This little amp will work with almost any capacitor running between pin 5 and the speaker, from 1.5 μF to as high as 1500 μF; bass response is much better as you increase the value of the capacitor. If this design seems to put out too much bass for the speaker you want to use (thus causing unwanted distortion), try lowering the value to 47 μF. If the amp yowls and screeches, picks up local radio all by itself, or otherwise misbehaves (weird hissing, thumping, etc.), then add a bypass electrolytic capacitor (like a 4.7, 22, or 47 μF cap), soldering the positive lead to pin 7 of the LM386 and the negative lead to the ground.
By default, the gain (amplification) of the LM386 is set to 20 (i.e., the output voltage is 20 times higher than the input). To get the maximum gain of 200, bridge pins 1 and 8 with a 10 μF electrolytic capacitor instead of a piece of plain wire (the negative leg of the capacitor goes to pin 8).
Throughout the project you were asked to keep some leads as short as possible. As a rule, any wire carrying an audio signal should be less than 8″ long, unless you use shielded wire (which we haven’t)—I like to keep audio leads under 6″, just to be on the safe side. This is to prevent interference from seeping into the circuit (simple amps are especially notorious for picking up local AM stations)—which brings us to a nice cheap-amp parlor trick.
We’re constantly awash in radio waves. AM radio encodes information (such as bloviating pundits and thirty-year-old Dolly Parton hits) by modulating the amplitude, or height, of these waves. The waves themselves are fairly energetic: If you string up a few hundred feet of thin wire—the kind you use to wind an electromagnet—solder it to the positive leg of an LED, and plant the other leg in the ground, the free-floating AM energy is often sufficient to drive the LED to a faint glow. So, why don’t we hear AM radio every time we pick up a phone or turn on any amp? Fortunately, those big, fat AM waves (which have wavelengths thousands of feet long) spend as much time being positive as negative and thus average out to zero (as shown on the left side of Figure 12-13); from a speaker’s perspective, this free-floating AM signal cancels itself out.
To make your Dirt-Cheap Amp into a simple AM radio, get a germanium diode (like a 1N34A) and a guitar cable. Plug the guitar cable into the amp, switch it on, and crank it up to 11. Now hold on to the positive lead of the diode, and touch the negative lead (the one marked with a stripe) to the tip of the cable. If there is a strong AM station in your area, you’ll hear it through the amp. In this simple circuit, your body (which functions as a naturally occurring tank circuit) acts as the antenna; the diode—which is called a detector in this context and is represented by the diode symbol in the middle of Figure 12-13—acts as a rectifier, cutting out the bottom half of the wave. Since the signal no longer averages to zero (canceling itself out), the sound is detectable by our amplifier. Fiat vox.
Peter Frampton’s signature hit is the 14-minute extended cut of “Do You Feel Like We Do?” on Frampton Comes Alive!. The song peaks with Frampton’s famous “talking guitar” solo. You can easily emulate this sound by gluing a funnel over the speaker of your Dirt-Cheap Amp. Run a flexible plastic tube from the funnel into your mouth, and put your mouth close to a plain old microphone (I’m not making this up; this Rube Goldberg hack is how Frampton got the effect originally). Plug an instrument into the gimmicked amp, crank it up loud, play, and use your mouth to mold the sound.