Preamp Controller Board

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

Not a kit board – comes fully populated, tested and ready to go.
Digitally controlled, analog audio preamp controller in the form of a populated 2.5 inches wide by 5.0 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 digital control of the analog LDRs via a software driven on-board microcontroller

Key Features

The LDR3x is a fully functional LDR based passive preamp volume controller with both remote & local control capability
Attenuates from approximately -60 to 0 dB via a custom 70 step attenuation curve
Able to auto-calibrate the LDRs ensuring accurate long term performance
Includes muting with smooth ramp down and ramp up
Single board handles 2 channel single-ended stereo
Two boards handle 4 channel balanced stereo input (2 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 (multi-function volume knob) with integral switch (see our Encode Module)
Status output than can drive a status LED
Interfaces with a 2 digit 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 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. Fortunately we were able to make these changes and also drop the the unit price from $259 to $199 ensuring that the LDR3x remains one of the best values in a audiophile grade attenuator available today!

Changes introduced with the V2.1 include:

Improved auto-calibration accuracy: Auto-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.

Precision pre-calibration process: Using precision temporary plug-in 0.1% Vishay Dale resistor modules it’s now possible to pre-calibrate the auto-cal circuit thus substantially improving the overall accuracy of auto-calibration. This further improves channel balance with commensurate improved stereo imaging and sound stage. We now do this as part of commissioning each V2.1 board prior to running auto-cal. This process is only recommended with new boards or after making hardware changes such as changing opamps. This process is also applicable to existing V2 boards but will require updated software (free download) plus the resistor modules. Tortuga Audio will soon be offering a relatively inexpensive 4-pack of pre-calibration modules as an option or as a separate purchase (the DIY’er can also build these themselves).

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.

Modified Auto-Calibration Circuit: Component changes were made to improve the accuracy and stability of auto-calibration.

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.

Key Features Introduced With Version 2

The V2 version of the LDR3x 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.

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.

Introduction of User Adjustable Input Impedance: User will be able to select up to 5 different user defineable input impedance settings between 5k and 50k and test the efficacy of each. Once selected, the LDR3x will have to be run through auto-cal at the new setting before a new impedance setting will be available. This has been discussed previously with respect to the V2 and will also be available to existing V2’s via software updates.

Unbalanced RCA or Balanced XLR Audio

Unbalanced (single-ended) Audio:

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

Balanced Audio:

Two LDR3x 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?

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

Technical Documentation

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

LDR3x.V2.1 Passive Preamp Controller