CW Audio Band Pass Filter

Recent listening to the bottom end of 40m during the CW section of the OC-DX contest really brought home how close CW signals generally are and just how useful a CW filter would be.

A little bit of research led me to the section titled “Inverting Band Pass Filter Circuit” at this site.

Specifically this circuit.

Playing around with the following equations,

I chose the following values.

R1 = 1.5kΩ
C1 = C2 = 220pF
R2 = 800Ω (series 120Ω and 680Ω)

This should mean that my pass band is about 482Hz to 904Hz, which should work pretty well for what I want.

I chose the TL071 as they were readily and reasonably cheaply available.

I breadboarded it up.


For reference,

  • Top blue horizontal is Vout+
  • Top red horizontal is Vcc+
  • Centre blue horizontal is Vcc-
  • Centre red horizontal is Input and Output Earth
  • Lower red horizontal is Input

So far so good.

Plugged in a stereo socket to lead off to one side of my 64Ω headphones and applied power.

To quote my High School Chemistry teacher (Hello Mr Potts), “Not a cracker”.

I did notice that the 9V battery I was using substantially dropped it’s voltage, so I tried it again with an ammeter in circuit.

WOW, it’s drawing 350-400mA.

I have no idea what is going on here and am suspicious that I’ve managed to kill the TL071.

At this point, I’d really welcome some suggestions.

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