2011/3/5 Tim E. Real <[email protected]>: > On March 5, 2011 02:22:52 pm Dominique Michel wrote: >> Le Sat, 5 Mar 2011 18:55:19 +0100, >> >> Giuseppe Zompatori <[email protected]> a écrit : >> > From: Stefano D'Angelo <[email protected]> >> > Date: 2011/2/27 >> > Subject: Re: [LAD] RDF libraries, was Re: [ANN] IR: LV2 Convolution >> > Reverb To: Giuseppe Zompatori <[email protected]> >> > Cc: [email protected] >> > >> > 2011/2/27 Stefano D'Angelo <[email protected]>: >> > > 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. >
From the paper: "We suppose the model is operating at audible fre- quencies, so it is not necessary to consider all the sec- ondary effects, like the Miller capacity." But the miller capacity is important for triode models, see here why: http://www.aikenamps.com/MillerCapacitance.html Aiken is a respected EE/tube amps builder... I was playing with their JCM900 sim many years ago, and never understood why they would model it instead of a JCM800 (much better amp)... Anyway I had a chance to test Serafini's commecral project "TH1" (http://www.overloud.com) and don't find the sims to sound too realistic. Just my 0.02€. Cheers, -Giuseppe > 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 > [email protected] > http://lists.linuxaudio.org/listinfo/linux-audio-dev > _______________________________________________ Linux-audio-dev mailing list [email protected] http://lists.linuxaudio.org/listinfo/linux-audio-dev
