Electronics > Beginners
Active twin t notch becomes an oscillator
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Ayax:
Hello everyone
First of all i need to say: sorry for my bad English.

My circuit is based on this
http://www.radio-electronics.com/info/circuits/opamp_notch_filter_2/opamp_twin_t_varq.gif

I used the active twin t notch filter with some modifications and got great results,
BUT sometimes the entire circuit becomes an oscillator.

The input recived the signal from a pasive instrument like a bass, guitar, even a dinamic microphone or
an active like keyboard, goes to a preamp and then to the notch.

Sometimes the oscillation happens when the signal input is too big like a beat, and it stops if you repeat the beat.

Sometimes it starts when connecting the power supply (a 9v battery).

I need to avoid this.. "parasitic oscillation"? "oscillation problem"?

I leave you images so you can see how i made the entire circuit and my modifications.

Thank you all in advanced

Benta:
 What frequency is the oscillation? Which opamps are you using?
Ayax:
The frequency is the same of the center of the filter, so in case of the "filter 1" it´s 120hz. I´m using the TL072.
Audioguru:
I have used many TL072 dual opamps powered with a 9V battery many times without problems. But its minimum supply is 7V which happens soon to a 9V battery.
Like most old Fet-input opamps, The TL07x and TL08x opamps have the "Phase Inversion" problem where the output goes as high as it can if an input becomes within a few volts from the negative supply that is 0V in this circuit.

Applying the battery and a high input level will cause the (-) input of the opamp to go near 0v and cause this problem that might cause oscillation.
mikerj:
Add some decoupling to your 4.5V rail splitter.
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