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Input Trim

FractalAudio

Administrator
Fractal Audio Systems
Moderator
The Axe-Fx II contains a parameter known as "Input Trim". This is just a straight gain control at the very front of the amp block. It has a range of 0.1 to 10.0 (-20 to +20 dB).

So what use is a straight gain control at the front? Doesn't the Input Drive do the same thing?

The short answer is "no". The long answer is "probably not".

On many amps the Drive knob, which may also be called Gain or Volume, has what is known as a "bright cap" across the physical potentiometer. This capacitor shunts high frequencies around the pot so that the Drive control is not a straight gain. It has an associated frequency response. As the Drive is turned down more high frequencies are shunted around the pot which results in a net treble boost. If the Drive is turned all the way down the treble boost is maximum, if it is turned all the way up the treble boost is zero.

The roots of the bright cap are due to manufacturers trying to compensate for different types of guitars. Guitars with single coil pickups tend to brighter but with less output. The user would then turn the Drive knob high on the amps. Conversely a guitar with humbuckers has more output but sounds darker. To compensate the user would typically turn the Drive down. This will result in a treble boost compensating for the darker response.

The Input Trim control allows one to fine-tune the amount of treble boost first and then adjust the amount of distortion. So it is probably more correct to think of the Drive control as a combination Drive/Treble control. With this in mind experiment with the Drive control combined with the Input Trim.

Indeed some manufacturers have actually implemented separate Drive and Trim controls on their amplifiers. For example the Fryette (VHT) Deliverance has two controls: a Gain knob and a Cut knob. The Gain knob has a bright cap across it while the Cut knob is just a straight volume adjustment. The purpose of these two knobs is exactly as described above.

The Input Trim is also useful in replicating the High/Low jacks on an amp. Some amps have High and Low sensitivity inputs. All of the amp models in the Axe-Fx assume the guitar is plugged into the "High" input. The Low input on an amp is simply 6 dB less sensitive. To replicate using the Low input simply set the Input Trim to 0.5 (which equals -6 dB).
 

Noriuky73

Forum Addict
Thank You Cliff for share this little gems !
This are great advices and help our knowledge of your beautiful black box !!

Sorry for the dumb question , so Input drive is drive/treble adjustment and Input Trim Drive without coloration ?

I assume by your words that it is ...

Cheers from Italy
 

Stratoblaster

Fractal Fanatic
The Input Trim is a go-to control for me on most of my presets, especially on amps with the a Bright Cap engaged. It works great with the Drive control; most of the time I like the overall tone where I set the Drive level but it's too dirty/distorted. Backing off the Input Trim fixes that right up. I also find it's especially useful when working with the jumpered Marshall amps with two Drive controls.
 
Actually, I'm a huge fan of the Input Trim. I have an IA switch on my controller set up to toggle the Input Trim down by a certain amount for each preset. When engaged, it simply drops the gain hitting the front of the amp and pulls out some distortion. Not exactly like rolling back the volume on your guitar, but very, very handy for less aggression on say the verse of a song and then easily and accurately toggling back to normal input trim levels (read: distortion) for the chorus.
 

Noriuky73

Forum Addict
Input Drive = Drive/Treble control.
Input Trim = Fine tune the amount of treble in the above.

Btw - Keep these pieces of info coming Cliff. Great idea!
Thanks for your reply Narzugon

I assumed , from Cliff statement , that if you would a drive ... Let's say at 7.5 without loosing the "treble" , we can set the input drive ... Let's say at 4 .0 gaining in "treble" and compensate with the input trim for the amount of " distorsion"

So I assumed that input drive si of course drive AND treble coloration instead input trim is only a DRIVE boost or CUT without coloration compensating for the drive maybe lost in search of "trebles "

Sorry for my poor English , hope you understand it
 

GM Arts

Forum Addict
Don't forget also that you can set the bright capacitor value to fine-tune the way this treble boost works.

As Cliff says, the bright works when drive is set to less than maximum, and has more effect as drive is reduced. So first set drive and input trim to roughly where you want them, before changing the bright capacitor.

Low capacitor values cause the bright effect to work only at very high frequencies, while increasing the capacitor value makes the treble boost work from lower frequencies (still in the treble range), making the "bright effect" stronger.

After adjusting the bright capacitor, you may wish to go back and tweak drive and input gain again.
 

Noriuky73

Forum Addict
Don't forget also that you can set the bright capacitor value to fine-tune the way this treble boost works.

As Cliff says, the bright works when drive is set to less than maximum, and has more effect as drive is reduced. So first set drive and input trim to roughly where you want them, before changing the bright capacitor.

Low capacitor values cause the bright effect to work only at very high frequencies, while increasing the capacitor value makes the treble boost work from lower frequencies (still in the treble range), making the "bright effect" stronger.

After adjusting the bright capacitor, you may wish to go back and tweak drive and input gain again.
Thank You Gm !!

I appreciate every single help for my quest for tonal "perfection" ...

Other dumb question... For bright capacitator you mean the bright switch in the first amp page? And if so , there is a way to change the value in some page ( sorry I'm not in front of my Axe ) ... When you talk about low or higher values you're talking about the bright switch on /off or about some adjustment in advanced parameters ?

I thank you in advance , I really hope to be able to tweak every single knob in the advanced parameters or at least understand his uses .
 

ak49climber

Inspired
So with Mike Fortin's Natas (now Randall Thrasher and Satan models) the High Frequency Gain is more or less a regular gain control and the "Gain" control is and input trim control then the Low Frequency Gain is like depth with some drive? Always wondered about his designs.
 

jakel

Veteran
Thank You Gm !!

I appreciate every single help for my quest for tonal "perfection" ...

Other dumb question... For bright capacitator you mean the bright switch in the first amp page? And if so , there is a way to change the value in some page ( sorry I'm not in front of my Axe ) ... When you talk about low or higher values you're talking about the bright switch on /off or about some adjustment in advanced parameters ?

I thank you in advance , I really hope to be able to tweak every single knob in the advanced parameters or at least understand his uses .
The bright switch on the first amp page enables or disables the bright cap. The value of the bright cap can be adjusted on the advanced parameter (I think that's what it's called) page. See GM Arts' post for good info on what happens when you adjust the cap value. I would add that as the value gets larger not only does the frequency go down (into the mids), but the gain is also increased. A Plexi is a good example of this. The cap value is normally quite high on these, so turning it on/off (with the bright switch on the first amp page) really affects the gain.
 

GM Arts

Forum Addict
Thank You Gm !!

I appreciate every single help for my quest for tonal "perfection" ...

Other dumb question... For bright capacitator you mean the bright switch in the first amp page? And if so , there is a way to change the value in some page ( sorry I'm not in front of my Axe ) ... When you talk about low or higher values you're talking about the bright switch on /off or about some adjustment in advanced parameters ?

I thank you in advance , I really hope to be able to tweak every single knob in the advanced parameters or at least understand his uses .
Yes, the Bright switch needs to be turned on for this pre-overdrive treble boost effect. This switch is on the PRE page on the Axe-FX and the Basic page in Axe-Edit.

The Bright cap value is on the ADV page on the Axe-FX and the Tone page in Axe-Edit.
 

GotMetalBoy

Forum Addict
This is great information. The next time the manual is updated, you should put a link to the Cliff Notes section of the the forum, so new customers will also benefit from this knowledge. I'm hoping my 2nd guitarist and even my bassist will be able to afford an Axe-Fx II sometime in the near future and I'll just be able to tell them to RTFM! & RTFCN! ;-)
 

Morphosis

Fractal Fanatic
Would it not be better to indicate Input Trim to -20db - +20db instead of 0.1 to 10 ? Also the "Boost" value to OFF and +12db (when i remeber correct this is the boost value?) instead of ON?

The explanation of this is superb. Till now, i just use Input trim to simulate the Lo-Input (0.500 in reference to Cliff), but now i feel much more free to use it. In fact oftenly i had to fight when rolling back the guitar volume with to much treble lost. So the trick would be to lower the Drive Input and compensate with higher Input Trim, and when rolling back the guitar level to clean up, the highs would be more boosted? Or did i have a error in reasoning here?
 

FractalAudio

Administrator
Fractal Audio Systems
Moderator
Would it not be better to indicate Input Trim to -20db - +20db instead of 0.1 to 10 ? Also the "Boost" value to OFF and +12db (when i remeber correct this is the boost value?) instead of ON?

The explanation of this is superb. Till now, i just use Input trim to simulate the Lo-Input (0.500 in reference to Cliff), but now i feel much more free to use it. In fact oftenly i had to fight when rolling back the guitar volume with to much treble lost. So the trick would be to lower the Drive Input and compensate with higher Input Trim, and when rolling back the guitar level to clean up, the highs would be more boosted? Or did i have a error in reasoning here?
That sounds more like your guitar needs a bright cap. Many guitars have a small treble bleed cap on the volume pot to combat high frequency loss due to cable loading.

Or use a cable with less capacitance or simply a shorter cable.
 
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