DIY "Lightspeed" Passive LDR Attenuator

Lightspeed Passive Attenuator

The Lightspeed Passive Attenuator designed by George Stantscheff (GeorgeHiFi on the audio forums) is by far the very best passive attenuator that I have heard. There is perfect clarity, a widened soundstage, separation of instruments, and seemingly infinite detail. What makes this passive preamplifier interesting is that there are no contact points in the form of a "wiper" as in the case of a potentiometer or "switches" as may be with a discrete stepped attenuator. The key components in the Lightspeed Attenuator are Silonex NSL-32SR2 Optocouplers (PDF - 34kB). The optocoupler device is a sealed unit which consists of a high performance LED that shines on a light dependent resistor (LDR).

Figure 1: Optocoupler (LED and Light Dependent Resistor)

The operating principle of the optocoupler is fairly simple. As the intensity of the LED varies, so does the internal resistance of the LDR (resistance decreases with increasing light). So in the case of the Lightspeed Attenuator, we change the volume (resistance) by changing the intensity of the LED. By using a series and shunt combination of optocouplers the attenuator can be configured to produces a constant input and output impedance, regardless of where the voltage control setting is.

It is worth mentioning that the Lightspeed Passive Attenuator is not for every audio system though. First off, it is a passive attenuator so there is no gain. Also, your source output impedance needs to be less than about 100 ohms which makes it suitable for use with several sources. The Lightspeed Attenuator works best with amplifiers that have input impedance greater than about 50k. If you would like to use the attenuator with a low input impedance amplifier best results are achieved by using a suitable buffer between the attenuator and amplifier.

DIY "Lightspeed Attenuator" - Passive LDR Volume Control

The schematic for the Lightspeed Passive Attenuator is shown below. George has chosen to share his Lightspeed Attenuator with the DIY community provided that it is only used for personal use only. Details about George's commercial offering of the Lightspeed Passive Attenuator are provided at the bottom of this page.

Figure 2: Lightspeed Passive Attenuator Schematic

The following will required to build a Lightspeed Attenuator.

5 VDC power supply.

100k dual log or linear potentiometer (quality does not matter as this only controls voltage and not the audio signal)

Four 100 Ohm 1/4W resistors

Two 1k to 5k multi-turn potentiometers

Two matched pairs of Silonex NSL-32SR2 Light Dependent Resistors (LDR)

Wire

Four RCA Jacks

A suitable enclosure

Miscellaneous Hardware: knobs, feet, switches ...

There are four leads from each optocoupler. The two shorter leads are for the LED portion. A white dot on the optocoupler capsule near the short leads marks the cathode of the LED (ie. V- or GND). The two longer leads on the opposite end of the optocoupler device are the LDR (there is no negative or positive).

Figure 3: Optocoupler Device Identification

If we take two of these resistor legs from separate optocouplers and tie them together we have formed the equivalent of a potentiometer. The top resistor has one free leg and one twisted leg. The free leg takes the signal from one channel of your source. The LDR leads that are tied together form the output go to the signal input of your amplifier. The last leg is connected to the signal ground. If we vary the value of the resistor going to ground then we vary the overall output impedance of the potentiometer. Your amp will like to see a steady output impedance. The value of the series resistor plus the value of the shunt resistor equals the output impedance of Lightspeed Attenuator. So as the series resistor increases or decreases in value we want the shunt resistor to do exactly the opposite.

For each channel, we need to vary the intensity of the LED inversely with each other. Using a 5VDC source and a 100k potentiometer (pot) we vary the intensity of the LEDs. A dual pot has 6 legs on it. 3 for each section. If you connect pins 1 and 2 together on the first section of the pot and connect pins 2 and 3 together in the same way on the second section we have just created two variable resistors that change in resistance opposite of each other. By connecting pin 3 of section 1 to pin 1 of section 2 and connecting that to the +5VDC from your power supply we can adjust intensity of the LEDs. The 100 ohm resistor is used to limit the current to the LEDs.

The 1k to 5k multi-turn pot allows us to make up for the matching differences between the optocoupler pairs. What's this? The Silonex Optocouplers resistance does not vary consistently from device to device. Consistency is very poor. In general, you will need to buy multiple optocouplers (25 or more) and measure their resistance at several different current draws. By plotting the results you can match optocouplers that 'track' in a similar fashion. When you find two that match, set them aside and begin to find another two that match each other. It is not necessary that all 4 match each other but the series resistors must match and the shunt resistors must match.

Photograph 1: Matching Optocouplers

For those of you interested in building a Lightspeed Attenuator, but do not wish to buy a large quantity of optocouplers and match them yourself, I am selling matched optocouplers. Here are the fine details about the matched LDRs for sale and you can contact me via email.

UPDATE - February 2010: You can now purchase matched LDRs for the project below or purchase an entire kit to build Uriah's new "Lighter Note" LDR attenuator at www.buildanamp.com

When the series or the shunt pairs of resistors do not match each other, they can be adjusted to match by using a 1k to 5k multi-turn pot in series with the one LED who's resistor is measuring a lower resistance. (Alternatively a potentiometer can be used in each channel). By adjusting the multi-turn pot we can balance the signal.

Step-by-Step Build Instructions

Connect pins 1 and 2 of section 1 on your dual 100k potentiometer. Connect pins 2 and 3 of section 2 on your dual 100k pot. Connect pin 1 of section 2 with pin 3 from section 1 on your dual pot. Supply the connection from step 3 with +5VDC Connect pin 1 of section 1 with two different 100 ohm resistors. Connect the 100 ohm resistors from step 5 to the LED anode of the two series optocouplers. Connect pin 3 of section 2 with two different 100 ohm resistors. Connect the 100 ohm resistors from step 7 to the LED anode of the two shunt optocouplers. Connect the power supply ground to the LED cathode of each optocouplers (the cathode is marked with a white dot on the optocoupler device). Choose a series and a shunt optocoupler pair and twist one leg of the LDR half from each together. Repeat step 10 with the remaining pair of optocouplers. Connect the free wire from one of your series LDR to the signal pin of your input RCA jack. Repeat step 12 with the other series LDR and the signal pin from the other input RCA. Connect the twisted LDR leads from step 10 to the signal pin of the RCA output. Repeat step 14 with the LDR leads from step 12 for the remaining RCA output. Connect the remaining free LDR lead on each series / shunt combination to the signal ground on both the input and output RCA jacks. Turn it on with no source or amplifier connected and turn the volume up in a few different steps. Lets say 7 o'clock, 10 o'clock and 1 o'clock. Measure the resistance of each series LDR at each step and record it. When you find that one LDR has less resistance than the other then this is the optocouplers which need a 1k to 5k multi-turn pot. Remove the 100 ohm resistor from the anode of the optocoupler LED and solder the potentiometer in place after shorting the wiper to one end of the potentiometer to make it a variable resistor. Alternatively a potentiometer can be used in each channels. Repeat the measurement parts of step 17 and adjust with the muti-turn pot until both optocouplers have the same resistance. To test the attenuator plug in your source to the attenuator and then into an amplifier with it's own volume control set at a reasonable level. If it is at full volume when you have the Lightspeed Attenuator turned down all the way, you need to reverse your wiring on your 100k dual pot as your series resistors are getting full light and your shunt resistors are getting no light so your volume is actually all the way up.

The photo below shows a Lightspeed Attenuator that I built to check the sonic quality of the matched optocouplers.

Photograph 2: Test Lightspeed Attenuator on Radio Shack 276-150 Protoboard

The circuit is simple and can be built on a protoboard. For those of you who have the ability to make your own printed circuit boards (PCB), here is a PCB image I came up with.

Figure 4: Lightspeed Attenuator PCB Images  with and without silk

Photograph 3: DIY Lightspeed Attenuator and Power Supply PCB

Many people have suggested using a regulated power supply, but I have found an unregulated switching supply to be fine. Since the current draw to the LEDs is small, you could also use batteries for the power supply. The 100k dual potentiometer is only used to adjust the voltage to the LEDs so it does not need to be high quality as it does not carry any audio signal. Of course you could use expensive signal wire RCA jacks if you like. Build it and it will amaze you.

DIY "Lightspeed Attenuator" - Passive LDR Volume Control

Giovanni Militano

A while back I purchased a set of matched Silonex NSL-32SR2 optocouplers from Uriah who organized a group buy for the optocouplers. Uriah was also kind enough to send me a couple of PCBs for the Lightspeed Attenuator (Figure 4). Please don't email him asking for PCBs as he does not have any available. Below is my construction log and comments about the Lightspeed Attenuator. Photograph 4 below shows a regulated 5V power supply on a stripboard and the Lightspeed Attenuator PCB. The power supply and attenuator follow the schematic shown in Figure 2 except that I used a 100uF capacitor instead of 10uF as that is what I had on hand.

Photograph 4: Lightspeed Attenuator PCB and Power Supply on Stripboard

The photograph below shows my test set up of the Lightspeed Attenuator and balancing of the output by adjusting the Bournes multi-turn 1k potentiometers.

Photograph 5: Testing and Balancing the Lightspeed Attenuator

For the enclosure I used an inexpensive Radio Shack 270-238 Aluminum Project Enclosure. The power source is an old 9V cell phone charger rated at 170mA which is much larger than required.

Photograph 6: DIY Lightspeed Attenuator in Project Box

The enclosure was lightly sanded and painted with black Tremclad rust paint. Finished photos are shown below. I was a bit sloppy with the painting, otherwise you can get a pretty good finish using just a foam brush and Tremclad. The box is light and tends to want to tilt backwards with the RCA cables attached so I have a big heavy knob on it now. Eventually I will touch up the paint job and use some better quality signal wire.

Photograph 7: Finished Lightspeed Passive Attenuator

The cost for my DIY Lightspeed Attenuator was about $40-50US, $30 of which went towards the matched set of optocouplers. If you are interested in higher quality volume controls, this is a very good project to consider. So, how do I like it? Well, I have another matched set of optocouplers on the way. To my ears, the Lightspeed Attenuator is the best sounding attenuator I have heard. It is hard to argue with no moving contact points in the signal path.

Lightspeed Passive Attenuator Clone

Here is Joe's DIY Lightspeed Attenuator built on a proto-board and in a Hammond chassis.

Photograph 8: Joe's Lightspeed Passive Attenuator Proto-Board

Photograph 9: Joe's DIY Lightspeed Passive Attenuator

Production Lightspeed Passive Attenuator

For those of you interested in purchasing a production Lightspeed Passive Attenuator from the designer George, you can get more information from his website.

Photographs of George's Production Lightspeed Passive Attenuator are shown below. Additional information is contained in the Lightspeed Attenuator Instructions (PDF - 33kB).

Photograph 10: Production Lightspeed Passive Attenuator

Should you have any questions or need help putting your attenuator together, ask them in the Lightspeed Attenuator / Audiohm Optocouplers / LDR thread on the forum.