Clio09, Of course in LSA the series/shunt photoresistor IS the volume control. The knob that you twist merely controls the control. I'm suggesting that by adding a companion active buffer, the LDR control could be directly compared to a conventional volume control(e.g. potentiometer, stepped attenuator, etc.) independent of systems matching. Chances are good that an LSA buffered in this manner would come out ahead of most active preamps. And by switching off the buffer function you would know immediately whether the problem is a mismatch.
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.
Showing 14 responses by dgarretson
"Amplifying the low-level stuff, stuff that might not get through (stage depth & other nuances) is important and not insignificant as we know. Is it possible a great active can get more of this information by way of design then a passive?" When I was building a replacement analog stage for my Sony CDP, I compared an entirely passive solution that used silver step-up transformers directly from the voltage DAC to output, to the same transformers with the addition of an active battery-powered buffer. In each case the CDP output was connected to an Atma-Sphere MP-1 tube line stage. Going into that experiment I expected to hear sonic trade-offs, with the passive winning on treble resolution & overall sonic purity, and the active winning on dynamics and LF control. In actuality the hybrid buffered approach won on every point. As a result I opine that if the impedance match is anything less than perfect(and who really knows for certain what is perfect?), a passive would benefit by being equipped with an active buffer on an A/B switch. There are several simple & inexpensive buffer designs(including one contributed to the Lightspeed DIY thread by Nelson Pass)that will do justice to a top-quality passive. In this scenario the comparison of LSA to other preamps becomes more of a contest between volume controls-- which is a critical and oft-neglected determinant of a preamp's performance. |
Thinking about those DartTZeel and Constellation preamps with LDR volume controls, I just replaced a four-deck Shallco series resistive attenuator in a differentially balanced tube preamp, with a 100K impedance LDR volume control. This is a relatively simple series/shunt arrangement, with fixed series resistors and a single LDR per channel as a variable shunt between phase and anti-phase of the balanced signal. One nice thing is that if limited to use as a shunt, the LDR only needs to operate through a range of 35R-10K ohms or thereabouts(similar to the range used in a passive) to provide a wide range of attenuation down to -70db in a 100K attenuator. It's still breaking in, but so far it's clear that an LDR volume control is superb. Thanks to George for popularizing their use! |
Indeed adding a buffer stage to an active preamp at input to the volume control allows the LDR to operate within its more linear range below 10K impedance. However in a balanced mode there is no escaping imperfect common-mode noise rejection if using LDRs in the Mk II switched ladder arrangement. However carefully the LDRs are matched, they always deviate from each other by at least 5%-- not close enough for optimal balanced operation. On the other hand, the simpler Mk I configuration ensures perfect CMRR-- provided that the shunt LDR is located between phase and anti-phase and not between each signal phase and ground. Any active tube preamp has at least several fixed resistors in signal path in addition to the volume control. Provided that the preamp can tolerate a shunt volume control, I can't imagine that one LDR more or less in place of one of many fixed resistors in signal path will matter much. Moreover, adding an input buffer just adds more components in the signal path and may thus introduce as many problems as it solves. I think this is an interesting development in the discussion. Combining the LDR technology with active balanced circuits offers relief from the usual SE vs. balanced, passive vs. active debate. Some proponents of Lightspeed tend to dismiss balanced circuits altogether. Personally I am a balanced guy from phono coil all through system to amplifier output, and am not ready to give this up. Moveover, adapting the Lightspeed to a high impedance phono stage source, presents further challenges a passive. IMO it's preferable to be able to combine these all these design constructs with minimal compromise, rather than to insist upon a purist approach that mandates exclusion of a particular construct. |
Augwest, IIRC the BBAM is a capacitive load with 40K input impedance. This could present a difficult load to an LSA passive. In my instance the challenge is compounded: I split the preamp's output between the downstream BBAM and a sub with 47K input impedance, yielding a combined load of 22K. With the AtmaSphere MP-1 preamp I can't hear any difference between the BBAM placed upstream or downstream of the preamp. However I bet that in your case placing the BBAM downstream of LSA will degrade performance. I replaced the stock volume control in the Atma preamp with a balanced shunt LSA Mk I type volume control that employs a TX2575 Z-foil resistor in the series position and a Lightspeed/Silonex shunt resitor. This was a major upgrade. Some preamp builders will argue that most of the performance gains to be had in a top-quality active preamp are made at the volume control. Based on this experiment I tend to agree. For a true apples-to-apples comparison it would be interesting to compare your LSA passive to an active preamp with an LSA-type volume control. The only current commercial product I'm aware of that employs an optical volume control is darTZeel. |
Clio09, Thanks for remembering. I replaced a balanced Shallco attenuator with a 100K balanced LDR attenuator in my AtmaSphere MP-1 phono preamp with great results. It is a relatively simple project, provided that use of LDR is limited to the shunt position in conjunction with a high-quality fixed series resistor such as TX2575. One of the many nice things about this application is that the useful range of attentuation across the full arc of the dual control pot can be varied to your desired gain structure by varying the value of the control pot. To obtain a range of attenuation between around -70db and -30db I used a 500K control pot with dual 50K trim pots for L/R balance. Depending on ambient and chassis temperature one must occasionally rise from the couch to trim channel balance. The improvement in sonic performance is well worth the effort. |
Grannyring, Feel free to PM me for the speaker cable recipe or for ideas about implementing LDRs in a volume control for an active preamp. [email protected] |
I have not heard LSA but have used the same LDR resistors to provide variable cartridge loading in a modified phono stage. LDRs are about as clean sounding as the best nude Vishay, Caddock, and tant resistors, so I imagine they would make a first class passive. Regarding the allegation of "forwardness," assuming that we are not talking about strident aggressiveness, the quality of forwardness in a top component is often a good thing in the sense that the piece sounds more alive, faster, dynamic and resolving. The listener is literally closer to the music, in fact the stage may extend both forward of and to the rear of the speakers. In this scenario depth-of-field cues are delivered through high resolution. Instruments appear layered in depth more by virtue of low-level cues than by soundstaging per se. The more astonishing hat trick may be when a system throws instruments outside the L & R speaker boundaries, or does a perfect job of imaging intentionally-recorded phase anomalies to the side of or behind the listener. Some of this is contingent upon room characteristics, but with great electronics it can be surprising how much of it can be pulled off independent of room. |
George, given the non-linearity of LDRs at high impedances, I don't think that it is practical to adapt your Mk II approach to a high-impedance volume control for an active preamp, whether balanced or single-ended. The Mk I approach with fixed series resistor and variable shunt LDR, may be the best that can be done in this regard. If the system is sensitive enough that in normal use the volume control is operated in the region between say -25db and full attenuation, then the Mk I approach will present a stable & benign input impedance similar to a traditional switched ladder. A respected designer of $15K active preamps recently opined to me that approaching SOTA, the quality of the volume control accounts for 85% of the performance of an active preamp. If one is committed to an active preamp, it's nice that a few hundred dollars and some relatively simple DIY gets the job done with LDRs. Clio09, my delay was due to the nagging fear that the failure of an LDR shunt LED will take the preamp to full volume, with disasterous conseqences downstream. A simple insurance bet is to parallel the LDR shunt with a fixed resistor, whose value is chosen to put a brake on max volume. To control the LDR LED segments, I use a 500K dual log pot as a master volume control for coarse adjustment, and a 50K log pot on each channel to trim balance. The LEDs are powered 5V by a Twisted Pear Placid current-shunt regulator kit. This set to pass 10ma to each LED. |
Hi George, I'm thinking of an DIY LDR attenuator to replace the stock 10K Alps dual pot in a Sennheiser HEV70 energizer for HE60 electrostatic headphones. I need to step down from 25Vdc to 5V for this application. The chassis is cramped and there is little room inside for a heat sink on the down regulator. An LM2596 looks like an easy fit in terms of low heat dissipation and compactness. Do you think this would compromise sonic performance relative to a LM7805 linear regulator? My concern about the LM7805 is overheating from the 20V step down. Thanks, Dave |
@audiolabyrinth Sorry, but you won’t be the first in the world! I replaced the stock Shallco series attenuator with a differentially balanced LDR control in my AtmaSphere MP-1 phono preamp. It is a successful upgrade, though not quite viable for commercial purposes, as the LDRs drift a bit with the heat of all those tubes. |
@chakster Wow, your reference to my post is from the deep archives. I recall trying the Lightspeed schematic together with battery-powered monolithic Nat Semi buffers. I ended up (and still employ) the Lightspeed schematic as a volume control in a tweaked-out AtmaSphere MP-1 tube preamp. TX2575 series resistors and Silonex LDR shunts in a differential balanced circuit. It sounds fantastic relative to the stock Shallco series attenuator. |