The combo preamps


Back in the 60's the good people at the Leslie headquarters in Pasadena, California, found yet another way to expand the market for the Leslie speaker: They developed a chrome wedge-shaped gizmo that would allow any audio signal to be connected to the Leslie speaker. Now, one *could* assume that the need for a 1/4" jack input came from guitar players of the flower-power-acid-progressive rock scene, and true - many of the really big names experimented with the Leslie guitar sound - Jefferson Airplane and Quicksilver Messenger Service are two West Coast names that spring to mind. (British bands of the more experimental scene with Pink Floyd as perhaps the greatest contributor, and of course The Beatles, used the Leslie for almost anything imaginable - drums, vocals, piano - you name it.)

I don't really buy that ;-)  - the Leslie guitar sound while now famous for its connection with the 60s was not the main factor here. I believe that the market for owners of so-called combo organs (Farfisa, VOX, Gibson etc.), accordions and  other types of reed-style instruments was a much more interesting potential buyers of Leslie speakers.

Before the Combo Preamp came out, people that wanted to mate a low-output signal with a Leslie cabinet would have to build their own interface and drive the Leslie with, say, a small guitar amplifier. With the new integrated design of Leslie Combo Preamp, utilising the newest solid state technology, the job was now much easier.

The new invention was a neatly designed stomp-box with two 1/4" jack inputs, separate volume control knobs, a foot controlled fast/slow button, a yellow light for tremolo indication, a red light for power and of course the 6 pin amphenol socket where the Leslie cable was plugged into. Also mounted on the box was a line cord both for supplying the Combo Preamp's internal circuitry with power and passing power on to the Leslie cabinet through the 6-pin connector. The first model, simply called Leslie Combo Preamp, was made to interface with the type 147 power amplifier. When the solid state ProLine Leslie models came along, a matching combo preamp was issued. Yet another three models were developed during the 70's to mate with new types of Leslie speakers. They differ in the way they interface with the cabinets, the number of control buttons, the way they are powered but they all provide the same basic element of signal matching and motor/rotor control.

The Leslie 147 is now in production again, as are other models with different specs, but the Combo Preamps have all been discontinued. One could with good reason claim that while the old tube Leslie designs are very versatile, and therefore still on the market, the early solid state design of the Combo Preamps would not be satisfactory by today's standards. Aftermarket Combo Preamps, on the other hand, can be bought at reasonable prices, offering a far better sound than the noisy, buzzy sounding transistor designs of the 60s and 70s. The Trek II model UC-1A is very well built and also good value for money. Other models from other companies are more expensive but may offer a wider range of tonal control and a tube-based design.

The intention with this web-page is to allow the reader, together with Uncle Harvey's Guide to Leslie Pin-outs, to determine which combo preamp that can be used with a given Leslie or the other way around - given a preamp, what cabinet model it will work with. The different models are listed below:


Combo Preamp Model Type # Use with Leslie speaker model Pin-out
Leslie Combo Preamp 7875 47/45, 147/145/247/245, 125/225 etc. 6W
Leslie Combo Preamp II 7370 760, 770, 825 ( and 700, 710)* 9
Leslie Combo Preamp De Luxe 7420 900, 910, 925, 950 9
Leslie Combo Preamp III 7880 330, 860, 820, 771, 815, 722HL, etc. 11
Leslie Combo Preamp IV 7884 330, 860, 820, 771, 815, 722HL, etc. 11

Type #'s refer to the 117V/60Hz standard US issues. The 240V/50Hz version of the original Combo Preamp has # 9875.

*Leslie 700, 710 need to be converted to single channel input with a TrekII crossover network in order to work with a combo preamp.

Leslie Combo Preamp FAQ

Q: Is there a Combo Preamp that will work with a Leslie 122?

A: No and yes. Since a Leslie 122 (and other Hammond-type models) were designed and sold to be used with console Hammond organs, where a combo preamp would never be needed anyway, such a model was never made. The Trek II model of today, however, can be used with any single channel Leslie including model 122/22H/31H etc.

: Can my Combo Preamp be modified to work with a different type of Leslie than originally intended?

A: Yes, in most cases. The first Leslie Combo Preamp can be made to work with 9-pin Leslies but you won't have the 'stop' feature unless you somehow mount a second foot-switch. A Combo Preamp II can be made to work with 11pin Leslies but the Leslie on/off function will not work. A Combo Preamp III can be used instead of a Preamp II if you make a pigtail with a separate power cord. Actually, the pigtail solution is often a good way to go. I must stress that AC mains voltages are often involved and these modifications should *never* be made unless you are absolutely sure what you are doing!

: Can I connect more than one Leslie to my Combo Preamp?

A: Yes, if you get a 'Y' connector or alternatively daisy-chain the speakers (older Leslies only) you can connect more than one Leslie. I would not recommend running power for more than two cabinets through the Combo Preamp, though. Combo Preamp III and IV for 11pin Leslies can control as many Leslies as you want, since AC power is not going through the Leslie cable. Trek II guarantees that the UC-1A can be connected to three cabinets drawing power at the same time (one 6W, one 6H and one 9 pin).

Q: If my Combo Preamp needs repair, where can I get the schematic?

A: The owner's manuals for the five Leslie Combo Preamp models can be downloaded from this web-site. They include schematics. See the links below.

Q: I have a Hammond organ with a 1/4" output that somebody made. When I connect this to my Leslie Combo Preamp there is a lot of noise and distortion.

A: This is because the output signal from any Hammond organ is many times as powerful as the input level accepted by the Combo Preamp. The proper way to connect a Leslie cabinet to a Hammond organ is to use a connector kit, which is often very simple. If the 1/4" way is the only practical solution I suggest that the signal from the organ is suitably attenuated with a voltage divider circuit consisting of two resistors: Take two resistors, 100k and 1k (0.5W) and connect them in series. Now connect the free end of the 100k resistor to the 'hot' signal coming from the organ and connect the free end of the 1k resistor to ground. Where the two resistors are joined together is where you should tap signal to the combo preamp. If you are handy you can fit the resistors inside the end cap on a 1/4" plug.


Download the combo preamp manuals here!

The manuals have been made available to me by Harvey Olsen of Chicago, IL, U.S.A.   The manuals have been carefully scanned by my friend Peter.

Click here to download the manual for the
Leslie Combo Preamp
Click here to download the manual for the
Leslie Combo Preamp II
Click here to download the manual for the
Leslie Combo Preamp III
Click here to download the manual for the
Leslie Combo Preamp IV
Click here to download the manual for the
Leslie Combo Preamp De Luxe

If you click here you can download an alternate version of the combo preamp II manual (for the console connector kit #7370).