John Larkin wrote:
On Sun, 24 Jan 2010 01:35:38 -0800 (PST), Morris Slutsky
wrote:
There's been a lot of discussion on AGA lately about the Marshall 2204
"Cascaded" preamp circuit and whether or not it sucks and how much. A
lot of people really do like the high gain preamp. But what sucks
about the cascaded Marshall circuit is that there's no way to switch
the high and low gain besides physically moving a cable from one input
jack to the other. That circuit was a long time ago, though, these
days everyone likes channel switching amps which use relays or other
switching logic to push the signal through different tube stages and
recombine them at the power amp. Which is complicated and no fun to
homebrew.
Anyway I came up with this circuit, which allows easy footswitching
with no relays or anything between a 'clean' input stage and a 'high
gain' input stage, with appropriate frequency response voicing. It's
built and working, as part of a homemade amplifier, and I really do
like the sound so far.
I don't know if it's original or not, in the context of a guitar amp,
but I think it's pretty neat.
http://img718.imageshack.us/img718/8...edfeedback.png
Reminds me of the "GE Circuit" which was used a lot as a
mic/phono/tape head front-end in the early transistor days:
ftp://jjlarkin.lmi.net/GEcircuit.jpg
John
I have seen similar circuits somewhere on the 'net where I think there
is positive feedback as well as negative feedback used. I guess the
positive feedback was used to kick up the gain of the crappy early
transistors. Here's a two transistor circuit from 1964 that uses "DC
positive feedback and AC-DC negative feedback":
http://www.freepatentsonline.com/3267386.html