Key features

The LDR3x.V21 is an a DIY preamp attenuator/controller board that uses light dependent resistors (LDRs) to implement stereo audio volume control. The V21 model is an enhanced version of the earlier V2. These enhancements are highlighted below.

• Not a kit – The V21 itself comes fully populated, tested and ready to go. It is not part of some greater kit and requires a DIY or OEM to integrate the V21 into an assembled functional preamp.

• Volume & preamp controller board – The V21 is a digitally controlled, analog audio volume and overall preamp controller in the form of a populated 2.5 inches wide by 5.0 inches long printed circuit board.

• Remote control – The common silver metallic Apple IR remote is employed as the remote control for the LDR3x.V21.

• Single-ended or balanced – A balanced preamp can be configured by using a pair of V21 boards.

• Passive or active preamps – The V21 by itself is an unrivaled passive preamp/controller but can also be incorporated into any active preamp design including both solid state and tube

• Attenuation range – Attenuates from approximately -60 to 0 dB via a custom 70 step attenuation curve

• Built-in LDR calibration – The V21 is able to automatically calibrate each LDR using a proprietary algorithm to force all 4 LDRs to match their performance. Matched sets of LDRs are not required.

• Smooth muting & volume ramping – Has volume muting with smooth ramp-up and ramp-down of volume during transitions.

• Input switching – Switch between 3 inputs (unbalanced or balanced) when mated with our IO3 relay board (or equivalent)

• Rotary encoder control – In addition to remote control, the LDR3x can be controlled manually via a rotary encoder with integral push button switch.

• Status LED light output – An output signal is available to drive an external status LED light

• 7 segment display – Interfaces with a 2 digit  7 segment DM1 Display Module to  display volume step & unit status

• Channel balance adjustment – Provides traditional left/right channel balance for fine tuning

• Optical isolation – Audio signals are not directly exposed to power signals within the LDR3x by virtue of the optical isolation inherent with LDRs.

• 12 volt trigger out – Provides a 12 VDC trigger out to turn other audio system components on/off.

• Flexible power supply – Runs off  of  most any 9-30 VDC power supply – regulated or unregulated.

What’s New In Version 2.1?

The V2.1 is an incremental update of the existing V2 platform in order to create a more accurate and robust preamp attenuator/controller through hardware optimization and software improvements.  

Changes introduced with the V2.1 include:

Improved calibration accuracy:  LDR calibration accuracy has been substantially improved approaching 0.1% matching between target and actual resistance level at each step of the attenuation range. This was largely accomplished through enhancements in the auto-cal software algorithm. Increased accuracy translates directly into improved stereo imaging and sound stage. Please note that this is also applicable to existing V2 boards via free software updates which can be download from our website.

Modified LDR Control Circuit:  Component changes were made to improve the accuracy, stability of LDR control while accommodating a wider range of LDR performance specs.

Socketed Op Amps: Sockets were added rather than permanently hard-soldering op amps into the board. This make it possible to try alternative compatible single-voltage quad op amps to optimize performance while also allowing for powering the board with higher voltage levels. Sockets allow the DIY user to experiment. Quad op amps are used in both the left/right channel LDR control circuits and also as a buffer in the auto-cal feedback circuit.

Potential Higher Power Supply Voltage: Although the stock op-amps currently used limit the supply voltage to a maximum of 15 VDC (12 VDC regulated recommended), there is potential to user alternative op-amps that can handle upwards of 36 VDC. While this will not change the performance of the LDR3x it will provide additional flexibility in the designing your DIY preamp or retrofitting the LDR3x into an existing preamp.

What’s Included?

• V2.1 Board – 2.5 by 5.0 inch populated and tested printed circuit board

• IR Receiver Module – Vishay TSOP31138 38kHz high performance IR Receiver that can be mounted into an optional DM1 module or otherwise located external to the board.

What Else is Required?

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:

• Apple remote – An apple remote and/or an encoder module

• 12 V DC power supply – A nominal 12 volt DC power source able to deliver up to 0.5 amp

• RCA jacks and wiring – for connecting the audio input/ouput to the V2 board

• Enclosure – some type of overall enclosure to house the preamp

Technical documentation

More details on the LDR3x.V21 can be found in the online product documentation section of our website.