Understanding All the Different Gain Controls

Input Drive is the Gain or Drive or Volume knob on the real amp.

The Bright switch controls the bright cap on the Gain/Drive/Volume control.
If an amp doesn't have a bright switch, the operation of of the models' bright switch is undefined.

Input Trim lets you adjust the range of gain of the amp. It allows you to reduce or increase the gain of the virtual amps input buffer.

Cliff: “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."

And: "The Input Trim control allows you to adjust the input attenuation without changing the frequency response. If you turn down the Input Drive and the model has a bright cap the amp will get brighter. Now you may like the brighter tone but wish there were more gain. Input Trim allows you to increase the gain without changing the tone. Conversely you may like the darker tone with Input Drive set high but wish there were less gain. In this case you can lower Input Trim. Most real amps do not possess an Input Trim control. Instead they usually have a switch or two input jacks that select between a high-gain and low-gain input".
 
that's how I got to the question I got to, Yek.
"If you turn down the Input Drive and the model has a bright cap the amp will get brighter."
...But if you disengage the bright cap, then it won't get brighter... so it is essentially a gain control (with no EQ change) like the Input Trim, right? What's the difference between Input Trim and Input Drive with bright cap disengaged?

Edit: I'm sure I've just missed some crucial basic fact, I just can't figure out what it is!
 
I'm curious about the AFS 1 model, is that a little different to most amps regarding the input drive? To my ears it seems to get brighter with more input drive rather than darker.
 
Rex said:
That's the opposite of what you asked in your earlier post, hence Cliff's answer.
Click to expand...
Forgive me if I mistyped earlier. I meant what I said most recently. Can you answer what I asked?

"if you disengage the bright cap, then it won't get brighter... so it is essentially a gain control (with no EQ change) like the Input Trim, right? What's the difference between Input Trim and Input Drive with bright cap disengaged?"
 
Hi Wolfenstein98k

Input Trim is there to boost/attenuate the incoming signal to the first gain stage. It does not affect the sound in any ways no matter if the bright switch is engaged or not.

An example: Many amps feature two inputs, a hi & a low input. Most of them just using a T-resistor network to attenuate the incoming signal by -6dB on the low input jack. To simulate this, you can turn input trim down to 0.5 But there is more, instead on the real amp were you could only attenuate, the axefx can also boost your signal. This is a handy feature if you got some really low output pickups on your guitar or just need a stronger input signal for your desired amp tones.

The input drive instead is placed after the first gain stage. Technically spken it's an attenuator for the first gain stage, since turned all the way up clockwise, there is no resistance between input lug and taper lug on the potentiometer, the signal from the first gain stage will pass directly to the next gain stage in your amp. Turn it counter-clockwise, you attenuate your first gain stage, higher resistance between input- and taper lug on the potentiometer, therefore the signal is attenuated.

The bright switch switches in/out a capacitor between input- and taper-lug of your potentiometer. As we learn from explanation above, larger resistance between input- and taper lug means more attenuation. A capacitor is technicially spoken a frequency-dependent resistor, means it bypasses the resistance between input- and taper lug of your potentiometer above a certain frequency, resulting in more treble sound because the higher frequencies were not that much attenuated by your Input drive settings as the rest of the frequencies are. The more you turn up your Input Drive settings, less effective would become your bright switch. The biggest effect from the bright switch you will get when the input drive is set between 4 - 6 (IMO), but this should be done by ear and by taste - not by numbers!

Since input trim stays before the first gain stage as some kind of a input sensitivity control, it does not affect the input drive and bright switch in a unwanted non-linear function.


Happy playing!

Cheers
Paco
 
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Wolfenstein98k said:
"if you disengage the bright cap, then it won't get brighter... so it is essentially a gain control (with no EQ change) like the Input Trim, right? What's the difference between Input Trim and Input Drive with bright cap disengaged?"
Click to expand...

Why not just try it?
Then you'll have your answer.
 
Wolfenstein98k said:
"if you disengage the bright cap, then it won't get brighter... so it is essentially a gain control (with no EQ change) like the Input Trim, right? What's the difference between Input Trim and Input Drive with bright cap disengaged?"
Click to expand...
With the bright cap disengaged, I would expect them to behave similarly. Whether their behavior is exactly identical depends on he individual amp design, and what frequency-dependent impedances so are inthe vicinity of the control. As joegold says, try it with your favorite amp models and see.
 
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FractalAudio said:
Input Trim:
The Input Trim control allows you to adjust the input attenuation without changing the frequency response. If you turn down the Input Drive and the model has a bright cap the amp will get brighter. Now you may like the brighter tone but wish there were more gain. Input Trim allows you to increase the gain without changing the tone. Conversely you may like the darker tone with Input Drive set high but wish there were less gain. In this case you can lower Input Trim.
Click to expand...

whuuuuuuuuuuuuut i thought it was just a boost, never touched it. IT NEVER ENDS :sweatsmile:

i guess it is just a boost. but before the treble bleed circuit. VIOLA
 
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brokenvail said:
Does the amp gain in global menus adjusts input trim?
Click to expand...

I believe so, yes.
I think it's an offset that's either added to or subtracted from every Preset's Amp Block's Input Trim (although the offset amount will not be visible in the Amp Block itself).
This allows you to use guitars with varying pickup output strengths w/o having to re-tweak a Preset for that specific guitar's output level.

I hope someone will correct me if I'm wrong.

Note: Instrument 1-2 In, on the first page of the I/O Menu does not change the gain at all.
Setting this correctly just optimises the noise floor of the Axe.
 
joegold said:
I believe so, yes.
I think it's an offset that's either added to or subtracted from every Preset's Amp Block's Input Trim (although the offset amount will not be visible in the Amp Block itself).
This allows you to use guitars with varying pickup output strengths w/o having to re-tweak a Preset for that specific guitar's output level.

I hope someone will correct me if I'm wrong.

Note: Instrument 1-2 In, on the first page of the I/O Menu does not change the gain at all.
Setting this correctly just optimises the noise floor of the Axe.
Click to expand...
I knew it was an off set I just wanted to confirm if it was off setting input trim
 
brokenvail said:
I knew it was an off set I just wanted to confirm if it was off setting input trim
Click to expand...
Fractal doesn't specify, so it's hard to know for sure. It's handy, though when swapping guitars that have significantly different output levels.
 
brokenvail said:
I knew it was an off set I just wanted to confirm if it was off setting input trim
Click to expand...

I don't know if that is *in fact* the correct description of what it does.
But I don't think it affects the tone like the Input Drive parameter in the Amp Block does.
Input Drive is also connected to the bright cap whereas Input Trim isn't.
 
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