try of 3 transistor audio amp build from 2014
A look at a 2014 effort at a possible discrete transistor replacement circuit for the LM386. You know the LM386, the chip everyone loves to hate:
https://wb9kzy.blogspot.com/2023/07/lm386-measurements.html
But it has a couple of benefits that people don't consider. First, low quiescent current - this is the current used when the chip is NOT amplifying, I think of it as a benchmark for battery life. The LM386 has a spec of 8 mA with a supply voltage of 6 V but the NJM386D was measured at 2.7 ma at 9 volts. The second benefit is up to 46 db of gain.
The first circuit for a discrete LM386 killer was developed from an old Heathkit GR-24 transistor radio schematic (just the audio amp is shown):
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| those voltage readings in the ovals are a nice touch |
The GR-24 was an AM only portable radio, powered by 6 D cells, not a lightweight, a 9V supply with a 3.2 ohm speaker. The GR-24 would have had a nice "tone", certainly better than the small 9V battery shirt pocket radios. I made changes of course, not having access to the exact same parts (2n2219 transistors with the little metal tabs were used on the output) but here is a photo of what I came up with:
The results: .1 V p-p is amplified to 4 V p-p where symmetrical clipping will begin. 8 ohm load, 9 volt supply, 8.1 ma supply idle (quiescent) current:
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so a voltage ratio of 40 or 32 db of gain - the quiescent current could probably be lowered with the addition of a pot in series with R16, the 6800 ohm resistor.
4 V p-p into 8 ohms is 250 mW, 4 V p-p into 3.2 ohms is 625 mW, so it's loud enough to be obnoxious. When clipping occurs it isn't that bad, it's symmetrical and possible smoothed a little by the transformer ?
Bottom line: it's a decent amp but not an LM386 killer, good gain however the quiescent current is a little too high, power output slightly low, but mainly the transformers are the main problem - they add expense, weight and are getting harder to source all the time so didn't bother to look at parts cost, size, number of parts compared to the LM386.
Best Regards,
Chuck, WB9KZY
http://wb9kzy.com/ham.htm
