Using LDRs for audio volume control is technically challenging because the relationship between control current vs. resistance within each LDRs is highly nonlinear, can vary considerably from one individual LDR to the next, and can also change over time as an LDR ages.
The conventional approach to dealing with these challenges is measuring a large collection of LDRs to find multiple matching 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 over time and allow channel balance to drift left or right. Moreover, if one of the matched LDRs fails, the preamp might be rendered unusable.
Calibration normalizes the performacne of each LDR so that regardless of their individual performance differences, they are made to perform uniformally. The ability to calibrate each LDR already installed in the preamp as needed eliminates the need to test and match LDRs in advance. It also ensures that the LDRs continue to perform optimally despite any drift that may occur due to the normal aging process of these analog devices.
Calibration is a closed loop measurment 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 permanent memory and then used to accurately control each LDR during normal operation.
The following points summarize LDR calibration.
- Built-in software driven process
- Requires no special or external equipment
- Only runs when preamp is otherwise offline and not operating normally
- Once started runs automatically without any user intervention
- Normalizes LDR performance
- Corrects for any drift as LDRs age
- Allows LDRs to be replaced if needed thus extending the life of the pramp indefinitely
Tortuga Audio introduced in-situ LDR calibration in May, 2014 with the release of its V2 preamp controller and was referred to initially as “auto-calibration”. Built-in self-calibration of LDRs is a unique feature of Tortuga Audio preamps.
What happens during calibration?
During calibration the 4 volume control LDRs are each run through 100 calibration steps thus there are 400 steps in a full calibration cycle. During each step the resistance of each LDR is measured and compared to a target value associated at each of the 100 attenuation steps. The target values are dependent on the specific input impedance value that the preamp is currently set at. If the input impedance is subsequently changed to a new value, calibration must be run again to build a new calibration table.
The 100 control settings for each LDR’s are stored in the controller’s EEPROM memory. These stored control settings are subsequently read from memory and during normal operation and are used to set the resistance level of the 4 attenuation LDRs (2 LDRs per stereo channel) to achieve the desired volume.
How to run calibration
Calibration is initiated by the user.
Detailed procedures for running calibration in your Tortuga preamp can be found in the Preamp Controls Section elsewhere in the online documentation. The procedure may vary depending on which preamp controller model (V2, V25, V3 etc. ) , firmware version and the type of display (7 segment or OLED) your preamp has .
When to run calibration
There’s no fixed requirement for when you should run calibration. You could literally go months or even years without ever needing to run calibration. However, we recommend running calibration once every few months to ensure optimal performance.
If your preamp’s channel balance seems to have drifted either right or left of center, running calibration is will most likely resolve the matter.
If the calibration process does not complete within a 15 minute period or otherwise stalls or goes into a repeating loop, this is a reliable indication that at least one LDR has gone out of specification and needs to be replaced. The LDRs come as easily replaceable plug-in modules.
Calibration best practices
Interconnect status during calibration
During calibration the preamp inputs and outputs are automatically disconnected within the preamp controller. The system is designed so that calbraton can proceed with all interconnect cabling left plugged into the preamp.
However, there may be times when disconnecting the interconnects can be advantageous during calibration. The reason for this is calibration is very sensitive to even modest ground loop currents and the preamp shares a common signal ground with all connected devices. When no sources or amps are connected to the preamp there’s only a single ground connection via the preamp’s power supply. This arrangement minimizes noise and ensures the best results.
Allow preamp to thermally stabilize
LDR’s are sensitive to ambient temperature changes. Therefore, you should avoid running calibration until the preamp has had time to stabilize at room temperature. If you just relocated your LDR preamp from either a hotter or colder location, you should wait at least an hour before starting a calibration cycle.
Avoid interrupting calibration cycle
Interrupting the calibration process won’t harm your preamp but it may result in less than ideal calibration results. If calibration gets interrupted, we suggest you rerun the process and allow it complete normally.
Run calibration twice at each new impedance level
If you establish a new input impedance level in your preamp it’s best to run calibration at least 2 times in a row. The first calibration pass at a new impedance level requires estimating certain values which are then more accurately determinable on subsequent passes.