Basic Tremolo Desing

[ppaappoo]

Hi, the other day I shink to use a tremolo without a led / ldr.
I thought in a triangular wave oscillator with a range from 0 to 4.5 v to feed the bias of a single om-amp booster.

Something like this: (the pads S1, 2 and S3 is the speed pot)



My question is how do I make the triangular wave amplitude does not exceed 4.5 v?

[ParlorCitySound]

Hey,
I'm sure someone else will chime in with a fix but I just recently built a triangle wave oscillator with a NE566n IC.

The datasheet:
http://www.datasheetcatalog.com/datasheets_pdf/N/E/5/6/NE566N.shtml

With a just a few parts it has a pot for amplitude. Might work nice for you.

Kevin

[Hides-His-Eyes]

Well, you use a depth control just like a volume control on the LFO output, and use a series resistor to limit the sweep. But I'm not sure this will do what you want it to.

[Taylor]

I could be wrong here, but I it doesn't seem to me like this will create a tremolo at all. You're just modulating the bias of the booster, so it's like mixing a DC wave on top of your audio.

The only way I could see this creating anything that's kind of tremolo-ish is if it bangs the signal into the power rails at the extremes of the LFO wave. Then you'd have a sort of rhythmic distortion.

The old tube amp tremolos often modulated the bias of the tubes to create trem, but (though I'm a total tube buffoon) I believe this works as a side-effect of the natures of tubes rather than that modulating bias inherently modulates volume.

Would love to be wrong here, since it's quite small and simple.

[Jazznoise]

I've had a similar idea but with feeding an oscilator ontop of DC into the collector of a transistor to vary the gain given.

Or is this simply lunacy?

[R.G.]

@Taylor: you're correct, it's a side effect of the tube bias changing the gain.

@Jazznoise: it works less well with transistors and you have to work differently.

In transistors, the low-current gain of the transistor increases with the collector current very nicely. You can make a simple voltage controlled amplifier by using an LFO to modulate the emitter current of a bipolar. However, the collector voltage with the modulated signal on it varies hugely.

What works better is to vary the emitter current of a differential amplifier, then use another differential amplifier to remove the common mode voltage changes from the collectors of the diffamp pair. That's the basis of many, many, many synth circuits. The cross-coupled dual complementary version of this is underneath some very sophisticated VCAs.

With a little thought and extension,  you quickly get to the OTA. I believe that this is how the OTA was developed. At least the path was similar if not the same.

[amptramp]

If you check out the Univox U-65 schematic here:

http://www.univox.org/pics/schematics/u65rn.jpg

you will see the tremolo section at the lower left.  It is simple, but it actually works reasonably well with a phase shift oscillator and transistor connected as a variable resistor.  Note that it has to work into the impedance of the input.

[R.G.]

Same mechanism (the equivalent collector-emitter resistance) is used in other effects, notably the EH Pulsar and some versions of the Thomas Vox repeat percussion.

It only works well on quite small signals, best under 50mV. Above that, the distortion rises rapidly. Single coil pickups may qualify, big bad humbuckers sound distorted. Not that that is a bad thing necessarily, it's just not what you set out to do. It can also introduce a pretty bad thumping sound, as there's a DC offset from driving the transistor. Generally this limits the speed of the LFO and the low pass of the signal path so the LFO thumping can be lowpassed out.

[Jazznoise]

Thanks, R.G! I'll have to read into this - hopefully with the summer approaching I'm going to get to put some theory into practice!

I'm assuming that this would also be a perfectly suitable means of creating ring modulation?

[Earthscum]

What works better is to vary the emitter current of a differential amplifier, then use another differential amplifier to remove the common mode voltage changes from the collectors of the diffamp pair. That's the basis of many, many, many synth circuits. The cross-coupled dual complementary version of this is underneath some very sophisticated VCAs.

So, is that the main basis for the feedback in a ladder-style VCF? I've been pondering over the functions of that part of the circuit for awhile. Most sites treat it as a given without much explanation.

[Taylor]

Thanks, R.G! I'll have to read into this - hopefully with the summer approaching I'm going to get to put some theory into practice!

I'm assuming that this would also be a perfectly suitable means of creating ring modulation?

It can also introduce a pretty bad thumping sound, as there's a DC offset from driving the transistor. Generally this limits the speed of the LFO and the low pass of the signal path so the LFO thumping can be lowpassed out.

If there is a thumping going on at LFO speeds, then it's very likely that the oscillator would be very loud at audio speeds, which makes a ring modulator fairly unusable (and that's if you think a ring modulator is usable to begin with!)