Modeling an Guitar Amp

by **gutoalvim** » Fri Dec 27, 2013 1:07 am

Hello my name is Guto and im completely new to all of this. Well, i started to research what was the best plataform to create a guitar amplifier simulation and i ended here. I am interested in simulating and already existing amp, these being the Lead Channel of the Mesa Boogie Mark IV or Mark IIC+ (I already have the schematics for it).
Is this possible in FlowStone? If so, what are the steps in creating such thing? Does anyone have any advice on doing it?

Thanks for the help!

Cheers

by **acg2010** » Fri Dec 27, 2013 4:25 pm

Guto, I am no expert and not even a guitar player myself but if you search the SynthMaker forums, there is a lot of good info on distortion and guitar amp sim info. My guitar amp sim project is here if you want to take a look at it:

viewtopic.php?f=4&t=1435#p5781

viewtopic.php?f=3&t=1654#p7484

It has been difficult to get direct help from people who actually create amp sims since they typically don't want to give up any programming or approach secrets. I spent a lot of time reading every thing I could about real guitar amps and cabinets on the internet and other sources to try and understand how they function to be able to try and model an amp. My project is just standard modules and some user created modules found on the forums. I also had a lot of help from people such as Trogluddite, Billv, and others here on the forums.

A good place to start on a guitar amp is to look at ways of creating distortion (waveshaper, gain, tangent functions, etc.). Then look at adding harmonics (odd, even). I found I needed a cab sim to tone down the higher frequencies generated by the amp.

Hope this helps some.

-acg2010 aka Ph D(J) Music

by **KG_is_back** » Fri Dec 27, 2013 7:16 pm

It certainly is doable in flowstone. When you have the schematic you're quarter way there already.
Some parts of the schematic are filters. Use Hilbert transform and Z-transform to create digital versions of those filters (if you can't do it find someone who can).
With the amplifier/gain/distortion stages it is somewhat more tricky. In real world the tubes and transistors distort the wave, but it is not a static wave shaping - it is program-dependent (the wave transfer function is dependent on many factors that can change quite fast: power load, overall power of the signal, interactions with other parts of the circuit). That is very hard to capture.

by **acg2010** » Sun Dec 29, 2013 4:22 pm

by **acg2010** » Sun Dec 29, 2013 4:56 pm

Here is some outside links regarding guitar amps and cabs:

A guitar preamp using biquads and a waveshaper
http://www.redfelineninja.org.uk/daniel/?p=184

Cabinet simulation using biquad filters
http://www.redfelineninja.org.uk/daniel/?p=171

Cabinet simulators
http://www.hexeguitar.com/diy-cabsims_e

graphic EQ as cabinet simulator?
http://www.diystompboxes.com/smfforum/i ... ic=48850.0

Of course, this is just the tip of the iceberg regarding info on the internet. But these are some of the ones I used for my attempt at a guitar cab sim.

As KG_is_back said, For a first approach, you can try and simulate the actual circuits of the amp schematic and there are already plenty of C++ code already out there for doing that (tube sim, etc.). A second approach to creating a guitar amp sim is to look at each main circuit or section of a amp (pre-amp, tone stack, gain, etc.) and try and simulate that portion with something that approximates the function with out trying to actually reproduce the exact circuit. A third approach is to actually measure the phase and frequency response of each circuit and try and simulate each with a combination of the two first approaches. My attempt at a guitar amp was approached more from following the second way listed.

For my cabinet simulation, I followed the example as discussed in the second link listed above and used the Biquad Designer software (listed below) to obtain the coefficients for the biquads to simulate the cab response:

http://www.arachnoid.com/BiQuadDesigner/index.html

Ideally if you could simulate the frequency response curve of the guitar cab, you could store the coefficients and biquad configuration (type of filters, i.e. high and low self, notch, bandpass, etc.) and be able to simulate/approximate a number of guitar cabs.

A lot of people also use IR (impulse response) to simulate cabinets and even to simulate different playing environments (large rooms, cathedral, etc.). This approach is a little more complicated because you must be a to perform convolution of the incoming guitar signal and the IR.

by **acg2010** » Thu Jan 02, 2014 5:20 am

More thoughts on approaches to modeling guitar amps:

Start by reading the Sound on Sound article " Modelling Plugins:

http://www.soundonsound.com/sos/aug10/a ... lugins.htm

The author Mel Lambert does a good job of discussing approaches (transfer function, virtual circuits, etc.).

Then read David Yeh's publications on modeling synth and guitar amp and tube circuits:

https://ccrma.stanford.edu/~dtyeh/papers/pubs.html

The papers cover digital audio effects and amp modeling.

Then you could look at using the circuit simulation software LTSpice to model and test amp circuits:
http://www.linear.com/designtools/software/

Then look at Duncan's Amp Modeling page here:
http://www.duncanamps.com/technical/ltspice.html

Then use Will Pirkle's vst C++ software, RackAFX to model and test ideas for the guitar amp:
http://www.willpirkle.com/

Additional C++ code for audio effects, synths, oscillators, tubes and amps can be found at:

http://www.musicdsp.org/

https://ccrma.stanford.edu/~jos/

Ronald Passion explains how to model a amp from a functional standpoint on the SynthMaker forum:

http://synthmaker.co.uk/forum/viewtopic.php?f=12&t=8574

the Simulanalog website has some good articles on amp modeling and tube modeling:

http://www.simulanalog.org/
http://www.simulanalog.org/tubestage.pdf

Additional internet references:

http://www.guitarampmodeling.com
http://quitte.de/dsp/caps.html
http://www.bteaudio.com/articles/TSS/TSS.html
http://www.guitarampmodeling.com/viewto ... =51&t=4543
http://www.ee.columbia.edu/~dpwe/pubs/G ... linear.pdf
http://www.music.mcgill.ca/~hockman/other/DAFX_book.pdf

by **tester** » Thu Jan 02, 2014 11:56 am

Nice resources and job done, thank you.

by **acg2010** » Fri Jan 03, 2014 5:19 am

Thanks tester.

If you really want to model a amp using the schematic then look at these references:

DSP Implementation of a 1961 Fender Champ Amplifier:

http://cegt201.bradley.edu/projects/pro ... Report.pdf

A 4-stage amp model is described in this article:

http://dafx09.como.polimi.it/proceeding ... per_29.pdf

Real-Time Digital Modeling of Analog Circuitry For Audio Applications:

http://www.wpi.edu/Pubs/E-project/Avail ... on_MQP.pdf

by **trogluddite** » Fri Jan 03, 2014 3:53 pm

Wow - lots of bedtime reading! Thanks for taking the time to collect those for us, agc.

by **acg2010** » Sun Jun 15, 2014 4:08 pm

FYI - Lots of literature references, thoughts, and ideas pertaining to modeling guitar amps and tubes compiled by mastermindjacquard at the guitarampmodeling forums at:

http://www.guitarampmodeling.com/viewto ... b6a#p95937

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