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The Leslie Effect Rotor Adapter

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5/1/1999 1:42 PM

The Leslie Effect Rotor Adapter

Mark - this one's for you!

I just posted a DIY project to make what I call a Leslie Effect Rotor Adapter (LERA), which converts an existing phaser or flanger to an audible illusion of a rotating speaker spinning up and down, like the motor speeding up and slowing down a real one.

This diddles with the speed control pot inside the existing effect.

The instructions are pretty much complete for the modestly accomplished DIY'er, build it on perfboard and start spinning.

5/2/1999 7:52 AM

Now here's something I've been dying to try!

I have one question before I perf-board the circuit.

Forgive my ignorance, but what decides your choice in op-amp for a circuit like this, or any circuit for that matter? I've never seen a project that uses an LM158 or LM2904, and I wondered why R.G. picked these, as opposed to, say half a 4558, or even a 741 for that matter.

What difference is made when you substitue op-amps in a circuit like this (that may not require low noise for audio application)?

thanks.

5/2/1999 12:23 PM

That is a good point. They were picked because their input common mode range includes ground.

Just any dual opamp will NOT work in this one, except by sheer luck, and the reason is that the opamp input needs to go all the way to ground. Most of the low noise audio opamps have an input common mode range that only goes down to a volt or two above ground. The so-called "single supply" opamps will work correctly with their inputs down at ground or even a few millivolts below.

The LM2904 is a dual version of the LM2902 quad single supply opamp, and the LM158 is a National dual single supply.

5/3/1999 1:24 PM
R.G.	I should also have mentioned that the LM324 works fine there, but using a quad is kind of massive overkill. The LM2904 is a dual version of the LM324. The LM358 works as well.

5/4/1999 5:51 AM
GFR	The CA3140 is a single opamp that can work with down to 0.5V below negative supply on the inputs.

5/5/1999 8:23 AM
R.G.	Yeah, there are a number of opamps that go down to ground; any with an input common mode range going to the negative supply will work.

5/3/1999 7:32 PM

You know my tastes too well, fella. Glad you're friend and not foe!

I might point out that this lil' honey has other uses too. To whit...

- A gradually increasing resonance on a phase shifter, flanger, etc.

- A footswitch selectable "rising grind", if the LDR is part of the gain-setting circuitry of an op-amp preceding a clipping circuit.

- A footswitch selectable "fade out", or "group solo boost" for folks who don't have a sound tech to do things like that for them at the sound board. Again, the LDR sets the gain of an op-amp stage, only in this case between the pre-amp/mixer output and the power-amp, or some loop connecting a submix.

Bear in mind that since the circuitry up to the LED is not carrying signal, you can situate the LED driver circuitry long distances away from the LED itself, and stick the LED/LDR module inside the audio device of your choice in the PA-rack, FX rack or whatever. So, even though it can easily accompany an effect in a decent sized stompbox, there is no reason why it has to be situated in the same place.

Nice work, RG. Funny how simple ideas can open up a world of possibilities. Craig would be proud of you.

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