I recently commented that I was building a discrete LM386 for a Direct Conversion receiver. I think I was exaggerating my project. In fact I just built a very ordinary push-pull amplifier using a 2N3904 and a 2N3906 as a complementary pair AF amplifier. This is part of an LM386, but there is much more inside that little chip. Dave went much further. His schematic is above.
Here is link to a higher definition image:
Dave writes:
Hello Bill -
Please find enclosed the LT spice circuit for the small audio amp that I developed. I have included numerous notes on the schematic as to component function, suggested values to tweak, etc.
My intention was to come up with a relatively low parts-count design while adhering to the classic three-stage topology [diff pair / voltage amp / voltage follower] that has been used as the basis for so many audio power amplifiers for decades. I'm sure some of the parts could be eliminated at the risk of possible stability issues, but a lot of that also depends on proper layout, length of speaker leads, speaker load impedance, etc.
In any case I thought it would be great if someone wanted to build up this design to see if it works in real life or whether any serious mods are needed to get it to behave (I have no illusion that the spice models are entirely accurate, nor is my analysis thorough).
It should be capable of at least 500mW into an 8 ohm load at 9VDC supply, and over a watt at 12VDC. It should handle a 4 ohm load although at reduced voltage swing on the output, and with increased output transistor heat dissipation requirements.
The output stage is a complimentary compound ("sziklai") pair which should, in theory, maximize voltage swing from a low supply voltage (as compared to the more traditional complimentary Darlington configuration.
The bias current is set by a fixed pair of resistors and could be quite different from the simulated value, so I would be careful on initial power-up to monitor the quiescent current draw. I would guess it need not be more than a few mA or so, and definitely less than 40mA or so. This can be made adjustable if desired.
Regards,
Dave
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So, does anyone out there want to give this a try? You could be helping to save generations of homebrewers from the indignity of using an integrated circuit!
Posts
I can't remember if you had it on here before, but Evil Mad Scientist made these discrete LM741's that I think would be fun to use in a radio: https://shop.evilmadscientist.com/productsmenu/762
ReplyDeleteI was kicking around the idea of doing the LM386 but I am having problems finding original (-ish) documentation of the chip. Only one low res die photo to boot.
ReplyDeleteHi Bill -
ReplyDeleteThanks for posting my schematic. I think Jenny List did a decent job with her link/posting on Hack-A-Day, although it seems from the comments that a number of readers thought from the title that it was going to be a discrete 80386 processor (!) I feel I should clarify to everyone that I hadn't actually looked at the published schematic for the LM386 when I set about designing the little amp, so it wasn't an attempt to reproduce an LM386 topology, but rather than to merely achieve similar functionality (and purely for fun!). I would love to see how this turns out when someone attempts a build and hear about what possible tweaks and variations might be incorporated.
Regards,
Dave
Unfortunately the schematics is too low quality too read all text. Can you upload that at full quality? Or better in svg or other Vector Graphics file.
ReplyDeleteTry clicking on the image. If you still can't read it, please let me know.
ReplyDeleteThe small front text is still unclear after clicking on the image. Any chance of an original file?
ReplyDeleteI'm having trouble posting the LTSpice model (.asc) but if you would send me your e-mail I can send that to you.
ReplyDeleteOK, with Dave's help I put up a link (above) with a high def (8 MB) image of Dave's schematic.
ReplyDelete