LDR adjustable impedance

Why change input impedance?

In some cases the default input impedance of a given audio device, like a preamplifier, may not be optimal and system performance may benefit from either raising of lowering input impedance. Or maybe not. It’s very dependent on the complex interactions between the components in an audio system.

Increasing the input impedance of a preamp will make it easier for the upstream source to transfer audio energy (i.e. the music) to the preamp because less current flow is demanded of the source. If your source happens to have a less than robust output stage, connecting it to lower impedance devices may overburden its ability to deliver the audio energy resulting in a loss of dynamics and poor bass. It may sound flat or muddy. Some source components may even require a downstream preamp with 50k or higher impedance to perform properly while 10-20k is more common.

The downside to increasing input impedance indiscriminately is that higher levels tend to increase noise/distortion. Again with the no free lunches.

There’s no simple formulaic approach to knowing what input impedance is optimal for your system. Changing the input impedance of your preamp 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.

What is adjustable input impedance?

Adjustable input impedance is not a common feature in audio equipment. Most audio equipment has a fixed input impedance that was established by the audio designer. Tortuga Audio’s LDR based preamps are unique in this regard.

Adjustable Impedance is a feature introduced by Tortuga Audio in early 2015 that 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 to find an optimal level for their source(s) and amplifier.

Changing input impedance

Impedance setting #1 is fixed at a default level in all Tortuga Audio preamps; typically 20k but may also be 75-80k in some units. All Tortuga Audio preamps are shipped with a default input impedance at setting #1. The default impedance level at setting #1 can not be changed.

The user may set up additional impedance settings with impedance levels between 1k and 99k. Older preamps allowed for up to 5 different impedance settings (#1 plus 4 additional). This was later increased to 10 including the default setting #1 plus 9 additional.

Once one or more additional impedance settings have been properly configured and initialized, the user is then able to instantaneously switch back and forth between impedance levels while listening to music. This flexibility allows the user determine which input impedance level is optimal for a given system.

It’s important to remember that each new impedance setting/level requires the preamp to be run through at least 1 cycle of calibration in order to establish a new attenuation table at the new impedance level. A second calibration pass is highly recommended for newly established levels. Calibration must be run for each setting/level individually since calibration only operates on the currently selected setting/level.

Please consult the preamp controls section for detailed instructions on setting up impedance settings, adjusting impedance levels, and running calibration to generate a new attenuation table.

Switching between impedance settings

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 because there is no defined attenuation table. Switching back to a defined setting/level 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.

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.

Please be patient and deliberate

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 level can be a bit confusing and the process and procedures may not seem intuitively easy. A careful read of this information on adjustable input impedance, together with information on LDR calibration and the related preamp controls is highly recommended.

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