[Notch filter musing] Why can't pirates recite the entire alphabet?

[EBK]

I finally got around to testing the circuit again.  It has an extreme amount of background noise, so I wouldn't want to record a demo, but more importantly, although sound comes through now, the notches are imperceptible.  I'll leave it on my breadboard a bit longer, and perhaps I'll mess with it a bit more sometime.

[Rob Strand]

The 200k pot doesn't help for noise.   You could drop that value down to 10k or 20k or so.   You would need to drop those added "DC fix" resistors as well.   I saw some designs with 1k and 2k but that's probably going too far, and it will create other problems.

If I get a chance I can check what value is good noise with spice but I don't expect big improvements over a 10k to 20k pot.

[Rob Strand]

This was quick to do but I only used a noiseless opamp so all the noise comes from the resistors.
I haven't transferred my opamp noise model to LTspice yet.

Noise starts to level off when the pot value is at about 10k ohm.
With a real opamp it might level off a bit higher due to the base-line noise.

                  
Noiseless Opamp (Q=5, 1kHz notch)                                               
                                               
rscale                0.03        0.05        0.1           0.3        0.5          1
Rpot [kohm]      6             10           20           60         100         200
                                               
Noise               dBV        dBV        dBV        dBV        dBV        dBV
A-Weighted:   -108.1    -107.47   -105.9    -102.6    -100.8    -98.1
CCIR-RMS:    -110.8    -109.5     -107.4    -103.3    -101.2    -98.3
                                               
Improvement in Noise over 200k
                            dB            dB          dB          dB           dB                             
A-Weighted:     10.08        9.32        7.87       4.57        2.71       0
CCIR-RMS:      12.41        11.14      9.01      4.95        2.89        0