The classic twin-T (or bridged-T) notch filter can be a royal PITA to set up because it requires three precision resistors and three precision capacitors; to get good notch depth they must all be precisely matched, and to tune the filter the three resistors all have be adjusted in unison while maintaining the exact ratio between them.
There are other notch filter topologies you might look into:
The
Wien bridge notch filter
The
Fliege notch filter
The
bridged-differentiator notch filter, which is very easy to tune, and
An
interesting notch filter I can't find a name for (see 1st diagram on page).
The Q of the bridged-differentiator notch filter can be increased by buffering its output with a unity-gain follower, and connecting the junction of Ra and Rb to the follower's output instead of to ground.
One caution about notch filters: a 50 Hz notch filter will ONLY remove the 50 Hz fundamental frequency, leaving its harmonics, and all other high-frequency noise, unaffected.