[LAD] Permafrost guitar amp, was Re: RDF libraries, was Re: [ANN] IR: LV2 Convolution Reverb

Stefano D'Angelo zanga.mail at gmail.com
Sat Mar 5 18:18:16 UTC 2011

Ciao Giuseppe,

2011/3/5 Giuseppe Zompatori <siliconjoe at gmail.com>:
> From: Stefano D'Angelo <zanga.mail at gmail.com>
> Date: 2011/2/27
> Subject: Re: [LAD] RDF libraries, was Re: [ANN] IR: LV2 Convolution Reverb
> To: Giuseppe Zompatori <siliconjoe at gmail.com>
> Cc: linux-audio-dev at lists.linuxaudio.org
> 2011/2/27 Stefano D'Angelo <zanga.mail at gmail.com>:
>>Well... they seem to have a lot of stuff there. :-)
>>However, I wonder how they do it... I think they are probably using
>>some black box modeling, since multiple nonlinearities+feedback in a
>>single system is very hard to model.
> They are very silent on this sadly, don't know what they are doing.


>>The kind of stuff I'm trying to do is accurately model a class A amp
>>with a single triode using white box techniques... to give you an idea
>>of what it sounds like see this:
>>http://www.youtube.com/watch?v=cdNtmaIdLdo - it is part of my MSc
>>thesis presentation (100.000 lire guitar, dated and slow laptop, cheap
>>speaker and cheap camera... only the sound card is good).
>>I guess you speak Italian (at least your name suggests that), so enjoy
>>my weird southern accent. :-P
> Very interesting, I tried compiling your thesis with permafrost to try
> this out (obtaining the source from the pdf has been hell BTW) but it
> bails with an "m_pi" undeclared input/output function...

Strange: m_pi should be declared as a constant in
/usr/share/permafrost/lib/m.pmf or similar location... Oops, perhaps I
missed a "import m;" at the beginning of amp.pmf. Could you try that?

BTW, I should have put the sources online for those who wanted to try
it... my bad.

> Anyway, are you limited to the simulation of a half triode with white
> box techniques?

It does a full simulation. What is not modeled is the grid current,
hence no blocking distortion.

> I think you should model at least both halves of a
> triode if you're after accuracy, a single triode amplifier won't even
> work in real life (I build tube amps, I know) ;)

In the sense that it sounds bad?

> Also class A amplifiers aren't very popular amongst guitar players
> (mainly because of their clipping behavior). You also want a
> multi-stage preamp with different filtering/biasing points between
> stages.
> You might think I am crazy but that's what you'll discover yourself by
> observing schematics to popular guitar amps.
> Here's a simple (early Fender-like) amp topology:
>                         Tube n. 1
>     -------------------------------------------------------
>                    Tube n. 2                           Tube n. 3 and
> 4
>     |                                                      |
>                        |
>  |
> 1st triode -> Tone stack -> post tone stack recovery triode -> P.I.
> (Phase inverter) triodes -> (at least 2) Pentodes -> O.T. (Output
> Transformer) -> Speakers
>                                    ^
>                                                              ^
>                                    |
>                                                               |
>           Presence
> pot<--------------------------------negative-feedback---------------------------------------------
> This is the easiest PP (Push Pull) class A/B amp I could come up with
> (sounds pretty darn good in real life). It has got a tone stack, 4
> tubes (2 triodes and two pentodes) and an OT/speakers, do you think
> this is feasible computational-wise with permafrost?

Yes, I know all of that. The problem is that each tube adds a
significant and difficult to handle nonlinearity, and it gets pretty
difficult to handle. With the current Permafrost you can model any
topology, but loops and conditions have to be implemented in C...
hence theoretically, yes you can, in practice it's really too hard and
the performance would be very poor.

I'm maybe going to work on this kind of stuff in the near future, but
I don't know yet what I will be doing in a couple of months. :-)

>>Well, they say guitarix has improved, yet the last time I was all but
>>satisfied with it. You may want to take a look at invada plugins, if
>>you haven't already.
> Invada has a simple generic tube drive function AFAIK, I still prefer
> the CAPS* amp over it as it's at least based on a real amp.

Ah, never looked into how those plugins work.



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