From benjamin.kuris@hp.com Tue Feb 24 11:43:54 2004
Subject:Looking for ears in Boston for a preamp design

I built myself an EL84 based leslie preamp out of a small monoblock
(Raymer made in Van Ness CA-- $50 on ebay) this past weekend to tide
me over while I have my speakeasy serviced and/or give me a smaller
option the my speakeasy classic.

It actually sounds quite good, I'm looking for some more ears to do
some listening tests. Then I'll probably release the design to the
masses.

Basic specs:
SMALL: 9x4x4!
1/2 12AU7 for gain (original design used 12AX7, w/2nd half for a
phone preamp application)
EL84 as output.
Output tranny is slightly bigger then the usual EL84 design, but
still open frame.
Single volume control (should really be called gain)
Single tone control (not doing anything right now)
~325V plate supply, lots of capacitance (200uF in first stage)
Negative feedback via preamp cathode (like in a fender princeton).
It runs quite hot...

The feedback lowers hiss and adds a bass emphasis which sound good to
me for organ-- eventually I want to add a switch to that for some A/B
comparison but I didn't have a matching part. Overall the design is
very hifi (not too surprising) but lets you overdrive the input of a
tube leslie (make sure it has load resistor enabled). The Bass
response is perfect!

Less is more-- signal passes through just 1 coupling cap and 1 pot.
The rest is all tube + output tranny.

My goal was to capture the sound of my A102 driving into my leslie
from G to GND off the preamp. This preamp does a great job and it
took minimal modification (add a 1/4" jack, wire in a leslie
connector, replaced one ceramic cap w/a orange drop)!

-Ben