"Auto" Impedance - 1st Active Block - Does an active Mixer Block Count?

sprint said:
Wow! I was trying out all kinds of convoluted ways to do something like this to force the input to low impedance just for classic vibe without giving up other patch functions that I want positioned before phaser block - this is perfect! Awesome info! Thanks for Sharing!
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I forgot about the 'Vibe'.
 
I'm so happy i looked at this forum post lol. I've only ever used the rear input, so now i'm going to switch to the front and have some fun! :cool:

Surprised though because nothing mentions that the input impedance is specific to the front input for input 1.
 
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KyleHowdy said:
I'm so happy i looked at this forum post lol. I've only ever used the rear input, so now i'm going to switch to the front and have some fun! :cool:

Surprised though because nothing mentions that the input impedance is specific to the front input for input 1.
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fyi - retested this today and see that the impedance value of 1st active block when input impedance set to "Auto" is reflected from both front and back input1 jacks. Back in February I was sure impedance changes were only getting reflected at front input, but, either it got fixed since then or I was seeing things. All good now - no difference front to back.
 
Moke said:
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... Not above. There are some other interesting things about how this is handled, but it would probably take a video to cover it all.

Example...
View attachment 78304
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Have to check this out. Do not want wah after Fuzz Face.
 
Moke said:
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... Not above. There are some other interesting things about how this is handled, but it would probably take a video to cover it all.

Example...
View attachment 78304
Click to expand...
Added this gem to the wiki: https://wiki.fractalaudio.com/wiki/index.php?title=Input_impedance
 
I checked out a few tonight.
The Bender Fuzz, Pi Fuzz, and Master Fuzz all were 22k.
The Fuzz Face and Rat stayed at 1M.
The BB Pre and Full OD were at 230k.
The Univibe was at 70k.
 
Are you sure about the Fuzz Face?
I thought it was known as one with really low impedance, whose tone was strongly affected by that.

Not so?
Is it being 1M in the Axe a bug?
 
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It has defaulted to 1M as far as I can remember?

From the article...

3.1 Fuzz Face Input Impedance:
Is equal to the input impedance of a common emitter stage. It can be calculated as:

Zin = Zin of PNP Common Emitter = rπ

rπ=(β+1)⋅VtIEQ=(β+1)gm=(70+1)0.0088=8Krπ=(β+1)⋅VtIEQ=(β+1)gm=(70+1)0.0088=8K

For this math calculation the feedback network is ignored but in practice, it will lower the input impedance to 5KΩ approx. The Fuzz Face has a very low input impedance that will change with the position of the RFUZZ potentiometer (between 5.2KΩ and 8.4KΩ in the simulation). So the feedback network has a negative impact on this parameter.

As a rule of thumb, Zin should be at least 1 MΩ. In other pedals with similar input stages like the one in the Big Muff Pi a series resistor is placed at the input in order to higher the impedance (at the cost of creating a voltage divider that reduces the available input signal).

The Fuzz Face low input impedance will load the guitar pickups. This is the reason why they do not respond well when they are placed after other pedals. A practical advice is to put your Fuzz Face first on the pedal chain, just after the guitar. The germanium transistor needs to see the inductance/impedance from the guitar pickups. If they see a buffer at the input, they tend to sound awful.
 
Don't remember the exact value, but it's definitely really low in Helix, low enough to really make a difference vs 1M. It's kind of the poster child for why modelers need to be able to control their input impedance in hardware.

Can we have an option, so we can choose to be authentic, or to not have our presets change?
 
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Dave Merrill said:
Can we have an option, so we can choose to be authentic, or to not have our presets change?
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We have an option for the ones that flip (ie pi fuzz, classic vibe) which is, for authentic, to put the block first active in line of sight of the input block (or set to 1M for not). I suspect face fuzz is an error.
 
sprint said:
We have an option for the ones that flip (ie pi fuzz, classic vibe) which is, for authentic, to put the block first active in line of sight of the input block (or set to 1M for not). I suspect face fuzz is an error.
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Yeah, I just found out about the straight line across from the input block thing, and moved the compressor and wah blocks up a row so they wouldn't count. Good trick.

But I agree, fuzz face is wrong. Hope it gets fixed. With the option :)
 
I just tested all the drives on Axe III mk1, firmware 17.02, this is what I got.

BB Pre 230k
Bender Fuzz 22k
Bit Crusher 1M
Blackglass 1M
Blues OD 1M
Box o' Crunch 1M
Compulsive Distortion HP 1M
Compulsive Distortion LP 1M
DS1 Distortion 1M
DS1 Distortion Mod 1M
Esoteric ACB 1M
Esoteric RCB 1M
Eternal Love 1M
Face Fuzz 22k
FAS Boost 1M
FAS LED-Drive 1M
Fat Rat 1M
FET Boost 1M
FET Preamp 1M
Full OD 230k
Hard Fuzz 1M
Heartpedal 11 1M
Hoodoo Drive 1M
Horizon Precision Drive 1M
Jan Ray 1M
M-Zone Distortion 1M
Master Fuzz 22k
Maxoff 808 1M
MCMLXXXI Drive 1M
Micro Boost 1M
Miss Boost 1M
Octave Distortion 1M
OD 250 1M
OD 250 Gray 1M
PI Fuzz 22k
Plus Distortion 1M
Rat Distortion 1M
SDD Preamp 1M
Shimmer Drive 1M
Shred Distortion 1M
Sonic Drive 1M
Suhr Riot Ge 1M
Suhr Riot LED 1M
Suhr Riot LED/Si 1M
Super OD 230k
T808 Mod 1M
T808 OD 1M
Tape Distortion 1M
Timothy 1 1M
Timothy 2 1M
Timothy 3 1M
Tone of Kings 1M
Treble Boost 70k
TS9DX+ 1M
TS9DX Hot 1M
Tube Drive 3-Knob 1M
Tube Drive 4-Knob 1M
Valve Screamer VS9 1M
Zen Master 1M
 
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Moke said:
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... Not above. There are some other interesting things about how this is handled, but it would probably take a video to cover it all.

Example...
View attachment 78304
Click to expand...
@Moke, any chance of that video on the "other interesting things about how this is handled"?
Or a quick explanation here?
 
Dave Merrill said:
@Moke, any chance of that video on the "other interesting things about how this is handled"?
Or a quick explanation here?
Click to expand...
It's been a minute. I can't remember what I had found at the moment. I may have taken some notes somewhere in my huge pile of notes..o_O:oops:

I see that the Face Fuzz seems to have changed? That could impact existing presets, and create tonal differences between certain Axe-Fx III and FM3/9 presets to be on the lookout for.
 
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Moke said:
It has defaulted to 1M as far as I can remember?

From the article...

3.1 Fuzz Face Input Impedance:
Is equal to the input impedance of a common emitter stage. It can be calculated as:

Zin = Zin of PNP Common Emitter = rπ

rπ=(β+1)⋅VtIEQ=(β+1)gm=(70+1)0.0088=8Krπ=(β+1)⋅VtIEQ=(β+1)gm=(70+1)0.0088=8K

For this math calculation the feedback network is ignored but in practice, it will lower the input impedance to 5KΩ approx. The Fuzz Face has a very low input impedance that will change with the position of the RFUZZ potentiometer (between 5.2KΩ and 8.4KΩ in the simulation). So the feedback network has a negative impact on this parameter.

As a rule of thumb, Zin should be at least 1 MΩ. In other pedals with similar input stages like the one in the Big Muff Pi a series resistor is placed at the input in order to higher the impedance (at the cost of creating a voltage divider that reduces the available input signal).

The Fuzz Face low input impedance will load the guitar pickups. This is the reason why they do not respond well when they are placed after other pedals. A practical advice is to put your Fuzz Face first on the pedal chain, just after the guitar. The germanium transistor needs to see the inductance/impedance from the guitar pickups. If they see a buffer at the input, they tend to sound awful.
Click to expand...
You're starting to sound like Cliff. :)
 
Moke said:
It's been a minute. I can't remember what I had found at the moment. I may have taken some notes somewhere in my huge pile of notes..o_O:oops:

I see that the Face Fuzz seems to have changed? That could impact existing presets, and create tonal differences between certain Axe-Fx III and FM3/9 presets to be on the lookout for.
Click to expand...
I swear I checked it some time ago and it wasn't 22k like it is now.
But then again, the memory molecules are getting a bit thin and wobbly around here...
 
Dave Merrill said:
@Moke, any chance of that video on the "other interesting things about how this is handled"?
Or a quick explanation here?
Click to expand...
I remember that there were certain blocks that did not break the auto-impedance 'Link' if put between the Input block and the Drive/Phaser block. But there didn't seem to be any obvious logic to which ones don't? Intended or overlooked, not sure?
 
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