Help with LFO!

[smoguzbenjamin]

I need a little help here... I'm going to be making the EA tremolo over the winter holidays, and I was thinking of adding clipping diodes in the feedback path of the LFO to clip the sawtooth (i think it is) output into a square-like waveform... Or is this a bad idea?  :roll: Might generate some funky sounds...

[idlefaction]

if you did it big muff style, with some clipping diodes and a capacitor, you'd probably find the thing would only put out a restricted range of signal.  if you used LEDs you could get up to maybe 2V p-p square wave out...  that's probably enough for many FETs.  you could use a low Vgs like J201.

give it a go!  sounds cool to me  

[smoguzbenjamin]

Hmmm, the EA trem LFO uses a normal tranny in it's oscillator. Give it a try though

[R.G.]

Putting a back-to-back diode limiter on it will produce a mildly distorted sine wave at a lower amplitude. Here's why.

A phase shift oscillator works by having some gain and a feedback network that converts an inverted output into a NONinverted output at some frequency by shifting its phase. The feedback network has some losses, so that in addition to having a phase shift of 180 degrees, there has to be enough gain to make up for the losses in the shift network.

In a phase shift oscillator, this is a delicate balance. The gain needs to be on the order of 15 to 30, depending on how similar the three phase networks that make up the feedback path are. When the oscillator starts up, any noise at the 180-degrees-shift frequency is amplified and builds up at that frequency into a sine wave that continues to grow until some nonlinearity limits it. In this context, "limiting" looks to the gain stage like a lowering of gain, so the thing ducks below the gain needed to sustain oscillation. The limiting is usually the power supply, but you can do the same thing with diodes at a lower level

You won't get a square wave, just a smaller sine wave. To get a square wave out, buffer the output with an emitter follower and use the clipper on the output of the emitter follower. The follower output does not get fed back to the input of the oscillator, so limiting its output does not affect the oscillator. You get a clipped sine wave to the extent that the phase shift output is bigger than the clipping diode threshold.

Mother Nature is very picky about the details on these things.

[Chico]

If you want a tremolo with a square wave lfo, try building the Anderton Tremolo at GGG ( I think).  It is a nice build and an interesting alternative to the EA (IMHO).  I have built both, and like each in the proper context.  The EA has a cool retro feel to it, but the Anderton allows you to switch between triangle and square wave lfo.  The square wave is cool for a really exagurated tremolo effect.  Also, the Anderton uses an optoisolator (or LED and LDR).  The EA modulates the volume using a jfet as a variable resistor.  So if you like disecting effects to learn the various ways to accomplish a given effect, its a great crash course to build both at or near the same time.

[smoguzbenjamin]

R.G. -> Sorry, not quite familiar with the terminology there... :lol: Emitter follower, do you mean like a second tranny on the emitter that amplifies the signal from the emitter, and clips that? A schematic would be greatly appreciated.