LDR.V3 preamp controller graphic

LDR.V3 Preamp Controller System In Development

April 20, 2017 Update

Development of the V3 has been suspended indefinitely. Instead, Tortuga Audio has released the V25 Preamp Controller which ultimately will be able to do most everything planned for the V3 but with a slightly different architecture.

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Next Generation Preamp Controller

Like the V2 before it, Tortuga Audio’s next generation preamp controller (the LDR.V3 or “V3”) will be a high performance audiophile analog attenuator that uses digitally controlled light dependent resistors (LDRs) in lieu of potentiometers, conventional resistors or transformers. Complete with input switching between multiple sources,  remote control, graphical display, and numerous other control options and features, the LDR.V3 Preamp Controller system can be used as a stand alone passive preamp or the front end of a conventional active preamp for a superior audio listening experience.

The LDR.V3 Preamp Controller system has been taking shape throughout 2016 and is an evolution of the V2 design which has been in production for over 2 years now. Key goals for the V3 include improving the sonic performance, adding key functionality, improving ease of use, and extending the range of potential applications.

The V3 is an ambitious project. We believe that getting it right is more important getting it out to market quickly. For this reason we do not have a projected release date for the V3 at this time although we are getting closer day by day. We are currently on our 4th iteration of prototype hardware and although there have been many changes in between each iteration, we believe the design is now close to its final form.

With the design process far along this is a good time to begin discussing the V3 publicly in more detail. To that end, what follows is a detailed description of the new LDR.V3 Preamp Controller system.

New System Architecture

The LDR.V3 Preamp Controller employs a new and different system architecture compared to its V2 predecessor. The V2 system combined digital and analog circuitry,  including audio, on a single printed circuit board and used a separate external input switching board. By comparison, the V3 moves the digital processor and all control interfaces on a Control/Display Board and moves the analog control and audio circuitry on to a separate LDR Attenuator/IO circuit board that includes self-contained source input switching.

LDR.V3 preamp controller system diagram
LDR.V3 Preamp Controller System Diagram

The V3  is designed from the ground up to be a multi-channel preamp attenuation/control system capable of handling up to 16 channels with 2 channels per each LDR Attenuator/IO Board for a maximum of 8 attenuation boards. Our current V2 system is limited to 4 channels which is adequate for single ended or balanced stereo attenuation only.

Moreover, whereas the V2 has a digital processor on each board, the V3 requires uses only a single processor on the Control/Display Board regardless of the number of channels thus greatly simplifying multi-channel integration. 

The V3 system minimizes the audio signal pathways and interfaces points by having the input switching integrated into each LDR Attenuator/IO Board where LDRs are used for both attenuation as well as input switching. All input and output connections are soldered (no screw terminals) with only short run hookup wire needed between the board and the audio jacks. Each LDR Attenuator/IO Board can accommodate up to 6 switchable stereo inputs with no additional off-board IO switching hardware required.

There are numerous others changes and additions in the upcoming V3 preamp controller system. The following sections describe both the LDR Attenuator/IO Board and the Control/Display Board in more detail.

LDR Attenuator/IO Board

In its current iteration of development the LDR Attenuator/IOI Board is a 2.85″ x 5″ printed circuit board. All electronic components are surface mount types. Board address (0-7) is set via 3 jumpers and connection to the Control/Display Board and possibly other Attenuator/IO Boards is via dual 2×5 pin headers. All plug-in LDR modules use 2×2 pin headers. Audio input/ouput signals are connected via solder pads only.

LDR.V3 LDR attenuator/IO board

120 Step 0-60 dB Attenuation via LDRs

The V3 has super smooth 120 step attenuation at 0.5 dB per step between -60 and 0 dB.  Analog attenuation is provided via light dependent resistors (LDRs) in a series/shunt LPad configuration.

Dual Low Noise Linear Regulators

Each LDR Attenuator/IOI Board is powered by the Control/Display Board. Each LDR board has its own dual low noise linear regulators that both isolate and step down control board power to both 4.5 and 3.3 volts to power the local devices that regulate the LDRs.

Up to 8 boards | 16 total independent channels

Anywhere from 1 to 8 LDR Attenuator/IO Boards can be connected to a single Control/Display Board providing up to 16 independent channels linked to a master volume control setting. Each channel can be trimmed +/- 6 dB (+/- 12 steps) relative to the master volume control.

Only a single LDR Attenuator/IO Board is required for single-ended stereo volume control. For balanced stereo, 2 boards are required wherein one board connects to the Display/Control board and the second connects to the first is a daisy-chain fashion via a 2×5 pin ribbon cable.

Multi-Channel Preamp Applications

The V3 system can be configured to provide superior quality multi-channel attenuation in the analog domain for systems with multi-channel active crossovers, DSPs, and/or multiple DACs. A typical application is downstream attenuation of of multi-channel DACs or similar scenarios involving 2, 3 or even 4 way active crossovers.

Integral Input Switching | 6 Inputs per Board

Input switching is done via plug-in LDR modules directly on the LDR Attenuator/IO Board without any additional external switching hardware. Each LDR Attenuator/IO Board can switch up to 6 single ended stereo input signals or up to 6 mono balanced audio signals (2 LDR Attenuator/IO Boards are required for balanced stereo).

Mono Mode

The LDR Attenuator/IO Board can be switched at any time to deliver monaural (mono) audio. When mono mode is enabled the left and right stereo channels are combined to deliver the same monaural audio in both channels. As with all other audio signal switching, the mono mode switching is done via an LDR and not a relay.

Polarity Switching

When configured for balanced audio attenuation, the V3 can switch audio signal polarity “on the fly”. Discriminating audiophiles will find this feature very useful in matching the compression and rarefaction of the sound wave playback with the polarity of the original recording. Note that polarity switching limits the number of switchable balanced input sources to 3 rather than 6. Polarity switching is not available with single ended audio.

Serial Communication Bus

The LDR Attenuator/IO Board employs 12 bit DACs (digital to analog converters) and ADCs (analog to digital converters) plus a digital I/O extender that are all controlled from the Control/Display Board over a dual channel SPI communications bus.

No Digital Processor | No Digital Activity Except During Control Inputs

Although the LDR Attenuator/IO Board has several digitally controlled (SPI) devices, the LDR Attenuator/IO Board has no noisy digital processor of its own. In fact there is no digital activity on the LDR Attenuator/IO Board during normal audio listening except for brief SPI communications during control changes such as when raising and lowering volume etc. after which all digital activity stops. The result is the blackest of black backgrounds with an extremely low noise floor.

Adjustable Input Impedance

Similar to the V2, the V3 has adjustable input impedance. The user can configure up to 5 different input impedance settings each with its own impedance levels. You can switch between the 5 settings while listening to live music or test signals to optimize the best impedance setting for your current equipment. This feature will be easier to set up and use with the V3 compared to the V2 or can be ignored entirely.

 All Switching via LDRs |  No Relays

All input and output switching is done with LDRs instead of conventional relays. A typical LDR has a turned-on resistance between 40-60 ohms which has a de minimis effect on the audio signal. When turned off LDRs have a resistance level in excess of 5 million ohms which effectively disconnects and isolates the audio signal from any other signal(s) present. Switching with LDRs is far smoother than with relays and significantly reduces the chance of clicks, pops or bumps due to switching. Best of all there are no mechanical interface points to degrade the low level audio signals when LDRs are used for switching and isolation.

Adaptable LDR Control Design

The unvarnished truth about LDRs is their performance curve (resistance vs. control voltage) can vary considerably from one LDR to the next even with the same make and model LDR. Moreover their performance curve will change over time. The V3 system has an adaptable control design that adjusts upper and lower control limits to fit each individual LDR. The result is finer control of the LDRs over the full attenuation range which translates into tighter left/right channel balance, improved stereo imaging and a more realistic sound stage.

Self-Calibrating LDRs With Diagnostics

The V3 builds on the auto-calibration feature introduced with the V2. Auto-calibration keeps the attenuation LDRs perfectly calibrated even as they age and and their performance curves shift over time. In the event an attenuation LDR requires replacement, the auto-calibration feature rebuilds the attenuation tables to match the performance of the new LDR(s).

The V3 takes this self-calibrating ability to the next level with fully automatic hands-off operation that takes place in the background when the preamp is turned off. If an LDR does go out of spec, built-in diagnostics will identify the faulty LDR and recommend replacement.

Modular Replaceable Plug-In LDR Modules

Much like tubes, LDRs are analog devices that can wear out over time. All LDRs come as compact plug-in modules that are easily removed and replaced by the user.

Control/Display Board

In its current manifestation the Control/Display Board is a 2″ x 4″ printed circuit board using mostly surface mount components plus numerous 0.1″ pin headers for connecting to control and display devices and the LDR Attenuator/IO Board(s).

LDR.V3 preamp bontroller board

Control Board Isolated From Audio Signals

All digital processing, control, interfacing and communications are handled by the Control/Display Board which has no direct interface with any audio signals and communicates with the LDR Attenuator/IO Board solely via a serial data bus.

Serial Communications Bus

The Control/Display Boards communicates with the LDR Attenuator/IO Board(s) via a high speed dual channel 4-wire Serial Peripheral Interface (SPI) which is only active during control changes (i.e volume increase/decrease, input switching etc.) and is otherwise inactive and silent. The dual SPI bus is connected in a daisy-chain fashion between boards when multiple LDR Attenuator/IO Boards are involved.

Powerful 32 bit ARM Cortex M4 Processor

The V3 system uses a single powerful high speed 32 bit processor with plenty of program expansion room for future growth. Does the V3 really need all that horsepower to do what it does? Not really. However, ARM Cortex processors have become the gold standard for embedded control and cost only a few dollars more than the older and far slower 8 bit hardware. We are making sure the V3 has the power and flexibility to further evolve with zero or only minor changes in the hardware configuration.

Multiple Display Options

New OLED Graphic Display

The V3 supports a new easy to read blue-on-black OLED graphic display that enables a more elegant menu driven visual control scheme compared to the dual 2-digit display modules used by the current V2 system. The V3 is easier to control and navigating through setup and options is  simpler and more intuitive.

The new primary display for the V3 is a 3.12 inch, 256 x 64 pixel resolution NewHaven OLED (organic light emitting diode) display module.  The OLED display connects directly to the Control/Display Board via a 22 pin header. This connection can optionally be made via a ribbon cable.

7 Segment Display Modules

The V3 retains the ability to use the same dual 2-digit 7-segment display modules used by the V2 system. These display modules connect to the V3 Control/Display Board via a 2×5 pin ribbon control cable similar to the V2. If desired, existing V2 units could be upgraded to the V3 with few if any changes.

Flexible Power Supply

The V3 system is powered by a single DC power supply connected to the Control/Display Board. The power source voltage is flexible and can be between 9-36 volts DC with 1 Amp or greater capacity. The Control/Display Board uses an efficient 1.5 Amp 300+ kHz switching step-down regulator to provide primary 5 volt power to all V3 boards. Low noise 3.3V and 4.5V linear regulators are used on both board types to further isolate and step down the primary 5 volt power before being used.

We plan to add the ability to use a separate ultra low-noise external 12 volt DC source (such as a battery)  to provide a parallel linear power to the Control/Display Board to be regulated down to 5 volts and passed on the LDR Attenuator/IO Board.

IR Remote Control

Similar to the V2, control via infrared remote is the primary method for controlling the V3. The V3 employs the elegantly simple Apple Remote (silver model) which uses a modified version of the NEC Infrared Protocol.

A complete listing of control functions is beyond the scope of this overview but will likely include most if not all of the current V2 control functions described here.

The V3 Control/Display Board has both a 3 pin header and a solder socket to connect a 38 kHz infrared receiver module. The infrared receiver module can also be connected to the V3  board via the master 7 segment display module which also has its own 3 pin IR receiver socket and conveys the IR receiver signal to the V3 board via the display modules own 2×5 pin control ribbon cable to the V3 Control/Display Board.

Rotary Encoder Control

A subset of control functions are available via a rotary encoder with integral pushbutton including turning the preamp on/off, muting, volume control, channel balance control and input switching.

The V3 Control/Display Board has a 4 pin header to interface with a rotary encoder for local control. A rotary encoder can also be connected to the V3 Control/Board via the 7 segment display module which has its own 4 pin encoder header and conveys the encoder signals  to the V3 board via its own 2×5 pin control ribbon cable.

WiFi Module For Fully Automatic Firmware Updates (IoT)

The V3 Control/Display Board accommodates a plug-in ESP8266 WiFi module that enables full IEEE 802.11 b/g/n Wi-Fi including WEP or WPA/WPA2 authentication. Once the user has entered their wireless name and password, this WiFi module is able to periodically query the Tortuga Audio server, download a firmware update if available, and have the V3 update its firmware without further involvement by the user.

The WiFi module also makes it possible to control the V3 via SmartPhones. Control via SmartPhones will not be available upon initial release and deployment of the V3 system but we plan to add this along with apps for both Android and Apple phones at a later date.

More broadly, the inclusion of WiFi capability positions the V3 to take advantage of the Internet of Things (IoT) as ubiquitous internet connectivity continues to evolve.

USB Port

Similar to the current V2 system, the V3 Control/Display Board has a USB port that enables firmware updates to be uploaded into the board using a simple USB cable together with an external PC based bootloader application.

12 Volt Trigger Out

A 12V trigger out is available for signalling other system components that the preamp system is on or off. Typically a trigger out is used to turn external components such as amplifiers on and off at the same time the preamp is turned on and off. A nominal 12 volt signal is output when the preamp is turned on and is removed when the preamp is off. The trigger out is rated at 250 milliamps and is fused accordingly to protect against shorts or overloads.

Auxilliary SPI Port

An auxilliary SPI port is available for future interfacing with other devices including potentially 3rd party devices. It’s a 4 pin SPI port that includes Select, Data Out, Data In, and Clock.


A 2 pin (Transmit/Receive) USART port is available for future interfacing with other devices including potentially 3rd party devices and to facilitate commissioning, maintenance and debugging.

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