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1.4.LDR Calibration

LDRs and Calibration

In May, 2014 Tortuga Audio released its V2 Preamp Controller which included self-contained LDR calibration. Originally referred to as auto-calibration or “autocal” it is now generally referred to as just “calibration”.  This built-in self-calibration feature is unique among all known audio products that use LDRs for volume control.

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. The current vs. resistance relationship 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 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 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 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.

  1. Calibration is a built-in software driven closed-loop measurment and LDR calibration process that doesn’t require any external equipment
  2. Calibration automatically calibrates the resistance values of the LDRs against a defined attenuation (dB) schedule composed of a finite number of volume control steps
  3. Calibration ensures the LDR preamp will continue to perform optimally as LDRs age or drift over time or if an LDR has to be replaced

Calibration and Adjustable Impedance

It’s helpful for the user to have an overall grasp of how the adjustable impedance feature interacts with the Calibration process.

The calibration process requires a known fixed impedance value in order to compute the target resistance values for each volume control step in the attenuation table.

Closely related to Calibration is the adjustable impedance feature inte

With adjustable impedance it’s possible to have up to as many as 10 difference impedance settings each with a different impedance level. Each of these impedance levels requires its own unique attenuation table and the only way to build those tables is to run calibration for each individual table. This has to done one at a time individually for each impedance setting/level. If the user makes a change to a given impedance setting/level, calibration must then be run immediately to build the attenuation table for that new impedance setting/level.

Calibration Recommendations

The following general guidelines apply to all Tortuga LDR preamps with built-in calibration.

How To Run Calibration

A calibration cycle must be started manually by the user.

The detailed procedure for running calibration in your Tortuga preamp can be found in the Preamp Controls section of the online documentation. The procedure may vary depending on whether you preamp uses the V2 or V25 preamp controller, your firmware version and in the case of the V25, whether your preamp uses the 7 segment display or the OLED display.

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 update the LDR attenuation table.  However, we recommend running calibration once every few months to ensure optimal performance.

If you preamp’s channel balance seems to have permanently shifted right or left of center, running calibration is warranted and in most cases will resolve the balance shift.

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 LDR has gone out of specification and needs to be replaced.

Unplug Interconnects

Experience has shown that calibration works more reliably and consistently when interconnects are unplugged 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 LDR Preamp To Thermally Stabilize

LDR’s are sensitive to ambient temperature changes. 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 calibratoin gets interrupted, we suggest you rerun the process and allow it complete normally.

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