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Lightspeed Attenuator / Audiohm Optocouplers / LDR
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Author:  Gio [ 18 Mar 2009, 21:20 ]
Post subject:  Lightspeed Attenuator / Audiohm Optocouplers / LDR

Update: Here are Uriah's detailed instructions for building a DIY Lightspeed Passive Attenuator (11 May 2009)

I got a set of matched Silonex NSL-32SR2 Light Dependent Resistors (LDR) from RiGuy who also sent me a PCB for the "Lightspeed Attenuator". Thanks Ri! ;)

This will be a build log and some listening notes. There is a lot more information about this circuit in the Lightspeed Attenuator thread.

Here is the schematic by GeorgeHiFi:

The PCB design:

Some construction photos:


It works!

Since the last photo, the enclosure has been sanded and painted with black Tremclad. Should be done in the next day or two and I will post final photos.

My circuit is identical to that shown in the schematic with the exception of the regulated power supply where I used a 100uF cap as I did not have a 10uF on hand.

As you can see from the test setup, I have not done any serious listening yet. However, initial impressions were positive! :up: I'll give a good listen when it is properly wired.


Author:  Gio [ 22 Mar 2009, 10:58 ]
Post subject:  Re: Lightspeed Attenuator / Audiohm Optocouplers / LDR

Hi Guys, finished up the LDR volume control.

I was a bit sloppy with the painting, otherwise the finish could have been much better.

The box is too light and tends to want to tilt backwards with the RCA cables attached. I might have a big heavy knob I can use to balance it more.

Power is from a 9V DC wallwart into a 7805. Very simple circuit that will easily fit on a Radio Shack protoboard.

I love it. It sounds great and is an improvement over a Alps Blue Velvet pot and a diy stepped attenuator with matched metal film resistors. I really love the idea of no contact points in the signal path (aside from RCA in/out).

Used passive like this, you want a low source impedance (typical CDP, not one with tube output stage) and an amplifier with a high input impedance (100k). For lower impedance I found it works better with a buffer.

Easy to implement this in a tube amp (typically high input impedance) and using the heater supply for the voltage. This could also be used as a good way to control a multiple channel amp (HT) with just one control point.

Cost for just the circuit and PS would be ~$40-50US, compared to an Alps Blue Velvet pot $15-20US and we know what a stepped attenuator can cost. If you are into higher quality volume controls, this is a good one to look at.


Author:  RiGuy [ 22 Mar 2009, 15:23 ]
Post subject:  Re: Lightspeed Attenuator / Audiohm Optocouplers / LDR

I would comment that its the best pot I have ever heard and I am really excited about it.
I would also say that while 100k and above are probably optimal they have been used by me in a MyRefC amp which has about 60k input impedance and have also been used by others in even lower impedance amps. You can use a buffer to run something else.

Author:  Gio [ 22 Mar 2009, 17:44 ]
Post subject:  Re: Lightspeed Attenuator / Audiohm Optocouplers / LDR

Hey Uriah, thanks for bringing this idea to my attention. For me too, this is the best potentiometer that I have tried.

Author:  Soundbrigade [ 24 May 2009, 07:08 ]
Post subject:  Re: Lightspeed Attenuator / Audiohm Optocouplers / LDR

U had 5 opto-couplers of the NSL-verson and feared I wouldn't be able to find good matches so I purchased some 20 LDRs and measured them.
The LDRs went into a tube (an old pen) with some tinfoil to eliminate light from the LEDs leak into the neighbor tube.
The very first trial shows I am on the right track, but I have som problems with the potentiometer. Got to check my supplies and find something that will have the right characteristics.

Some pics:

Here are the LEDs and LDRs mounted on a small board :

Author:  RiGuy [ 24 May 2009, 08:40 ]
Post subject:  Re: Lightspeed Attenuator / Audiohm Optocouplers / LDR

its hard to find /DRs with similar resistive characteristics. Also I think there is something special about the Audiohm LDRs soundwise. You couldnt get more DIY than your version! Keep us posted on how you solve your issues. I am guessing that something is not matching right. Could even be the leds.

Author:  Soundbrigade [ 24 May 2009, 11:03 ]
Post subject:  Re: Lightspeed Attenuator / Audiohm Optocouplers / LDR

I was careful when matching the LDRs, so I have no fear that I will have too much unbalance between the channels. Beside I can add a trimmer to adjust the LEDs.
Else I think that the LDRs I use don't differ from the ones used in the readymade modules.

Author:  Gio [ 24 May 2009, 17:48 ]
Post subject:  Re: Lightspeed Attenuator / Audiohm Optocouplers / LDR

Wow, DIY optocouplers! :)

It looks like the ends may have openings. Are you sure light is not getting in from the ends? You could try some duct seal or something similar. Also, does the distance of the LED from the LDR seem to be a factor?


Author:  georgehifi [ 25 May 2009, 03:16 ]
Post subject:  Re: Lightspeed Attenuator / Audiohm Optocouplers / LDR

Ah memories these diy LDR's take me back 30 years when I first made the Lightspeed Attenuator, I used old LDR's out of Leica camera lightmeters, mounted in a brass tube, one each end, then a led through a hole in the middle of the tube, one led for both channels.
I think this is also the way Mark Porzilli did the Melos SHA Gold Reference pre in the late 90's, all doomed to going out of balance within a few short weeks. Thanks god for the NSL32SR2s it is now in over >170 Lightspeed Attenuators that I have made and not one return for the 5 years they been out there all over the world.
Cheers George

Author:  Soundbrigade [ 25 May 2009, 12:41 ]
Post subject:  Re: Lightspeed Attenuator / Audiohm Optocouplers / LDR

Gmilitano: I will use the Georgehifi's schematic as template and will make adjustments so I roughly have the same level from both units.
The LEDs are set approx 15mm away from the LDRs. I imagine that most emitted light will end up in the LDRs and I will try to seal off the ends of the tubes to avoid stray light leak into the LDRs.

Much of the work here is to solve a problem where I today have long wrires to and fro in my amplifier. I'd say I prefer some not 200% matched LDRs that 1 meter of cable running through the HV PSU. :lightbuilb:

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