# Tunable notch Filters for audio frequency

Dear Team,
I have a requirement of tunable notch filter for which I tried two IC's LTC1060 & LTC1067. Below is my requirement description
Requirement description: Tunable notch filter from 1KHz to 5KHz.

Problem/Issue: Whistling noise in the audio frequency band 1KHz to 5KHz
Solution required: To identify and eliminate the noise in the frequency band of 1KHz to 5KHz.
To have a practical solution to identify the noise frequency, we planned to have LTC1060 or LTC1067 in which the notch frequency can be tuned using which we can identify the whistling noise frequency.
So we have taken the circuits as mentioned in the datasheets and tried to simulate the same in LTSpice. The response is not as what is specified.
Please support to get the required tuning of notch frequency.
Problem faced during simulation:
1. LTC1060: The simulation did not show any notch in the graph.
2. LTC1067: I could get a notch but it is not at the frequency as specified in the datasheet which is 1.02KHz, but as per simulation I got it at 4.04KHz. Also as I change the clock frequency, there was no change in the notch frequency. But as per datasheet it is a clock tunable notch filter. Kindly explain
Please let me know where I have gone wrong in the circuit or simulation. Also if I need to modify the circuit to my requirements what are the changes which needs to be taken care off.
The circuits which I have simulated are in the respective datasheets LTC1060 Pg-18 & LTC1067 Pg-20
Awaiting your earnest response.
Kindly do the needful at the earliest.
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• Thanks, Philip. You really helped a lot.

The problem now is the dual supply. Do you see it possible to use 2x LTC3388-3 for achieving +/-5V from a single 9V supply and then use two LDO for powering the system with +/-4.5V for audio application?

• The LTC1067 notch filter can be operated from a single 9V battery and then ac couple the output. Use a 5V LDO for the the LTC6990 VCO circuit.

A switching regulator is not recommended for a low noise signal circuit.

• Yes, that's why my question. But how to redesign the circuit and also I should have a single supply for the LTC6240

• Hi Philip, thanks again! This really helped.

BTW: I see you are creating the notch subtracting a BPF from the original signal and you are adjusting the gain of the original signal in the rest.  Isn't this possible to adjust the gain of the BPF instead? I'm asking this because with the actual design, I'm changing the gain outside the notch zone.

Thanks again, Julian