Phono Pre-amp

Basic hum filter circuit in the DC power for opamps is an LC circuit made up as a Pi or Twin T.

The simple circuit I'm using is inserted immediately after the diode bridge. It consists of
- series 1/2 ohm resistors on each rail.
- a parallel filter cap -- 10mf polypro -- across the rails.
- the common mode choke 20mh so that any ac component in the lines is stomped on
- another filter cap forming a dual pi filters
- a 43 volt zener (1N5367BG) across the rails to protect from overvoltage.


Twin T

Low noise preamp notes.

Twin T Notch filter design.

F = 10^6/(2 πRC) = notch frequency
where R = resistance in ohms = (√2)(ZO) typical; ZO = source/load impedance in ohms; and C = capacitance in microfarads.

Cool web site with passive low pass information:
http://age-web.age.uiuc.edu/faculty/qzhang/teaching/ABE425/2006%20Lec05_SC_AmpFilt.ppt#322,25,Design of High Pass Active Filters

And another cool site.

http://www.ee.calpoly.edu/~taufik/coursesh/ee212/Filter2.pdf

Library of Congress call number for a filter book...
621.3815324\HOR

COMMON MODE CHOKE

See schematic in 6.15(d) of this book sample...
http://books.google.com/books?id=m86wSQDKKJEC&pg=PA141&lpg=PA141&dq=bifilar+inductor+noise+%22common+mode+choke%22+-globalspec+-patent&source=web&ots=R9K120nQMO&sig=NLHSBr5ps2LEJ09sgLd8d3D9cBM#PPA141,M1

Practical transformer handbook by Irving Gottlieb.