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"The Warpspeed Control Engine© is a Silonex optocoupler-based volume control. Optocouplers have two components in one package about the size of a TO-92 transistor. One side is a light-emitting diode (LED), the other a light-dependent resistor (LDR) whose resistance varies as the light intensity of the LED is changed. On the Warpspeed the light intensity is regulated with a voltage-controlled current source (VCCS) supplying power to the LED. This design takes advantage of the classic Wheatstone Bridge and connects four optocouplers with the LEDs as the bridge elements.

"The voltage ratios of the LED bridge are kept in balance with the common connection of the LED cathodes and the help of two precision VCCS acting in tandem to deliver precise voltages in 1mV steps. With well-matched optocouplers this precision translates to <0.3dB resolution per 1mV step. A +12V battery supply is highly recommended, say Panasonic's VRLA LC-R127 R2P1 SLA with 0.25" Faston 250 male terminals for use with the DC power cable that comes with the unit. A similar battery with at least 5Ah also works. Moderate to high-gain amplifiers from say x 4.5 (13dB) or more can be liberated from preamplifiers, additional gain stages and some of the unwanted baggage that comes with those.  
simplified single-ended schematic

"Differences with other light-based volume controls are in exactly how the LED section is controlled to shine light into the LDR. I chose to use dual voltage-controlled current sources working in tandem and the LDRs are operated below their 50mW spec for longevity. The tricky part that requires much time and patience is matching of the optocouplers. That is mandatory to maintain stereo channel balance. This VCCS supplies down to <1uA of current and accepts an input of 6 to 24Vdc from a power source, most preferably a bank of alkaline battery cells. The LDRs are simply connected as a series/shunt voltage divider with varying resistances to control audio volume levels. Using 12 x 1.5V alkaline D cells connected in series and parallel for dual banks of 9V, I estimate battery life—depending on daily use and making sure it is off when not in use—to be about 1 to 1½ years."

"All switching and controls—inputs, balance—are done in the LED domain for all models. The LDRs are then directly soldered into the connectors to keep the signal path short and pure. Additionally the battery supply contributes less noise/ripple than even a really clean DC supply from rectified AC."

Available Warpspeed configurations include:
• V4 - 1-input/2-output (paralleled) stereo RCA - $450
• V4x2 - 2-input/2-output(paralleled) stereo RCA - $600
• V4x3 - 3-input/2-output(paralleled) stereo RCA - $750
• V8 - switchable RCA (1-in/2-out paralleled) or XLR (1-in/1-out) stereo; no RCA/XLR conversion - $675
• V8.1 - 1-in/1-out XLR stereo - $600
• V8+4 - switchable RCA (1-in/2-out paralleled) or XLR (1-in/1-out) stereo; independent RCA and XLR optocoupler circuit; no RCA/XLR conversion - $800
• V12 - 1-in/2-out RCA with separate level controls for high, mid/low bi-amping and a master volume control - $850
• V16 - TBD 1-in/3-out RCA with separate level controls for high, mid, low tri-amping and a master volume control - $TBA
• V24 - multichannel up to 6ch with level control for each channel and master volume control; RCA or XLR - starting at $1.000
• Balance control option on all V4 models except V4x3
• xW1 - single L/R balance control standard on all V8 models - $100
• xW2 - dual L/R balance controls - $200
LDR response at typical listening levels

Like phone sex the Warpspeed believes that the best turn-on comes from no physical contact. Don't touch that signal. Instead of a wiper across resistive carbon, switched resistors or switched transformer or autoformer secondaries, here light triggers volume changes. Since the technology itself is nothing new but applications in commercial hifi seem presently restricted to a few über-priced preamps, one wonders why*2.

When the Warpspeed's original DIY soul succumbed to the lure of minor capitalism, that same basic but still rare tech became available to audiophiles of smaller wallets who didn't care to roll their own. A year later its creator had grown confident enough—or just curious?—to ask for a formal assessment. Naturally this would involve going against established players to set up a David vs. Goliath or underdog vs. crowned champ mechanic. Before we go all juvenile on the bookie, let's remember that passives despite their philosophical virginity aren't exactly universal.
basic balanced schematic

Whether passive trumps active depends. A perfectly voiced SET into well-matched speakers for example can very much benefit if one doesn't wish to dilute its flavor by one iota. A very fast transistor amp tending toward textural/tonal leanness meanwhile might benefit from the signal conditioning a superior active will provide. It's a function of what one owns and means to achieve. Passive bliss rests on favorable conditions including the most obvious - sufficient gain. In technical terms the Warpspeed is a pass-through variable voltage divider. It shifts the burden of providing current on your source. Here 2Vrms or more and low output impedance are advantages but common with most current DACs. Very long interconnects are far from ideal and high load input impedances—say 50k up—are better than lower because it is now your source and no longer the preamp which actually drives the input stage of your amplifier.*3

*2 Constellation's earlier mentioned tertiary circuit which compensates for thermal drift affecting the LDR's resistive value might hint at one reason why this solution is rare in precision hifi. To work as precisely as claimed might require a bit more than DIY enthusiasm and the raw part(s). Allan's concern for longevity of the parts requiring specific safeguards adds another possible reason. Finally there's the need for extensive matching of the parts. This would rely on bulk orders to start with a sufficient number and match down to very tight tolerances. It suggests that these parts measure 'all over the place'. And that invites questions as to at least this vendor's manufacturing quality or whether LDRs were ever meant as true precision parts.

*3 "The Zout starts off around 30Ω fully counter clockwise. Full mute on my system is around 60Ω. Midpoint of the 3-turn control pot will range around 500 thru 900Ω. The actual value depends on where the LDRs match up. The review unit is at 550 to 600Ω. At 6dB attenuation or 1V from a 2V source it will already be loud to very loud on typical systems. So a 10kΩ Zin of a power amp will be quite happy with the Warpspeed. I use mine with a bi-amp setup having an aggregate 15kΩ. An Aussie DIYer happily uses his kit on his Zenv9 (FirstWatt F3 clone) having a 9kΩ Zin. The W'Squid operates the optocoupler beyond the Silonex-specified response of 40Ω to 5MΩ. Beyond is 30Ω to 40MΩ where my ohm meter tops off.

"Yet the LEDs are kept at SOA which mystifies and riddles the Fictionaires [Allan's term for the heathens or disbelievers - Ed]. This character gives the W'speed the advantage of a full 120dB attenuation range making it usable with low, typical or high gain/sensitivity systems. In my system, very very loud will have the Zout at around 1.5kΩ to 2kΩ. Full clockwise though will give you 40MΩ or infinity on average ohm meters - obviously best done disconnected. Input impedance is of course the opposite. It starts out at infinity, then 40MΩ when my meter starts to measure, then follows the series anti-log response adding the shunt response as you proceed along the above graph. Zout is the shunt log response."

Socket assignation for V24 multi-channel version

I love having Bent Audio's autoformer-based Tap X in my hardware mix. Being remote controlled over 61 x 1dB volume steps plus remote input switching, balance and polarity, it lacks no creature features but in the right scenario—as recently with RWA's Liliana monos—this top-notch passive preamp often turns out to be my best option. In the DIY-gone-commercial passive sector, the last blip on the radar had been the Promitheus TVC from Malaysia. By 2012 LDR volume controls were making the loudest buzz*4. Then the Warpspeed said pretty please for a review. Did the buzz have any bite?

*4 In the very affordable sector there's also Ed Schilling's The Truth preamp. It replaces the optocoupler with photo cells and claims "input impedance too high to measure, output impedance of a few ohms, bandwidth to 60MHz, slew rate of a few hundred V/microsecond and the ability to drive 10 meters of interconnects with ease."