Suggestions for simple audio notch filter

[Learner]

Hi guys,
I am looking for a simple notch filter with minimal component, can anyone suggest any?

I have built a 2 op amp notch filter(inverting amplifier driving RC filter with a buffer feeding back to the + ve terminal of the inverting amp to control the bandwidth), the first one works OK but when I modularised them in series I will run into oscillation at some extreme setting. The sweeping frequency is not terrible wide in this particular circuit eg from 300hz~15Khz. I am using 50k ohm dual gang linear pot for the frequency control and 100 k ohm for feedback (bandwidth)damping control, I'll post the circuit layout if anyone wants to see it?

I was wondering if there is anything else I can use besides building a state-variable filter and sum the output HP and LP, since it takes too many parts when you start modularising the circuit.

I am looking to build a 5 band notch filter with independent Q and frequency control, does not require a huge frequency sweeping range but would prefer to have high Q of up to 100.

Thanks!

 

[flatulent]

Look for a "twin T network" reference such as http://www-k.ext.ti.com/SRVS/Data/ti/KnowledgeBases/analog/document/faqs/tt.htm

This is a six element totally passive network that can be a notch. Being passive, it should not oscilllate.

 

[Learner]

Hi flatulent,
Thanks for the link, that's actually the first notch filter I built using the twin T network which I found from the IC op-amp cook book by Walter Jung.

I used a dual gang variable resistor to control R1 and R2 which is = R as the control for Fo, it caused a lot of oscillation and instability I guess I should of used a variable resistor in series with R3 for the Fo control like sugeested in the link. Wonder if anyone knows what would be the Fo sweep range using this method? Would've been very helpful if it was given on the link but I think it will cause a lot of oscillation/instability too if it exceed a certain limit.......

Therefore, I think the best way is to keep R3 in ratio with R constant at all time for circuit stability. Since R3 = R/2 so I think I should probably to use a 3 pole rotary switch to control the Fo and keep R3 = R/2 constant at all time, as a absolute necessity....

I just wish I knew that earlier instead of building serveral circuits and start all over again, now the other headache is should I use switch cap or switch resistor on the rotary switch.......

Back to square 1 and start unsoldering to begin recycle the parts, again!! :x