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Input impedance

From Axe-Fx II Wiki
Revision as of 12:52, 15 July 2019 by Yek (talk | contribs) (Variable input impedance)
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Available on which products

  • Axe-Fx III: variable, Input 1 (front and rear)
  • FM3: fixed at 1 Megaohm
  • Axe-Fx II: variable, front input
  • FX8: variable, instrument input
  • AX8: fixed at 1 Megaohm
"The XL+ front input circuit is identical to the AX-8 except the AX-8 doesn't have the variable impedance circuitry." source

Variable input impedance

Variable input impedance changes the analog circuitry of the input jack to interact with the guitar pickup. It recreates the way that some classic effects “load down” pickups, causing a change in frequency response.

Input Impedance is also referred to as: Input Z.

In Auto mode, the impedance is set automatically, based on the first non-bypassed effect to follow the input.

"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
"It's not virtual. It physically switches in a capacitor." source
"You can simulate the effect of a guitar into a Fuzz-Face using the Input-Z feature." source
(firmware Ares) "Duplicating the behavior of a fuzz pedal requires that the pedal be the first non-bypassed block following the Input 1 block and the input block impedance must be set to Auto since fuzz pedals load down the guitar’s pickups."
"You can use the Input Impedance control to lower the input impedance and flatten the response. Try 220K, that will be close to your passive DI." source

Variable impedance values

  • 1MΩ
  • 1MΩ + Capacitor. Use this to simulate a long guitar cable (source)
  • 230kΩ
  • 230 kΩ + Capacitor
  • 90 kΩ
  • 90 kΩ + Capacitor
  • 70 kΩ
  • 70 kΩ + Capacitor
  • 32 kΩ
  • 32 kΩ + Capacitor
  • 22 kΩ
  • 22 kΩ + Capacitor

Buffered signal and variable input impedance

Using a buffer before the instrument input disables the variable impedance feature.

"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
"A wireless acts as a buffer. Hence changing the input impedance will not be able to load down the pickups." source

Fuzz, buffers and impedance

Read this: Fuzz, buffers and impedance

Guitar volume and impedance

"When you roll your guitar volume down you increase it's output impedance dramatically. It can be upwards of 100K ohms. The self-noise of a 100K ohm resistor is quite high. Now amplify that (a lot) and you'll hear the noise. If you continue to roll the volume down you'll notice the noise goes away because the output impedance decreases." source

Prevent thump

Engaging some effects, like Wah or Phaser, can cause a "thump" in the sound. This may be caused by impedance switching.

To solve this, change Input Impedance from "Auto" to "1M".

The above doesn't apply to Fractal Audio processors with a fixed (not adjustable) input impedance.

More information