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The Truth About True-Z

Only published comments... Apr 06 2010, 10:18 AM by Chris Townsend

Some people have asked what True-Z is; others have wondered if True-Z is just marketing hype. As one of the inventors, I can tell you the effect it produces is very real and measureable. And even though what True-Z does is essentially quite simple, it's a key piece of the guitar tone puzzle that has often been overlooked.


The Basics


It’s well known that electric guitar pickups "react" differently depending on what they are plugged into. The impedance (capacitance and inductance) of the pickup, combined with the input impedance of the guitar amp or effect pedal, form a resonant circuit that shapes the frequency response of the guitar. Changes in the input impedance of the amp or pedal that the guitar is plugged into can have an enormous effect on these resonant characteristics.  Producing a flat frequency response is not what’s important here.  Instead the key is allowing guitarists to get the tones they know and love, which all involve considerable frequency response shaping from the amp or pedals.


Typical guitar modeling processors have a fixed input impedance, which can limit the accuracy of the modeling. Eleven Rack's True-Z circuitry is designed to solve this problem by automatically changing the input impedance to closely match the impedance of the first active effect in the Eleven Rack signal chain. For example, if you're plugging into a wah pedal which is connected to an amp, the wah determines the impedance; but, if you entirely bypass or remove the wah, the amp determines the impedance that your guitar sees. Eleven Rack achieves the same result by dynamically changing the input impedance as needed. The impedance values have been predetermined based on measurements of the actual analog effects being modeled, so there’s no guesswork required to figure out which values to use.


Of course, if you’re in the mood to experiment, you can try any combination of impedance and effect. Manual mode allows you to set the impedance to whatever fixed value you want, from 1 Meg ohm to around 20k ohms, so you can dial in the sound to taste. Higher impedances result in an increase of the natural resonant peak of the pickup and an overall brighter output. Lower impedances result in a darker sound and the natural resonance of the pickup being damped.


Frequency Response Fun


For my fellow technology lovers, here’s a frequency response plot of a typical single coil guitar pickup. The green line is with a 1 Meg ohm input impedance and the red line is with a 70k ohm impedance:



As you can see, there is about a 10dB difference at around 5 kHz between the 70k ohm load and the 1 Meg ohm load! Obviously, that’s quite significant.


Fuzz pedals are an example of an effect with very low input impedance (usually around 20k ohms). This impedance dramatically rolls off the high end and totally removes the resonant peak of the pickup. The "fuzz sound" is dependent on this lack of resonance, because otherwise you end up with more of a typical overdrive or distortion sound. Here are some audio examples which illustrate the difference:


Fuzz with True-Z off (1 Meg ohm load):


Fuzz with True-Z on:


Analog wah pedals also typically have a relatively low input impedance of around 70k to 90k ohms. A wah is a resonant filter, and using it with a pickup with a strong resonant peak might be problematic, because when the frequency of the wah resonance lines up with the frequency of the pickup resonance you get one massive resonance, but only at one pedal position. This lower impedance is helpful because it tames the natural resonance of the pickup, so primarily the resonance of the wah comes through. As you can hear in these clips, the result is smoother and less harsh:


Wah with True-Z off (1 Meg ohm load):


Wah with True-Z on:


Tube amps are an especially interesting case, because their input impedance decreases considerably at high frequencies. This occurs because tubes inherently act like capacitors. True-Z can also add capacitance (that's what the "+ Cap" means in the True-Z control) to closely match the impedance of an actual tube amp. This capacitance directly changes the response of the pickups. The resonant peak of the pickup is lowered and the high end is rolled off somewhat, which removes some of the "brittleness" you might otherwise get with a generic DI. And again, because the response of the pickup is changed in a way that's dependent on the pickup you are using (due to its particular inductance, capacitance, and resistance), it is not something you could "fix" with EQ before the amp.


Here’s another frequency response plot of a typical single coil guitar pickup. The green line is with a 1 Meg ohm input impedance and the red line is with True-Z tube amp impedance enabled (1 Meg ohm + Cap):



Now, Put True-Z to the Test


While these graphs and numbers are fun, nothing can replace a real-world hands-on test. If you own or get a chance to demo Eleven Rack, here’s how you can easily hear the difference of True-Z for yourself.

First, create a preset with the wah first in the signal chain. You'll see that this will automatically set the True-Z impedance to "90k + Cap". Now if you go to the input block you can override the automatic impedance and set it to 1 Meg ohm, which is approximately the impedance of a typical DI. You can save these as two separate presets, so you can quickly switch between them while you play. Notice how the wah sounds “harsher” with the 1 Meg impedance. You can also do the same test with the fuzz or the amp as the first effect.


When we set out to provide the best effects and amp emulations, we looked at the whole picture of everything that could affect recreating the tones. Trying to simulate the effect of impedance with DSP is practically impossible, because every pickup has a different resonant frequency and there is no means of knowing ahead of time which pickup will be used. True-Z is one of the reasons why your guitar will sound and feel like you expect it to when plugged into Eleven Rack. If you spend some time experimenting with True-Z’s impedance settings, we hope you’ll agree that the effect of True-Z is quite significant and even essential to achieving realistic modeling.

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About Chris Townsend

I am the Lead Guitar Products Architect at Avid.  I created most of the amp and FX algorithms in Eleven Rack and the Eleven Plug-In.  On a typical day I can be found coding algorithms while simultaneously strumming the guitar.  My wife discovered I was a true engineering geek when she saw I was reading Inside the Vacuum Tube while on vacation.

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