> Le Sat, 5 Mar 2011 18:55:19 +0100,
> Giuseppe Zompatori a écrit :
>
>> From: Stefano D'Angelo
>> Date: 2011/2/27
>> Subject: Re: [LAD] RDF libraries, was Re: [ANN] IR: LV2 Convolution
>> Reverb To: Giuseppe Zompatori
>> Cc:
linux-audio-dev@lists.linuxaudio.org
>>
>>
>> 2011/2/27 Stefano D'Angelo :
>> >
>> >
>> > 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. :)