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

Dominique Michel dominique.michel at vtxnet.ch
Sat Mar 5 19:22:52 UTC 2011

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 :

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:

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

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. :) 

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:

My 2c


> >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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