The unused op-amp might just be sitting there unconnected, minding its own business and not doing anyone any harm, but it might also be causing trouble. You just don’t know.
If you ignore it, and it turns out to be the cause of the problem, you’re going to feel pretty silly.

 

I actually tried ~uF capacitors as decoupling but seems to be too large and vanished the whole signal...

Something wrong there.
You could decouple with 10000uF and it wouldn't stop it working.

 

The Instructable does not clearly show which part sweeps the frequency.
It does not sweep it enough.

 

The Instructable does not clearly show which part sweeps the frequency.
It does not sweep it enough.

Awesomeeeeeee
Man! How did you simulated that?
Can I do that also using Multisim?
What is the frequency you set for AC Input?

 

Most of us use the free LTspice simulator. I have never used Multisim.
It was set for frequency sweeps: 1) 100Hz to 1GHz and 2) 10Hz to 100GHz.
You can see that the circuit sweeps from 400Hz to about 1800Hz.

 

The Instructable does not clearly show which part sweeps the frequency.
It does not sweep it enough.

one thing only!
have you forget the 10uF capacitor right after AC input?

 

The capacitor is not necessary if there’s no DC offset in the signal.

Bob

 

The output capacitor blocks any DC and it and its load resistance determine the low frequency response.
Here is the input and output signal at the 10uF output capacitor feeding a 100k load and with a 0.1uF output capacitor:

 

have you forget the 10uF capacitor right after AC input?

Nope. Didn't forget. Doesn't matter.

The 10 uF cap and 80 K resistor form a high pass filter with a corner frequency of 0.2 Hz. This is invisible to the variable filter function of the rest of the circuit.

ak

 

I showed that the 10uF output capacitor driving 100k ohms makes no difference to audio frequencies and the 10uF input capacitor feeding 80k ohms also makes no difference to audio frequencies.

 

Hey everyone
I'm here to proudly announce that all your help and suggestions worked and we implemented the circuit successfully and it was awesome...
I added some decouplong cap around 10nF and removed first and last 10uF as you guys mentioned they were only for DC filtering which I didn't include as there is no DC I suppose...
Further, I tested the circuit first whithout the second OpAmp being wired... it was workeing fine though...
But just to make sure it wont play and oscillate later, I buffered it to the first OpAmp I used in the schematics shown...
Thank you all you were great help and support
Remind me if I have missed something to mention about the test we made

Cheers

 

The +4.5V and -4.5V are DC and must have at least 100uF capacitors parallel to them.

 

The +4.5V and -4.5V are DC and must have at least 100uF capacitors parallel to them.

You're right
I'll keep that in minf and next time I will test this