The top resistor has one free leg and one twisted leg. If we take two of these resistor legs from separate optocouplers and tie them together we have formed the equivalent of a potentiometer. The two longer leads on the opposite end of the optocoupler device are the LDR (there is no negative or positive).įigure 3: Optocoupler Device Identification A white dot on the optocoupler capsule near the short leads marks the cathode of the LED (ie. The two shorter leads are for the LED portion. There are four leads from each optocoupler. Miscellaneous Hardware: knobs, feet, switches. ![]() Two matched pairs of Silonex NSL-32SR2 Light Dependent Resistors (LDR).100k dual log or linear potentiometer (quality does not matter as this only controls voltage and not the audio signal).The following will required to build a Lightspeed Attenuator. Details about George's commercial offering of the Lightspeed Passive Attenuator are provided at the bottom of this page.įigure 2: Lightspeed Passive Attenuator Schematic George has chosen to share his Lightspeed Attenuator with the DIY community provided that it is only used for personal use only. The schematic for the Lightspeed Passive Attenuator is shown below. 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.ĭIY "Lightspeed Attenuator" - Passive LDR Volume Control The Lightspeed Attenuator works best with amplifiers that have input impedance greater than about 50k. Also, your source output impedance needs to be less than about 100 ohms which makes it suitable for use with several sources. First off, it is a passive attenuator so there is no gain. It is worth mentioning that the Lightspeed Passive Attenuator is not for every audio system though. 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. So in the case of the Lightspeed Attenuator, we change the volume (resistance) by changing the intensity of the LED. As the intensity of the LED varies, so does the internal resistance of the LDR (resistance decreases with increasing light). The operating principle of the optocoupler is fairly simple. The optocoupler device is a sealed unit which consists of a high performance LED that shines on a light dependent resistor (LDR).įigure 1: Optocoupler (LED and Light Dependent Resistor) The key components in the Lightspeed Attenuator are Silonex NSL-32SR2 Optocouplers (PDF - 34kB). ![]() ![]() 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. There is perfect clarity, a widened soundstage, separation of instruments, and seemingly infinite detail. 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.
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