From Axe-Fx II Wiki
The information on this page supplements the official manuals.
Variable Input Impedance: supported by which Fractal Audio devices
- Axe-Fx II: yes.
- FX8: yes.
- AX8: no (fixed at 1 Megaohm).
What is Input Impedance?
It adjusts the way the device interacts with the instrument.
- "Some old stomp boxes, i.e. Univibe, fuzzes, etc. have a low input impedance. This low input impedance will load down a guitar's pickups and change the frequency response of the pickups. You cannot simulate the effect of pickup loading after signal acquisition since the pickup parameters are an unknown quantity. So if you want to exactly model one of these old stomp boxes you need to replicate the impedance loading. The 11R and the Axe-Fx II do this by switching in resistors and capacitors on the inputs that change the input impedance to match the device being modeled. I do not believe that input impedance affects feel in any way. This is simply impossible. I remember people saying they could feel the input "pushing back". Silly. It will, however, definitely affect the sound as the frequency response of the guitar will be altered. The resonant frequency of the pickups will shift downwards and the Q will be altered as well. Now... it is debatable whether this alteration in frequency is actually desirable. If you want to exactly duplicate the sound of a vintage effect then, yes, it is desirable. However, most, if not all, modern effects feature buffered inputs to prevent pickup loading. This is done because typically we don't want to load the pickups. When the pickup designer is designing a pickup he is assuming the pickup will not be loaded. He chooses the winding, magnets, etc. such that the pickups have a desired frequency response into a very high impedance (i.e. 1 Mohm or greater) load. Variable input impedance is a nice feature if you want accuracy but it's not that big of a deal and certainly doesn't make one device better than another. Personally I prefer the sound with it turned off." source
- "The pickups see a load. That load is the impedance of the cable and the impedance of the amp input. The typical input impedance of a tube amp is a resistance plus a small capacitance to ground (120 pF or so). The Axe-Fx input simulates this input impedance. Some effect pedals present a different load. The Axe-Fx replicates this by switching in different load resistors and capacitors when a model of that effect is first in the effects chain." source
- Javajunkie (about Auto Impedance): "The impedance will set itself (physical switch) depending on the first ACTIVE effect. Only a few of the effects will change the default of 1MOhm and only if the are active not bypassed. This is just how it works. The effects of the impedance change is EASILY measurable." source
Where to adjust Input Impedance
Axe-Fx II and FX8: Input Impedance is in the GTE/IN menu.
AX8: not supported because of the hardware design.
"The XL+ front input circuit is identical to the AX-8 except the AX-8 doesn't have the variable impedance circuitry." source
Input Impedance and buffers
"A buffer will render the impedance stuff ineffective. It will also add (maybe considerable) noise which may defeat the low-noise advantage of the front input." source
Use Input Impedance to prevent "thump"
Engaging some effects, like Wah or Phaser, can cause an audible "thump". This may be caused by impedance switching. To solve this, change Input Impedance from Auto to (i.e.) 1M.
- "The thump isn't the wah, it's the Input Impedance changing. If your preset is set to Auto for the Input Z, when you engage the wah the impedance switches. This causes a slight thump if you are playing since the load on your guitar changes. If you want to avoid this, turn Input Impedance to 1M OHM." source
- "Set the Input-Z on the preset to 1M rather than Auto. The reason the Wah engages abruptly is because it switches the input impedance. Setting Input-Z to 1M overrides the impedance switch (and, frankly, sounds better IMO)." source