March 1971 Popular Electronics
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
An RF Cafe visitor wrote
to ask that I scan and post this article on the
Simulator, which appeared in a 1965 issue of Popular Electronics. "What
the heck is the Leslie effect?" you are probably asking, as did I. Basically, it
is a mechanism for artificially creating the "wobbulating" effect of a pipe organ
in a large echoing environment. Inventor Donald Leslie worked for the Hammond organ
company and developed an electromechanical contraption that rotated a baffle in
front of speakers to create the effect. Commercial electronic Leslie Effect products
were sold back in the 1960s and 1970s when high fidelity (hifi) stereo equipment
was the "in" thing, like computers were in the 1990s and cellphones are now. There
were a lot of electronics hobbyists that loved to build projects printed in magazines
like Popular Electronics, especially since at the time many things could
be built more cheaply than buying a finished product off the store shelf. Electric
guitar players were big adopters of Leslie effect generators, and were used by lots
of top musicians. You can buy some of the vintage devices - both mechanical and
electronic - on
eBay if you're interested.
All-Electronic Organ Accessory That Sounds Like a Leslie
The true Leslie speaker system is a cumbersome and costly organ
accessory. The almost identical sound effect can be achieved by inserting the device
described here between the keyed tone generators and the power amplifier in your
electronic organ. Through use of a wobbulated bandpass filter, the Leslie effect
is reproduced by the main amplifier and speaker. Adjustments on the simulator permit
the operator to duplicate roughly the acoustic effects of vibrato, tremolo, and
The special effects Leslie speaker system is a popular addition to any organ
- acoustic or electronic. Usually a "Leslie" system refers to a mechanical means
of obtaining a vibrato-like sound effect - a gentle undulation of intensity at a
rate of 8 to 12 Hz. Even in modern electronic organs, the Leslie uses a massive
rotating diffuser to disperse the sound from an extra speaker. It is effective,
but also massive, noisy, and costly. Described here is a system which achieves nearly
the same results with an adjustable all-electronic simulator.
While cost and size are definite advantages, perhaps the best part of the electronic
Leslie Effect Simulator is its versatility. With the controls cranked buck, the
Simulator adds an interesting, subtle effect to conservative music. But if you're
a wild man, you can really twiddle the knobs and wail! Control adjustments can produce
anything from "super" bass or treble boost to sounds listeners describe us "shimmering,
bubbly or out of sight!"
Theory of Circuit Design. While the frequency shift, or vibrato,
effect of a Leslie speaker system (see box) is expensive to generate electronically,
the total effect can be convincingly simulated simply by placing a bandpass filter
between the musical instrument and its amplifier and sweeping back and forth across
the bandpass. This is the principle employed in the Leslie Effect Simulator circuit
shown in Fig. 1.
At the heart of the Simulator is an active bandpass filter composed of R14, R15,
Q2, and C7 through C9 in the feedback loop of the amplifier/buffer combination made
up of Q3 and Q4. Transistor Q1 and its associated components form a low-frequency
phase-shift oscillator, the output frequency of which can be set from between about
4 Hz to 12 Hz through the use of speed control R4. The signal from Q1 is attenuated
by weight control R8 and applied to the gate of Q2 to change the source-to-drain
impedance of the FET and, consequently, the center frequency of the pass band.
Photoelectric system I1/LDR1 is used to bypass the Simulator when the system
is not in use. Closing a footswitch plugged into J3 powers I1 which, in turn, illuminates
LDR1. Once illuminated, LDR1's internal resistance drops and forms a signal bypass
loop around the filter circuit.
Construction. Since only low frequencies are involved in the
operation of the Leslie Effect Simulator, parts layout during assembly of the project
is not critical. Just adhere to the general rules of neatness and good soldering.
In particular, keep signal leads as short as possible.
Leslie Effect Simulator schematic.
Fig. 1. Footswitch plugs into J3; when closed, footswitch powers
I1 which illuminates LDR1. With LDR1 illuminated, input signal at J1 goes directly
Leslie Effect Simulator parts list.
Fig. 2. Actual size printed circuit board etching and drilling
guide is shown above. At right is components placement and orientation guide. Note
orientations of flats on transistors and plus sign on rectifier assembly.
Input and output jacks J1 and J2 should be located close to each
other if separate jacks are used to allow LDR1 to be mounted between them as shown
Neatly dress control and jack wiring to one side of circuit board
and lace together with cable ties or lacing cord. Secure power transformer to chassis
with 4-40 machine hardware; add 1/4" spacers when mounting board in place.
Current limiting resistor R24 and I1 are mounted on terminal
strip fastened to side panel in line with LDR1 when cabinet is assembled. If side
panel is metal, use four-lug terminal strip and do not connect R24 or I1 to mounting
In prototype, top of case slopes downward. Top, front, and rear
panels are painted flat black to contrast with rich tones of walnut side panels.
Controls and jacks are lettered in white, using dry-transfer lettering kit.
Begin assembly by etching and drilling the printed circuit board, carefully following
the actual size etching guide provided in Fig. 2. (If you prefer, you can obtain
a ready-to-use circuit board from the source listed in the Parts List.) Once the
board is prepared, mount the parts in their respective locations, paying particular
attention to the orientations of diodes, transistors, and electrolytic capacitors.
Use a low-wattage soldering iron to solder the component leads to the circuit board's
foil pattern. It is also a good idea to heat sink the leads of the solid-state component
to prevent heat damage.
After all components are mounted on the board, solder in place the primary and
secondary leads of power transformer T1 and pieces of hookup wire sufficiently long
to reach the front panel controls when the project is fully assembled. Then carefully
check the foil side of the board, particularly around the transformer connections,
for solder bridges. If any exist, reheat and remove any excess solder to eliminate
You can use just about any type of case that suits your fancy to house the circuit.
If you wish to duplicate the case shown in the photos, all you need are some 22-gauge
sheet aluminum, lumber, glue, and fasteners. No special tools are needed for forming
and fabricating the metal parts.
The top, front, and back of the case are made from a single sheet of aluminum,
machined on the front and back panels to accommodate the controls, jacks, and entry
for the line cord. It is then bent to shape to form a friction fit over the side
panels. While you're at it, you can also cut to size the bottom plate, using the
same sheet aluminum.
To make the side panels, you will need one walnut and one white pine panel for
each. Cut the walnut pieces 1/4" longer in their length and width dimensions than
the height and depth dimensions of the metal pieces. The pine pieces should be 3/8"
shorter in both dimensions than the length and width of the walnut pieces. Now,
make a "sandwich" of the pine and walnut pieces with white glue and wire brads,
centering the former on the latter. This done, smoothly sand and hand rub paste
wax on the outer face and edges of the walnut panels to bring out a dull sheen.
Then use short wood screws to fasten the bottom plate to both side panels and temporarily
set the assembly aside.
Next, paint the front, top, and back assembly with a color to contrast with the
dark shade of the walnut panels. When the paint has thoroughly dried, use a dry-transfer
lettering kit to label the controls and jacks. Mount the controls and jacks in their
respective holes; then pass the free end of the line cord through its entry hole
and secure it to the rear panel with a strain relief.
Referring back to Fig. 1, connect and solder the free ends of the wires coming
from the circuit board to the lugs of the appropriate control and jack lugs. Tin
the free ends of the line cord and solder them to the hole locations marked AC on
the board. Now, interconnect with lengths of hookup wire the ground leads of J1-J3
and connect and solder the leads of LDR1 directly to the signal lugs of J1 and J2.
Neatly dress the leads along one edge of the circuit board.
Now, mount R24 and I1 on a three-lug terminal strip (no lugs grounded). Position
the assembly near LDR1 so that when the lamp is lit it will illuminate effectively
the LDR. Mount the assembly in place with 4-40 machine hardware. Connect this assembly
via one wire to the positive side of the power supply on the circuit board.
Finally, mount the circuit board with 4-40 hardware and spacers, and power transformer
T1 with 4-40 hardware only. The project is now ready to be tested.
Setup and Use. Plug the line cord of the Leslie Effect Simulator
into a 117-volt ac outlet. Connect an input and amplifier to J1 and J2, respectively,
and a footswitch to J3. Turn on the system by rotating R8 clockwise just past the
click. Close the footswitch to test the bypass circuit; I1 should immediately come
Temporarily cover the sensitive face of LDR1 with a piece of black electrical
tape to keep ambient light from interfering with the adjustments to be made. Advance
Accent control R-20 to about two-thirds of its clockwise rotation and set weight
control R8 fully counterclockwise - but do not click the power off. The maximum
effect of Tone control R11 occurs over about one-quarter of its travel. The extra
travel is useful in some effects when the weight control is fully advanced. Adjust
R12 so that the most sensitive area of the Tone control is at the center of the
Tone control's travel. You can check out your settings by striking a chord and noting
the action of the Tone control as it is rotated.
Trimmer potentiometer R22 should be swept over its entire range to check the
gain of the Simulator. It should then be set so that there is minimum change in
volume level as the Simulator is switched in and out of the system (by operating
the footswitch). While adjusting R22, be sure to remove the tape from over the LDR
to permit switching out the Simulator.
To a certain extent, Accent control R20 changes the overall gain of the Simulator.
It should be adjusted for unity gain at the accent setting you intend to use most
often or for whatever compromise suits your taste. When both internal adjustments
(R12 and R22) have been made, uncover LDR1 and assemble the case.
In use, the best way to get the feel of the controls of the Simulator is simply
to play with them. However, a few simple hints will get your started. First, to
obtain the Leslie Effect, set the Accent control to approximately the center of
its travel and rotate the Weight control a small fraction of a turn clockwise. Set
the Tone control to the center of its travel and adjust the Speed control as desired.
Now, when the instrument plugged into J1 is played, you should get an effect
that is something like a tremolo, except that there will be a touch of sweeping
pass band in the background. If the effect is not pronounced enough to suit you,
advance the Accent control.
For super bass/treble boost, turn up the Weight control as far as it will go
without turning off the Simulator. Advance the Accent control all the way. Now the
Tone control can be rotated clockwise for treble boost and counterclockwise for
bass boost. Somewhere between the two extremes, the amplifier might break into oscillations,
but this can be readily remedied simply by backing off on the Accent control slightly.
Advancing the Weight control past its midpoint and setting the Tone slightly
treble of center can produce an effect quite similar to reverberation if the Accent
control is advanced to the point that just causes oscillation when a note is struck.
If, during the operation of the Simulator, you notice a loud ac hum level, try
reversing the ac line cord plug. This should effectively curb the hum loop.
Beyond the very rough hints outlined above, familiarizing yourself with the Leslie
Effect Simulator will depend on your experimental nature. You will certainly want
to experiment to determine just what the Simulator is capable of doing. Go to it.
The "Leslie Effect"
In principle, the Leslie speaker system is nothing more than one or more loudspeakers
mounted at the end of an arm which is rotated by means of a motor. (Other variations
use a fixed speaker and employ a rotating "paddle" or baffle, but the principle
is the same.) As the loudspeakers swing around in an arc, several things happen
to the sound. First, a Doppler shift in the apparent pitch of the sound is caused
as the motion toward and away from the listener takes place. Next, a variation in
sound level is produced as the speaker alternately faces toward and away from the
listener, Finally, there occurs a great variety of effects which stem from changes
in the acoustics of the system enclosure and the room in which the system is being
Posted March 25, 2019