LDRx Passive Preamp

Summary

The LDRx is an audiophile grade passive preamp that combine advanced LDR (light dependent resistor) technology with elegant contemporary styling designed to satisfy the most demanding audio enthusiast. The LDRx comes standard with 3 RCA inputs and dual (parallel) RCA outputs. An optional 4th and 5th input are also available.

The LDRx is housed in attractive, high-quality contemporary enclosures that combine the warmth of eco-friendly renewable bamboo together with black-anodized brushed aluminum panels milled from high grade stock.

The elegantly simple Apple Remote provides remote control, along with a rotary encoder with integral push-button located on the front panel. Volume level and status information are provided via a pair of digital displays that are invisible when the unit is off.

Subjective Performance

Characteristics – Colorless, transparent, neutral, articulate, clear, fuzz-free, clean, natural, high resolution, great mid-range, solid bass. These are words and phrases commonly used by both customers and reviewers to describe the LDRx passive preamplifier sound.

Compared To Other Types of Passives – The consensus of customers and reviewers is you can expect performance  that is superior to conventional passive preamps utilizing even the best audiophile grade potentiometers, stepped resistive attenuators or even most transformer based volume controls.

Compared To Active Preamps – Independent reviewers have found that the LDRx passive preamps match the performance of even the finest high-end preamps costing many thousands of dollars more.

Key Features Under The Hood

Optically Isolated Passive Attenuation – The LDRx is a true passive preamp insofar as there’s no amplification or buffering of the audio signal notwithstanding the presence of a power supply.  Audio signals are optically isolated from both the power supply and the LDR control circuitry.

3rd Generation LDR Technology:  The LDRx is built with our 3rd generation LDR3x.V2.1 preamp controller which employs microprocessor controlled analog light dependent resistors to provide audio attenuation and input switching with unparalleled audio quality.

Auto Calibration: Similar to tubes, LDRs are analog devices subject to aging and drifting over time. Unlike tubes, the LDRx employ self-contained auto-calibration to keep the unit in precise calibration. This provides added assurance that the LDRx will hold its value over the long haul.

No Relays In The Audio Signal Path:   The LDRx uses LDRs for audio input switching. Auto Calibration does employ relays but these are physically disconnected from the audio signal path during normal operation.

Software Updates via USB: If warranted, software can be easily updated by the customer via a USB port located on the rear panel. Software updates can downloaded from our website and uploaded into the LDRx from a laptop or desktop PC. Eventually, we hope to make the possible from a Mac as well.

Replaceable LDR Modules:  While the useful life expectancy of LDRs are estimated to be upwards of 50,000 hours, just like tubes, some may fail earlier or go sufficiently out of spec over time to warrant replacement. And just like tubes, our LDR Modules are easily replaceable ensuring the lasting value of your passive preamp investment.

High Quality Power Supply: Control power is provided by AC mains first passing through a high performance EMI filter/choke that feeds a low EMI toroidal step-down transformer. An oversized DC filter supplies power to a multiple high performance switch mode DC regulators used to provide extremely quiet and stable 12 V and 5 V power to the LDR3x.V2 preamp controller board and peripherals.

An Outside To Match The Inside

Stylish Enclosure: The LDRx is housed in an attractive, high-quality contemporary enclosure that combine the warmth of eco-friendly renewable bamboo together with black-anodized brushed aluminum panels milled from high grade stock.

Cardas RCA Jacks: The LDRx comes standard with Cardas GRFA-L RCA jacks. These are beautiful beefy jacks that are rhodium over silver plate over high quality brass. Too bad they’re on the back and not the front!

Controls: The elegantly simple 7-button Apple Remote provides remote control, along with a rotary encoder with integral push-button located on the front panel. Volume level and status information are provided via a pair of digital displays that are invisible when the unit is off.

Options & Customization

Because we fabricate and assemble most of our products ourselves, we can offer the flexibility of numerous options, upgrades and customizations that are rarely available with mass-produced audio products. Some of these are listed below. We welcome you inquiries regarding potential upgrades or customization to your LDRx. Depending on the specifics, shipping is always subject to parts availability, the number of custom units in the queue, and may be delayed between several days to several weeks to accommodate your request.

Number of Inputs:  Select from 3,  4,  or 5 RCA inputs for a nominal incremental cost.

RCA Jacks:  Beyond out standard offering of Cardas jacks, we can accommodate a relatively wide range of makes/models of inputs jacks although pricing will be subject to our ability to source these at either OEM or retail prices. Please contact us to discuss options.

Solder: Our standard solder is lead free silver with a minimum of 2.5% silver content.

Hookup Wiring:  Our standard wiring is high quality Milspec teflon coated 26AWG silver coated copper wire. We also offer optional 26 AWG Neotech UP-OCC solid copper for point to point wiring within the signal path.

Specifications

• Preamplifier Type: Passive unity gain without buffer | attenuation via light dependent resistors (LDRs) in an LPad configuration
• Attenuation Range: –60 dB  to 0 dB in a 70 step attenuation schedule (see impedance/attenuation diagram below)
• Compatible Amplifiers:  Works best with tube or solid state amps having input impedance of 20k ohms or higher
• Nominal Input Impedance: 20k ohms (see impedance/attenuation diagram below)
• Control System: Software based (updateable) 8 bit digital microcontroller driving 12 bit digital-to-analog volume control
• Software :  Updateable via download from website and upload via USB cable from PC
• Auto Calibration:  User enabled/disabled self-contained auto calibration of LDRs to correct for drift/aging
• Replaceable LDR Modules: Similar to tubes, LDR modules can be easily unplugged and replaced if warranted
• Display:  Dual numerical displays (blue) providing volume and status information
• Control Methods & Functions:  Remotely  controlled via the elegantly simple Apple Remote | Front panel control via rotary encoder with integral push button switch
  • Power on/off toggle
  • Volume raise/lower
  • Mute/UnMute (with smooth ramping of volume)
  • Channel balance adjust & reset
  • Display brightness raise/lower
  • Channel input select
  • Auto calibration start/stop
  • Remote pairing (enabled via encoder bush button only)
• Input Switching:  Audio signal switching done via LDRs – no electromechanical relays in the signal path
• Power Supply: 
  • IEC type AC socket with removeable cable
  • Switchable 120 or 240 VAC supply
  • EMI noise supressor/choke on AC entry
  • 10 VA low noise toroidal transformer
  • Oversized low ripple DC filter
  • Stable low noise 5V and 12 V DC via dual switch mode power regulators
  • Power Draw:  On: < 0.4 amp @ 12 VDC | Off: < 0.25 amp @ 12 VDC
• 12 Volt Trigger Out: Heavy duty chrome plated through-hole threaded phone jack provide 12 volt signal when unit turned on
• USB Port:  For updating control software if warranted.
• Point-to-point Wiring:   All internal point-to-point wiring of power & audio I/O to PCB  utilizes milspec teflon insulated silvered copper wire. Neotech UP-OCC solid silver wire available for hookup of the audio signal path.
• Inputs/Outputs: 
  • Inputs: 3, 4, or 5 unbalanced RCA inputs
  • Outputs: 2 parallel unbalanced RCA outputs
• Audio Connectors: Cardas GRFA-L rhodium over silver plated brass RCA jacks
• Custom Enclosure:
  • Poly oiled bamboo side panels
  • Front, rear, top and bottom milled from high grade aluminum stock
  • Attractive brushed and black anodizing on all visible aluminum parts
  • Dark acrylic front panel accent/inlay in front of display modules
  • MDF hockey-puck feet with heavy duty non-slip black polyurethane rubber | it stays where you put it
• Size (shipped package): Width -14″ | Depth – 14″ | Height – 6″
• Size (unit): Width – 12.5″ | Depth – 10.5″ | Height – 3.75″
• Weight (shipped package): ~11 lbs
• Weight (unit): [TBD]
• Items Included: Preamplifier | Apple Remote | Power Cord | USB Cord