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AF filter using opamps - substitution of opamps

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I once made a filter for the same purpose: emphasize a single morse code signal amid a mix of other keyers.

I decided on a biquad bandpass filter. It used 3/4 of a 324 quad op amp, plus a few caps and resistors. Fixed frequency around 600 hz. Worked well. To get optimum response I only needed to turn the BFO knob on my SW radio, to match my biquad filter frequency.

The article says something about using 741 op amps in early versions. The schematic has handwriting saying 741 and 324. Not known as being low noise.

The author used 074 type. FET input. I once used a couple FET input op amps (TL084) in projects. They worked for a while, then fried and I didn't understand why. Later I found they have latchup problems when the input voltage is allowed to get too close to the supply rails.

As you build and test the project, you might want to use individual ICs, as well as IC sockets. Especially if you use expensive op amps with finicky fet input specs.

You can start out using cheap op amps and change to the low noise fet type later.

I believe the filter will perform pretty much the same with any general purpose op amp.
 
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    neazoi

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Thank you, this is very informative.
I think I am going to try the single opamp versions, since it will be easier to align the components and also it will be easier to add more stages or less, to achieve variable bandwidth. Will something like the cheap ne5534 prove better opamp?
 

Yes, the 5534 is often quoted as a low-noise high-performance op amp.

At first I thought I could get by with a one opamp bandpass filter. But the graphs for selectivity made me realize I needed higher Q. This was because morse was usually broadcast in just a few spots on the SW radio. Often I could hear 2 or 3 keyers very close together. So I went for the biquad.
 

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