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These are the archives from Mark Longo's original Hammond List, 1994-97

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Leslie 147 vs. 122


I've found it interesting how many discussions have referred to the 122 as
being the ultimate Leslie and the 147 is often passed over.  I actually own
both and either will do a virtually identical job.  The differences are
primarily in the organ interface.  Both have the same speaker components
and motor assemblies and both drive about 40 watts.  The difference is in
the methods used for getting it done.

The 122 uses a true differential signal and DC control to switch the motor
relay.   The audio signal running down the Leslie cable (5-pin) (organ
terminals "G-G") is transmitted differentially meaning that the signal
"difference" is what's presented to the amplifier.  In fact, Leslie 122s go
one step farther and carry this process through in a differential push-pull
configuration to the output tubes. The advantage to this is that any common
mode noise presented about the cable transmission line is cancelled (i.e.
there is no "difference" to amplify) and only the lovely Hammond audio
signal is heard.  This is an issue because the Leslie cable is carrying 110
AC to power the Leslie amp and motors, and the motor switching, being
inductive, is a great source of electrical noise as impulse currents ride
down the Leslie cable to their ultimate power source back at the organ.
The differential mode and the low impedance audio path all work to
eliminate any extraneous noise from being amplified.  The motor switching
relay actually is controlled by superimposing a DC "carrier" signal on the
audio line.  Being DC in nature it, again, has no "difference" to amplify
so it is not part of the audio produced.  Since transformers cannot
transmit DC, the path to the amplifier doesn't even "know" it's there.  The
DC is siphoned off the audio conductors and it controls the current flow
through a separate tube on the amp that carries the relay coil current and
switches the motors.  If you resign yourself to accepting that 110 AC will
be carried through the Leslie cable (as all but a few combo amps do) this
is definitely the way to go in theory and in practice.

The 147 will do the job just as well.  If all good techniques are in place
for filtering noise spikes with capacitors across all the switch contacts
etc. per stock, you should have no trouble.  Granted, some of these
components are old and may need to be replaced.  The reason for the concern
is that the 147 runs audio as single ended as opposed to differnetial, so
you present its amplifier with a "hot" audio signal and a ground.  Since
Leslie cables are not shielded, your ability to pick up noise and amplify
it is much greater since a noise spike will not appear as common mode but
as "real" signal!  One should be sure to keep the impedance of the audio
path low.  A load resistor is typically found on the audio input pins of
the 147 of about 8 ohms, sometimes switchable to 16 if desired.  The relay
switching is actually performed by applying 110ac to the relay coil so this
provides another transient for the audio path to pick up.  However, if all
is in top shape, you will hear no difference.  In fact, a 147 amp will
squeeze out a touch more power because the single ended configuration in
the amp is a little more efficient.  To control the relay in a 147 requires
6 pins officially but, since one of the relay wires is one side of the 110
AC line, it is in fact redundant and 5-pins can do the job with a little
modification.  If you really want to rig things up to be trouble free, you
can replace the stock relay with solid state relays.  These typically
operate on a small switching voltage, say 5 volts, which you can tap off of
your preamp with a small auxiliary supply or scale down from your existing
122 switching supply.  Your favorite EE buddy from college can whip up a
quick design for you from your preamp schematic.  The solid state relays
have the distinct advantages of:
- no moving parts to wear out
- no contacts to bounce or arc or burn out
- no mechanical "click"
- Turn on and turn off at zero crossing.
The latter advantage is the big one with respect to noise.  Since AC power
cycles at 60 Hz from zero to peak voltage, timing of the motor start can be
a significant factor in terms of noise generation.  If the contacts close
on a mechanical relay at a peak time in the AC voltage, the contacts will
arc as the current through the motor jumps the gap and creates a noise
transient.  On a solid state relay, the "contacts" (which are really a
solid state TRIAC) wait for the voltage to reach zero at the close of the
next AC cycle before "contacting".  Zero voltage means zero current which
means no arcing which means no noise.  As you may have surmised, this is
what I've done to both my 122 and 147.

Lastly, and perhaps most importantly to note, if a 122 is desired and a 147
is all that can be found, it is a relatively painless task to convert the
147 into a 122.  All the major components are there and with a little amp
chassis work and a few miscellaneous parts, you can have a 122.  The label
might still say 147 but you'll know.  Just be sure to tell the guy you sell
it to if you do!

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