LDR V25 Preamp Controller


The LDR V25 preamp controller (the “V25”) is a software driven audio volume controller and source input switcher for high performance passive and active audio preamps.

The V25 is Tortuga Audio’s 4th generation attenuator using light dependent resistors (LDRs) for a level of sound quality that once you experience it you won’t want to go back to conventional potentiometers, stepped attenuators or even transformer based volume controls.

The V25 preamp controller board enables the audio hobbyist, professional designer or OEM to build an exceptional passive audio preamp that rivals the performance of some of the best high end preamp equipment available today regardless of price. When mated with a downstream preamp gain stage and/or buffer, the V25 becomes the heart of a very high performance active preamp.

The graphic OLED Display Module (sold separately) integrates with the V25 to provides an easy and intuitive user control experience through an interactive menu driven control interface designed around  the 7-button Apple remote.

Graphic OLED Display

OLED disply module with installed interface board, all cables, rotary encoder and installed IR Receiver module. Save $50 off display when purchased together with the V25 preamp controller.

Input Expansion

The base V25 model comes equipped with a 3 stereo inputs plus a mono switch with the ability to add up to 3 additional inputs for a maximum total of 6 inputs.

Mini Component Kit

The mini component kit includes the remaining items recommended for a complete basic V25 system excluding enclosure and input/output jacks
* Apple remote
* 12 VDC power supply (external)
* Panel mount power jack for power supply
* USB cable with panel mount jack for firmware updates

+ $65.00
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SKU: LDR.V25 Category: Tag:

V25 Controller Board Overview

The V25 Preamp Controller (Rev A) was first released in April, 2017.

The V25 is a digitally controlled passive analog 2 channel audio attenuator with integral input switching that uses light dependent resistors (LDRs) for both attenuation and input switching.

The V25 is classified as a “passive” attenuator insofar as there is no active processing of the audio signal and no direct connection to or manipulation of the audio signal by a power supply. Despite the sophistication of the V25 design, all the the audio signal “sees” is a voltage divider composed of 2 LDRs in a classic series/shunt L-Pad configuration that emulates a potentiometer.

The V25 board is shown below with annotation of the key components and control interface points.

V25 preamp controller port - annotated

V25 Board Versions

There are currently 3  hardware versions of the V25 board designated as A, B and C with A being the original board as shown above. The version letter is printed on the V25 board on the left side under the name of the board. The differences between these versions relate to the display interfaces and the USB interface. These differences are summarized in the table below. All 3 versions utilize the same firmware which recognizes these differences.

CriteriaVersion AVersion BVersion C
Handles 7 segment display (J3)YesYesNo
Handles OLED display (J9)YesYesYes
OLED header (J9) interface typeParallelSerial (SPI)Serial (SPI)
Serial (SPI) interface voltage to 7 segment display header (J3)5V3.3VN/A
USB header VBus pin has 5V powerNoYesYes

V25 Update – June,  2018

The following changes to the V25 became effective starting June 1, 2018.

Base Model Now Includes 3 Inputs plus Mono Switch – While keeping the base price unchanged, the base model V25 now includes switching between 3 inputs (was previously single input) . In addition, all V25 boards now include the Mono switch for switching between stereo and monaural listening modes.

Graphic OLED Display Module  – OLED display module - front viewThe V25 is now fully operational with the latest OLED Display Module (sold separately) which provides a highly interactive, easy to use, intuitive, menu driven user control interface for configuring and controlling the V25 via both remote control and encoder.

Major firmware update  – The V25 firmware has undergone a major update incorporating both the new OLED interactive controls plus numerous “under the hood” improvements.

V25 Specifications

Attenuation typePassive series/shunt (L-Pad) voltage divider using two (2) light dependent resistors (LDRs) per channel | total of 4 attenuation LDRs for attenuation.
Attenuation Range-60 db to 0 db in 99 in ~0.5 dB steps plus 1 additional muting step.
GainUnity (1x) gain | no amplification, processing or active manipulation of the audio signal | audio signal optically isolated from control system
BufferingNo input or output stage buffering
Input impedanceNominal 20k by default. User may configure up to 9 additional impedance settings each adjustable between 1k and 99k and switch between the settings with live music to optimize input impedance for their system.
Number of Channels2 independent channel stereo
Multi-channel Attenuation2 or more V25 boards can be daisy-chained to provided mutl-channel attenuation with 2 channels per board
Channel Balance AccuracyWithin 0.5 db over full attenuation range
Channel IsolationEach channel 100% isolated and independent from the other within the V25 board | audio signal grounds must to tied to a common star ground point external to the board
Audio Signal GroundNo connection of signal grounds to power ground within controller, however, audio signal ground must share common ground with power supply via an external star ground point in order for LDR self-calibration feature to function properly.
Audio Inputs | Single Ended1 board accommodates up to 6 switchable single-ended stereo (2 channel) inputs
Audio Inputs | Balanced2 boards accommodate up to 6 switchable balanced stereo (2 channel) inputs. With 2 boards it's possible to configure a mix of single-ended and balanced inputs.
Audio Input Switching/IsolationInput switching is done with LDRs | typical 50 ohm resistance when on | in excess of 5 megaohms when off | no relays in signal path.
Audio Output Switching/IsolationOutput signal switched on/off with LDR. Isolation between inputs and output when switched off in excess of 10 megaohms.
Total Harmonic Distortiontypicallay less than 0.5%
Power Input7-30 volts DC rated at 0.5 amp via J1
Power Regulation2 stage regulation | 5 V 1st stage low noise switch mode regulator followed by 3.3 V low noise 2nd stage linear regulator
ProcessorARM M3 Cortex 32 bit microcontroller running at 72 MHz
Control Input | RemoteAccepts control inputs from infrared silver Apple remote or equivalent remote using 38 kHz NEC protocol and Apple commands | infrared commands received via infrared receiver module attached indirectly via display module or directly to J3 pins.
Control Input | EncoderAccepts control inputs from rotary stepped encoder with integral push button switch | encoder attached indirectly via display or can be attached directly to J3 pins.
Control Input | UARTAccepts control inputs from external device using UART serial protocol and specific command codes (available upon request).
Display | OLEDGraphic OLED display with an intuitive, easy to use, menu based control scheme. Connects to the V25 controller board via both J3 2x5 header and J9 2x7 header. Requires display jumper across pins at J7.DISP.
Display | 7-Segment (legacy)Original dual 2-digit numerical 7-segment LED type display with adjustable brightness | each 2-digit display is a physically separate module connected via ribbon cable | connects to board via J3 2x5 pin header
Display | Status LEDStatus LED light | connects to board via J7
Mono ModeOutput switchable between stereo and mono mode via LDR switch | applies on to single-ended audio
FirmwareCustom interrupt driven application code written in the C language
Firmware UpdateFirmware can be updated via a resident bootloader that communicates with an external PC based HID bootloader application. User can download firmware update files from company website.
USB PortStandard 5 pin USB port allows processor to communicate with external HID bootloader application for updating firmware.
WiFi ModuleFuture ability to plug in a simple WiFi module via J6 that will allow the V25 to be controlled via smartphone app.
Trigger OutOutputs a DC voltage signal when unit is turned on. Signal can be used to turn other devices on/off that are equipped with a Trigger Input. Trigger voltage level is equal to DC voltage level supplied to the board.
Printed Circuit BoardHigh quality nickel immersion gold solder pads with blue solder mask
Ambient RequirementsBoard must be operated in a stable, dry indoor environment only with ambient temperature within 60-90 degree F
ROHSCompliant or exempted
Physical DimensionsLength: 5.7 inches
Width: 2.7 inches
Corner Holes Center: 0.145 inches from edges
Corner Holes Diameter: 0.15 inches
Top clearance: 0.7 inches
Bottom clearance: 0.5 inches

V25 Compared to the V2?

  • Overall smoother, quieter, more resolving attenuation – it’s just better!
  • Improved built-in LDR calibration algorithm
  • Completely redesigned hardware & software
  • Code rewritten in C
  • 8x larger application memory
  • 2 stage low-noise power regulation
  • Faster 32 bit ARM vs slower 8 bit PIC processor
  • 6 DAC channels vs only 2 for improved LDR control
  • 100 step attenuation vs. 70
  • Graphic OLED display with simple, intuitive interactive menu
  • Provision for future WiFi module
  • Can daisy chain multiple boards for multi-channel control

What’s Included?

  1. V25 board with 3 switchable stereo inputs (up to 3 additional inputs can be added at additional cost)
  2. Monaural mode switching LDR
  3. Mating slip-on power connector with wire crimps
  4. Updated firmware

What Else is Required?

To implement a fully functional passive preamp requires the V25 board plus the following additional items that are sold separately by Tortuga Audio and/or third parties.

  1. Apple Remote
  2. Graphic OLED Display Module assembly including Encoder
  3. 12 V DC power supply (0.5 amp or higher)
  4. RCA/XLR jacks
  5. Wiring and enclosure

Changelog & Product Roadmap

Click on the accordion link below to view content.

Technical Documentation

More detailed technical information V25 can be found in the product documentation section of our website.

Weight0.18 lbs
Dimensions5.7 × 2.7 × 1.0 in
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