Dead-Bug Bullfrog
A one-chip LM386 speaker amp, point-to-point · By Onion Madder · Mess O' Pedals
Every noise box in this series ends the same way: “now get it out of the box.” A raw logic chip can't push a speaker - straight into 8 Ω you get a whisper. The LM386 is the fix: a tiny 8-pin audio power amplifier that takes a line-level signal and swings a small speaker to a genuinely rude volume, on a battery, with a handful of parts. It's small and it is startlingly loud - hence the frog. Built dead-bug, of course: legs flattened out flat and every part soldered straight to them. No breadboard, no PCB.
This is the amp the whole dead-bug series has been pointing at. The Choir, the Cicada, the Screamer - all of them end with “feed it into an LM386.” This is that LM386. Build it once and every other box gets a voice. Same vape-pack power, same method.
⚠ Before you start tap to expand / collapse
Hot iron, obviously. 350°C of don't-grab-that. Ventilate, don't breathe the flux smoke, park it in its stand every time.
Polarity matters on four caps here - the gain cap, the bypass cap, the big output cap, and the supply cap are all electrolytics. Backwards electrolytics bulge or pop. The stripe marks the minus leg.
Mind the supply ceiling. The LM386 runs about 4V to 12V. Two or three vape cells (7.4–11.1V) or a 9V battery are all fine; don't feed it much past 12V.
It will get loud. Turn the volume down before first power-up, and keep the speaker away from your ear while you test.
If you power it from a salvaged lithium pack, the usual cell cautions apply - see Do You Want to Survive the Vapocalypse?
How it works, in one breath
The LM386 is a power op-amp with its feedback baked in. Feed a small audio signal into the + input (pin 3), ground the − input (pin 2), and it comes out of pin 5 big enough to move a speaker. Its gain is fixed at 20 by an internal resistor - but bridge pins 1 and 8 with a capacitor and you bypass that resistor to jump the gain to 200. A coupling cap on the output blocks DC from the speaker, a little Zobel network (a resistor and cap to ground) keeps it from breaking into supersonic howl, and that's the whole amplifier. One chip, one speaker, loud.
What you'll need
New to the parts? Each line notes how to spot it - the printed marking, the package, or the polarity clue.
- 1× LM386 (LM386N-1, -3, or -4) low-voltage audio power amp, 8-pin DIP - spot it: LM386 printed on top; a notch or dot marks pin 1. The -4 suffix takes the highest supply and gives the most output.
- 1× 10K audio-taper pot for volume. Spot it: pots marked A = audio/log taper (use this for a natural-feeling volume knob); 10K or 103 for the value.
- 1× 0.1 µF ceramic - input coupling (printed 104). Blocks DC from whatever you plug in.
- 1× 10 µF electrolytic - the gain cap across pins 1–8. Spot it: the stripe marks the minus leg; the + goes to pin 1. Leave this out and the amp runs at gain 20 (quieter, cleaner).
- 1× 10 µF electrolytic - the bypass cap on pin 7 to ground (+ to pin 7). Optional; it knocks down supply hum.
- 1× 220 µF electrolytic - the output coupling cap. Big can; the + goes to pin 5 (the output), minus to the speaker.
- 1× 10 Ω resistor + 1× 0.047 µF cap (printed 473) - the Zobel network, wired in series from pin 5 to ground. This is the anti-howl part; don't skip it.
- 1× 100 µF electrolytic + 1× 0.1 µF ceramic - supply decoupling across V+ and ground, close to the chip.
- 1× 8 Ω speaker (0.5W or more). A 4 Ω speaker is louder but works the chip harder; 8 Ω is the safe default.
- 1× SPST toggle for on/off - a 2-lug switch.
- 1× Power source - two or three vape cells in series (7.4–11.1V), or a 9V battery.
- 1× LED + a series resistor (~1K–10K) as a power indicator. Spot it: the longer leg is + (anode).
- Solid-core hookup wire, soldering iron, cutters, multimeter.
Quick reference - the whole build on one card
Pinout plus every connection at a glance. The steps below walk through it slowly; this is the bench card.
Notch marks pin 1. Keep the chip label-up and this map reads exactly as you see it. Signal in the +IN (pin 3), out the OUT (pin 5), and the two GAIN pins (1 & 8) are how loud it gets.
Power
- Pin 6 → V+ (4–12V)
- Pin 4 → ground
- 100µF + 0.1µF across V+ ↔ ground
Input
- Signal → 10K vol pot → 0.1µF → pin 3
- Pin 2 → ground
Gain
- Pins 1–8 open = gain 20 (quieter)
- 10µF across 1–8 (+ to pin 1) = gain 200 (loud)
- Pin 7 → 10µF → ground (bypass, optional)
Output
- Pin 5 → 220µF (+ to pin 5) → speaker → ground
- Pin 5 → 10Ω + 0.047µF → ground (Zobel)
Step 1 Presolder everything, then mount the panel parts
Same discipline as the rest of the series: tin every leg and every wire end at the bench before the chip comes out. Mount the volume pot, the LED, the toggle, and the speaker (or a jack to it) in the enclosure first, so the chip drops into a finished harness. Snap the pot's anti-rotation tab off if your box has no hole for it.
Step 2 Flatten the legs — and leave it label-up
Bend all eight legs out flat so they splay sideways. Those flattened legs are your tie points. Do not flip the chip over - flipping mirrors the pinout. Leave it label-up, notch where you can see it, and the map above matches the chip exactly.
Step 3 Power and decoupling
Hook up the two power pins and drop the decoupling caps right across the chip. The LM386 pulls hard current spikes when it's driving a speaker, so good decoupling is what keeps it clean - a big 100 µF for the spikes and a small 0.1 µF for the fast stuff, both as close to pins 6 and 4 as you can get them.
| Connect | To | Why |
|---|---|---|
| Pin 6 | V+ | Supply positive (VS) |
| Pin 4 | Ground | Supply negative |
| 100 µF (+ to pin 6) | Pin 6 → Pin 4 | Bulk decoupling |
| 0.1 µF | Pin 6 → Pin 4 | High-frequency decoupling |
Step 4 Input and volume
Signal comes in through the volume pot and a coupling cap into the + input (pin 3). Ground the − input (pin 2). Wire the pot as a divider: input to one end, ground to the other, wiper to the cap, cap to pin 3.
| Connect | To | Part |
|---|---|---|
| Signal in | Volume pot (outer lug) | 10K audio taper |
| Other outer lug | Ground | — |
| Pot wiper | Pin 3 (+IN) | via 0.1 µF coupling cap |
| Pin 2 (−IN) | Ground | — |
Step 5 Set the gain
This one decision is how loud it gets. The internal gain is 20 with pins 1 and 8 left open. Bridge them with a 10 µF cap (+ leg to pin 1) and the gain jumps to 200. For a noise-box amp you almost always want the loud option.
| You want | Wire | Result |
|---|---|---|
| Loud (default here) | 10 µF across pins 1–8, + to pin 1 | Gain 200 |
| Cleaner / quieter | Leave pins 1 & 8 open | Gain 20 |
| Somewhere between | 10 µF + a resistor in series across 1–8 | Gain 20–200 |
| Less hum | Pin 7 → 10 µF → ground (+ to pin 7) | Bypass cap (optional) |
Step 6 Output, speaker, and the anti-howl network
The output on pin 5 sits at about half the supply voltage, so it needs a coupling cap to keep that DC out of the speaker: a big 220 µF, plus leg to pin 5, minus to the speaker, and the speaker's other side to ground. Then the part people skip and regret: the Zobel network, a 10 Ω resistor in series with a 0.047 µF cap, from pin 5 to ground. It tames the speaker's rising impedance and stops the amp oscillating into an ear-splitting supersonic howl.
| Connect | To | Part |
|---|---|---|
| Pin 5 (OUT) | Speaker (+ side) | via 220 µF, + to pin 5 |
| Speaker (other side) | Ground | 8 Ω |
| Pin 5 (OUT) | Ground | 10 Ω + 0.047 µF in series (Zobel) |
Step 7 Power up
Wire your SPST toggle in the V+ line. Turn the volume all the way down first. Flip it on, plug a signal into the input - a phone, a Choir, a Cicada - and bring the volume up slowly. It should get loud fast and stay clean until you push it into crunch at the top. A little grit at full tilt is normal and, honestly, part of the fun.
Voltage note tap to expand / collapse
The LM386 runs about 4V to 12V. More volts means more headroom and more volume before it clips. Two or three vape cells in series (7.4–11.1V) or a 9V battery are all in range; keep it under 12V. A single 3.7V cell technically works but is right at the floor and will sound weak.
Step 8 If it won't behave
- Loud squeal or supersonic howl? The Zobel network is missing or mis-wired. 10 Ω in series with 0.047 µF, from pin 5 to ground. Also keep the input wires away from the output/speaker wires.
- Hum or buzz? Add or reseat the decoupling caps at pins 6/4, and add the 10 µF bypass cap on pin 7. Star-ground everything back to one point.
- Quiet even at full volume? Check the gain cap across pins 1–8 (+ to pin 1) - without it you're stuck at gain 20.
- No sound at all? Confirm pin 6 at V+ and pin 4 at ground, the signal actually reaches pin 3, and pin 2 is grounded.
- Distorted and hot? Supply too high, speaker impedance too low, or the output cap in backwards. Check the 220 µF polarity - + to pin 5.
Going further
- Bass boost. The datasheet's classic trick: a 10K resistor and 0.033 µF cap in series from pin 1 to pin 5 lifts the low end. Great for making a tiny speaker sound bigger.
- A real input jack. Add a 1/4″ or 3.5mm jack before the volume pot and the Bullfrog becomes a standalone practice amp for anything, not just your noise boxes.
- Build it into the noise box. Instead of a separate amp, drop the LM386 stage straight onto the end of a Choir or Cicada so the whole instrument is self-contained and loud on its own.
- Two of them = stereo. The chip is cheap; a matched pair gives you a little stereo pair for spread-out, ping-ponging patches.
Build this one and the whole series wakes up. Every box that used to whisper into a jack now has its own loud little voice. Show me what you make.
The LM386 application circuit - gain cap, output coupling cap, and Zobel network - is straight out of the chip's datasheet and forty years of hobbyist use. This write-up, wiring, and words are my own. Build it, bend it, make it yours.