[sdiy] Notch Filter for Logon's Human-Capacitor Filter

[Aaron Lanterman]

Hi gang: One of my students, Logan, has been trying to post a question to 
the list, and it seems to be going into a bit bucket. He's been receiving 
mail from the list, so he's not sure what the trouble is...

He's basically built a Theremin-inspired Sallen-Key filter where you, the 
operator, are one of the capacitors. How cool is that?

Anyway, since I seem to be able to forward to the list, I told him I'd 
forward it for him... see below.

- Aaron

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     I'm a student in Aaron Lanterman's synthesizer class. I'm working on a 
circuit that allows the user to control the cutoff frequencies of a high 
pass and low pass filter through capacitance with an antenna. However, one 
of the problems I have encountered is 60 Hz wall voltage being coupled 
into my circuit through the antenna. To me, the easiest solution seems to 
be putting a 60 Hz notch filter at the output of the circuit. In an effort 
to avoid inductors, I found this circuit which can be downloaded as a PDF 
from this link. www.national.com/ms/LB/LB-5.pdf or can be found quickly 
through a search engine by searching for "High Q Notch Filter." I am 
trying the circuit configuration in figure 1 in the pdf file. However, my 
simulations with PSPICE are showing me a frequency response that is hardly 
a notch filter at all. Note, I am using a TL084 Op Amp rather than the 
LM102 used by National Semiconductor. It has been suggested to me that the 
input impedance of the TL084 is too low, but even when I used the "ideal" 
Op Amp the simulation seems to fail. I would greatly appreciate any ideas 
from anyone on why this might be happening.

Thanks,

Logan Snow