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

On March 5, 2011 02:22:52 pm Dominique Michel wrote:
> Le Sat, 5 Mar 2011 18:55:19 +0100,
>
> Giuseppe Zompatori <siliconjoe@gmail.com> a écrit :
> > From: Stefano D'Angelo <zanga.mail@gmail.com>
> > Date: 2011/2/27
> > Subject: Re: [LAD] RDF libraries, was Re: [ANN] IR: LV2 Convolution
> > Reverb To: Giuseppe Zompatori <siliconjoe@gmail.com>
> > Cc: linux-audio-dev@lists.linuxaudio.org
> >
> > 2011/2/27 Stefano D'Angelo <zanga.mail@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. :)
>
> 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
>
> My 2c
>
> Ciao,
> Dominique

There's also some info at simulanalog.org, on the articles page, such as
 "A complete model of a tube amplifier stage"
http://www.simulanalog.org/tubestage.pdf

Some of the articles talk about why many tube sims are not good,
 why they fail to take into account IM distortion etc.

I really like their very good plugins, especially the JCM900.
If someone could make a LADSPA or LV2 version of these, I would be very happy.

Tim.

>
> > >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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Received on Sun Mar 6 04:15:02 2011