LDR3x.V2 Passive Preamp Controller


The V2 version of the LDR3x has been replaced by the V2.1

Find our more about the V2.1 here. 

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With the V2, you can now build a world class preamp rivaling the best of the best! And with auto-calibration and replaceable LDR modules, your investment in high performance audio will last a lifetime.

The LDR3x.V2 is an a DIY passive preamp controller board designed around light dependent resistors (LDRs) that allows the DIY’er to build a passive or active preamp with exceptional audio performance that can be controlled with a remote.

LDRs are being increasingly recognized by audio enthusiasts as THE best technology for volume control. And Tortuga Audio’s LDR3x.V2 is the most advanced state-of-the-art LDR passive preamp design available.

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SKU: LDR3xV2 Category:
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What’s New In Version 2?

The V2 board was released on May 7th, 2014 as the successor to the very popular V1 board with the following improvements and differences:

Auto Calibration: Version 2 will no longer be dependent on specialized external test equipment and software tools for initial calibration and tuning of the LDRs. All of that is now self-contained in the board itself. In the event one or more LDRs age differently and drift out of calibration, the LDR3x.V2 can adjust each LDR’s attenuation table to bring the unit back into precise calibration. This also mitigates dependence on having to use matched sets of LDRs in each build. Also, owners are no longer dependent on us for storing and maintaining their calibration data should updates be needed in the future.

Replaceable LDR Modules:  LDRs are similar to tubes insofar as they are nonlinear analog devices. They have an internal LED light source, they don’t behave exactly the same from one to the next even if the same make/model, their performance characteristics can change over time, some fail early on, and they eventually age and die (although 50,000 hours is a long life). Wouldn’t it be nice to just yank a bad one out, and stick in a nice new one for around $20? Done!

Increased Precision:  Whereas the V1 used 8 bit digital pots,  the V2 board employs both 12 bit DACs and ADCs. This gives the LDR3x.V2 16 times greater precision in adjusting the resistance values of the LDRs. This means more precise channel matching over the full attenuation range and that translates directly into improved stereo imaging and sound stage.

Quieter Power Supply: Version 2 replaces a linear voltage regulator with a precision low noise switching power supply module with superior ripple rejection. The result is a darker background between the notes and improved articulation and expression of sound. What does this mean? It now sounds even better!

Software Updates via USB: Version 2 uses an on-chip USB bootloader which allows software updates to be handled via a USB connection to a PC. Thus, software updates can downloaded from our website out and owners can upload the latest software into their V2 board from their laptop/desktop PC. Eventually this will also be doable from a Mac as well.

Fewer Control Interface Options:  The downside of all these cool improvements is fewer DIY control interface options although control via both IR remote and manual encoder remain unchanged. Control via discrete channel select inputs and analog pot are no longer available.

The LDR3x.V2 – What Is It?

The LDR3x.V2 is an a DIY passive preamp controller designed around light dependent resistors (LDRs) that allows the DIY’er to build a passive or active preamp with exceptional audio performance that can be controlled with a remote.

  • Digitally controlled, analog audio preamp controller in the form of a populated 2.5 inches wide by 4.7 inches long printed circuit board
  • Controllable via the Apple IR Remote (not supplied) (in the future will also be controllable via smartphones with IR capability)
  • Allows the DIY audio enthusiast to build their own exceptionally transparent and articulate balanced or unbalanced audio preamplifer
  • Rivals the performance of even the best high end preamp costing 10-100 times more than the LDR3x.V2
  • By itself is an unrivaled passive preamp/controller but can also be incorporated into any DIY active preamp design
  • Utilizes light superior sounding dependent resistors (“LDR”) in lieu of conventional potentiometers or stepped resistive attenuators for volume control
  • Precision control of the LDRs and all other functionality of the LDR3x.V2 is done via a software driven on-board digital microcontroller

What Does It Do?

  • The LDR3x.V2 is a fully functional LDR based passive preamp volume controller
  • Attenuates from approximately -60 to 0 dB via a custom 70 step attenuation curve
  • Continually auto-calibrates its LDRs while turned off  (and connected to power)
  • Includes muting with smooth ramp down and ramp up
  • Accepts one (1) single ended unbalanced stereo input (single V2 board)
  • Accepts one (1) balanced stereo input (requires dual V2 boards configured as master/slave)
  • Can switch between 3 inputs (unbalanced or balanced) when mated with our IO3 relay board (or equivalent)
  • Fully controllable via infrared remote
  • Fully controllable via an encoder with integral switch (see our Encode Module)
  • Status output than can drive a status LED
  • Can be interfaced with the  7 segment DM1 Display Module to  display volume step & unit status
  • Provides traditional left/right channel balance for fine tuning
  • Optical isolation between audio and power/control signals – pure resistive pass-through attenuation
  • Provides a 12 VDC trigger out to turn other audio system components on/off
  • Runs off  of  most any 12 VDC power supply

Unbalanced RCA or Balanced XLR Audio


A single LDR3x.V2 board (paired with an optional  input relay module) accommodates up to 3 unbalanced (single-ended) audio inputs.


Two LDR3x.V2 boards (each paired with an optional input relay module) can be configured to accommodates up to 3 balanced audio inputs. One board is designated as the Master (the right channel) and the other is designated as the Slave. This is done via the setting of jumpers on the board together with a 3 wire control cable between the 2 boards.

What’s Included?

  • Tortuga Audio 2.5 by 4.7 inch printed circuit LDR3x.V2 Board populated with components
  • Vishay TSOP31138 38kHz high performance IR Receiver which can be inserted/soldered into the V2, a DM1 module or otherwise located external to the board

What else is Required & What Options Are Available?

To implement the most basic LDR3x system, in addition to the LR3x.V2 board itself,  the DIY’er will need to supply the following additional items:

  1. Apple Remote and/or Encoder Module – (both sold by Tortuga Audio)
  2. 12 V DC power supply (0.5 amp or higher) – (sold by Tortuga Audio)
  3. RCA jacks and wiring for connecting the audio input/ouput to the V2 board –  (sold by Tortuga Audio)
  4. Typically you’ll also want an enclosure  to house the preamp

Tortuga Audio offers the following options which are all available for purchase from our online store:

The Apple Remote
The Apple Remote
Encoder Module
Encoder Module
LDR3x power supply - front view
PS12.5 Power Supply Board
LDR3x IO3 Relay Board
IO3 Input Relay Board
DM1 Display Module - Front View
DM1 Display Module
Cardas GRFA-L RCA jacks
Cardas RCA Jacks

LDR3x.V2 Technical Documentation

More details on the LDR3x.V2 can be found in the Product Documentation section of our website.

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