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georgehifi 25th May 2006 07:13 AM

Lightspeed Attenuator a new passive preamp
6 Attachment(s)
I've tested and listen to just about every concievable passive potentiometer (volume control), Bournes, Alps Blue Velvet, Alps Black Beauty, Penny&Giles, Dact Switched resistors, and transformer (TVC) based volume controls.

Nothing has equaled the transparency and dynamics of sound than that of a series/ shunt LDR arrangment in the attachement, it has no contacts in the signal path, I believe that is the secret.
The only stipulation that all these passive controls including the series/shunt LDR need I believe is,
1: Low source impedence (cd player) <1kohm
2: High input impedence (amplifier) > 33kohm.

Cheers George

To explain a little the 5 photo attachments.

1st: Is the basic led/ldr and how it works in the Lightspeed Attenuator, it mimics a 10kohm series shunt logarithmic potentiometer.

2nd: Is the Lightspeed Attenuator stereo version, complete circuit diagram, note how the signal goes through nothing but one soldered resistor, no sound robbing light contact of a metal wiper on a resistive track in a volume control (potentiometers) in the signal path.

3rd: Is the same but a dual mono, good for room or system imbalances, (for some reason owners of both say the dual mono has the edge sound wise) I believe it's the fact you can precisely centre the image regardless of recording or environment influences.

4th: Is the buffer Nelson Pass designed a few years back for the Lightspeed Attenuator (later to be known as the B1 Buffer) for it to be able to drive low input impedance amps of less than 33kohm.

5th: Is the $5 led tester you can get on Ebay. With this you can quad match your own set of NSL32SR2S(sorted) Silonex led/ldr's you'll need get about 15 pieces to get 2 sets of quad matched led/ldr's for two x stereo Lightspeed Attenuators. So go halves with a buddy and you can have one each.
Match the LDR resistances using a DMM, powering the LED from 1mA to 20mA, give them time as they need a few minutes to warm up.

PS: You can skimp on the cheaper NSL32SR2 but you have to buy almost twice the amount, as their low resistance is not as low and constant, they cannot get to quietest minimum volume level as the NSL32SR2S (sorted) can.

Cheers George

gfinlayson 25th May 2006 07:27 AM


I'm looking to build a high-quality passive preamp in the near future. I like the LDR idea. Can you provide any more information on the design and components used?



georgehifi 25th May 2006 07:50 AM

Use a dual 100k log or linear pot to supply the voltage to the leds, one gang of the pot increases the series ldr's led voltage while the other gang in reverse lowers the voltage on the shunt ldr's led, this way you have a constant input and output impedence of the passive, regardless of where the voltage control setting is. I believe it's the perfect passive volume control.

You can put an LRD in one end of a tube and a Led in the other end x 4, you have to sort through a few ldr's to get 4 that track together for good channel balance, the last thing you want is to have a balance control, but even that's possible if you use two single 100k led voltage pots instead of a dual.

Cheers George

st2_998 25th May 2006 01:34 PM


Originally posted by georgehifi

The only stipulation that all these passive controls including the series/shunt LDR need I believe is,
1: Low source impedence (cd player) <50ohms
2: High input impedence (amplifier) > 100k.

Thanks George, I'ma very bad at electronics, so I'll try to sort it out. For example, I don't remember if LDR increases or lowers resistance with increasing light.
This type of control perhaps should be put after a buffer (attenuating buffer's self-noise) but I understand that impedance matching would be wrong, isn't it?

georgehifi 25th May 2006 09:42 PM

You should look for a LDR who's led on resistance is around 50ohm and dark resistance around 1megohm.
Also the balance control that I said would be 2 x 100k single pots for the leds is wrong, it would have to be 2 x 100k dual pots.
Also you could have a LDR in each end of a 1inch tube and the LED through a drilled hole in the middle, that way you'll only need 2x tubes for a stereo control.
There are combined Led/Ldr packages out there, but they are very expensive and you still need to match them.
And forget about using buffers before and after the LDR control, been there done that they only add their own colouration to the sound. A CD players output drive is plenty capable of driving a amp to clip, all you need with the LDR Control is to keep the interconects to the amp as short as possible, <2mts and to use ones that are <100pf per foot.

Cheers George

gfinlayson 25th May 2006 09:56 PM

George, do you have any manufacturer's part numbers for the LDRs? The lowest values I can find are about 2 kohm when illuminated.

pinkmouse 25th May 2006 10:26 PM

So the Heretical line stage isn't good enough 'eh Graeme? :)

Interesting topic guys, looks like a good way of controlling lots of channels for a HT setup.

georgehifi 25th May 2006 11:16 PM


Originally posted by gfinlayson
George, do you have any manufacturer's part numbers for the LDRs? The lowest values I can find are about 2 kohm when illuminated.

They are out there, you'll just have to do your homework to find them.
I make these LDR Volume controls ("Lightspeed Attenuator") as a source of income, I started them 30years ago, but back then the ldr's were way out tracking wise and changed their resistace curve with use, you needed 100 to match up 4, then I shelved the idea. In the late 90's Melos bought out their flagship preamp the SHA-Gold-Reference $20kUS which used the ldr's (hailed as the best sounding preamp ever, probally because of the ldr volume), still unreliable, nearly all that were built came back to them, maybe this is what sent them broke.
But now there are ldr's which can be 100% reliable(you still have to sort them 10 to get 4 to track), you will just have to search for them like I did, then you will keep it under your shirt like I am, because if I tell everything it will effect my sale of my
"Lightspeed Attenuator" units.

Cheers George

gfinlayson 26th May 2006 07:27 AM


The heretical line stage is very good indeed; transparent, detailed, very smooth. I'm just curious to compare it with a good passive. The LDR passive seems a very simple, low cost way to try it out.

I'll probably end up building a heretical line stage anyway, if you're happy to share the design.


powerbecker 27th May 2006 02:16 AM

here is some usefull information about LDRs:

The lowest values I can find are about 2 kohm when illuminated.

Normally with only 10 lux. With more brightness they easy get down to 50E.

please can You tell how equal in which range Your stereo-attenuator works?


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