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

Stefano D'Angelo zanga.mail at gmail.com
Sun Mar 6 09:57:46 UTC 2011

2011/3/5 Dominique Michel <dominique.michel at vtxnet.ch>:
> Le Sat, 5 Mar 2011 18:55:19 +0100,
> Giuseppe Zompatori <siliconjoe at gmail.com> a écrit :
>> 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>:
>> >
>> >
>> > Ciao Giuseppe,
>> >
>> Ciao Stefano,
>> Taking this email to a new thread.
>> >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...
>> Anyway, are you limited to the simulation of a half triode with white
>> box techniques? 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) ;)
>> 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?
> With triodes, the preamp will include at least 2 stages (like the 2
> valves of an ECC81). With pentodes, you get higher gain and can make a
> complete guitar amp with 2 tubes like in the Fender Champ :
> http://www.drtube.com/schematics/fender/champ-5c1-schem.gif
> Also, be aware that each manufacturer make compromise between the sound
> quality and the manufacturing costs. As example, a well-know mark is
> using cheap power transformers, and when at full volume, the sound will
> be very bad because half of the distortion you will ear is due to
> saturation into the power transformer. As a consequence, those
> amplifiers are widely used by jazz musicians but almost never by rock
> musicians. The laters will also often blow the power transformers.
> Another well-know mark is using good but small output transformers, as
> well than better power transformers than the precedent one. The
> consequence is than the sound is very good at full volume, but a rock
> or blues musician will often blow the output transformers.
> Generally speaking, a common source of non linearities, and often
> ignored, is due to a non adequate driver stage. Tubes are by design
> made to work best at high impedance. To get a low output impedance,
> you need a transformer. But that's expensive hardware, especially in
> class A. Preamp and driver stages are class A stage, when the output
> stage is generally a class AB2 push-pull where the grid can become
> positive in respect to the cathode.
> Common tubes for such a push-pull are 6L6. According to the datasheets,
> such tube can take 0.35 watts and the driver output impedance must be
> lower than 500 ohms:
> http://www.mif.pg.gda.pl/homepages/frank/sheets/021/6/6L6.pdf
> It is no one single guitar amp I know on the market with a driver stage
> that can provide such a power to the output stage, and this is a major
> source of non linearities when the output stage is driven at high
> volume.
> It would be simple to design such a driver : one tube like a
> triode mounted EL82 in a class A stage with a high quality driver
> transformer. But it will cost too much money, so no one single
> manufacturer I know is using such a design. It is too bad because such
> this sound just terrific, the sound is clean and fully saturated at the
> same time, and its dynamic is outstanding. :)

Indeed in the video linked in the other thread I was explaining that a
huge part of the sound is due to the output transformer and to the

The prototype plugin I developed for that and linked here has a
parameter called "Output transformer quality" that controls the amount
and frequency of resonance due to the output transformer, which is
modeled as being linear for computability/performance reasons, but
still it shows how much the whole output is influenced by that thing.

The main strength of that plugin, indeed, is showing how feedbacks
from each part of the electro/mechanical/acoustic system have
influence on all other parts. In this sense, I don't think there's
anything more accurate.

In my very little free time I'm working on improving individual
components (especially the triode) and adding missing parts (eq,
loudspeaker enclosure, etc.)

> Be also aware than the valves models used by simulation software like
> spice are good for consumer audio equipments or sonorisation stages, but
> than they don't take in account the saturation that a guitar amp will
> provide. In other words, they are completely out of business in regard
> to correctly simulate a guitar amp at high volume, or even a simple
> preamp stage in saturation. For that, a much better approach would be
> to use models based on the constant current family of curves:
> http://www.agsrhichome.bnl.gov/AP/ap_notes/ap_note_97.pdf

True. I'm now investigating an hybrid physics-based/interpolative
approach for modeling the tube that was described in a journal article
by three Italian professors, in order to accurately model high

Its parameters are extracted from those curves.


> My 2c
> Ciao,
> Dominique
>> >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.
>> >Stammi bene,
>> >
>> >Stefano
>> Anche tu!
>> -Giuseppe
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