7 Nov
2002
7 Nov
'02
11:33 a.m.
This is from the music-dsp list, it seems relevant to what people were
discussing with SPICE emualtion of tube circuits.
- Steve
----- Forwarded message from "Sergio R. Caprile" <yogurthu(a)arnet.com.ar>
-----
Date: Thu, 07 Nov 2002 11:05:32 -0300 (ART)
From: "Sergio R. Caprile" <yogurthu(a)arnet.com.ar>
Subject: RE: [music-dsp] Transformer emulation revisited
To: music-dsp(a)shoko.calarts.edu
X-Mailer: XFMail 1.5.2 on Linux
On 06-Nov-2002 music-dsp-digest wrote:
----- End forwarded message -----
I've been too lazy to sit down and calculate
the typical Preamp hi-freq
boosting from schematic component values, need to do that sometime.
I did some Spice simulations some time ago, as far as I remember it was more a
sort of low-freq roll-off: single pole hi-pass between stages and poor resistor
decoupling.
Even if the preamp has hi-freq boost that
emphasizes higher-frequency
distortion, a downstream saturating transformer might "sound good" by
selectively adding extra harmonics to the low notes? From one view, the
transformer might be "undoing" the preamp's hi-freq emphasized distortion.
But the two might work synergystically to make a more complex sound than a
simple broadband fuzz?
Since there is non-linear processing involved, the transformer is unable to
undo the preamp hi-freq distortion, but it adds a different color. The main
reason to hi-pass before distorting stages was to avoid that cutting rumbling
sound or excessive bass, well, that was before grunge and Metallica and...
There was no tone control setting that would
completely straighten out the
lines in the tri wave, even with the amp running clean. There were
frequency-dependent effects in distorted tones which made the waves even
more difficult to interpret.
The Fender tone network (later adopted and modified by Marshall and almost
everyone, including Roland) is designed to color the signal, there is no
"clean"
position as in a regular Baxandall network. There is an intentional notch at
250/500 Hz that can be softened by the mid control and two bumps at 100Hz and
5KHz.
I am not much into tubes, but most of the preamp stages where intentionally
biased so the tubes could easily be overdriven. Some Marshall have "feature"
capacitors running across different stages, I mean, the schems say: for model A
this capacitor, model B no capacitor, model C, change to this value. Pretty much
of the sound seems to have come out of sorcery ;^), or trial and error btw.
James:
You said you simulated a simplified tone control
Did you simulate the Fender/Marshall network ? I tried to get the transfer
function but it gets a bit complicated and I always give up before bilinear
transform, and don't want to split the circuit as that might remove all the
interactions along the controls.
Regards
--
--------------------------------------------------------------
Sergio R. Caprile, Electronics Engineer, Bs.As., Argentina
http://www.geocities.com/scaprile
--------------------------------------------------------------
dupswapdrop -- the music-dsp mailing list and website: subscription info,
FAQ, source code archive, list archive, book reviews, dsp links
http://shoko.calarts.edu/musicdsp/