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.
- 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.