Active twin t notch becomes an oscillator

[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.

[SuzyC]

Your problem is in feedback to the unsymmetrical twin-t you've created. It is no longer even close to a twin-t, just another malformed phase shift oscillator just waiting for a kick for it to ring into oscillation.

You can stop the oscillation by replacing R5 with a 100K pot to make the feedback adjustable  (just like it is shown in the basic design example)and then you can set  the level of feedback until your circuit is stable again.

In a twin-t symmetry is everything, you've distorted that principle.

 Unless you have very low max Vdd opamps, that resistor in series with the battery is just wasting power.