A Balanced Passive Preamp

The LDR1B.V25 Balanced Passive Preamplifier is a remote controlled balanced audiophile grade passive preamp (attenuator) with 1 single XLR input and 2 parallel XLR outputs. The LDR1B provides true balanced attenuation with independent attenuation of both phases in each stereo channel. Also, being a true passive preamp, the LDR1B is a unity gain preamp with no amplification or buffering.

The LDR1B.V25 is an updated version of our popular LDR1B.V2 model and its design closely parallels our single-ended LDR3.V25. The updated LDR1B incorporates our new and improved V25 preamp controller that uses LDRs (Light Dependent photoResistors) for exceptional audio performance.

Enthusiasts who value a dead quiet preamp with a deep dark background between the notes, exceptional transparency, and pleasing dynamics, will find the LDR1B.V25 to be a very satisfying preamp rivaling many high end peamps at a fraction of the cost. And it comes with a simple remote control so you can relax and enjoy your music in comfort from your favorite listening spot.

The LDR1B.V25 comes in black or silver anodized front panel and uses high quality Neutrik XLR jacks with Neotech OCC solid copper point-to-point hookup wire.

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Specifications

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Criteria	Description
Attenuation type	Passive series/shunt (L-Pad) voltage divider using two (2) light dependent resistors (LDRs) per channel | total of 4 attenuation LDRs for single-ended attenuation & 8 for balanced attenuation
Attenuation Range	-60 db to 0 db in 99 ~0.5 dB steps plus 1 additional muting step.
LDR Modules	All LDRs are easily replaceable plug-in modules
Gain	Unity (1x) gain | no amplification, processing or active manipulation of the audio signal | audio signal optically isolated from control system
Buffering	No input or output stage buffering
Auto Calibration	Built-in calibration algorithm that calibrates the four attenuation LDRs at each of the 99 attenuation steps to compensate for any drift or aging or in the event an LDR needs to be replaced. Pre-matched LDRs are not required | Enables adjustable impedance feature
Adjustable Impedance	Nominal 20k by default | User may configure up to 9 additional impedance settings adjustable between 1k and 99k each | Once additional settings are initialized user may switch between the settings with live music to optimize input impedance for their system.
Number of Channels	2 channel stereo
Channel Balance Adjustment	+/- 10 dB channel balance adjustment range in 0.5 dB increments
Channel Balance Accuracy	Within 0.5 db over full attenuation range
Channel Isolation	Each channel signal is 100% isolated and independent from the other | audio signal grounds are tied to a common star ground point
Audio Inputs	Single Ended Models - 3 switchable single-ended Cardas RCA stereo inputs Balanced Models - 1 Neutrik XLR inputs
Input Switching	Input switching via LDRs | typical 50 ohm resistance when on | in excess of 5 megaohms when off | no relays in signal path | total of 6 LDRs used for input switching
Output Isolation	Output signal is switched off and isolated when AutoCalibration is enabled | Switching is done using LDRs | no relays in signal path | Isolation between inputs and output when switched off in excess of 10 megaohms | total of 2 LDRs used for output isolation
Total Harmonic Distortion	1% or less
Power Supply	US : Nominal 12 volts DC linear regulated "wall wart" rated at 0.5 amp International: Nominal 12 volts DC switching "wall wart" rated at 1.0 amp
Power Regulation	2 stage regulation | 5 V 1st stage via low noise switch mode regulator followed by 3.3 V low noise 2nd stage linear regulator
Processor	ARM M3 Cortex 32 bit microcontroller running at 72 MHz
Controls | Remote	Accepts control inputs from silver Apple infrared remote or equivalent using 38 kHz NEC protocol and Apple commands | infrared commands received via infrared receiver module located on right display module.
Controls | Encoder	Accepts control inputs from rotary stepped encoder with integral push button switch | encoder attaches to right display module
Control Modes	* Volume & balance adjust mode (default) * Input adjust mode * Impedance adjust mode * Display adjust mode * Max initial volume at input change adjust mode * Autocalibration enable mode
Display | Current	Dual 2-digit numerical 7-segment LED type display with adjustable brightness |
Display | Future	Possible future upgrade to graphical OLED display at additional cost for display | future option will require a firmware update
Mono Mode	User can switch between stereo and mono mode via the remote | switching uses an LDR switch
Firmware Updates	User can update firmware using PC based HID bootloader application (no MAC option) | User can download the bootloader application and updated firmware files from company website | Firmware is custom interrupt driven application code written in the C language provided in hex format
USB Port	Type A female USB port allows processor to communicate with external HID bootloader application for updating firmware | USB cable provided
WiFi Module | Future	Future ability to plug in a simple WiFi module to allow control via smartphone app | will require WiFi module, firmware update, and upgrade to future OLED display
Trigger Out	Outputs a 12 V DC signal when unit is turned on. Signal can be used to turn other devices on/off that are equipped with a Trigger Input.
Enclosure	Heavy wall extruded aluminum case with black powder coat finish | Brushed anodized aluminum front and rear panel | Front panel with dark grey acrylic inlay in front of visual display | Rear panel with laser etched labeling | Black oxide socket screws with washers attach front and rear panel to case | no-slip urethane feet
Printed Circuit Board	High quality nickel immersion gold solder printed circuit boards with blue solder mask
Solder	Lead free solder with 3% or higher silver content
ROHS	Compliant or exempted
Fuses	No internal or external fuses
Ambient Requirements	Preamp must be operated in a stable, dry indoor environment only with stable ambient temperature within 60-90 degree F
Dimensions	~ 3.25 inches tall | ~6.0 inches wide | ~ 9.0 inches deep
Weight	~ 4 pounds | ~6 pounds as shipped

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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 Passive Preamps

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 Tortuga passive preamps match the performance of high-end active preamps costing many thousands of dollars more.

Key Features Under The Hood

True Balanced Attenuation

Each phase of each channel is independently attenuated. There is conversion, conditioning, or processing of the audio signal aside from each signal phase passing through an L-Pad type voltage divider composed of 2 LDRs in a classic series-shunt configuration.

Optically Isolated Passive Attenuation

There’s no amplification or buffering of the audio signal notwithstanding the presence of a power supply used solely for photoresistor control and input switching. Audio signals are optically isolated from both the power supply and the LDR control circuitry.

4th Generation LDR Technology

Built with our most advanced V25 preamp controller which employs microprocessor controlled analog Light Dependent photoRresistors  (LDRs) to provide audio attenuation and input switching with unparalleled audio quality.

Built-in LDR Calibration

Employs self-contained calibration of the 4 attenuation LDRs to maintain accurate attenuation schedule and channel balance and ensure that the preamp will hold its value over the long haul as the LDRs age.

Replaceable Plug-in 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 socketed and easily replaceable ensuring the lasting value of your passive preamp investment.

No Relays In Signal Path

LDRs are also used for audio input switching in lieu of relays. Auto Calibration does employ relays but these are physically disconnected from the audio signal path during normal operation.

Firmware Updates via USB

If warranted, control system firmware can be easily updated by the customer via a USB port located on the rear panel. Firmware updates can downloaded from our website and uploaded into the preamp from a laptop or desktop PC (not currently compatible with MACs).

Key Changes in the V25 Model Compared to V2

• Overall smoother, quieter, and more resolving attenuation | 100 vs.70 steps
• Major improvements to LDR controls and auto-calibration of the LDRs
• Faster more powerful processor with expanded memory for future features/upgrades
• Quieter power supply design with low noise 2 stage DC-DC regulation
• Input switching integrated into main control board to minimize interface points
• Provision for future OLED graphical display & WiFi/Control module