Why doesn't this circuit work?

[egasimus]

Long story short, I have this on my breadboard - a simple low-pass filter and comparator based on a TL082. It is powered by a 9V battery.

However, when I plug my 24 ohm headphones into the output, all I get is squared off mains hum if I touch the input or input ground while my foot is touching the floor. I've tried plugging in a guitar to the input, as well as a line-level output, with similar results.

So, what's wrong with it? Needs a buffer in front? Has trouble driving 24 ohms? TL082 isn't suitable for this application? Building stuff without understanding many circuit fundamentals is hard...

[sault]

Sounds like something isn't grounded properly. What are you grounding to? Does it work when you don't touch it?

[egasimus]

Nope, it doesn't work no matter what I do. I've also tried plugging in the (grounded) output of my soundcard, and that doesn't cut it, either.

[amptramp]

You almost have noiseless biasing with the two 10K's at the top, but there is no bypassing to ground.  The capacitor that should be to ground on the input filter is taken to this point and drives the noiseless bias divider.  Return the 22 nF at the op amp input to ground and add a bypass to ground for the biasing supply and it should work.

[egasimus]

Added a 10u to ground, and connected the 22nf cap to ground instead of Vb - still the same. By the way, why should the 22n be connected to ground and not to Vb? In my understanding, Vb is virtual ground for most of the circuit...

[misterg]

You need feedback on the right hand op-amp (wire or resistor from output to - input).

Andy

[egasimus]

I actually need it to work as a comparator

[Gurner]

Added a 10u to ground, and connected the 22nf cap to ground instead of Vb - still the same. By the way, why should the 22n be connected to ground and not to Vb? In my understanding, Vb is virtual ground for most of the circuit...

Your vb virtual ground isn't as low impedance as true ground.

What are you doing with that 47nf cap from the -ve pin to the +ve pin? (ie what's its intended purpose?)

[misterg]

I actually need it to work as a comparator

when I plug my 24 ohm headphones into the output, all I get is squared off mains hum if I touch the input or input ground while my foot is touching the floor. I've tried plugging in a guitar to the input, as well as a line-level output, with similar results.

Sounds like it's working exactly as designed, then.

Andy

[egasimus]

Gurner: The only 47n cap I see is part of a standard active filter topology I saw on the Net.

misterg: well, why does it hum when I touch the strings of my guitar, but doesn't play anything when I actually strum them? With these values, the cutoff frequency is supposed to be 494 Hz...

[misterg]

misterg: well, why does it hum when I touch the strings of my guitar, but doesn't play anything when I actually strum them? With these values, the cutoff frequency is supposed to be 494 Hz...

I'm not sure what you want to achieve, but what you've got is the inverting(-) input of the 2nd op amp referenced to 1/2 supply voltage and the non-inverting(+) input connected to the output of your filter circuit, also referenced to 1/2 supply voltage. There is no feedback, so the op-amp is operating in comparator mode. Every time  (+) is greater than (-) the output will saturate at 1-2 volts below supply voltage. Every time (-) is greater than(+), the output will saturate 1-2 volts above ground.

The open loop gain of a TL082 is typically 100Volts out per 1mV in. Your output voltage swing is (of course) limited by your supply rails to say 1.5 V to 7.5V, about +/-3V. The change in input voltage that this represents is +/- 0.03mV, or about +/- 30 microvolts!

(Your guitar pickup will have an output of tens to hundreds of millivolts)

The output of any op amp has an offset voltage - for zero input, the output isn't exactly zero. The TL082 is pretty good for this at +/-15mV, but that is still 500 times bigger than you need for the 1st op amp to saturate the second.

I would guess that the circuit picks up every bit of hash, noise & am radio stations and oscillates like crazy at frequencies you can't hear (the TL082 is good for a few MHz) every time you put a signal near it that overcomes the output offset voltage on the first op amp. I think what you're hearing is the large hum signal modulating this, but this is guessing.

Andy

[DavenPaget]

He wants the OPA to operate as a comparator .

[misterg]

He wants the OPA to operate as a comparator .

I'm pretty sure it is... 

Andy

[slacker]

Has trouble driving 24 ohms?

This is probably the problem, stick an ouput cap between the output of the opamp and the volume pot and plug it into an amp, that should work.
You could have a look at this, which is basically the same thing and known to work http://www.aronnelson.com/gallery/main.php?g2_view=core.DownloadItem&g2_itemId=39358&g2_