Audio Modulator Noise

[cloudy1]

I'm working on a guitar effects pedal. I need to fluctuate a resistance from a higher to lower resistance and back. I'm currently using an op amp sine wave generator into an LED with
a photo resistor attached. The problem is the op amp is putting an audible click every time it
creates a new wave. I've tried running the op amp generator off a separate power supply but
the noise is still there. When I plug my bench generator in to produce the sine wave there is no noise. I know my bench generator creates waves digitally. I need the circuit to be as small as possible to fit in a stomp box. Is there any way I can filter out the clock noise? Or is there a better way to fluctuate resistance? Have been trying for months to fix this. Any help would be appreciated. Thanks

 

[davenn]

hi there
welcome to the forums

lets see the full circuit diagram of what you are doing
and maybe a sharp bright pic or 2 of your construction so we may have a chance of seeing where you may be going astray

Dave

 

[cloudy1]

I don't have a circuit design. I'm a musician that trying to build an effects pedal for my own use. I don't have the electronics skill to design and build my own circuits so I'm using pre existing circuits. What I'm trying to do is take a distortion unit and have the distortion go from clean to total distortion and back again. Since it's a distortion unit the gain is real high so it amplifies any noise/ glitch in the circuit. This is my problem. The modulation signal is putting an audible glitch in the distortion. How can I get rid of this?

 

[davenn]

I don't have a circuit design. I'm a musician that trying to build an effects pedal for my own use. I don't have the electronics skill to design and build my own circuits so I'm using pre existing circuits. What I'm trying to do is take a distortion unit and have the distortion go from clean to total distortion and back again. Since it's a distortion unit the gain is real high so it amplifies any noise/ glitch in the circuit. This is my problem. The modulation signal is putting an audible glitch in the distortion. How can I get rid of this?

but you are saying that are building something .... you have an opamp, LED etc

before you build anything its a really good idea to produce a circuit of what you are going to build ... something to follow
How would you expect to get around a foreign country if you didnt have a map ?

with out circuits, its pretty impossible for anyone else to try and figure out what you are doing and to try and help you sort out any problems
So we need as much info from you as possible

Dave

 

[KrisBlueNZ]

As Dave says, we NEED to see a circuit diagram.

If you have something there that you're experimenting with, we need the circuit diagram or "schematic diagram" of it, and the modifications you've been making to it.

If you can't trace out the circuit of the existing pedal, you can at least draw up a diagram of the bits you're trying to add to it.

You can also post photographs of the existing pedal, and your new circuitry, and how they fit together. A picture is worth a thousand words and will often enable us to recognise a problem that's obvious to us but not to you. So it's nearly always worthwhile to take some pictures and post them here.

 

[cloudy1]

I've drawn up a crude cad drawing of what I'm doing. It's just like a tremelo unit but instead of the volume going up and down the gain of the distortion goes up and down. The circuit works and sounds cool but I can't get rid of the ticking sound from the op amp generator. The op amp generator I got off of ebay. Thanks

 

[KrisBlueNZ]

Does this click happen once in every cycle of the sinewave generator? Does it always happen at a particular point in the cycle? If so, which part?

Have you looked at the LED? Does it increase and decrease smoothly in brightness? Set the modulation oscillator to a very slow frequency and watch the LED carefully.

You said the modulation oscillator is powered from a separate isolated supply. Have you used batteries, with no connection at all to any of the other circuitry?

Have you disconnected the original gain potentiometer in the stomp box? Were only two terminals of that potentiometer used, or were all three terminals used? What is the resistance value marked on the potentiometer? What is the type number of the LDR?

Can you take a photo of your whole setup and post it here?

 

[Rusty]

I dont understand what you are doing and what you want to accomplish.
But a wild guess: Your photoresistor is picking up light from the LED as a train of pulses and sending it to the signal path which results in a repeating clicking noise. If the clicking is at the same frequency as your generator, then this is probably the problem..

 

[cloudy1]

ok one more time. What I want is identical to a tremelo unit but instead of volume fluctuation
I need gain fluctuation. This is not rocket science. I need a circuit the will vary the gain of a distortion pedal up an down. How can I do this? Thanks

 

[Yoa01]

Easy. Just made this up for ya:
http://img189.imageshack.us/img189/7874/386amp.png
It's a standard variable-gain LM386 amplifier, but uses a FET to induce voltage control to the gain control resistor, thus causing fluctuating gain. Input your sine wave, and you should be good to go.

This is more designed as a standalone unit. To control an existing pedal, simply use the resistor+FET scenario with the gain pot.

 

[davenn]

Easy. Just made this up for ya:
http://img189.imageshack.us/img189/7874/386amp.png
It's a standard variable-gain LM386 amplifier, but uses a FET to induce voltage control to the gain control resistor, thus causing fluctuating gain. Input your sine wave, and you should be good to go.

This is more designed as a standalone unit. To control an existing pedal, simply use the resistor+FET scenario with the gain pot.

sine wave from where ?

 

[KrisBlueNZ]

ok one more time. What I want is identical to a tremelo unit but instead of volume fluctuation

Perhaps you would save the attitude until after you've responded to all the questions I asked you in my previous post on this thread?

 

[cloudy1]

Thanks that's just what I was looking for! Made the circuit, plugged it into my distortion and it works great. No clicking!!! Yeah. Thanks.

 

[BobK]

Okay, I see what you are trying to do. The problem is that the LED only conducts in one direction, so, fed with a sine wave, it is off 1/2 the time (actually more than that because it needs about 2V before it conducts). The on off threshold is the click you are hearing.

Does the fucntion generator have a DC offset function? If so, you should turn it up to the point where the LED never goes fully off. Otherwise you would wire the LED with two resitors, one going to the signal generator and the other to a battery to provide DC offset.

Bob

 

[Yoa01]

sine wave from where ?

He said he had an OpAmp-based sine wave generator in post #1

However, cloudy, I'm glad the idea worked for you, but these are nice people and just want to help. I'd recommend taking Kris's advice. I learned that the hard way.

Good advice Bob! I continually forget about my diode's voltage thresholds and it creates a lot of problems, especially when building guitar circuits.