M3 sound enhancement

[Wim Rijnsburger <rijnsburger@ecn.nl>]

Hello All (and especially Al),

The active members of this list know that I am always trying to enhance
the sound of my (poor mans) chopped M3 with 760 Leslie. I have
done several modifications on the pre-amp (posted on this list
under the subject "How to B an M" and was very enthousiast about the
result of recapping the tone generator. In my opinion it is step 1
to be done when you are looking after the real sound. After the
recapping job, I thought that nothing could be done to enhance the
sound further.

And then I read a message from Al (Goff) how they put B3 pre-amps
with M3 TG and manuals to get more portable on the road organs.
Why should they do that, I thought. The electronical differences
between the input stages of my AO-29 pre-amp and the B3's AO-28
pre-amp seem so minimal, and are so easy to correct (I'v changed
R20 to 2M2 and R6 to 5M6 to get the same frequency response as the
input stages of the B3, at least theoretically).

One day I gave it another look.
1. There was a small difference in design of the intermediate
   amplifier (the stage before the expression pedal).
2. The M3 has a push-pull type amplifier designed to drive a loudspeaker,
   where the B3 has a single ended class A type output stage.
3. The M3 output level capacitor controls the overall negative feedback
   of the complete 4-tube power amplifier, whereas in the B3 only
   the feedback of a 1-tube stage after the expression pedal
   is controlled.

Another day (uhhh.. night), I gave it a try.
1. I modified the design of the intermediate amplifier to resemble the
   B3's design:
       R29 (.27M) -> 0 Ohm
       R28 (3.3M) -> 3.3M || 4M7
       C9  (20n)  -> 20n || 12n
   That made a lot difference in sound! Much more brightness in the
   lower keyboard range and keyclick.
2. I removed C20 and rewired the connection of the volume control
   capacitor C23 from the output transformer to the anode of V4b,
   just as in the B3 is.
   The organ spitted even more than it did already. And the setting
   of the control influenced the frequency response much more, just
   as described in earlier messages on this subject from Al and others.
   So now I decided not to sleep until all other differences were
   gone, or until my ears became satisfied.
3. Because there was no overall negative feedback over the power
   amplifier stages anymore, I could now safely remove a number of
   frequency correcting components:
       R40 (100k) -> 0 Ohm
       C22 (220p) -> removed
       C27 (1n)   -> removed
   The result: a really Groove Holmes screaming upper octave!
4. With the optimal setting of the volume control capacitor with
   respect to frequency response, I could not get the output amp
   in overdrive. I need a little overdrive there, because I do
   have a 760 Leslie (solid state).
   So I changed the phase invertor stage of the power amplifier
   to get more gain and overdriving one side of the push-pull
   end stage:
       R41 (100k) -> 100k || 1uF
   Now the amplifier saturates just enough to get a little bit
   growl, no rattles or sharp edges!
5. Now I could not be stopped anymore. I remembered a message on this
   list about differences between the AO-28 and the AO-28-1 amplifier.
   I saw that mine was comparable to the AO-28 and decided to
   change it to resemble the AO-28-1. This modification affects
   the percussion circuit, giving it a more marimba type of sound.
   I like it!
6. Finally I remembered a message about various chorus control.
   I added a 22k trim-potentiometer in series with the vibrato
   delay line and the chorus/normal switch. I experimented
   with various settings and now I can choose between
   3 different chorus levels. The original no-chorus vibrato is
   lost, but I did not like that sound at all.
7. I got to bed....

Maybe this inspires other M3 owners?

Greetings, Wim.

PS. The message about differences between the AO-28-1 and AO-28
    amplifier is a little misleading, because there are other
    differences in the percussion circuitry, that were not
    mentioned there! If you make these changes, you have to do
    them all!!

-- 
Wim Rijnsburger <rijnsburger@ecn.nl>
Netherlands Energy Research Foundation, ECN
P.O. Box 1, 1755 ZG  Petten, Holland
phone: +31 224 564097   fax: +31 224 561407