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Project Notes

#095 LittleGem/A386

A basic Smokey-inspired LM386 guitar power amp.

Notes

The “Little Gem” amplifier by runoffgroove is one of numerous LM386-based amplifiers descending from the original Smokey amp. The “A386” is a “Little Gem” variant by General Guitar Gadgets that basically packages the circuit with a few more frills like a conventional pedal.

The Smokey is about as basic as you can get - an LM386 and two caps, see my Smokey build here.

Little Gem/A386 takes some of the suggestions by Dave Stork and enhances the circuit in a few ways:

  • adds variable gain control ranging from about 25 to 200 (the original Smokey was fixed at 200)
  • adds input AC coupling with a 10nF capacitor and removes the 47μF bypass capacitor
  • adds output-side master volume control
  • adds an output-compensating Zobel Network
  • increases the output coupling capacitor from 47μF to 220μF to improve low-end performance
  • A386 modifications add a power indicator, power-source selection and reverse-current protection

I didn’t have anything as small as a 25Ω rheostat for output volume control, so it uses a 22Ω resistor in parallel with a 500kΩ trimpot which gives a range of about 0-21Ω.

The breadboard build as described below is pretty temperamental due to interference and the possibility of dodgy connections. But even on a breadboard it’s good enough to say it “basically works”:

  • seems to lose the higher frequencies so the clean sound is pretty flat
  • the overdrive is surprisingly warm but with the fizzle of electronic distortion
  • it can’t handle very high gain without farting out and oscillating heavily
  • it drives some 6Ω speakers to surprisingly loud levels - enough for the neighbours to hear!

I might put the components on a PCB next to get a better gauge of the true performance.

Construction

Breadboard

The Schematic

The Build

Credits and References

About LEAP#95 OpAmpAudio
Project Source on GitHub Return to the LEAP Catalog

This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is my personal collection of electronics projects, usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting (IMHO!).

The projects are usually inspired by things found wild on the net, or ideas from the sources such as:

Feel free to borrow liberally, and if you spot any issues do let me know. See the individual projects for credits where due. There are even now a few projects contributed by others - send your own over in a pull request if you would also like to add to this collection.