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