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.
What is Attenuation?
Attenuation is the opposite of amplification.
When you amplify an audio signal you are increasing its average voltage level. When you attenuate an audio signal you reduce its average voltage level. Volume control and attenuation are terms that are often used interchangeably.
Attenuating an audio signal (i.e. reducing its voltage level) is most commonly done using a voltage divider.
The common potentiometer (“pot”) is nothing more than a type of voltage divider.
How Does Volume Control Work?
Potentiometers used for volume control will have a fixed resistance rating that is typically 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. The output signal of the pot comes from the wiper.
The resistance above the wiper is the series resistance (Rseries) and the resistance below the wiper is the shunt resistance (Rshunt). 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. Putting all this math together the resulting attenuation expressed in decibels is defined as: dB = 20 x log(Ratio) or if you like 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 minimum (maximum attenuation).
Volume control with an LDR is conceptually similar to a potentiometer.
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. With a pot all this is done mechanically whereas with LDRs it’s done electronically and thus can be manipulated in ways that you can’t with a pot.
Input Impedance with LDRs
Music is alternating current/voltage (AC), not constant direct current/voltage (DC). With DC electricity we use the term resistance. With AC electricity we use the term impedance. Thus, the term “impedance” is a way of talking about resistance when the voltage is composed of an ever changing voltage over a wide frequency spectrum as is the case with music.
In the section above we defined Rseries and Rshunt. The sum of these 2 values is defined as he nominal impedance of both types of volume control.
If we want to maintain a target 20k ohms input impedance, the combined resistance of the series and shunt LDRs must always equal at least 20k ohms. With LDRs we can’t quite achieve this in practice. With LDRs the Rseries and Rshunt values can never be exactly zero.
For practical reasons such as longevity, we limit the minimum resistance level of our LDRs to 100 ohms and the maximum resistance to 100k ohms. In order to also achieve a full attenuation range of 60 dB within this limited range we have to let the impedance level rise when volume is between 0-22%. At higher volumes the impedance is able to remain constant.
This is illustrated in the graph below of a nominal 20k LDR attenuator. The input impedance (green line) starts at 100k at zero volume, drops down to 20k by step 16 (-46 dB) and than remains constant over the remaining attenuation range. Note that the impedance is the sum of Rseries (red line) and Rshunt (blue line) the ratios of which define the attenuation level in dB (black line) at each step.
The output impedance of potentiometers and LDR attenuators is neither fixed nor is it independent from the input impedance. As is clear in the above graph, the output impedance (yellow line) varies with each step in the attenuation.
The output impedance is actually the parallel resistance of the series and shunt LDRs. The output impedance starts at ~100 ohm and remains relatively constant until step 16 and then begins to increase along with the shunt resistance. Output impedance peaks at 25% of the nominal input impedance corresponding to step 59 which is at exactly -6 dB of attenuation.
Used as passive attenuators there’s no avoiding the varying output impedance shown. As a practical matter this rarely poses a problem in real world audio applications.
What Is Adjustable Input Impedance?
Adjustable Input Impedance is a feature introduced in January 2015 that allows the user to change the default 20k input impedance of Tortuga Audio’s preamps to any level between 1k to 99k ohms.
Using this feature is completely optional. In most cases leaving the setting at 20k is ideal. But for those who wish to explore potential improvements, trying alternative input impedance levels is possible with Tortuga Audio preamps.
Why Would I Want To Change Input Impedance?
Some combinations of source (DAC, CD transport, phono stage etc. ) and amplifier may benefit from changing the input impedance of the passive preamp.
Adjustable Impedance allows the user to set up five (5) different input impedance levels and switch between these while playing music without interruption. This flexibility will allow the user to fine tune and select the passive preamp impedance level which is optimal for their source(s) and amplifier.
Increasing the preamp’s impedance will result in a higher overall effective impedance as seen by the source. A higher impedance will make it easier for the source to transfer audio energy (i.e. the music) to the amp because it requires less current from the source. If your source happens to have a less robust output stage, connecting it to lower impedance devices may result in loss of dynamics and poor bass. Some source components may require say 50k or higher impedance to perform properly.
Why not just set the default preamp input impedance to 100K and be done with it? You may also ask why don’t all amplifiers have 100k versus say 20k input impedance? Raising input impedance higher than necessary can raise distortion levels. This is particularly true of LDR passive preamps where high voltage drops across the LDRs raises their distortion level – although even at 100k input impedance our LDR preamps still sound fantastic.
Input impedance should be sufficiently high to ensure optimal performance but no higher. Raising the impedance even higher probably won’t improve performance and may in fact raise distortion. More is not always better.
There’s no simple formulaic approach to knowing what input impedance is optimal for your system. This is something you have to arrive at through a bit of trial and error using various impedance levels.
How To Set Up Each Impedance Setting
Impedance setting #1 is 20k by default. All Tortuga Audio preamps are shipped with this default impedance. User may change this impedance level to any value between 1k and 99k. Similarly, impedance settings #2 through #5 can each bet set up each with its own impedance level. Multiple settings allow for switching between setting levels while listening to music so you can determine which is optimal for your system.
When you first receive your preamp, only impedance setting #1 will have been configured. Settings #2 through #5 are empty. If you switch to impedance settings #2 through #5 without first configuring them you will get no output from the preamp. This is the #1 most common mistake people make with our preamps.
Changing the impedance level of a given impedance setting number (1-5) must be done for each impedance setting individually per the steps below. For example, this means you have to follow the entire procedure listed below for impedance setting #2 and only #2. Once you’ve completed the procedure for #2, you can then choose to change the impedance for some other impedance setting number. If you wish all 5 settings to be available, you must setup each setting number separately.
Below you’ll find the procedure for changing the impedance setting number and/or impedance level. The procedure may differ slightly depending on type of controller board in your preamp.
Instructions For V25 Preamps
|Step||Description | For V25 Preamps Only|
|1||Turn on the preamp|
|2||Put Preamp In Impedance Adjust Mode - Press the Mode Button on remote 2 times (once only for single input LDR1.V2 or LDR1B.V2) to display the Impedance Adjust values. The left display shows the impedance setting number (1-5). The right display shows the impedance level (1k-99k) associated with the impedance setting number. |
|3||Select Setting Number - Use the Raise/Lower buttons to select the impedance setting # (1-5) shown in left display that you wish change or set up initially.|
|4||Adjust Impedance Level - Use the Left/Right buttons to adjust the impedance level shown in the right display to desired value (1k-99k)|
|5||Save Changes - Press the Enter button to save the current setting number and associated impedance level. This returns unit to normal volume control mode.|
|6||Run AutoCal To Build New Attenuation Table - Press the Mode Button on remote 5 times (4 only for single input LDR1.V2 or LDR1B.V2) to switch to AutoCal Adjust Mode. Both displays should show zero values. Press the Right Button 3 times in a row to start AutoCal.|
Instructions For V2 Preamps With Firmware Versions 2.2.x
|Step||Description | For Firmware Version 2.2.x Only|
|1||Turn on the preamp|
|2||Press the Mode Button on remote 2 times (once only for the LDR1.V2 or LDR1B.V2) to display the Impedance Adjust values. The left display shows the impedance setting number (1-5). The right display shows the impedance level (1k-99k) associated with the impedance setting number. |
|3||Use the Raise/Lower buttons to select the impedance setting # (1-5) shown in left display that you wish change or set up initially.|
|4||Use the Left/Right buttons to adjust the impedance level shown in the right display to desired value (1k-99k)|
|5||Press the Enter button to save the current setting number and associated level|
|6||Turn off the preamp|
|7||Enable Auto-Cal by pressing the Enter button. The Auto-Cal process will populate the attenuation table based on the new impedance level for the selected step. |
|8||Repeat steps 1 through 7 for each setting. Each setting number must be configured individually per all the above steps.|
Instructions For V2 Preamps With Earlier Firmware 2.1.x
|Step||Description | For Firmware Version 2.1.x Only|
|1||Turn on the preamp|
|2||Press the Left button on the remote. The right display will show the currently selected impedance setting (1-5).|
|3||Use the Raise/Lower buttons to select the setting set you want to change or set up.|
|4||Press the Left button again on the remote. The right display will now show the impedance level (1k - 99k) associated with the currently selected setting.|
|5||Use the Raise/Lower buttons to adjust the impedance level to desired value (1k-99k)|
|6||Press the Enter button to save the current setting number and associated level|
|7||Turn off the preamp|
|8||Enable Auto-Cal by pressing the Enter button. The Auto-Cal process will populate the attenuation table based on the new impedance level for the selected step. |
|9||Repeat steps 1 through 8 for each setting. Each setting number must be configured individually per all the above steps.|
Using Adjustable Impedance While Listening To Music
Once 2 or more impedance settings have been set up, you can switch between them in real time while listening to music.
Please note that if you switch to an impedance setting that is not defined, the audio output will shut off. Switching back to a defined setting number will switch the music back on.
Consult the instructions above to access the impedance level adjustment display and use the Raise/Lower buttons to switch between 2 or more settings with different levels. You may notice a qualitative difference between levels or you may not. Much depends on the specific equipment in your system and its sensitivity to impedance bridging ratios.
Based on feedback we’ve received from customers and our own experience with adjustable impedance the optimal setting is one that provides sufficient impedance bridging (ratio of amp input impedance to source output impedance) between the source and the amp. Increasing the impedance bridging ratio further usually does not provide additional benefit and can actually have a negative impact on sound quality.
LDRs and Calibration
Using LDRs in audio applications is technically challenging because the relationship of current vs. resistance within each LDRs is both highly nonlinear and can vary considerably from one individual LDR to the next. It can also change over time as an LDR ages.
The conventional approach to dealing with these challenges is testing 100 or more LDRs to find matching pairs or multiple pairs. Even when initially matched, LDRs are known to shift their calibration curves over time as they normally age. This can degrade the stereo imaging and sound stage and allow channel balance to drift left or right. Moreover, if one of the matched LDRs fails, the preamp might be rendered unusable.
Tortuga Audio has developed proprietary technology that overcomes these challenges thus making LDRs practical for commercial audio applications. Software driven digital control of the analog LDRs is an essential part of this technology.
In May, 2014 Tortuga Audio released its V2 Preamp Controller with adaptive self calibration call auto-calibration or “autocal” for short.This is unique among all known products using LDRs in audio applications.
Autocal eliminates the need to test and match LDRs. It also ensures that the LDRs continue to perform optimally despite any drift that may occur due to the normal aging process of analog devices. Autocal is a closed loop system employing both DACs (digital to analog converters) and ADCs (analog to digital converters) to calibrate each LDR against a multi-step attenuation schedule. Calibration results are stored in memory and then used to accurately control each LDR during normal operation.
In summary, autocal:
- Is a self-contained software driven self calibration process that doesn’t require any external equipment
- Automatically calibrates the resistance values of the LDRs against a defined attenuation schedule
- Ensures that the LDR preamp will continue to perform optimally as LDRs age or drift over time or if an LDR has to be replaced
Running Auto Calibration
For Version V2/V21 Hardware
The following summarizes how the current version of AutoCal works for preamps with the V2/V21 boards. Earlier versions of the firmware may behave slightly differently.
|Turn Preamp Off||manual||Preamp must be plugged into power but turned off|
|Start AutoCal||manual||AutoCal must be manually started. Once started the AutoCal cycle will run to completion and shut off by itself.|
|display||Left display will show which LDR is currently being calibrated. Right display will show which step (1-70) is currently being calibrated. If connected, the Status LED will blink rapidly during AutoCal.|
|sequence||LDRs are calibrated in the following sequence: #1-right series, #2-right shunt, #3-left series, #4-left shunt. The series LDRs start at step 70 and count to 1. The shunt LDRs start at step 1 and count up to 70.|
|disconnect||Audio inputs/outputs are automatically disconnected when AutoCal starts and reconnected when AutoCal stops|
|duration||A complete AutoCal cycle typically takes around 10-15 minutes but may run faster or slower. A very slow (> 30 minutes) cycle may indicate that an LDR has developed a problem and may need replacement.|
|via remote||Press the Enter(center) button on the remote to start AutoCal|
|via encoder||Turn the Encoder approximately quarter turn to the right to start AutoCal|
|Stop AutoCal||automatic||Once started, the AutoCal cycle will run to completion and shut off by itself (recommended)|
|manual||AutoCal can be stopped at any time but we recommend allowing the calibration cycle to finish on its own|
|via remote||Press the Enter button on the remote. You may have to press the Enter button more than once|
|via encoder||Turn the Encoder approximately quarter turn to the right|
|Skip LDR||manual||You can force AutoCal to skip to the next LDR (nor recommended)|
|via remote||Press the Right button to skip from current LDR to the next LDR. If already on the 4th LDR, will shut off AutoCal.|
|via encoder||(not available)|
|Frequency||If your preamp performs well there’s no compelling reason to run AutoCal. If you wish to run AutoCal periodically, we suggest once a month or once a quarter.|
Auto Calibration Recommendations
We recommend the following:
When To Run Autocal
There’s no fixed requirement for when you should run autocal. You could literally go months or even years without ever once running Autocal. However, we recommend running autocal once every 1-3 months to ensure optimal performance.
Keep Source/Amp Turned Off During AutoCal
During AutoCal, the V2 disconnects the audio signal inputs and outputs from the V2 board. You do NOT have to disconnect your amp from the V2. However, we recommend you turn off both your source and your amp prior to running AutoCal as a precaution. If your amp remains on when AutoCal starts and stops, sometimes you may notice a brief pop or click sound coming from your speakers when AutoCal starts and stops.
Allow Your LDR Preamp To Thermally Stabilize Before Starting AutoCal
LDR’s are sensitive to ambient temperature changes. Therefore, you should avoid running AutoCal until the V2 has had time to stabilize at room temperature. If you just relocated the V2 from either a hotter or colder location, we suggest you wait an hour before starting AutoCal.
Avoid Interrupting AutoCal Once It’s Been Started
Interrupting AutoCal won’t harm your preamp but it may result in less than ideal calibration results. If AutoCal gets interrupted, we suggest you rerun AutoCal to completion.
The following specifications are common to all Tortuga Audio preamps as of May 2017.
|Preamplifier type||Passive audio attenuator using light dependent resistors (LDRs) in a series/shunt (L-Pad) configuration |
|Gain||Unity (1x) gain (no amplification)|
|Input impedance||20k by default adjustable between 1k and 99k|
|Audio Signal Isolation||No active manipulation of the audio signal. Audio signal optically isolated from LDR controls but may share common ground.|
|Total Harmonic Distortion||1% or less|
|Channel Balance||Within 0.5 db over full attenuation range|
|Attenuation Range||-60 db to 0 db in at least 70 discrete steps plus muting|
|Fuses||No internal or external fuses|
|Control system||V2 - 8 bit PIC microcontroller running at 4 MHz|
V25 - 32 bit ARM microcontroller running at 72 MHz
|Circuit boards||Nickel immersion gold PCBs with blue solder mask|
|Solder||Lead free solder with 3% or higher silver content|
|ROHS||Compliant or exempted|
|Ambient Conditions||Stable, dry indoor environment only with ambient temperature within 60-90 degree F|
|Trigger Out||Outputs a 12 VDC signal when unit is turned on. Signal can be used to turn other devices on/off that are equipped with a Trigger Input.|
|Encoder Control||Rotary stepped multi-function volume control (front panel) with integral push button switch|
|Remote Control||Infrared control via standard silver Apple remote | 38 kHz NEC protocol|