Adjustable impedance with LDRs

Tortuga Audio introduced adjustable impedance to its LDR preamp controller/attenuators in early 2015 as an optional feature. Tortuga Audio makes no claims guaranteeing the outcome of changing the controller’s impedance level. This feature is being provided solely as an exploratory option for those curious and wishing to explore possible outcomes.

Adjustable inpt 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 preamp controller/attenuators are unique in this regard.

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. Impedance setting #1 is fixed at 50k and cannot be modified. Settings #2 through #10 are initially disabled and are entirely optional and can be ignored.

Once two or more impedance settings are properly configured, the user may switch instantaneously between different input impedance levels while playing music or running test signals. The user is free to explore and possibly fine tune their preamp’s impedance level to find an optimal level for their source(s) and amplifier.

Input & output impedance & why it matter?

Impedance is another word for resistance and is usually expressed in units of "ohms". In audio, impedance often involves complex relationships between simple fixed resistance plus dynamic resistance from capacitors and inductors that vary with the frequency of the audio signal. Impedance is also another way of talking about how hard audio equipment has to work to do its job. And the harder you have to work, the harder it is to be good at your job!

Figure 1. potentiometer & LDR type voltage dividers

Impedance of a typical volume control voltage divider is shown in the figure above as the mathematical sum of the "series" and "shunt" resistance. Most volume controllers are in the range of 10k-100k ohms. Tortuga Audio typically sets the default input impedance of its LDR volume controllers in the 20-50k range.

Audio equipment typically has both an input and an output impedance.

Input impedance tells us how hard we have to work to get a signal into a device. Devices with high input impedance place minimal current demands on connected upstream components. This is good thing and usually translates into a better sounding system because the equipment pushing the audio signal doesn’t have to work as hard.

Output impedance tells us how the output audio signal holds up under load wherein a lower output impedance is almost always better.

A device with low output impedance is like a strong, steady water main that delivers constant full pressure regardless of whatever you connect to it. The signal arrives at the next component at the intended volume and with the intended sound, regardless of what that component is or how long the cable is.

Conversely, a device with high output impedance is more like a garden hose fed by a weak pump. A weak pump struggles to maintain pressure (voltage) as flow (current) increases. Plug it into one amplifier and it sounds fine. Plug it into another and the volume drops, the treble softens, or the bass gets flabby. Long cables make the problem worse, because a weak source cannot push high frequencies through them cleanly. The essential point is this: low output impedance means the device’s sound does not change based on what you plug it into.

Input vs. output impedance

The relationship between input and output impedance are inextricably linked by physics. While most volume controllers have a fixed input impedance, output impedance is typically not fixed but is instead dependent on the input impedance and the attenuation level.

Let’s unpack this by taking a closer look at the figure below that shows the relationship between impedance and the attenuation level of one of our earlier LDR3x.V2 LDR attenuators. This attenuator has 70 volume steps over a 60 dB range with a 20k nominal input impedance.

ldr3x.v2 attenuation and impedance curves

dB schedule (black line): The dB schedule is the attenuation schedule. This is our defined goal. All the rest falls out from this schedule. This particular example is a linear fixed dB per volume step except for a reduction in the schedule slope to lessen the incremental sound pressure changes as you approach full volume.
Input impedance (green line): The green line shows the input impedance as volume is adjusted between step 1 (minimum volume) and step 70 (maximum volume). Note the fact that the impedance doesn’t actually remain fixed at 20k but ramps up to 100k as the command step falls below 15. This is an artifact of the math of attenuation plus the limitations of LDRs. Simply put, we can’t maintain a fixed input impedance of 20k with a 60 dB control range. Doing so would require us to operate outside of the acceptable LDR window of minimum 100 ohms and maximum 100k ohms. To live within these constraints we allow the impedance to increase from 20k to 100k when volume is low. In practice, this has no discernible impact on the sound quality.
RSeries (red line) & RShunt (blue line): Recall that by definition, the sum of RSeries and RShunt is equal to the input impedance which is to say the green line equals the sum of the red line and the blue line. The LDR resistance schedule for RSeries and RShunt is derived from governing voltage divider equations that are not covered here.
Output impedance (yellow line): The yellow output impedance curve is by far the more interesting and non-intuitive of the these curves. Starting at minimum volume, note how output impedance is at its lowest and remains unchanged as input impedance falls from 100k to the target 20k. Once the input impedance reaches 20k, the output impedance climbs in sync with the increasing volume. But as the RSeries begins to drop quickly towards full volume, the output impedance hits a peak at -6 dB and then drops very rapidly reaching the same low level at both ends of the volume control range.

Output impedance - a deeper dive

The fact that most volume control devices do not have a fixed output impedance has real-world implications that are worth noting for those interested in understanding this topic more fully. Here are some additional points to consider regarding non-constant output impedance.

Observations

Typical volume controls do not have a fixed output impedance
Output impedance changes every time you turn the knob
At full volume and at full mute, an output signal pushes easily through when impedance is minimal
Output signal weakens as the volume increases into the middle of its range
Output is weakest (impedance highest) around 6 dB below full volume, which is a fairly loud setting

Why it matters

Sound can change with volume - Because the control’s ability to drive the next component varies with the setting, the tonal balance and the sense of dynamics can subtly shift as you adjust the volume. A system that sounds right at one level may sound a little softer or duller at another.
Outcome depends on connected equipment - A passive volume control relies on current drive of its source, the input impedance of the amplifier, and the length and quality of cables to the amplifier. Any or all of these can show up as lost treble, flabby bass, or reduced volume.
Higher impedance volume controllers are more affected. While increasing input resistance is gentle on the source feeding it, the price for doing so is a proportionally higher worst-case output impedance. Choosing a design impedance value is always a compromise between being easy on the source and being strong enough to drive the attached power amp with authority.

How active beats passive

Performance independent of attached equipment - Conventional active preamp performance is rarely dependent on the impedance of attached upstream or downstream equipment.
Performance independent of volume level - Input and output impedances remain fix and optimal so the system sounds the same no matter where the volume knob sits.

How passive beats active

A well matched passive attenuator operating between a robust low impedance source and a high impedance power amplifier, will have excellent performance, be dead black quiet between the notes, and lack nothing in tone or dynamics compared to any active preamp regardless of price.

Why change input impedance?

Conventional audio preamps have a fixed input impedance, chosen by the designer as a compromise. One of the benefits of using LDRs in lieu of fixed resistors or potentiometers is the ability to adjust the overall input impedance of the LDR attenuator. This is done entirely within the software and requires no additional hardware.

Adjustable input impedance lets the user resolve that compromise for their own system. The benefits fall into the following categories:

Matching the source - Every source component has its own output impedance and its own preference for how heavily it is loaded. Tube outputs, some DACs, and phono stages have relatively high output impedance and want a light load, meaning a high preamp input impedance, or they lose level and bass. Many solid-state sources are happy driving a heavier load and can sound more dynamic doing so. An adjustable input lets you give each source what it wants instead of hoping the fixed value is close enough.
Better current drive to the amplifier - As we discussed earlier, the input and output impedance are unavoidably linked in a passive volume control. The attenuator’s worst-case output impedance is a fixed fraction of its total resistance, so lowering the input impedance also lowers the output impedance. That makes the preamp stronger at pushing the signal through cables into the power amp, which preserves treble and dynamics. Adjustability lets you trade a little source loading for better amplifier drive, or the reverse, depending on which side of the system is more sensitive.
Lower noise - Lower impedance circuits generate less thermal noise and are less prone to picking up hum and radio interference. If the source can tolerate a heavier load, dropping the input impedance quiets the system.
Compatibility with multiple sources - A listener with several sources, or one who changes equipment over time, does not have to accept a setting that suits one component and compromises another. The preamp adapts rather than forcing a new purchase.
Tuning by ear - Because the ideal impedance value depends on the specific combination of source, cables, and amplifier, no designer can know the optimal impedance in advance.

Making input impedance adjustable turns impedance from a hidden fixed spec into a control the listener can use to find the best sound for their system.

Adjusting input impedance

Impedance setting #1 is fixed at a default level in all Tortuga Audio preamps; typically 50k 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 configure up to 9 additional impedance settings #2 through #10 each with its own impedance level 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 or pushing through test signals.

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.