[linux-audio-dev] Re: [ann] unmatched - a LADSPA amp tone

[Stuart Allie]

re: tube amp simulations for guitar...

from the music-dsp mailing list...

Just thought I'd mention that the upsampling/nonlinear
processing/decimation process that Line 6 performs was also descibed in Hal
Chamberlin's Musical Applications of Microprocessors (1985 edition), pg.
120:

"One modification technique that does not always work well when done
digitally is nonlinear waveshaping. Since clipping and other waveshape
distortions are likely to generate strong high-frequency harmonics, alias
distortion can become a problem. If necessary, the distortion operation can
be done at a much higher sample rate at which the alias distortion is less
of a problem, then digitally low-pass filtered to less than half of the
system sample rate, and finally resampled."

------------------------
and from http://www.metaltronix.net/basic%20tube%20FAQ.htm

[how do i] have a smoother, less buzzy distortion?

- Use a lowpass filter somewhere inside the amp in the signal path to cut
higher harmonics; perhaps a capacitor to ground from the final preamp tube
grid or plate -or- 
- Use series grid resistors to cut the high frequencies in and after
distortion stages 
- Use a lowpass filter after the amplifier and before the speakers to cut
out some of the higher overtones. 


How do I get...
* Blues distortion? 
Made by overdriving preamp and power tubes a little, enough to just start
compressing the peaks of the waveforms, and not very much high frequency
content, by electronically cutting highs or running the signal into a
speaker cab that acoustically cuts highs. 
Guitar Player magazine ran a construction article on this very topic,
modifying a Fender Bassman to be the "Ultimate Blues Machine". The article
ran in 1995, authored by John McIntyre. 

A recently voiced although intuitively applied idea in distortion is that
tube distortion sounds best when each successive distortion stage is
overdriven by less than about 12db. This has the effect of keeping the tubes
inside the area where the signal is more compression-distorted than clipped.
That is what those resistive divider chains between distortion stages are
for inside those distortion preamp schematics. Mesa's distortion preamps are
another good example. 

Overdriving a tube stage too much gives you harsher clipping, not the
singing, sweet distortion we want. To really get sweet, crunchy distortion,
keep each stage that goes into distortion no more than 6-9db into
distortion. 

* Marshall/metal/Boogie/etc. distortion? 
Made by massively overdriving preamp tubes until the original waveform is
massively compressed and clipped. Usually followed with a moderate amount of
high frequency cut to remove some of the "insect attracting" overtones
generated in the clipping process. There is commonly some output tube
overdrive in this process, too. 

* Good distortion at low(er) volumes? 
overdrive preamp tubes until you get the clipping you want, then feed a
limited amount of this to a power amp stage to get the loudness you want.
This is how master volume controls work. 

-------------------------------------

All fuel for thought I hope.

Cheers,
Stuart