Ugly Circuit Boards - for experienced electronics makers: Video posted Jan 12, 2025

Video posted Jan 12, 2025 :: https://youtu.be/-is1NowfrtA

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Above — Schematic of the so simple AF presented in the video.

Above — The so simple PA stage in a test bench setup: 1KHz signal generator, 8 Ω dummy load and DSO.

Above — Clean output FFT of the so simple PA when driven to 288 mW signal power across the dummy load.
 

Above — 2 users requested a higher power version with (hopefully) the same parts count. I employed 2  packaged Darlington emitter  followers -- the TIP122 / TIP127 pair instead of the 2N4401 / 2N4403 followers.

 

Above — I had to add 2 diodes to the PA bias stack, plus an additional DC filter capacitor  which increased the parts count by 3 items. The 22 Ω DC low-pass filter is now moved on the other side of the NPN transistor to avoid a big voltage drop across that resistor with the higher DC current drive of the 'big guns' PA pair. A 470 µF bypass cap gets added to the DC supply of the TIP122 to low-pass filter the output followers. 

 Above —  Clean output in time domain with the alternate so simple PA  driven to 700 mW signal power across the dummy load.

 
 
Above —  Clean output FFT of the alternate so simple PA when driven to 700 mW signal power across the dummy load.

Addendum Dec 6,  2025

Above —  Another, even better PA. Rather than drive 2 successive complimentary emitter followers pairs per standard, I went with the complimentary feedback pair Q2|Q4 and Q3|Q5. The gain of the finals is 1.x – not too high so that it creates oscillations above 100 KHz.

They’re easy to drive, very stable, and do not require a current source to drive fairly well. I went with an adjustable bias transistor with a trim pot to carefully control Vbias. I decided on a relatively low 15 mA quiescent currrent.

If I increase vBias to a quiescent current ~ 50 mA , I can reduce the ~2 KHz harmonic by 3-4 dB. All scope shots are at 15 mA quiescent current. It’s always fun to get a swing of 10 VAC peak-to-peak in a ~12 VDC single DC supply audio PA. This audio distortion performance beats all the commercial Ham radio receiver AF PA amps I’ve ever tested, or read test reports about -- even with 15 mA quiescent.

Further, output networks such as the Zobel [ 10 ohm resistor in series with a small cap shunt to ground ] , plus a feedback network across the 18K resistor to reduce closed loop gain at higher frequencies should help when running a speaker.

Reducing the entire PA stage ‘s voltage gain by lowering the feedback resistor value (from 18K) also lowered distortion on my FFT. Distortion is an inverse function of the closed loop gain. 

Above — FFT of the complimentary feedback pair driven to ~ 3 Vpp ( 146 mW). 
 

Above — FFT driven to ~9 Vpp or 1.34 Watts at ~1 KHz.

Above — FFT driven to 10 Vpp or 1.63 Watts at ~1 KHz. It's always fun to get 10 Vk-pk on a ~12 VDC single supply power supply.  HiFi grade.