# What would be a good way to....

Discussion in 'Electronic Design' started by Motty, Oct 14, 2003.

1. ### MottyGuest

Here's the deal: I am designing and building a ring modulator for a
guitar effect. I have two signals - the guitar - and a function
generator output signal (Max038 chip) going into a Burr Brown
multiplier chip. The problem I am facing is that I need a way to turn
off the output from the circuit when the guitar is not producing a
voltage (or current). Since the output is two signals mixed together
I don't want the function generator signal to show up on its own (when
it is not being multiplied by the guitar signal). This would just
produce a tone at the output. Hopefully this makes sense.

I was thinking of some type of transitor switch circuit, but I would
have to go back and look at my books really hard. Does anyone have a
suggestion as to how to design a voltage controlled switch? The raw
output of the guitar is a +,-200mV signal at the MAX. Usually it is
in the 10-100mV range centered around ground.

2. ### Jan-Erik SöderholmGuest

In a multiplication, if one of the factors are zero
the product (=output) will also be zero, not ?

Or maybe the multiplier isn't perfect...

Jan-Erik.

3. ### Jeroen VriesmanGuest

As Jan-Erik stated, your multiplier is not perfect, but anyway:

Switching the output signal on and off would create clicks, I would use an
NE570, with the guitar signal into your rectifier, and the VCA after the
multiplier. With a capacitor, and a current limiter at the output of your
rectifier it's done.

4. ### Michael BlackGuest

But the mixer should be balanced, or else you'd be bothered with
the oscillator signal and the guitar signal being at the output all
the time. Remove the guitar or oscillator signal, and there should
very little signal at the output of the mixer.

Maybe you didn't catch that, and anticipate a problem that isn't
there.

If you notice too much output with only one signal to the mixer,
then you might want to look at a different mixer, or see if there
is a way of balancing the mixer through an external DC bias.

Also, perhaps layout is a factor. If the oscillator is strong enough,
and layout is wrong, the oscillator might be getting around the mixer
through inductive pickup on the wires. For that matter, if the power supply
line is not bypassed properly, the oscillator signal could be flowing on
the voltage supply line(s).

I seem to recall some IC mixers that did not work so well if the signal
levels were too strong. So that might also be a factor to watch.

Michael

5. ### N. ThorntonGuest

Another option is to use your tone to just control the gain of a
circuit. Using a FET is a very easy way to do this.

Regards, NT

6. ### MichaelGuest

Maybe the multiplier is fine, and the problem is the hiss from your
guitar when your not playing, if thats the case you can get a pedal
which does a noise gate. Maybe if the idle noise was quiet enough this
noise gate might work on the output of your multiplier?