From: balon@cup.hp.com (Tom Balon)
Subject: Re: Tube Emulator
Date: Tue, 10 Nov 1992 19:40:39 GMT
Distribution: alt.guitar
Organization: Hewlett-Packard, Cupertino, CA

In article <BxGv7A.6ru@cmptrc.lonestar.org> neal@cmptrc.lonestar.org (Neal Howard) writes:
>In article <TILL.92Nov5171340@acid-rain.lucid.com> till@lucid.com writes:
>>   Date: Thu, 5 Nov 1992 17:56:31 GMT
>>   From: tjs@eecs.umich.edu (tim stanley)
>>   Organization: University of Michigan
>>
>>   In the November 2, 1992 issue of Electronic Engineering Times, on page
>>   31, there is an article title "Chip sounds like an old Fender".
>>
>>Thanks for the pointer;  I'll have to check it out.
>>
>>   It is about a guy named Eric Pritchard who claims to have developed a
>>   solid state replacement for the 12AX7 vacuum tube.  Measurement show
>>   an "emulation error" of only .01%.  The device is called the Tube
>>   Emulator and is marketed through a company Pritchard started called
>>   Deja Vu Audio.  The circuit (presumably the actual emulator itself) is
>>   currently realized in a discrete version.
>>
>>[...]
>>
>>   P.S.
>>   This post is FYI only.  In other words, technical flames directed at
>>   me about the credibility of this device > /dev/null.
>>
>>The device is entirely credible.  Hell, I was able to come up with an
>>FET/bipolar circuit that emulated the first 12AX7 stage of my Fender Twin
>>to within 1% (with a voltage subtraction comparison).  A single FET comes
>>darned close if you bias it correctly.
>>
>>What isn't credible is the emulation error spec -- the parameters of
>>individual 12AX7 tubes vary all over the place.  But if he's averaging the
>>curves of a number of tubes and using that for his metric, it's certainly
>>reasonable, though a bit misleading.
>>
>>Accurate emulations of a tube stage, or anything else for that matter, are
>>not difficult for a competent electrical engineer.  
>
>There's one very important parameter that no known solid state device can
>come close to what a tube amplification stage can do: the slewing rate.
>Tube amps easily reach slewing rates of 75 to 100 volts per microsecond
>and solid state devices (yes, even FETS) are hard pressed to reach 25 volts
>per microsecond. The high slewing rates (ability to "accelerate" from lower
>frequencies within a complex waveshape to the higher frequencies -- my layman's
>definition, if anyone else has a better or more descriptive one, please post)
>of tube-type amps is one of the main reasons for their outstanding clarity over
>solid state amps.
>
>-- 
>=============================================================================
>Neal Howard   '91 XLH-1200      DoD #686      CompuTrac, Inc (Richardson, TX)
>     "To do is to be." - Socrates      "To be is to do." - Aristotle
>                       "Doo bee doo bee doo." - Sinatra
>=============================================================================



	Perhaps op-amps from long ago had problems with slew rate, BUT
	new op-amps are much faster. The AD827 I have in my CD player for
	example has a 300V/us slew with a 120 ns Settling time bandwidth
	is 50MHZ. If that ain't fast enough for ya, how about a AD9610 
	high-speed op-amp, it has a 3500V/us slew ! 

	Personally I don't care about trying to build a solid-state device
	that's "Just as good as a 12AX7", I'd prefer to just use a 12AX7.

	Regards,
	 Tom

    +-----------------------------------------------------------------------+
	T.A.S.            		
	Tube Amp Service  	      "On the trailing edge of technology."
	3455 Homestead Rd #14
	Santa Clara, Ca. 95051  (408) 243-7745 PST
    +-----------------------------------------------------------------------+
Newsgroups: alt.guitar