An Innovative Approach to Passive Preamplifiers
For the past several years I have had the privilege of using the Lightspeed Attenuator in my system. The Lightspeed Attenuator, or perhaps more appropriately for the time period, the Light Dependent Resistor (LDR) volume control, was designed by George Stantschleff in early 1974. Back in May of 2006 George started the now long running thread on DIYAudio (accessed by clicking HERE) that discusses all things LDR and morphs into a full blown thread for the Lightspeed Attenuator and various member DIY projects based on George's schematic. Even Nelson Pass participates in the thread and contributes his own schematic.
What makes the Lightspeed Attenuator sound so dynamic, transparent, and clean from top to bottom? Perhaps an explanation from George Stantschleff himself would be appropriate:
"The best way to describe the Lightspeed Attenuator is that it is like just one fixed soldered resistor that is in the signal path between the source (cd) or phono stage, and the power amp. This resistor is soldered in place and has no contact points that potentiometers pots (potentiometers) or switched resistor (dact type) or TVC (Transformer) volume controls have. Now this resistor LDR (light dependant resistor) has the ability to change it's resistance according to how much light LED (light emitting diode) is shone on it, hence it can control the volume without any mechanical contacts needed in the signal path.
It is believed that any light weight contacts (switches and especially pots and their wipers) have a diode rectification effect on the AC music signal, trying to rectify it which is very bad. The Lightspeed Attenuator (LDR) system totally addresses is diode rectification effect. This is why it is so transparent and totally honest to the source, not adding any colorations or distortions to the source, it is just like being able to plug your cd players fixed output directly into your poweramp yet having control over the volume.
Ever wonder why when after having cleaned your RCA plugs and sockets on your interconnects, your system always sounds better. This is a similar type of contact albeit a much better one than the feather weight one inside a pot and the wiper contact of a switched resistor type control, because an RCA has a much larger and stronger pressure contact area, but still benefits by keeping it clean. I hope this has given you a bit more insight to the Lightspeed Attenuator and it's revolutionary way of controlling volume."
The simplistic design of the Lightspeed Attenuator exemplifies the concept of, "straight wire, no gain," and is true to the source. However, there are some requirements for one to consider to ensure the Lightspeed Attenuator will be compatible in your system. These include:
- A high voltage output source. For digital sources greater than 2 volts output.
- A source component with a low output impedance, less than 200 ohms.
- An amplifier that is sensitive, 1.5 volts or better (generally 1 volt is ideal).
- An amplifier that has a high input impedance, greater than 47k ohms.
Also, please take into account that in keeping with its simplistic design, the Lightspeed Attenuator can only accommodate a single source and does not include any additional inputs or input switcher. If you have multiple sources you would like to connect to the Lightspeed Attenuator this input switcher comes highly recommended and is very economical: DECWARE RSB6. The Lightspeed Attenuator comes equipped with only a single attenuator. However, for those who would like a channel balance adjustment feature, for an additional charge the Lightspeed Attenuator can be configured with dual attenuators.
The Lightspeed Attenuator uses a Wall-Wart power supply that will be recommended by George Stantschleff based on your countries power requirements and must be purchased separately by the customer due to Australian export regulations. However, some owners like myself use a battery power supply. George can recommend those options as well. Here in the US this is one I recommend: DIEHARD PORTABLE POWER 750. In addition to the Diehard Portable Power 750 you will need a cable adapter like this one: POWER PLUG.
Electra-Fidelity Audio is not a retailer for the Lightspeed Attenuator, but we highly recommend them if your system meets the requirements. For additional information, pricing and shipping costs send an email inquiry directly to: firstname.lastname@example.org.
Also, if you are the DIY type you can download the schematic and build your own by clicking HERE.
For those of you who visit the Audiogon forum there are a couple more threads you may wish to follow: