Path: utzoo!attcan!uunet!nancy!wsrcc!wolfgang
From: wolfgang@wsrcc.uucp (Wolfgang S. Rupprecht)
Newsgroups: sci.electronics
Subject: Re: TUBE DESIGN
Message-ID: <1990Oct21.171736.9918@wsrcc.uucp>
Date: 21 Oct 90 17:17:36 GMT
References: <13b593cad136270a1244@canremote.uucp> <57@sierra.STANFORD.EDU> <271c8c45-4b1.2sci.electronics-1@vpnet.chi.il.us> <1990Oct20.220252.1269@zoo.toronto.edu>
Organization: Wolfgang S Rupprecht Computer Consulting, Washington DC.
Lines: 48

>In article <271c8c45-4b1.2sci.electronics-1@vpnet.chi.il.us> cgordon@vpnet.chi.il.us (Crash Gordon) writes:
>>Hardcore audiophiles claim that a tube amp sounds better than solid state.

In sci.electronics you write:
>Yeah, it's so hard for a transistor circuit to reproduce all those warm
>fuzzy nonlinearities.

;-)

Actually the truth is probably that it is much easier to design a
*bad* transistor amp than a bad tube amp.

The real problem is that a transistor stage with high gain and no
local loop feedback (ie. no emitter resistor) is *very* nonlinear.
Much more so than a tube stage constructed similarly.

Many of cheap transistor amps run each stage at close to open loop
gain of the transistor.  They then fix the strong nonlinearities with
a feedback loop around the whole amp.  This is what I call the Phase
Linear approach - shove a 741 in there to get .00000000000000001 %
distortion.  It works quite well for carefully selected test
conditions, that is ones that leave the every stage biased in a linear
enough region of its curve (eg. 1khz sine wave).  Unfortunately the
first stage, the one that does the error difference subtraction, is
very easy to overload if we task it with the chore of multiplying any
error by 10**5.  How does one overload the input stage?  Simplest case
inputting a square wave.  For the time that the output slews from low
to high the amplifier is *not* modelable as a linear system.  Any
small change in input will not result in *any* change in the output.
This is the "transistor" sound.  The sound of a bone-headed designer
that just learned which end of a transistor went to which rail. ;-)

The solution?  Do a good job on feedback on each stage, and keep each
one of the stages linear by using lots of local feedback.  Don't try
to patch it all up in the end.

Tube amps all have very limited gain in all the stages, and little or
no global feedback.  This is all becaues tubes have very limited gains
at best.  Notice how they force you to do a good job.

You still need to add the "tube distortion curves" is you want the
transistor amp to sound like real tube amp while clipping. You can
probably use a large bag of diodes and resistor ladder to model this. ;-)

-wolfgang
-- 
Wolfgang Rupprecht    uunet!{nancy,usaos,media!ka3ovk}!wsrcc!wolfgang
Snail Mail Address:   Box 6524, Alexandria, VA 22306-0524