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Build an AM, CW and SSB Regenerative Receiver for Amateur Radio

Build an AM, CW and SSB Regenerative Receiver for Amateur Radio

5. Audio Frequency Board

AF circuit breadboard
AF circuit breadboard
Above — AF circuit breadboard
Regen#5 features an op-amp plus discrete component AF board. I love analog design and the exquisite control that choosing discrete parts offers. My goal =  low distortion + low noise from AF input to loudspeaker.

Preamplifier

Nothing special here. A 0.1 µF input bypass rolls off noise and prevents local AM signals from entering, getting rectified and amplified by the AF chain. I opted to not include a multiple pole low-pass filter chain for dissecting CW/SSB pile ups, or a tone control circuit, but in a keeper-grade radio, I would add 1 or both.

I don’t run band-pass AF filtering in my receivers, but that’s another option. With all the free, online software, designing good AF filters has never been easier.
You may boost the gain of the op-amps by increasing the feedback resistor value. Too much gain on strong signals may cause a spasm of feedback and create celestial noises.



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PA

I’ve discussed this circuit before on this blog post.
Except now, I’ve solved a distortion problem caused by closing the loop in the op-amp connected to the power followers. In high gain feedback amplifiers, it does not take much time delay or phase lag to cause oscillation at high frequencies near the upper end of the bandwidth.  The small 22 pF feedback capacitor lowers the closed loop bandwidth so that there is insufficient gain at high frequencies for oscillation to occur.

FFT of the audio amplifier board.
Above — FFT of the audio amplifier board. Swinging 10 Vpp with the strongest harmonic at – 57 dBc between a 12.2 to 0 VDC single-supply rail provided 1 of the happiest accomplishments in my fledgling amateur radio designer career. I hope to better this 1 day, but that will prove difficult.

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