What do the knobs do?

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What do the knobs do?

Postby fho » Fri Jul 28, 2017 2:20 pm

Ok ... tongue in cheek title ... but:

Is there a "wiring diagram" of the Guitarix Preamp? From just fiddling with the knobs this is my current understanding of their functions regarding the underlying tube model:

  • Pre Gain: Volume knob to adjust the input level before any processing is done.
  • Master Gain: Volume knob to adjust output level of preamp block. But is it before or after the second tube when you have a pre- and master tube selected?
  • Drive: Also adjusts the level of the signal. Used to "drive" the tube harder ... probably the first (ie preamp) tube? How does this differ from Pre Gain?
  • Clean/Dist: Not sure about this one. Is there a second, clean "channel" that this knob blends to and from? Judging from the fact that the signal is still distorted when completely clean it doesn't seem like it.
  • Bass Boost: Boost bass. At which frequency? Before or after the tube? (These are not available in the LV2 version so I guess they effect the signal after the preamp)
  • Presence: Boost upper mid range. At which frequency? Before or after tube?
  • Reverb: Don't really care about this one.

Those are the knobs in the preamp module. Master Volume is there too but that one adjusts the output after everything. The other modules knobs are intuitive. I only wonder if the Cabinets Level is another standard volume knob or does it add some colorization?

Just to be clear: I am asking for the technical functions and "wiring" of everything. I can fiddle with the knobs and get great tones, but I kind of want to know what everything is doing so it's less guess work.

Apart from that... great work on everything! Guitarix has successfully replaced my practice amp at home for about half a year now ... if only I could get it into a reasonable package for live use ;)
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Re: What do the knobs do?

Postby brummer10 » Fri Jul 28, 2017 5:50 pm

Hi fho

welcome to the guitarix forum.
First, it's good to hear you like what you get out of guitarix. :D

To your questions,
fho wrote:Pre Gain: Volume knob to adjust the input level before any processing is done.


Technically, pre-gain adjust the volume after the first half and after the second half tube. Before the "master tube", but not before the signal pass the amp. So, it is meant to drive the master tube harder.
fho wrote:Master Gain: Volume knob to adjust output level of preamp block. But is it before or after the second tube when you have a pre- and master tube selected?

It's after the second tube, so after the amp stage, to adjust the output level of the amp simulation.
fho wrote:Drive: Also adjusts the level of the signal. Used to "drive" the tube harder ... probably the first (ie preamp) tube? How does this differ from Pre Gain?

Drive, is a cubic non-linear distortion unit before the amp stage, it could be used to drive the first two half tube's harder, with already added harmonic distortion. And of course, it will add volume to the signal.
fho wrote:Clean/Dist: Not sure about this one. Is there a second, clean "channel" that this knob blends to and from? Judging from the fact that the signal is still distorted when completely clean it doesn't seem like it.

This one blend between the pre-distortion unit and the clean input.
fho wrote:Bass Boost: Boost bass. At which frequency? Before or after the tube? (These are not available in the LV2 version so I guess they effect the signal after the preamp)

Bass Boost is wired behind the amp and boost the signal around 120Hz.
fho wrote:Presence: Boost upper mid range. At which frequency? Before or after tube?

This is as well behind the amp stage, it's convolution based, hard to say which frequency range is boosted, it act a little bit like a dynamic EQ.
fho wrote:Reverb: Don't really care about this one.

Lol, understandable.

fho wrote:Those are the knobs in the preamp module. Master Volume is there too but that one adjusts the output after everything. The other modules knobs are intuitive. I only wonder if the Cabinets Level is another standard volume knob or does it add some colorization?


Cabinet is a convolution effect, Level controls the "volume" of the convolution kernel. So, it add gain and colourisation.

regards
hermann
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Re: What do the knobs do?

Postby fho » Sat Jul 29, 2017 8:18 am

Ah great! So I got most of it wrong then :D

So Drive is more like a distortion pedal in that regard.

Pre-Gain I got completely wrong, just to make sure I understand it correctly: So normally the first half of the tube is not overdriven (when not using Drive or a boosting pedal, in real hardware it would just be there to impedance match/buffer the signal). But increasing the pre-gain drives the second half of the tubes (both of them) harder.

I read up on Presence circuitry again. Apparently it's normally implemented as a filter in the feedback loop (presumably of the power stage). When I chose a model with feedback in it's name, does this interact in any way? I would assume that for simplicity reasons it's a dedicated processing block.
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Re: What do the knobs do?

Postby brummer10 » Sat Jul 29, 2017 9:24 am

fho wrote:So Drive is more like a distortion pedal in that regard.


Yes.

fho wrote:Pre-Gain I got completely wrong, just to make sure I understand it correctly: So normally the first half of the tube is not overdriven (when not using Drive or a boosting pedal, in real hardware it would just be there to impedance match/buffer the signal). But increasing the pre-gain drives the second half of the tubes (both of them) harder.


The first half tube is indeed the impedance match/buffer, then Pre-Gain controls the volume before the second half tube, to drive this one ( the second half) a bit harder so that it start to distort/overload. As well it control the volume after the second half tube, before the master tube, to drive this one even harder.
fho wrote:I read up on Presence circuitry again. Apparently it's normally implemented as a filter in the feedback loop (presumably of the power stage). When I chose a model with feedback in it's name, does this interact in any way? I would assume that for simplicity reasons it's a dedicated processing block.


The feedback models been realized a bit different then usual, here the feedback loop is integrated into the first 2. half tubes, were the second half feedback into the first half, and the master tube feedback into the second half.
All amp models been separate units stacked together in a multi-plug. So it's relative easy to define amp-models with different staging, without interference with each other or any following or previous unit(plug).

The Presence implemented as a convolution filter, helps here as well.

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hermann
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Re: What do the knobs do?

Postby fho » Mon Jul 31, 2017 9:07 am

The Presence implemented as a convolution filter, helps here as well.


How does this work then? I thought convolution was not able to emulate non linear effects (ie distortion) and from what I gathered *presence* is distortion on a frequency range (caused by lack of feedback to the tube).
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Re: What do the knobs do?

Postby brummer10 » Wed Aug 02, 2017 3:14 am

fho wrote:How does this work then? I thought convolution was not able to emulate non linear effects (ie distortion) and from what I gathered *presence* is distortion on a frequency range (caused by lack of feedback to the tube).


You are right, in that sense it isn't really a Presence. It just happens that the "effect" of it comes mostly close to a presence control.
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Re: What do the knobs do?

Postby PelzR » Sat Aug 12, 2017 7:26 pm

fho wrote:Reverb: Don't really care about this one.

But I'd like to read something about it. When playing with guitarix I use the room reverb quite often. What exactly does the function do (what are the "settings")? When raising it (like 10 o clock) it still sounds good without messing around with the frequencies (like phase shifting).

Is the LV2 Plugin "Room reverb" the same thing or something different?
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Re: What do the knobs do?

Postby fho » Sat Dec 02, 2017 11:49 pm

Also continuing on this thread:

What does the Feedback plugin do? I looked at the source ... just to find out that I have no idea how faust works :-)

From fiddling around it seems it really takes the signal from the plugins position and feeds it back into the beginning of the chain?

Does it make sense then that it sounds a lot "brighter" when I increase the Feedback knob?
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Re: What do the knobs do?

Postby brummer10 » Sun Dec 03, 2017 4:10 am

fho wrote:From fiddling around it seems it really takes the signal from the plugins position and feeds it back into the beginning of the chain?


No, not really.
The feedback plugin takes the signal from the end of the plug, multiply it with a short delay of itself, invert it and feed it back to the begin of the plug, not the chain.
fho wrote:Does it make sense then that it sounds a lot "brighter" when I increase the Feedback knob?

This leads to a add operation when the knob is on negative values, and to subtraction on positive values. So positive or negative feedback. Indeed, it influence the brightness of the sound.

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