A basic Smokey-inspired LM386 guitar power amp.
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.
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.