DIY Lightspeed Passive Attenuator

Alps Blue Velvet, Bournes, TKD, Alps Black Beauty, DACT Stepped Attenuators ... If you are familiar with some of these names, you likely have an interest in high quality passive attenuators and you should continue reading. The noted items have a few things in common - first off, they are potentiometers that are often used as attenuators or volume controls in audio equipment. As a volume control, they also have another thing in common - they all used some form of "contact point" to attenuate audio signals. In the case of a potentiometer, this is typically a "wiper" which slides along a conductive material forming an adjustable voltage divider. If you are passionate about audio reproduction, you are fully aware that these "contacts points" can be the source of unwanted noise. Now it certainly does not make sense to spend money on high quality interconnect cables only to run the signal through a dirty potentiometer that you infrequently spray with with a contact cleaner like Deoxit.

When people talk about the "sound of capacitors" someone will inevitably say that the best sounding capacitor is "no capacitor" - that is if it can be eliminated. So following that principle, one could argue that the best sounding attenuator is one with no contact points in the signal path. Enter the Lightspeed Passive Attenuator.

The Lightspeed Passive Attenuator uses optocouplers to establish the level control and this is done with no "contact points" through the attenuator. The optocoupler is a LED and Light Dependent Resistor (LDR) is one housing. The operation principle of the Lightspeed is simple - by adjusting the intensity of the LED you can change the resistance of the LDR. By using a series and shunt combination of matched optocouplers the attenuator produces a constant input and output impedance, regardless of where the voltage control setting is. Hmmm, an attenuator with constant input / output impedance and no contacts points - great idea!


Uriah has put together a detailed set of instructions to build a DIY Lightspeed Passive Attenuator. I've built one of the attenuators for myself (photo below) and I think it's great! It is a remarkable improvement over all the passive potentiometers that I have tried. The attenuator is very simple to build, so if you have an interest in higher quality volume controls take a look at the instructions.


The attenuator design could easily be scaled upward to include support for multiple channels and used as an effective way of controlling several of channels for a Home Theater setup. For full information, see the DIY Lightspeed Passive Attenuator.

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