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.
pubul57
George is the expert and I'm sure he will give you the definitive answer, but I think I read somewhere that LDRs work a little different and that I'm not sure it actually improves as you get further into the rotation as is normal with most attenuators. George? Al? Anthony?
A couple things. One, the sensitivity of your amp is quite low, 770mV (and if you have the 350mV option extremely low). The input impedance is a very compatible 180k ohm. I looked at your system page and was wondering if you are using the Metric Halo LIO-8 as your source. I did look at the specs of that piece and the output impedance is quite low at 5 ohms, which is compatible with the LSA. The spec I couldn't find is the output voltage of the unit. What I did find was:

Maximum I/O Levels (Balanced)
Peak Line Output @ 0 dBFS (no jumper/low power) +18.5 dBu
Peak Line Output @ 0 dBFS (output jumper/low power) +22.0 dBu
Peak Line Output @ 0 dBFS (output jumper/high power) +24.5 dBu
Analog Send Max Output +21.5 dBu
Line In Max Input +24.5 dBu
Output Impedance 5 Ω

Monitor Controller
Nominal FS output (Balanced) Output Jumper Off -19.0 dBu
Nominal FS output (Balanced) Output Jumper On -12.0 dBu
Maximum Output Same as Line
Gain Range -96 dB to +30 dB
Gain Precision ±0.05 dB
Gain Step 0.5 dB

I'm kind of curious of what the effect of the peak line output and gain range are all about. It's a bit foreign to me but this is a pro audio unit so it appears they are stating things a bit differently. Could be too much output from the source and it doesn't take a whole lot to drive your amp to full output.

The LSA doesn't have the same mood swings as your typical resistor based passive, but it doesn't maintain the same output impedance across the volume control. The big thing in your favor is the 180k input impedance of your amp. You are likely to be well above acceptable ratios no matter where you are on the volume control. I think worse case scenario is the output impedance of the LSA is around 8k ohm, so you have a very good cushion given the high input impedance of the amp. It would be nice to have a bit more flexibility before you hit your desired volume level, but I don't think you are missing anything at the level you're at.
Kevinzoe
Am I missing anything by not having the volume at a higher setting (11-1 o'clock for example)? Besides getting louder, will the sonics improve as I turn the volume up louder?Kevinzoe

Unless you have very high capacitive interconnects (more than 200pf per foot) you will not hear a difference, even with 200pf per foot = 600pf for 1mt, the -3db point will be at 38khz at the worst position (highest output impednace) of the Lightspeed output impedance.
If you use better interconnects 100pf per foot = 300pf per mt, the -3db point will be at 76khz.
I use interconnects that are 40pf per foot = 120pf per mt, this with the Lightspeed is -3db at 190khz. and so on.
Cheers George
Clio09 - thanks for taking the time to do your homework before your great answer above. While I intend to purchase the LIO-8 DAC I am currently using an antiquated Sony 5 disk player (CDP-CA7ES) which has an output impedance and voltage of 10K ohms and 2volts respectively.

Knowing these specs now, does it change anything in your answer Clio09 or am I still 'safe' in that I'm not missing anything sonically by having the volume control low at 8-9 o'clock?

Off topic - was the DIY Skyline Diffuser write-of of mine helpful / useful for you?
Just looked at the specs of the Sony CDP-CA7es and the 10k for the output is the min load recommended, I would say with this figure it has an output impedance of around 200ohm which is fine.
You will not do any harm to anything using the Lightspeed at 8 or 9 o'clock, the only detriment is the range you have is minimised, sound will be the same with good interconects at 9 o'clock or at 12 o'clock and with 180kohms input impedance this is a great high input for any passive.
Cheers George