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1.3.Adjustable Impedance

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.

Adjustable Impedance allows the user to configure up to 10 different input impedance settings each with its own defined impedance level between 1K and 99k.  Once properly configured, the user may switch instantaneously between different input impedance levels while playing music. This flexibility will allow the user to explore and possibly fine tune their preamp’s impedance level which is optimal for their source(s) and amplifier.

Why Would I Want To Change Input Impedance?

Some combinations of sources (DAC, CD transport, phono stage etc. ) and amplifier may benefit from changing the input impedance of the passive preamp.

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 a 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 higher 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.

There’s no simple formulaic approach to knowing what input impedance is optimal for your system. Changing the input impedance may make no difference at all in some systems. In other systems it may reveal an optimal setting. Finding the optimal input impedance level, if there is one, can only be determined through a bit of trial and error.

Adjustable Impedance & Calibration

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. 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.

How To Set Up Each Impedance Setting

Impedance setting #1 is 20k by default. All Tortuga Audio preamps are shipped with this default impedance. Older preamps with earlier firmware allowed for up to 5 different impedance settings with each setting set to an impedance level between 1k and 99k. Newer preamps with more current firmware allow for up to 10 different settings.

In newer preamps with more current firmware, setting #1 is locked at 20k and is the default impedance setting whose level can’t be changed. Users may optionally choose to enable any or all impedance settings #2 through #10 with each setting having its own impedance level.  Once properly configured, multiple impedance settings allow the user to switch between impedance levels while listening to music so you can determine which is optimal for your system.

The detailed procedure for configuring a new impedance setting/level or changing an existing impedance level may differ slightly depending on the age of the preamp, whether the preamp uses V2 or V25 preamp controller, the firmware version and whether the preamp uses a 7 segment display or the newer graphic OLED display.

Please refer to the Preamp Controls section of the online documentation where you can find impedance adjustment procedures specific to your preamp model.

While detailed procedures may vary between difference preamp models, the following guidelines are applicable to all Totuga preamps with adjustable impedance.

  1. New preamps only have impedance setting #1 configured and set to 20k by default.
  2. All other settings in new preamps are not configured and if switched will result in no output from from preamp
  3. You can only set up and change the impedance level of one impedance setting at a time and such changes do not become effective until the preamp is run through calibration at the new impedance setting/level.
  4. You need to run calibration on each new setting/level at least 2 times before it’s likely to be work properly
  5. Left/right channel imbalance is the most common indicator that a given impedance setting/level requires another round of calibration

Switching Between Impedance Levels 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.

You may or may not notice a qualitative difference between different impedance levels. Much depends on the specific equipment in your system and its sensitivity to impedance bridging ratios.

Based on customer feedback 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 may in some cases actually have a negative impact on sound quality.

A Final Word Of Advice

Exploring adjustable impedance is something you should only do deliberately and with a patient attitude. Configuring a new setting for the first time or changing an existing impedance setting/level can be a bit confusing, may not be intuitive, and the process and procedures are not necessarily super simple. A careful read of the above information together with a close review of the topic of Calibration is highly recommended. Finally, you should read and re-read the detailed preamp controls information regarding impedance and calibration specific to your model of preamp before proceeding.

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