in ergonomics for the time being. Reading about the Warpspeed I find the designer has this thing he calls a Squid in the circuit, but I think it is commonly known as the VCCS. IIRC an idea he got from Nelson Pass. I'm a little fuzzy on that though. Maybe George can clarify. |
I read that as well and find #4 interesting. Wasn't the whole idea of using LDRs and optocouplers in George's design to take the switch out of the equation. In other words a $5 switch would work just as well as a $200 switch. Regarding #1, this is true in my system, but not sure what the point is. It doesn't degrade the sound. As for #3 I guess by virtue of the fact that the Warpspeed uses a SLA battery, the designer must feel battery power is better. While I liked using a battery power supply with my LSA, I can't say it was better, just a little different, but I could never really pin point why. I now use the battery PS on one of my LSA's and the linear PS on the other. So for me clarification on #2 and #5 would be helpful. |
George, in the Warpspeed DIY thread, the poster identified some area we he thought the LSA could be improved: What I found interesting at the start of the Warpspeed thread was the identification of a few "problems" with the LSA and attempt to improve upon the LSA: 1. the inability to adjust to complete silence 2. on low/high volume level settings, power levels on the LEDs endanger/shorten/toast the life out of the optocouplers 3. the need to improve the power delivery to the optocouplers 4. the need to improve on quality of the volume adjustment pot 5. the Lightspeed, simple as it is, still has a number of variables/design factors that affect performance What is your view on these issues. On 5, we wonder if perfomance can be improved or you feel that after 30 years of sorting this thing out we are pretty much at the performance limit - obviously some ergonomic issues exists, but might not be addressable without compromising ulitmate performance. |
I bought a Lightspeed about a year back. It works really well with both the PSAudio DL-III and PWD. It lets a lot of the more subtle details come thru, and with less edginess. Please note that you need a low-output-impedance source and a high-input-impedance amp to really take advantage of the LS. Both of the PSAudios have 100 ohm outputs, and my amp has 220k ohm inputs with 20dB of gain. Altogether, given the impedance and gain issues, the LS is certainly worth auditioning. That it costs less than $500 makes it a no-brainer. |
09-28-11: Banquo363Not necessarily. It depends on how the two impedances vary as a function of frequency. If neither varies significantly over the frequency range, there would be no adverse effects, just a slight gain reduction. I couldn't find any indications of the output impedance vs. frequency characteristics of your Allnic H-1200, but more often than not the most significant output impedance variation of a tube-type line-level component will be an impedance rise at deep bass frequencies, due to the output coupling capacitor that is commonly used. That would cause deep bass frequencies to be attenuated, assuming the input impedance of the destination component is reasonably flat as a function of frequency. What will attenuate high frequencies is a combination of high output impedance (at high frequencies) plus high cable capacitance (or, in theory, high input capacitance of the destination component, but that generally doesn't occur in practice, and I assume does not occur with the LSA). Regards, -- Al |
Banquo363Am I right in believing that non-ideal impedance match between phono stage and LSA results in a attenuation of the high end? Perhaps it hasn't bothered me because of my old ears?Banquo363 No, the slight impedance (resistance)mismatch you have will compress slightly the dynamics, and if ever the output impedance becomes higher than the input impedance then you will attenuate the whole signal, as it then becomes a voltage divider. What attenuates the highs is lots of cable capacitance. Cheers George |
In most cases high end roll off is the symptom. As we age, typically our high frequency hearing regresses anyway. You're not that old though, not that I could tell anyway. Maybe you just find the sound pleasing, for whatever reason, as I did when I used the LSA with an amp with 21k ohm input impedance. Sometimes we take these ratios and such to an Nth degree when in reality maybe it doesn't make as much difference as we think. |
Banquo363 The 1.2kohm for the phono is very high and should not be an "ideal" match with the Lightspeed, but it will not be harmful to anything either. I have quite a few customer with dac's and phono stages with output impedances this high with tube output, and they say they couldn't care if it's not a 100% match as they are over the moon with the sound that they are getting with them into the Lightspeed Attenuator. Cheers George |
The HF filter that capacitance of an interconnect that is 300pf for it's total length when with the Lightspeed is 76khz -3db 200pf is 114khz -3db and for 100pf it is 227.5khz -3db. As you can see even the 300pf is fine as it is still only -3db at 76khz. This is why I recommend 100pf max per foot and the use of 1mt length with that maximum capacitance. But most good quality interconnects are well below 100pf per foot which is even better. This also applies to high output impedance tube preamps with their interconnects, especially when say a 30kohm output impedance preamp is used, if it has an interconnect with 300pf for it's length, it works out to be -3db at 17.5khz which cuts into the audio band quite severely. Cheers George |
In the system I have that features the LSA my speaker cables are 18' long. The brand is OCOS. They are made by Dynaudio and are known for their performance in long runs. I use 1m interconnects. As for capacitance on the interconnects. Anything under 60pF/ft. should be fine. I'd venture to say you could go higher if you use 1/2m cables. There are a lot of fine cables out there that meet these requirements. The best method, if it is possible, is to place the passive volume control at the amps input. Ralph offers this option on the Atma-Sphere line for the M-60 series on up. However, given the design of the LSA, I do not believe it would be feasible to place it at an amps input. I am curious though if the LSA can be hardwired from source through amp. |
My record for misspellings in a single comment. George, is there any issue other than capacitance related to cable length? That is, would a 1/2 meter of a quality cable almost always sound better than 1 meter even if it were slightly more capacitive; is something going on with length other than aggregate capacitance? Are any of these differences even relevant a 1 meter or less of IC? |
