Help with Op-amp distortion circuit

[Servant07]

I am new to this and have only built one pedal so far but I am doing some playing around with some online tutorials to learn more about distortion circuits. I built a non-inverting op-amp circuit on my breadboard using a TL072 and my problem is that it only works with the guitar volume up to about 3/4 full, any thing past that and the circuit produces no sound. And ideas what is causing this?

[Frank_NH]

Do you have a schematic of your circuit?

[Servant07]

The site i'm working off from is here: http://www.generalguitargadgets.com/how-to-build-it/technical-help/articles/design-distortion/

And the circuit I am working with is this:


C1 = 47nf
C2 = 470pf
As mentioned above I am using a TL072.
(-) pin 2
in (+) pin 3
out pin 1
pin 8 to 9V
pin 4 to ground

[induction]

It's not biased correctly. You either need a bipolar power supply, or you need to bias the non-inverting input to somewhere near 4.5 V and add input and output caps.

Looks like maybe you didn't read the page you linked down to the section on power supplies.

Read the rest of the article, and try building something like the example circuit at the bottom, or build the MXR Microamp instead (it's very similar but doesn't have the clipping diodes). There are a few extra components, but they're pretty necessary. If you read the whole article, you should be able to figure out what they're for. If not, just ask us.

[Servant07]

Thanks, in my head I was thinking I would gradually build the circuit up testing each section as I go. Not going to work that way I see. Anyways, I added this part to my test circuit on my breadboard.



So with this added am I now suppose to be getting 4.5V at the input? I am getting 4.5V at the top of 1M resistor, but not at the bottom where it goes to the input.

[induction]

You need an input cap between the input signal and the 1M. The input wire is connected to ground via your pickups, and the bias voltage node sees that path as a voltage divider, and drops the voltage accordingly. The input cap will prevent this from happening.

You will also need an output cap for similar reasons.

[induction]

In other words, follow your link and go to the schematic under 'My example designs' and build that, but you can leave out S1, S2, C1, C6, D1, D2, D3, D4, and R6. All of them could be considered optional. The rest need to be there for the circuit to work reliably. For R4, you can keep your 10k, or use the 100k pot. You also need the power circuit you showed above, of course.

The optional components are all used for tone shaping. Since you're working on the breadboard, you can always put them in if you want to hear what they do.

[Servant07]

Just finished building the circuit without the tone shaping parts like you suggested and it works great. Getting 4.3V at the input pin now.

Thanks for the help, really appreciate it. Now it's time to play around with tone controls and some clipping diodes.

[induction]

Glad you got it going. Now the fun begins.