Wednesday, May 10, 2017

LESLIE 147, THE MAGIC ROTATING SPEAKER SYSTEM

LESLIE 147 TUBE POWERED ROTATING SPEAKER SYSTEM
by Mark King for proworkshop.com
THE REAR OF STOCK LESLIE 147, UPPER AND LOWER COVERS REMOVED
PRELUDE

I bought a Leslie 145 brand new in 1972 (identical to 147 except a shorter wood-cabinet). It contained the 40 watt tube amp, came with the weird thick brown cable which is 30-feet long (the only choice). It also came with a good looking chrome preamp box, the whole thing was around $650; that was a lot of money for me. More than my car! But it had an amazing sound and I carried that thing around to many band jobs over the next 25 years. One bitterly cold winter night it almost killed me when I slipped on some ice while unloading the beast after a band job. I retired it from live performance use after that gig, it was just too big and heavy to haul around.
I foolishly sold that 145 to a friend in 1998. He still has it and it still works. I wish I had kept it but since I could not get that one back, I went looking for the next best thing, a real honest to goodness Leslie 147, the ultimate combo organ amplifier.

The Leslie speaker mechanisms of yesteryear are incredible in their simplicity and magical in their tonal delivery. I have personally pissed away hundreds of dollars on various Leslie simulators, trying to capture that elusive midrange bandpass, the movement, the magic that only the real thing delivers.
LESLIE 147 WITH REAR COVERS REMOVED
WHAT IS A ROTATING SPEAKER SYSTEM?

Do the speakers actually rotate? No, instead the sound output from the internal drivers is rotated.

TWO-WAY SPEAKER SYSTEM

The Leslie 147 (like its cousin the model 122) is a two-way speaker system featuring a horn loaded compression driver and a 15" woofer. These components are driven by a 40-watt tube amplifier.

The horn is driven by a custom made high frequency compression driver. The 15" woofer and it's accompanying enclosure accounts for the majority of space the entire speaker-cabinet takes up. The woofer enclosure includes special internal porting to enhance bass reproduction. The crossover frequency is 800 Hz and the overall impedance of the entire speaker system is 16 ohms.

At the top of the Leslie 147 the plastic physical horn shaped device is located in a separate compartment; it is the plastic horn which rotates not the driver and the electrical connections. The horn looks like two horns facing opposite directions from each other but only one of the horns actually emits sound, the other is for weight to balance the assembly and keep it rotating smoothly.
LESLIE 147 HORN, DRIVER BELOW, ROTOR MOTORS LEFT
Below the entry point to the rotating horn mechanism is a high frequency compression driver firing straight up into the throat of the rotating horn. The horn driver assembly is firmly mounted to a horizontal wood plate that separates the horn compartment from the low frequency driver enclosure.
BELT DRIVE TO LOWER ROTATING BAFFLE
The 15" bass speaker is physically located in a compartment in the middle of the Leslie speaker and faces straight down. The speaker is separated from the top horn area and the bottom amplifier compartment by 3/4" baffle boards. The 15" speaker sends it's sound down through a hole in the baffle board into a rotating drum-like assembly which spins and directs the sound. The lower rotating drum is covered in fabric to make it more aerodynamic and reduce drag, this also allows it to achieve its highest speed more quickly.

The plastic horn at the top and the fabric-covered drum at the bottom of the Leslie do all the rotating, the physical drivers with their electrical connections do not rotate.

TWO MOTORS = TWO SPEEDS + ALL POINTS IN BETWEEN

A Leslie 147 spins the rotors at two speeds referred to as high and low. How the two speeds are achieved is an amazing engineering design. The high frequency horn is driven by a stacked pair of motors, one motor provides the high speed when it receives AC power and the other motor provides the low speed when it receives power, both motors are never powered at the same time. An ingenious centrifugal clutch mechanism disengages the slow speed motor when the high speed motor is activated.
STACKED PAIR OF HORN ROTOR MOTORS
The rotating drum in front of the 15" woofer output is spun by a stacked pair of motors just like the ones that spin the high frequency horn around.
CLOSE VIEW OF STACKED PAIR OF LESLIE 147 LOWER ROTOR MOTORS
A brown power and control cable feeds into the amplifier chassis of the Leslie 147. This cable is typically terminated at the organ end with a preamplifier box which provides 1/4" instrument inputs, input volume controls, high-low speed selection and AC power input. The 30-foot long Leslie connecting cable attaches to the amplifier and preamplifier by a six-pin octal plug assembly on either end. AC power, high-low speed switching and audio signal all travel through the Leslie connecting cable. The audio line is unbalanced, 60-cycle AC power induces noise into the audio line of the Leslie control cable, this accounts for a lot of the low frequency hum a stock Leslie 147 amplifier produces.
LESLIE 147 POWER AMPLIFIER, 6-PIN OCTAL INPUT
An electrical relay inside the power amplifier chassis responds to signals from the brown remote input cable. This relay turns power on to either the high speed or low speed rotor motors.

40-WATTS OF TUBE POWER

The amplifier on a Leslie 147 is vacuum tube driven with an overall power output rating of 40 watts RMS. A pair of 6550 power tubes are transformer coupled into the speaker system. It is a fairly simple amplifier design capable of being powered "on" for extended periods of time. There are only two adjustments on the amplifier, one is a switch which connects a dummy load across the inputs for special applications and the other is a continuously adjustable master volume control.
LESLIE POWER AMP, NOTE TWO CONTROL KNOBS AND WHITE/BROWN ROTOR MOTOR AC POWER CONNECTIONS
WHAT MAKES THE 147 SPECIAL

The big walnut Leslie speaker-cabinets are most often associated with Hammond organs since they were so often paired together. The Hammond B3 organ featured a balanced audio output so it required a special Leslie model to direct connect the two pieces together. The Leslie model 122 was designed to mate the balanced output from the B3 organ to the balanced input on the amplifier chassis.

In the 1960's lighter weight combo organs from a variety of manufacturers became very popular and their owners wanted the famous Leslie sound. The Model 147 was the answer for this new and growing market segment. Various combo preamplifier designs which attach to the Leslie 147 control cable provide the necessary 1/4" unbalanced inputs to plug in a Vox or some other portable organ. A footswitch on the combo preamplifier provides the artist with a selection of high or low speeds.

The high frequency driver in a 147 has a purposely tailored frequency response that rolls off above 8,000 Hz. This roll off is one of the things that accounts for the unique sound and character produced by the system.

The industrial design and reliability of the Leslie 147 is proven out by the fact that so many of these units are still functioning, they are highly sought after and typically sell for several times their original purchase price. The internet has created a vast resource of replacement components so that any part of a 147 can be repaired or replaced.

THE LESLIE PRO-CONTROL SYSTEM


Here in the Proworkshop recording studio our Leslie 147 has been outfitted with a custom Pro-Control box. The Pro-Control allows the internal Leslie amplifier to be completely bypassed and disconnected. Any external power amplifier capable of driving a 16-ohm load may be used to drive the Leslie 147 speakers by using the Pro-Control speaker input. The speaker input jack on the Pro-Control box directly feeds the Leslie crossover network input. This opens up whole new worlds of spinning Leslie sounds.

In our recording studio we have a variety of tube guitar amplifiers that are capable of driving 16-ohm loads, any of these are capable of providing much greater tone control and adjustment than the stock Leslie 147 amplifier. Surprisingly, our Marshall TSL-100 sounds excellent for clean sounds but when you're ready for extreme grinding overdrive nothing beats the Mesa Boogie Dual Rectifier. This amp has lots of bass performance so it can really work the Leslie 15" speaker, it sounds fantastic driven by our Hammond XK3 organ.
HAMMOND AND LESLIE IN PROWORKSHOP STUDIO

The Pro-Control box takes care of switching the Leslie motors from high or low speed using a modern relay inside. This relay is driven by an internal 12-volt power supply which isolates the user from the dangerous 110 volt AC power used to drive the rotor motors.
PRO-CONTROL MOUNTED TO LESLIE 147
A 1/4" jack on the side of the Pro-Control unit works with any standard on-off footswitch to control the high/low speed switching using the isolated and safe low-voltage control system.

A male three-pin XLR on the side of the Pro-Control box provides enhanced remote control over the  Leslie 147 speed switching. The XLR connects the main unit to the Pro-Remote, a custom footswitch which provides "power-on" and "high speed" LED indicators.

Any standard three-pin XLR microphone cable may be used to connect the Pro-Remote to the Pro-Control main unit mounted on the back of the Leslie 147. The control signals are all filtered low-voltage DC so they may be sent down a standard analog microphone snake. The Pro-Remote allows the Leslie 147 speaker to be located in one of our isolation rooms and controlled by a performer in the main performance area. 

A standard 3-pin female XLR on the Pro-Remote is provided to connect back to the male input on the Pro-Control box. A 1/4" input jack on the Pro-Remote can accept any standard on-off pushbutton as another speed control option.

The Pro-Control system was designed in such a manner that absolute minimum modifications to the Leslie 147 are required. The main unit mounts to the Leslie using one of the stock screws that holds on the rear of the speaker-cabinet on so no additional holes need to be drilled in the Leslie.

The installation process is quick and easy.

Remove the back, unplug the Leslie speaker output cable from the power amp.

Cut the red and black wires inside the woofer baffle area so nobody will ever see them, these are the two wires that feed audio into the Leslie crossover network. The Pro-Control comes with a black and red twisted pair which gets connected to the crossover input either by soldering or using crimp on connectors (ours is soldered). Coil up the original stock speaker plug and wire, store this inside the Leslie if you ever want to return it to stock operation.

Reinstall the back cover on the 147 and use one of the screws to feed through the mounting hole in the back of the Pro-Control head unit. The Pro-Control is mounted on the back of the Leslie 147 directly above the stock amplifier. Two small extension cords hang below the Pro-Control box, one is white and one is brown.

Locate the four AC plugs on the Leslie amplifier, two are white (the slow speed rotors) and two are brown (the high speed rotors). Plug the two white plugs into the white outlets and the two brown plugs into the brown outlets which exit from the Pro-Control box.
LESLIE MOTORS CONNECTED TO PRO-CONTROL UNIT
The Pro-Control unit has an AC power cord which needs to be connected to a source of 110 volt AC power. A toggle switch on the side of the Pro-Control turns power on and off. A green LED indicator light on the side of the unit tells the operator whether power is on or not. When power is on the Leslie rotors should be spinning. If no control switch is plugged in the Pro-Control defaults to low speed.

Pro-Control offers several advantages over the stock system. Hum and noise are virtually eliminated since audio is no longer flowing side by side with AC power in the same cable. The stock Leslie brown control cable is only 30-feet long and it's not easy or recommended that you extend its length. The 6-pin octal plugs on the Leslie cable are obsolete and somewhat fragile. Pro-Control eliminates the stock brown control cable completely (put it away safe somewhere, these are getting more scarce and going up in price).

The stock Leslie 147 has no electrical ground reference, this type of construction is no longer allowed by modern safety regulations. Pro-Control is fully isolated so the user is never exposed to any dangerous voltage or grounding issues.

With Pro-Control the stock Leslie amplifier is no longer connected or used. We left ours mounted in the Leslie speaker-cabinet so we would know where it is if we ever want to resume using it or to sell the Leslie speaker to a collector.
MOST STUDIOS FACE THE BACK OUT FOR EASY MICROPHONE PLACEMENT
OLD VS NEW

The Leslie brand has been bought by the Hammond-Suzuki company. They have started producing Leslie speakers again and these look similar to the old ones but the drive mechanism is completely different. In the original models the two stacked motors provide unique braking response when switching between the two speeds. 

On newly designed Leslie speakers a single motor controls each rotating element and electrical braking is used to slow down the components in a way that performs sort of like the original. Optical sensors provide feedback about the actual speed of the rotating elements in the new design. The information from the optical sensors is used to apply reverse braking energy to slow down the rotation when going from high to slow speed. 

New Leslie designs also include controls for the user to customize the rate of change between speeds. The connecting cables are different for the new range of Leslie speakers since it's now illegal to combine the AC power into the same cable. 

Do the new designs match the old ones for sound quality? Yes they do make the classic Leslie rotating speaker sound and it's a better world that there are some new Leslie speakers available rather than none.

No they don't sound exactly the same, the original design utilizes mechanical braking when changing from high to low speed. The mechanical braking on the original models creates a unique swirly sound as the drivers change speeds. The effect is similar on the new Hammond-Suzuki Leslie speakers but not identical. The simplicity of the original mechanical braking action is now replaced with electronic speed control that approximates the original but does not duplicate it. 

I evaluated samples of the modern design and found the change from High to Low speed to take longer than on the original. No matter how I adjusted the speed controls I could never get it to sound right to my ears. After listening to the original design for over 45- years and hearing it on countless albums the vintage Leslie performance is ingrained in my DNA. An approximation is just that, only an approximation, it's not exactly the same because it's a completely different rotor drive mechanism. While the new design offers features the original can't match, the original offers speed changes with a unique sound which the new system can not duplicate. 

LOCATING THE PROWORKSHOP LESLIE 147

I looked for an original early 1970's Leslie Model 147 in good mechanical condition on Craigs List for two and a half years. One finally popped up in Orlando, near enough I could drive there and pick it up in my van (shipping one of these heavy beasts is prohibitively expensive). With the addition of the Pro-Control remote system it's the best Leslie speaker system I've ever used.

Real spinning and grinding Leslie sounds have returned to the Proworkshop studio and everyone here is very thankful. Long live the Leslie speaker system.

Good music to you!

LESLIE 147 WITH PRO-CONTROL INSTALLED
NOTE: The Leslie Pro-Control and Pro-Control Remote are custom built products from the laboratory at proworkshop.com. Visit the proworkshop web site to contact us for ordering and purchase information.

LESLIE PRO-CONTROL SCHEMATIC
PROCEED AT YOUR OWN RISK! 
WE ARE NOT RESPONSIBLE IF YOU ELECTROCUTE YOURSELF

We've provided the schematic for those who would like to build their own Leslie Pro-Control system. We don't offer internet or email support on this. Proceed at your own risk, there is lethal voltage in this box and it can kill you. You've been warned. Be careful, please don't hurt yourself. 

THE FOLLOWING COMPONENTS IN PRO-CONTROL CAME FROM AMAZON

The power switching relay and socket. It's a DPDT relay with a 12-volt DC coil. The current draw does not really matter since it's typically very low (.1 amp or less) and our power supply delivers over 1 amp of current.

The snubber diode across the relay inputs squelches pops when power is removed from the relay. It is a 1-amp 1000 PRV part. Don't get the polarity wrong or you'll blow the power supply. Don't use a low voltage part either, that reverse kick from the relay coil (as it relaxes) has no current but it's a big voltage, the diode shorts it out and makes it disappear noiselessly.

Lights are 12-volt panel mount LEDs. The ones I like to build with already have the required current limiting resistor installed to make them work on 12 volts DC, very convenient. They are very bright.

12-volt DC power supply. The one I used is typically used to drive TV cameras 24/7 so it should be reliable in this application. It has tightly regulated output, light weight, compact and cool running. I measured the output with my Fluke meter at 12.1 volts on both of the units I bought for this project (one to use and one spare). The power supply can deliver over 1-amp of current, all that for under $8.00. It's a very good time to be doing DIY electronics.

On-off power switch. Ours used one section of a DPDT toggle switch which is a handy configuration I like to keep in stock.

The isolated 1/4" Switchcraft jacks. These help keep the low voltage system completely isolated from the Pro-Control chassis. 

White and Brown extension cords. These were chopped and used to provide matching color coding outlets for plugging the speakers in to the Pro-Control unit. The cut off piece with a male end from the white extension cord was used for the power inlet cord to the Pro-Control headend. 

OTHER COMPONENTS

We had the Hammond aluminum enclosure in stock, it was purchased at Fry's in California. They can be ordered from many electronic parts suppliers.

The XLR connectors are standard Neutrik parts we keep in inventory.