Input block

Input block

Axe-Fx III — 5 blocks, including Input USB, 4 channels
FM9 — 4 blocks, including Input USB, 4 channels
FM3 — 2 blocks, 4 channels
VP4 — input section, no channels
AM4 — input section, no channels

Axe-Fx II, AX8 — 1 block, no X/Y
FX8 — n/a

About the Input block

The Input block is the spot where the digitized audio signal enters the grid. The Axe-Fx, FM9 and FM3 have multiple inputs. The VP4 and AM4 have a single input.

The Input block also provides these functions:

• Noise gate
• Input impedance (not on FM3 and AX8)

On the Axe-Fx III, FM9 and FM3, the Input block can also operate as an effects return in an effects loop. See below.

Position on the grid

Axe-Fx III, FM9 and FM3 

The Input blocks can be positioned anywhere on the grid

Axe-Fx II, AX8, FX8, VP4 , AM4

The Input block is fixed at the start of the grid

Input source

The Input blocks are linked to the hardware analog input ports. See Audio in and out for more information.

The Setup menu on the Axe-Fx III, FM9 and FM3 allows inputs to be linked to digital input.

Additionally, the Axe-Fx III and FM9 have an Input USB block. This gets its input signal from USB Outputs 7+8 from a computer. It can be placed anywhere on the grid.

Note that the Multiplexer block can connect directly to a physical input port, without requiring an Input block in between.

Input impedance

Impedance of instrument ports

The impedance of the instrument ports on the devices:

Variable

• Axe-Fx III (front)
• FM9
• VP4 (L/Mono only)
• AM4
• Axe-Fx II (front)
• FX8

Fixed (1 Megaohm)

• FM3
• AX8

Auto/Variable impedance

Auto, or variable, input impedance changes the analog circuitry of the input jack to interact with the guitar pickup. In Auto mode, the impedance is set automatically, based on the first non-bypassed effect to follow the input. This simulates the way some classic effects “load down” pickups, causing a change in frequency response. Input impedance is also referred to as “Input Z”.

The FM3 and AX8 do not support auto input impedance.

Input impedance is in particular relevant when using fuzz pedals. Fuzz, buffers and impedance contains more information.

Forum member Sprint investigated which effect block types can trigger a change in impedance in “Axe-Fx blocks that can influence Input Impedance”.

Drives

BB Pre (230k)
BB Pre AT (230k)
Bender Fuzz (22k)
Colortone Booster (90k)
Colortone OD (90k)
FaceFuzz (22k)
FullOD (230k)
Gauss Drive (230k)
Master Fuzz (22k)
OD One Overdrive (230k)
Pi Fuzz (32k)
Pi Fuzz - Ram's Head (32k)
Pi Fuzz - Russian (32k)
Pi Fuzz - Triangle (32k)
Super Fuzz (32k)
Super OD (230k)
Treble Boost (70k)

Phasers

Classic Vibe (70k)
Naughty Rock (90k)

Forum member Moke tells us we can get around the “1st active block in the chain” thing if needed, by putting any blocks in front of the Drive(s) that you want ignored, one row above. The algorithm only looks at blocks to the right of the Input block in the same row, or below, not above.

FRACTAL AUDIO QUOTES

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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.

[1] 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.

[2] It's not virtual. It physically switches in a capacitor.

[3] You can simulate the effect of a guitar into a Fuzz-Face using the Input-Z feature.

[4] 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.

[5] The first block must be engaged for the impedance to change. If it is bypassed the impedance will revert to 1M.

[6] Variable impedance is only used for pedal modeling. Tube amp models are always 1M (no capacitor). The reason it was not included in the FM3 was cost and space. The input impedance of the FM3 and Axe-Fx III without any additional resistance or capacitance modifications is identical and replicates the input impedance of a tube amp. Variable input impedance for our products is used only when modeling certain pedals. I can't speak for other products.

[7] The front input has Auto-Z technology. The rear doesn't. If the preset/scene has a block that lowers the input impedance the inputs will sound different.

You can get around the "1st active block in the chain" thing if needed, by putting any blocks in front of the Drive(s) that you want ignored, one row above. The algorithm only looks at blocks to the right of the 'Input' block in the same row, or below.

[8] When you roll your guitar volume down you increase its 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.

The Auto Impedance values are:

Auto
1MΩ
1MΩ + Capacitor. Use this to simulate a long guitar cable [9]
230 kΩ
230 kΩ + Capacitor
90 kΩ
90 kΩ + Capacitor
70 kΩ
70 kΩ + Capacitor
32 kΩ
32 kΩ + Capacitor
22 kΩ
22 kΩ + Capacitor

Important: a buffered device before the instrument input disables the auto impedance feature. This includes pedals with buffers (such as BOSS pedals) and wireless transmitters.

FRACTAL AUDIO QUOTES

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[10] 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.

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

Audible thump

Engaging some effects, like Wah or Phaser, can cause a 'thump' in the audio. 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 devices with fixed (not adjustable) input impedance (AX8, FM3).

FRACTAL AUDIO QUOTES

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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.

[12] 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).

Adjust the level of instruments

The Level parameter in the Input block has a modifier. This lets you adjust presets to the output level of a specific guitar, using a pedal or switch.

Noise gate at the input

Each Input block has its own noise gate. Read this: Noise gate

Effects loop

On the Axe-Fx III, FM9 and FM3, combining an Output block and Input block creates an Effects Send/Return loop. Read this: Audio in and out

Global blocks: Input 1 only

The Input 1 block can be used with Global blocks (Axe-Fx only). The other Input blocks do not support this.