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Srajan Ebaen
Financial Interests: click here
27" iMac (3.4GHz quad-core IntelCore i7, 16GB 1.333MHz RAM, 2TB hard disc, 256GB SSD drive, ADM Radeon HD 6970M with 2GB of GDDR5 memory); PureMusic 1.87; Amarra 2.3; Audirvana Plus; April Music Eximus DP1; Esoteric/APL Hifi UX1/NWO-M; Audiophilleo 2
Preamp/Integrated: ModWright LS-100 with Psvane CV-181T tubes, Esoteric C-03, Bent Audio Tap-X
: First Watt SIT1 monos, ModWright KWA 100SE
Speakers: Aries Cerat Gladius
Cables: Complete loom of Zu Audio Event, KingRex uCraft USB cable with UPower battery supply, Stereo
lab Tombo Trøn BNC/BNC coax
Stands: Artesania Esoteric double-wide 3-tier with TT glass shelf, , 2 x ASI HeartSong amp stand
Powerline conditioning: 1 x GigaWatt PF2, 1 x Furutech RTP-6
Sundry accessories: Extensive use of Acoustic System Resonators, noise filters and phase inverters
Room size: 5m x 11.5m W x D, 2.6m ceiling with exposed wooden cross beams every 60cm, plaster over brick walls, suspended wood floor with Tatami-type throw rugs. The listening space opens into the second storey via a staircase and the kitchen/dining room are behind the main listening chair. The latter is thus positioned in the middle of this open floor plan without the usual nearby back wall.
Review Component Retail: $675 V8; $100 for xW1 balance control option so unit as reviewed $775
Sunday, November 21, 2010 7:14 PM. Subject: passive volume controls. "You might remember me from diyAudio. This is Blues. I regularly visit your website for equipment reviews. I came across your search for passive preamps and your decision to go back to one form of these. I recently posted an optocoupler-based version named Warpspeed at the preamp section of the forum. Here is the link. I have plans to offer this as a partly built kit without the chassis/hardware but was thinking about building another and sending it to you for review. The very first one I made is a keeper. I'd be very uncomfortable sending it halfway across the globe. I have no time table for this but let me know if you are interested and I'll place it in the front burner. Have you looked into similar designs like the Lightspeed? As you will see, the advantages of the StereoKnight vs. passive pot at low volume levels as mentioned in your review is also very apparent on the Warpspeed." - Allan Flores

2 out, 1 in. 1 too many. Add volts for this to make sense. Most contemporary sources output 2V. Most modern amps are driven to full output with 1V or less. We're thus generating superfluous gain only to throw it away again. Add a preamp with 20dB of gain. Now gain poisoning is fact not fiction. This isn't news to advocates of passives. But a volume control in an otherwise empty box with perhaps switches and wiring for source selection must still make more decisions. Without an active buffer, impedance matching will become problematic depending on what type of source and load impedance the passive must bridge. Poor impedance interactions affect the frequency response of the speaker.

What about the attenuator? Should it be a resistive pot, a ladder type, a multi-tapped transformer or autoformer? The LDR attenuationists*1 argue that a light-dependent resistor triggered by a variable LED without physical contact makes for the ultimate. Nelson Pass would argue that sans buffer, impedance matching remains critical. Hence the always generous designer made available a DIY circuit to address this issue. The Lightspeed attenuator promoted by George Stantscheff has opted to not incorporate it. Allan Flores meanwhile concedes Pass' point and set out to address it in his own way. Enter the Warpspeed Control Engine©. It began as pure DIY project but now is an à-la-carte turnkey product with many built-to-order options. It claims to address what Allan disliked about prior LDR attenuators - their inability to manage complete mute; that extremely low/high volume settings led to power levels on the LEDs which endangered, shortened the life span or altogether fried the optocouplers; and his perceived need to improve power delivery to the optocouplers and the quality of the volume adjustment pot.


*1 This modest project isn't alone in this. Consider the $60.000 Constellation Audio Altair preamp. "Instead of the mechanical pot found in most high-end preamps, it uses optically controlled resistors that can be adjusted to an accuracy of 0.1dB. To maintain Altair’s perfect positive/negative signal balance and left/right channel balance in all operating conditions, we use 48 of these resistors, an expensive but undeniably superior design." For somewhat greater detail we have this quote from Robert Harley's The Abso!ute Sound review:

"... When you adjust the Altair's volume knob, you are changing a digital code that feeds a DAC whose current output is a function of the digital code driving it. The DAC's current output flows through the LED, with the amount of current flow proportional to the LED's brightness. In this way a digital code determined by the volume control's setting adjusts the current output of a DAC which determines the LED's brightness which determines the resistors' value and thus the audio signal level... To ensure that the light-dependent resistors are perfectly calibrated, a tertiary circuit measures the Altair's internal temperature and continually re-calibrates the DAC's current flow so that the resistors have precisely the correct value at all times." The Altair even uses LDRs as input selector 'relays'. The Swiss darTZeel NHB-18 preamp appears to exploit a similar light-sensitive volume control solution with 100 LEDs per channel.

As Allan put it, "the Lightspeed, simple as it is, still has a number of variables/design factors that affect its performance". The Warpspeed claims to solve all the above and add these bonuses: low volume levels are more engaging (music details are as apparent as at moderate/loud levels); low volume listening is perfectly in sync with Nelson Pass' monotonic amps where distortion decreases at lower levels; wider range of volume settings; battery power means less AC-power related artefacts on optocouplers; precise power delivery and control at safe levels on optocouplers; by design keeps optocouplers' power-level ratio at/near equilibrium; peace of mind at any volume level; works with a wide range of sources and power amps regarding output/input impedances.

21 June 2012 06:50, 2012. "It has been a year and a half but here I am. The Warpspeed started out in kit form but I took it to the next level by offering fully built units for almost a full year now. Kits are no longer available. I am more comfortable supporting units made the way I design and build them. It took a while for it to catch on with audiophiles but I can say it now has gathered some momentum with modest success.

"The options expanded from the basic 1-in/2-out V4 entry-level model to cater to many audiophile switching and level-control requirements. The WarpSquid  with dual VCCS controller is the basic building block. The design is flexible and adaptable enough to expand to my latest V24 6-channel Warpspeed available in RCA or XLR versions. All models are battery powered and still  hand-built to order. This summer I'm afforded some time to build a unit to send for review if you are available. The unit I am planning on building is a V8 switchable between RCA (1-in/2-out) and XLR (1-in/1-out), with an xW1 transWarp L/R balance control but no RCA/XLR conversion." 
WarpSquid & 6ch/V24 Kraken
the XLRs are a standard pin 2 (+) hot, pin 3 (-) cold and pin 1 audio circuit GND
22 June 2012 02:06, 2012. "I knew you as of the 2-ch purist sort but felt the need to point out the multi-ch versions as this hasn't been done before with LDRs as far as I know. Having a balanced design is a first as well I believe, excepting the high-enders at Constellation and darTZeel. While some of the players and DIYers are still debating how to go about controlling the optocouplers in stereo unbalanced mode, I have forged ahead. I have kept the design from hawk eyes in DIY by potting it. This I hope stays a mystery to them. Which takes us to my concern about publishing detailed internal hi-def pictures. If it won't be breaking your review policy, is it okay if I provide internal pics so I'm sure they don't reveal too much?

"Remote control is an option for volume only. It is expensive (+$600) partly because I chose not to compromise on the precision multi-turn wirewound pot I use. It requires a redesign to accommodate Alps, Noble or the like which are of lower precision and quality. For this reason many have asked for remote but no one has ordered one thus far. The unit I will be providing won't have remote.

The unit will come with a 4 x 9V battery harness and a DC power cable that has female 0.25-inch terminals that mate with male terminals on rechargeable SLA batteries. You can choose to use either battery type or try both."