Building a better Leslie 147 preamp

[Rhodesplyr]

The original Leslie 147 Combo Preamp is a solid-state design from the late 1960s, and they tend to be noisy. Particularly annoying is noise due to interaction of the 500k volume pots and the input stages when the pots are set anywhere at the midpoint. (I understand why this happens due to noise in the input circuit; it's not a dirty pot problem.) Even when rebuilt with metal film resistors and lower-noise transistors, it doesn't have a great noise floor.

I'm attaching a schematic because I've been trying to brainstorm an alternative to this circuit. Because I have plenty of experience with tubes, I could do it easily that way, but I'd pay a price in size and heat. I know much less about what ICs are appropriate for this application. Primarily what I was wondering was what opamp chips would be good choices for the input circuit. I realize that the power supply would have to be redesigned as bipolar for many of them. Would a TLO72 be a good choice? (I would assume that the TL072 would be driving some other output device.)

Leslies require a fairly large voltage swing to drive them to full power, a good deal hotter than standard line level. The input of a Leslie 147 is a 10k pot into a 12AU7, so assume a 10k load. There is also a cable, sometimes 30 feet, between the preamp and the Leslie, so there is some cable capacitance to consider in terms of stability.

Thanks in advance for any suggestions.

[Mrsices]

Interesting ... because the stock combo pre-amp for the 147, 125, etc. only uses a preamp - I can run my guitar, keys, etc. into a leslie 125 using the combo pre-amp with no issues.

[twiggybush]

That preamp schematic looks noisy. I know what your getting at. I have a C3 and Leslie 720. The AO-28 amps push something in the range of double line level. That is what Leslies were set up to accept as input. So.... How about a step up transformer? Neutrik has a line of mic input trannys for $10 ea. Before purchasing a mic tranny try an xlr/1/4" impedance matching adapter to see where that goes. Another thought that came to mind is a headphone amp. I've seen single tube schematics for this floating around. A step up transformer will give the cleanest results. You did not say what keyboard you have. I wondered if it had speakers? If so you could tap into the speakers.

[soundguruman]

No, you will never get too good a signal to noise ratio from early transistor designs.

You need to check the Fender / Leslie designs.
That's what Robin Trower liked.

Stevie Ray Liked that a lot too.

You don't see much Fender Leslie stuff these days, but it was pretty cool.
https://www.google.com/search?q=Fender+Leslie+schematics&client=firefox-a&hs=dbU&rls=org.mozilla:en-USfficial&channel=sb&tbm=isch&tbo=u&source=univ&sa=X&ei=hq oPU9mCIcWX2QXgrIBQ&ved=0CCoQsAQ&biw=1420&bih=825#facrc=_ &imgdii=_&imgrc=668AmOPfhAmN4M%253A%3BfmK7VgCWEDnUmM%3Bh ttp%253A%252F%252Fprowessamplifiers.com%252Fschematics%2 52Fthumbs%252FfenderLeslie_122r-Amp.gif%3Bhttp%253A%252F%252Fwww.prowessamplifiers.com%2 52Fschematics%252Ffender%252FLeslie_122r-Amp.html%3B400%3B258

http://en.wikipedia.org/wiki/Fender_Vibratone

The Fender Leslie speaker cabinet, complete with psychedelic paint job, and built in black lights.

[guitician]

The circuit doesn't look necessarily noisy, maybe it just needs low noise transistors.

[Rhodesplyr]

The circuit doesn't look necessarily noisy, maybe it just needs low noise transistors.

Low-noise modern transistors and metal film resistors definitely help, but they don't eliminate the noise that peaks at the volume pot midpoint.

I'm a professional Hammond/Leslie tech, and I've worked on several Leslie 16/18/Vibratone cabinets. They have no active electronics in them other than that required to switch motor speeds. Basically, they are extension cabinets with speakers and rotating drums in them. The parts often get lost, but, they were sold with a simple crossover and switches that send the mids to the Leslie and route any remaining lows and highs back to the guitar amp's internal speakers or regular speaker cabinet. You were supposed to get a chorus effect of the the Leslie combined with the regular stationary speaker.

The reason I posted is that I have a somewhat trashed original Leslie Combo Preamp, and before I rebuild it in stock form, I wanted to brainstorm a better approach using the existing chassis and transformer.

[guitician]

A 500K volume control is kind of a standard value that was used in them days, but I would think a smaller value would have less thermal noise, if that's the noise your referring too. And I think the transistors produce less noise when run from a lower source impedance. If it results in less output, adjust the gain of the devices to compensate.

[Rhodesplyr]

A 500K volume control is kind of a standard value that was used in them days, but I would think a smaller value would have less thermal noise, if that's the noise your referring too. And I think the transistors produce less noise when run from a lower source impedance. If it results in less output, adjust the gain of the devices to compensate.

Yes, I'm sure one of the original design goals of the preamp was to allow guitar players to plug in directly without having to use a pre-preamp. A later Leslie preamp uses JFETs in the front end for high input impedance, but those designs also have noise issues.

[guitician]

Is the noise there with the guitar plugged in and the volume turned up? The loading of the guitar will bring the noise down. I dont know how the organ loads it, but the guitars pickup is a lot less than 500K ohms. A 100k pot may work fine for the organ, but would suck some of the guitars highs.

[Rhodesplyr]

Mid range of the volume pot's travel is the absolute key point here. You're correct that the noise decreases via the loading of the source with the volume pot at maximum.

There is noise at the mid range of the pot's travel with nothing plugged in. There is noise at the mid range of the pot's travel with a source plugged in. I've been over the circuit with a guy who used to work designing semiconductors, and he verified that the noise was a combination of resistor white noise at room temperature, shot and flicker noise, and the noise of the input transistors themselves, all of which is at a maximum impedance to ground with the volume pot at the midpoint. That noise can be reduced somewhat with modern transistors and metal film resistors, but it's still annoying.

I've been messing with these for years, and that's why I'm interested in a fresh approach. There's only so much that can be done with the original design.

Maybe I should just do it with tubes where you get the inherent advantages of high input impedance, high input/output isolation, and no need for base-biasing current.

[guitician]

OK, sorry, I should have re-read the original post. Are op-amps not good enough? They make some really nice ones now. They have single supply dual ones at TI...
Operational Amplifier (Op Amp) - Precision Amplifier - TLE2142 - TI.com
http://www.ti.com/product/ina163
http://www.ti.com/product/opa445

[Enzo]

Plain old vanilla 4558 dual op amps worked just fine for a zillion guitar amps and PA mixers. No reason they can't serve now, though most folks have moved on to 4580 by now. Then again TL072 are still cheap and work fine. 5532 was always the go-to low noise IC, but really, I often find 4580 just as quiet. I mean nothing against fancy op amps, even the expensive ones are only a couple bucks, but commodity parts will do the job on hundreds of products in any music store, so why not just steal a circuit. I work with Peavey a lot, but any brand will do. Look at the input channel of some keyboard amp or for some solid state guitar amp. What you see there could easily be adapted.

[Rhodesplyr]

Thanks, Enzo. Do you think that one of these op amps would have low enough output impedance to drive a ~30 foot cable without high-frequency loss or stability issues? Or would I need an emitter follower at the output?

[Enzo]

Beats me. What do they use to drive FX loops? What sort of outputs are on overdrive pedals?

[J M Fahey]

Thanks, Enzo. Do you think that one of these op amps would have low enough output impedance to drive a ~30 foot cable without high-frequency loss or stability issues? Or would I need an emitter follower at the output?

Any Op Amp will have lower output impedance than any reasonable emitter follower.