Input Impedance

trancegodz

trancegodz

Fractal Fanatic
I read about input impedance in the Wiki and have been experimenting with the various options.

https://wiki.fractalaudio.com/wiki/index.php?title=Input_impedance

The default for the Input Impedance block is “Auto” for the Axe FXIII. It is fixed at 1 Megaohm on the FM3.
Do you guys prefer it set to Auto or 1M and why?

When should you use one of the other settings instead of Auto?

I tried them all out with my Stratocaster and you can definitely hear a big difference in the sound with each choice.

If you were to plug into a real Fulltone Compressor before going into the Axe FX, or if you were to plug into a real Fuzz Face before going into the Axe FX, what should you set the Input Impedance on the Axe FXIII to? Auto, 1M, or something else?

I use the volume control on my Stratocaster and roll it off to 7 often. When I do this the notes cut off completely if I have the Input gate turned on. How should you set the Input Gate to prevent this?
 
trancegodz said:
I use the volume control on my Stratocaster and roll it off to 7 often. When I do this the notes cut off completely if I have the Input gate turned on. How should you set the Input Gate to prevent this?
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Lower the threshold on your gate to let quieter signals through. The Threshold is the minimum signal level where it decides "This is playing and not noise". So I'd go to your worst case scenario (guitar volume down and light playing) and adjust the threshold until you still get all your playing through and your sustain isn't choked out too quickly.
 
For a while I played around with setting the impedance to specific values, particularly in relationship to the type of amp tone I was looking for. So for more modern tones I used either auto or 1M, while older and lo-Fi sounds were assigned a much lower impedance. It made for a nice tone mellowing for amps like the Gibson or tweed deluxes that mirrored my ‘48 Supro; a darker and fuller tone that brightened up with more distortion. When I owned Bogners, Marshalls, and a handful of other modern builds, I never experienced that same high end roll off.

But multiple firmwares later, I’m using other methods to simulate those differences between models (e.g. controllers attached to the tone of an amp model).
 
I am also very interested in a good answer, a solution. - "Input Impedance"
But!
No one said the solution!
Can a specific example be requested?
I think a lot of people care!
Thank you very much!
 
Most tube amps have a high input impedance. The difference settings are more for replicating the low input impedance of certain effects like vintage Fuzz and Wah pedals. I believe the old Boss CE-1 unit had a somewhat low input impedance too.

The Auto setting only uses the first block after the input block. If that block is bypassed, it reverts to 1 Meg. If it's enabled, it mimics the impedance of the selected model in that block. Most modern pedals and amps will have a high input impedance to maintain signal strength and high end clarity.

This also only really has an effect if you are using a passive guitar. Guitars with active pickups and/or preamps will typically have a much lower output impedance, so changes in the input impedance of the next device will have much less effect.

If you want the input impedance to mimic the active state of the first block after the input block, use the Auto setting. Otherwise choose a manual setting that sounds good for that particular guitar and tone.
 
mr_fender said:
The Auto setting only uses the first block after the input block. If that block is bypassed, it reverts to 1 Meg. If it's enabled, it mimics the impedance of the selected model in that block.
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It actually uses the value of the first active block after the Input block, not only the first block.

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Maybe that's what you were saying?
 
I have it set to 1M. Live, I have a buffer between the guitar and Axe-Fx, so I want to have a consistent sound and take the impedance out of the equation.
 
Correct, the first non shunt block after the input block. Not just the one right next to it. However if you have a bypassed block between the "low impedance" block and the input, it will revert to 1 Meg. So for example, if you have a bypassed comp block before a drive block set to fuzz face, the bypassed comp block determines the impedance, not the fuzz. You can think of each block as having a perfectly silent buffered bypass.

https://forum.fractalaudio.com/threads/possible-bug-impedance-auto-function.153096/post-1821026
 
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Yeah the wording in the manual is not super clear. I always thought it was the first active block in the chain until I saw the above post from Cliff.
 
I am fairly certain that it uses the first "active" block meaning bypassed blocks are treated as true bypass and therefore do not affect the input impedance. This is one of the features that made me leave the Helix for Axe because the Helix does it the other way first block regardless of bypass.
 
Does anyone here have an Axe-FX III? The input block edit menu shows the current impedance while set to "auto". Assuming there's no bug with the displayed vs. actual impedance, you can check how things really work by looking at that.
 
On the hardware... If I exit the edit screen and re-enter the edit screen, the Auto Impedance displays the correct actual Impedance. If I bypass or activate a block, I have to repeat that process to update the auto impedance value. This was supposed to be fixed a while back to update in real-time?

The actual Impedance (while in Auto) has never been displayed/updated at all in the editor? Would love to see that updated in real-time while switching scenes to more easily verify what's happening.

I have done some experimenting trying to figure out the routing and bypass rules around the Auto-Impedance 'link' to the five drive models that have this function. It is a bit confusing?

There seems to be a couple of blocks (so far) that break that 'link' that shouldn't? The Send and Return blocks do this, which seems like a bug to me? There are a couple of other odd anomalies too that are a bit harder to explain that may also be bugs?
 
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Hi Moke,

On the Axe FXIII itself under Input 1 the impedance doesn't change at all here with the Fuzz Face on in the first block when it's set to Auto. No matter how many times I exit out and back in of the input block. It always shows Auto 1M. I tried a bunch of different fuzz's and drives and all of them showed Auto 1M when they were on. That doesn't seem right. The only effect I have found so far that shows any difference from Auto 1M is the Univibe which showed Univibe Auto 70k when it was on.

From the WIKI:
Many fuzzes depend on a direct connection between the guitar pickup and the input of the pedal. A modeler gets in the way because its input is always buffered. Thuis means that a modeled fuzz will react differently than the actual pedal.
"The Fuzz in the Axe-Fx reacts as though there is a buffer in front of it (because there is). It's a limitation inherent to all modeling products. I modeled it using a nominal source resistance. I forget what I used for the source resistance but it was probably around 100K ohms. To really simulate it you would need a controller to simulate the changing output impedance of the guitar." source
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"You can simulate the effect of a guitar into a Fuzz-Face using the Input-Z feature." source
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"The problem is that the signal is buffered and there is no way to model the interaction of the pickups with the circuit." source
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(firmware 4.00 for the Axe-Fx III) "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."
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Forum member AustinBuddy recommends setting Input Impedance to 90K in presets which have a fuzz model as the very first (non-bypassed) block. This doesn't apply to processors with a fixed input impedance.
 
For straight up rock 1m is fine, but for cleans and blues I play with it.
 
trancegodz said:
Hi Moke,

On the Axe FXIII itself under Input 1 the impedance doesn't change at all here with the Fuzz Face on in the first block when it's set to Auto. No matter how many times I exit out and back in of the input block. It always shows Auto 1M. I tried a bunch of different fuzz's and drives and all of them showed Auto 1M when they were on. That doesn't seem right. The only effect I have found so far that shows any difference from Auto 1M is the Univibe which showed Univibe Auto 70k when it was on.

From the WIKI:
Many fuzzes depend on a direct connection between the guitar pickup and the input of the pedal. A modeler gets in the way because its input is always buffered. Thuis means that a modeled fuzz will react differently than the actual pedal.




Forum member AustinBuddy recommends setting Input Impedance to 90K in presets which have a fuzz model as the very first (non-bypassed) block. This doesn't apply to processors with a fixed input impedance.
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I expected the Fuzz Face model (all of the fuzzes actually?) to alter the impedance too? But most don't... My tests today show that only five Drive models currently do this.
  1. BB Pre
  2. Full OD
  3. Super OD
  4. PI Fuzz (the only fuzz?)
  5. Treble Boost
 
Odd, since the BB Preamp, Fulltone Fulldrive 2, and Boss SD-1 all have a 1 Meg input impedance per their websites.
 
The Big Muff and Treble booster make more sense, especially the Treble Booster. Rangemasters have very low input impedance like Fuzz Faces usually in the 10k-15k range IIRC.

Vintage style Wah pedals also often have pretty low input impedances too.
 
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