Light Dependent Resistors (LDRs)
An LDR is the marriage of a photoresistor with a light emitting diode (LED) in a small sealed package approximately the size of an M&M candy. Each LDR has 2 pairs of wires. One pair connects to the LED while the second pair connect to the photoresistor.
The resistance of an LDR varies in proportion to the brightness of the LED which is in turn dependent on the amount of electrical current running through the LED. As the LED becomes brighter the resistance of the photoresistor decreases.
Through precise control of input current, LDRs can smoothly regulate resistance over a wide enough range to provide effective high performance audio volume control.
LDRs optically isolate the control circuitry from the audio signal passing through the photoresistor. The audio signal only encounters a variable resistance that is regulated by photons (i.e. light) and not electrons.
All Tortuga Audio preamps utilize LDRs to control volume (i.e. to attenuate the audio signal).
Why Do We Use LDRs for Volume Control?
We use LDRs because the resulting sound quality is amazing. Adjectives like clear, open, unveiled, articulate, and uncolored all apply to the LDR. All while maintaining excellent bass and overall dynamics even in a passive preamp.
If LDRs are so great why then why don’t all preamps use LDRs? Reasonable question. Here’s why not.
The simple reason is LDRs are notoriously difficult to use for volume control. They are inherently nonlinear which means they act differently at different levels of resistance. They are also not consistent from one to the next even within the same make and model from the same production batch. Their performance curves may even drift slightly over time. LDRs also have higher distortion characteristics than most other alternatives although not enough that it really matters subjectively.
For all the above reasons most audio designers have avoided taking on the challenge of using LDRs.
Tortuga Audio took on this challenge several years ago and has not looked back. We overcame these technical challenges. We are currently on our 4th generation of our LDR preamp control technology.
How are LDRs Used for Volume Control?
Volume control with an LDR is conceptually similar to a potentiometer so lets look first at how the common potentiometer (“pot”) is used for volume control.
Potentiometers used for volume control typically have a fixed resistance between 10k and 100k ohms. The audio input signal connects at one end of this fixed resistance and the other end is connected to ground. The output signal from the pot is a third connection to what’s referred to as the “wiper” which slides along the pot’s fixed resistor. This is shown in the diagram below.
The variable resistance above the wiper (the output) is the series resistance (Rseries) and the variable resistance below the wiper is the shunt resistance (Rshunt). The sum of Rseries plus Rshunt equals the rated impedance (10k, 50k, 100k etc. ) of the pot.
The resistance Ratio is defined by the formula: Ratio = Rshunt/(Rshunt + Rseries). This Ratio also happens to be the voltage ratio of Vout divided by Vin such that Vout = Vin x Ratio. A pot’s attenuation level at any point along the wiper can be expressed in decibels which is defined as 20 x log(Ratio). Putting all this together your arrive at dB = 20 x log (Vout/Vin) where Ratio = Vout/Vin = Rshunt/(Rseries + Rshunt).
Thus, when Rseries is zero the volume is maximum (no attenuation) and when Rshunt is zero the volume is zero (maximum attenuation).
With LDRs, attenuation is achieved by varying the resistance levels of 2 separate series and shunt LDRs to achieve specific resistance ratios that correspond to specific dB attenuation levels. While a pot does this mechanically, the resistance levels of both the series and shunt LDRs must be done electronically. Doing so reliably and repeatedly can be a challenging design problem.
Apple Remote Pairing
What Is Remote Pairing?
Remote pairing is simple process to “teach” your Tortuga preamp how to talk to your Apple or Tortuga (legacy) remote.
Do I Need To Pair My Remote?
Chances are the answer is NO! A remote is included with each Tortuga Audio preamp and that remote will already have been paired with your preamp.
If you purchase a Tortuga Audio DIY premap controller without also purchasing a remote then you will have to purchase your Apple remote separately and pair it with your controller.
When Is Remote Pairing Necessary?
Every Apple remote has an internal ID number. There are 256 possible ID numbers. The purpose of the ID number is to allow the use of different remotes for different products. You may have an Apple remote to your Apple TV in addition to your Apple remote for your Tortuga preamp. Without ID numbers it would be chaos with both devices reacting to any remote. Usually, pairing is only required with a new remote. Rarely it may also be require after a major firmware upgrade.
Can I Change the ID of my Apple Remote?
The answer is yes but the only reason for doing so is if you have two Apple remotes for two different devices and both remotes happen to have the same ID number (one chance in 256). The Apple remote ID can be changed by simultaneously pressing both the Menu and the Enter/Center key on the Apple Remote for 5+ seconds and then releasing. This will change the ID number to some new value. After changing the ID you will then have to pair the Apple remote with your Tortuga preamp.
Does Remote Pairing Apply to my Tortuga Preamp?
Starting with version 1.0.9 of the LDR3x.V2 firmware the Apple Remote can (and sometimes must) be paired with the preamp unit in order for the preamp to recognize and respond to the Apple Remote’s commands. This is only relevant to the LDR3x starting with the V2 model and also with the LDRx, LDRxB, LDR3.V2 & LDR3.V2K models. Prior to version 1.0.9, the Tortuga preamps would respond to any Apple or Tortuga legacy remote without pairing which occasionally created conflicts with other equipment.
How Do I Pair My Remote With My Preamp?
Pairing your remote with your Tortuga preamp is a fairly straight forward process that can be accomplished in less than a minute. Below you’ll find a step by step guide on pairing your remote.
|1||Turn on the preamp||Press/release the Encoder button or briefly connect/release J2-ES to J2-G on the LDR3x board|
|2||Enable “pairing” mode||Press in & hold the Encoder button for at least 20+ seconds (suggest 30 seconds to be safe) and then release. If you have a DIY LDR3x preamp controller board without an Encoder, you can emulate the Encoder press/hold by connecting J2-ES to J2-G (ground).|
|3||Confirm preamp is in pairing mode||Upon release of the Encoder button (see step 2 above), the DM1 display module and/or the Status LED will start blinking rapidly.|
|4||Pair the remote||By pressing any key on the remote except the Menu(Power) key. Pressing the Center/Enter button is preferred. This will stop the rapid blinking and return the preamp to normal volume control mode.|
|5||Confirm the pairing||Press the raise/lower keys on the remote. If the pairing was successful the preamp should now react normally to the remote. You are done!|
“I press/hold the Encoder push button for 20 seconds and release and the display blinks for only a second or so and then stops” – This means you didn’t hold the Encoder push button in long enough. If you hold it in for greater than 10 seconds but less than 20 it resets the preamp’s channel balance which is indicated by brief (1-2 seconds) blinking.
Firmware Update Instructions
Updating your preamp’s firmware requires the use of a bootloader program running on a PC together with the latest firmware “hex” file. You download both the bootloader and the firmware file from our website. Then you connect your PC to your preamp via a USB cable. To execute the actual upload process you run the bootloader program, apply power to your preamp and then use the bootloader to upload the firmware hex file into your preamp.
Once set up, the firmware upload/update process typically takes less than 15 seconds.
Detailed instructions on the firmware update process are provided herein.
What is the Bootloader?
The (mikro)Bootloader is a small PC based software application that you use to establish USB communications with, and upload new firmware into, the V2/V25 preamp.
Unfortunately the Bootloader is a PC/Windows only application. There’s no MAC version currently available. We are not anti-MAC, but the supplier of our programming tools stubbornly refuses to provide a MAC version of the Bootloader program while providing no reasons for not doing so. Go figure.
The Bootloader program is a simple executable file (mikroBootloaderUSBHID.exe). Unlike conventional software, it does not have to be installed/registered into your PC. The program file simply has to reside somewhere on your PC. We suggest you download it to your PC desktop. The first time your run this program (by double clicking on it) it will create a small text file named “settings.ini” in the same directory. No special drivers are needed. It doesn’t install or change anything in your PC. If your PC has a USB port, you should be good to go. We know the Bootloader works with Windows 7, 8 and 10. It may also work with older versions of Windows but we haven’t tested that. To remove the Bootloader program from your PC simply erase both the mikroBootloaderUSBHID.exe and settings.ini files from whatever directory you put them in.
Download The Bootloader
About USB Cables – Important!
The #1 problem that people encounter when trying to update their firmware is the inability to make a connection. In most cases this is caused by their USB cable.
Most of us have several USB cables laying around because we have cell/smartphones and other devices that use USB cables for charging. Unfortunately, many of those USB cables are for charging only and lack the ability to transmit data.
Charging only USB cables will not work with the bootloader and firmware update system!
You need to use a proper USB “data” cable that has the conductors for the D+ and D- USB data signals.
Firmware updating will not work without a proper USB data cable.
Firmware Update Procedure – Short Version
Here’s the short version of how this works.
- Remove power from your preamp (unplug the power cord)
- Download the updated firmware file from our website to your PC
- Download the Bootloader program from our website to your PC
- Connect your PC to the preamp via the USB cable provided with your preamp
- Start the Bootloader program
- Apply power to your Tortuga preamp (this starts a 5 second timer during which you must do the next step)
- Click the “Connect” button on the Bootloader program
- Browse for and select the firmware (hex) file you downloaded in step 1)
- Click on the “Begin Uploading” button on the Bootloader program.
With step 6) the actual uploading of the firmware takes less than 5 seconds. You’ll see a progress bar. It will be obvious when it’s done. When it’s done you can disconnect the USB cable from the preamp and proceed to use the preamp normally.
The information that follows below elaborates on the above steps.
Step 1 – Preparation
Remove Power From Your Preamp
This is key and often confuses people so read carefully. The Bootloader is only able to connect to your preamp during the 5 seconds after power is applied. If the connection hasn’t been made during that 5 second start-up period you have to remove power and try again. By “connection” we don’t mean physically connecting the USB cable. “Connection” means step 4.
Download Firmware File & Bootloader Program
The latest firmware file and the Bootloader program can both be found here [link to be added].
The firmware file is referred to as a “hex” file because it’s a text file wherein the code is formatted using a specific hexadecimal protocol and file uses a .hex file type suffix.
The Bootloader program is a small Windows executable (.exe) application file. You only need to download it once and keep it handy somewhere on your PC.
Connect The USB Cable
All finished Tortuga preamps require a USB cable with type A male connectors on both ends. The cable is supplied with all finished Tortuga Audio preamps. Connecting the cable does nothing by itself. Your PC should not react.
DIY Boards – Micro-B Socket
If you have an older V2 LDR3x DIY board with a micro-B socket board soldered to the J8 header on the board you’ll need a cable with a male A end and male micro-B USB end. An LED light on the micro USB connector breakout board on the V2 will light up once it’s connected to your PC.
DIY Board – Tortuga USB Connector
If you are using a Tortuga USB connector cable with the female end connected to the J8 header make sure you have the female end connected such that the red wire is next to microcontroller end of J8 header and not the end with the white dot.
Step 2 – Start the Bootloader program
When the Bootloader program starts it will look more or less like the pic below. At this point it won’t be doing anything except waiting.
Step 3 – Establish the USB link to the preamp
Connecting power to your preamp starts the 5 second bootloader connect window.
You should notice the following changes to the mikroBootloader program once power is applied to your preamp:
- The little USB symbol to the right of “1 Wait for USB link” should turn red
- The Device window should display “LR3x.V2 Board” or “LDR.V25” or similar
- The MCU Type window should display “PIC18” or “STM32F” or similar
- The History Window should say “Waiting MCU response…”
At this point you will have 5 seconds to establish a USB link between your PC and preamp by pressing the “Connect” button next to “2 Connect to MCU”. If all goes well, the “Connect” button text will change to “Disconnect” and the History Window will now say “Connected” as shown in the pic below. If you run out of time, don’t panic, just remove power again, wait a few seconds and then reapply power to the preamp which restarts the 5 second Bootloader timer.
Step 4 – Choose the firmware (.hex) file
Select your firmware file by pressing the “Browse for Hex” button next to “3 Choose HEX file”. This will open a file dialog box. Find your downloaded .hex file stored on your PC and click the “Open” button. At this point the firmware is ready to be uploaded into the preamp board.
Step 5 – Upload Firmware To Preamp
Press the “Begin uploading” button next to “4 Start bootloader” to upload the firmware into the preamp. The previous firmware build will first be erased and then the updated software build will be written to the preamp’s microcontroller chip. This erase/write operation is very fast and should take less than 5 seconds to complete.
You should get a “Success” message. Click the “OK” button to close the message box. Note the new messages in the History Window. Upon a successful write, the Bootloader program will automatically disconnect from preamp and the preamp will reboot which takes 5 seconds.
You are now done and can close the Bootloader program, disconnect the USB cable, and enjoy your updated preamp.
V25 Board Overview
The V25 Preamp Controller (Rev A) was 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.
|Attenuation type||Passive 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.|
|Gain||Unity (1x) gain | no amplification, processing or active manipulation of the audio signal | audio signal optically isolated from control system|
|Buffering||No input or output stage buffering|
|Input impedance||Nominal 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 Channels||2 independent channel stereo|
|Multi-channel Attenuation||2 or more V25 boards can be daisy-chained to provided mutl-channel attenuation with 2 channels per board|
|Channel Balance Accuracy||Within 0.5 db over full attenuation range|
|Channel Isolation||Each 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 Ground||No 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 Ended||1 board accommodates up to 6 switchable single-ended stereo (2 channel) inputs|
|Audio Inputs | Balanced||2 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/Isolation||Input 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/Isolation||Output signal switched on/off with LDR. Isolation between inputs and output when switched off in excess of 10 megaohms.|
|Total Harmonic Distortion||1% or less|
|Power Input||7-30 volts DC rated at 0.5 amp via J1|
|Power Regulation||2 stage regulation | 5 V 1st stage low noise switch mode regulator followed by 3.3 V low noise 2nd stage linear regulator|
|Processor||ARM M3 Cortex 32 bit microcontroller running at 72 MHz|
|Control Input | Remote||Accepts 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 | Encoder||Accepts 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 | UART||Accepts control inputs from external device using UART serial protocol and specific command codes (available upon request).|
|Display | 7-Segment||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 | OLED||Graphical OLED display (future) | connects to board via both J3 2x5 header and J9 2x7 header | future option that will require firmware update, optional OLED display plus setting display type jumper on J7.DISP.|
|Display | Status LED||Status LED light | connects to board via J7|
|Mono Mode||Output switchable between stereo and mono mode via LDR switch | applies on to single-ended audio|
|Firmware||Custom interrupt driven application code written in the C language|
|Firmware Update||Firmware 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 Port||Standard 5 pin USB port allows processor to communicate with external HID bootloader application for updating firmware.|
|WiFi Module||Future ability to plug in a simple WiFi module via J6 that will allow the V25 to be controlled via smartphone app | will require WiFi module, firmware update and fully operational OLED display|
|Trigger Out||Outputs 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 Board||High quality nickel immersion gold solder pads with blue solder mask|
|Ambient Requirements||Board must be operated in a stable, dry indoor environment only with ambient temperature within 60-90 degree F|
|ROHS||Compliant or exempted|
|Physical Dimensions||Length: 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
The V2 introduces Auto Calibration as well as replaceable LDR Modules. Auto Calibration is a self-contained system wherein the V2 fine tunes the attenuation table of the 4 primary LDRs based on each LDRs actual measured performance. This ensures that the LDRs continue to perform optimally over time despite potential performance drifting as these analog devices age normally. Even if one LDR fails prematurely or degrades beyond the reach of Auto Calibration, it’s a simple matter to plug in a new LDR Module similar to tubes.
Preamp Controller Board
The LDR3x.V2 (the “V2”) is a preamp controller in the form of a populated 2.5 inch by 4.8 inch printed circuit board that forms the core of a passive preamp or as the attenuator/controller of an active preamp. The V2 builds on the experience and success of the V1 board.
Insofar as the V2 lacks a power supply, controls or enclosure it is neither a complete finished audio premap nor is it a comprehensive kit.
Once connected to an external power source (not included), the V2 can accommodate one (1) single-ended audio input without any additional equipment and up to three (3) inputs when interfaced with an external input relay board (not included).
The V2 is sold with an uninstalled infrared receiver module required for remote control. The IR receiver module can either solder to the V2 board, located external to the board, or mounted to the DM1 Display Module master board.
The V2 can be controlled by the Apple Remote or Encoder or both. Neither are included with the V2 and must be purchased separately. Control inputs that are enumerated and explained in sections below.
The V2 is part of the Tortuga Audio family of LDR based passive preamps.
Unbalanced Audio (single board)
A single V2 board operates as a standalone passive preamp for single-ended unbalanced inputs. By itself it can handle a single input. When mated with the Tortuga Audio IO3 input switching board it can switch between 3 different inputs.
Balanced Audio (dual boards)
By using a pair of boards, the V2 becomes a preamp for balanced audio systems. One board is designated the Master and the second configured as the Slave board via an on-board jumper. The Master V2 interfaces with the Apple Remote and/or Encoder and sends volume level and other commands to the Slave V2 via a 3 wire control cable.
Dual Mono (dual boards)
Similar to the balanced audio described above, a pair of V2 boards can serve as left/right channel mono preamps wherein only a single channel of each board is used. As with mono amps, this approach can further enhance channel separation and thus raise the bar on your audio experience. Each mono V2 preamp board can be located immediately adjacent to its amp thus minimizing interconnect distance. A simple 3 wire cable between the pair of V2 boards is all that’s needed for control.
LDR3.V25K Kit Overview
Get Familiar With Your Kit
Before proceeding with actual detailed assembly, we strongly encourage you to read all of information below.
Check Your Parts List
Every kit should include a parts list that was used by Tortuga Audio to make sure we included all the parts in your kit shipment.
If you haven’t already done so, we encourage you to go through that checklist to make sure you can find and identify all the parts/wiring. If you believe we left something out please contact us and we’ll be happy to ship you the missing item(s).
Your kit is actually a fairly straight forward build since all the circuit boards come fully populated and tested. Most of the work involves mechanical assembly followed by some wiring. The wiring is probably the most complex part but if you move deliberately through the instructions it should all be clear.
The kit is composed of the following sub-assemblies:
- The V25 Controller Board with integrated input switching
- The display assembly and its mounting bracket
- The mounting board to which items 1) and 2) are to be mounted
- The rear panel including the RCA jacks, power jack, trigger jack, and USB jack.
- While not an assembly as such, the enclosure box and front panel
The Mounting Board
The Mounting Board is a 5.73″ x 8″ epoxy fiber board make of the same material as a typical printed circuit board. The Mounting board slides into the bottom grove of the Enclosure Box.
The Mounting board comes with 8 pre-drilled holes as shown below (except yours doesn’t have the writing on it). Note the hole patterns to orient the top and front of the mounting board.
Three items get mounted on the Mounting Board: 1) The Display Bracket holding the Display Modules; 2) The Controller Board; and, 3) The Star Ground.
The Enclosure Box (the “Enclosure”) is a robust extruded aluminum box that is 3″ tall by 6″ wide by 8″ deep. It is powder coated with a flat black finish. The Enclosure has a series of internal slots on the sides into which you can slide the Mounting Board. The Mounting Board must use the bottom most pair of slots.
Four (4) peel-and-stick black urethane feet are provided that are to be mounted on the underside of the Enclosure approximately 1 – 1.5 inches in from each corner. Once in place these have excellent long term adhesion and will be difficult to remove and move so be careful when attaching them.
Fasteners & Display Bracket
The collection of kit fasteners and brackets are shown below followed by a brief description of each.
The Display Bracket is a gray plastic L-bracket with 6 pre-drilled holes. The leg of the bracket with 2 holes is used to mount the Display Bracket to the Mounting Board. The remaining 4 holes in the other leg are for attaching the Display Modules to the Display Bracket.
Display Bracket Fasteners
There are 2 zinc coated screws with nuts that are used to attach the Display Bracket to the Mounting Board.
Display Nylon Fasteners
The white nylon fasteners are used to attach the 2 Display Modules to the Display Bracket. There are four (4) threaded rods, 8 main nuts, and 4 spacers. In some cases we may substitute 4 additional nuts in lieu of the spacers.
There are eight (8) panel screws with washers. These are black socket head sheet metal screws. These are used to attach both the front and rear panels to the enclosure.
There are six (6) aluminum standoffs along with twelve (12) zinc coated standoff screws. The standoffs are used to mount the Controller Board to the Mounting Board and to make the Star Ground.
Front Panel & Encoder Knob
The front panel is a heavily tinted clear acrylic 1/4″ thick panel. When the preamp is turned off the dark acrylic panel significantly limits your ability to see the internal components and for all practical purposes looks like a black panel. When the preamp is turned on the numerals from the two Display Modules are clearly visible while everything else still remains unseen.
The front panel attaches to the Enclosure via 4 socket head screws. There is hole and recess in the front panel through which the Encoder is mounted (Encoder not shown in pic below). The black knurled Fender Telecaster control Knob mounts on to the Encoder as the final assembly step.
The rear panel is a black anodized aluminum panel as shown below. It accommodates 10 RCA jacks (6 inputs, 4 outputs), the USB cable, the 12 VDC power input jack and the 12 V Trigger jack.’
The rear panel connects to the Enclosure via 4 socket head screws with washers.
The kit includes 10 standard RCA jacks as shown below. You may have opted for the Cardas upgrade (not shown here).
Power & Trigger Jacks
The power jack is a 5.5 mm style barrel jack with a 2.1 mm pin. It accepts the barrel plug from the external “wall wart” style 12 VDC power supply provided with the kit. The 12 V trigger jack is a mono 3.5 mm phone jack that provides a 12 VDC signal whenever the preamp is turned on.
Both of these jacks are mounted in the Rear Panel.
Internal USB Cable
The Internal USB Cable is provided solely to enable updating of the preamp’s firmware which is done via a USB connection to a PC. This 12 inch cable is mounted internally on the Rear Panel and connects to the J8 header on the Controller Board.
Wire & External USB Cable
The last part of the kit includes a bag with various wires plus an external USB cable. Like the Internal USB Cable, the external cable is for connecting the preamp to a PC for purposes of updating the firmware.
While there’s no hard and fast rules when it comes to using the wire provided you will find that color/lengths of wire provided will make most sense if your use white wire for audio signal hookup, the red and black wire for power and the green wire for ground connections.