#095 Little Gem

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

Credits and References

• my Smokey build
• Little Gem & Little Gem MkII
• General Guitar Gadgets A386 Amp
• Smokey Amps - official site
• Smokey Amp Analysis - excellent circuit analysis of the original Smokey amp.
• Observations on the Smokey circuit by Dave Stork
• LM386 datasheet