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Hornet Amp

The one-chip speaker amp, soldered down for good.

The Dead-Bug Hornet on copper. One LM386, a volume pot and a speaker: the part every other build on this site eventually asks for, because a 555 or a CMOS chip will drive a small speaker badly and an LM386 will drive it properly. Build it once and every noise box you own gets a voice.

The Hornet layout drawn as a stripboard diagram, titled Hornet - LM386 amp, 14 by 12 stripboard. Twelve orange copper strips lettered a to l run left to right with fourteen columns numbered above them. A pale rectangle marked IC1 LM386 sits left of center with four gray X marks through the strips beneath it where they are cut. Outlined rectangles standing upright are the resistors R1 and R2, small outlined boxes the ceramic capacitors C1, C5 and C7, and pink circles the electrolytics C2, C3, C4 and C6. Five straight lines are the wire links J1 to J5. Holes carrying wires off the board are labeled in the margins: LED plus, 9V, GND, IN, SPK plus, SPK minus and LED minus. A parts list runs underneath and a credit line reads Made with Copper Bottom. Below it the same board is drawn a second time from the solder side, mirrored, with the column numbers running right to left, every solder point as a dark dot, the cuts as X marks, each chip as a dashed outline with pin 1 boxed and the off-board pads labeled on the margin they now sit nearest.
The whole build, drawn in Copper Bottom. Fourteen columns by twelve strips, four cuts under the chip, five wire links. The file below is the build document; this picture is generated from it, so the two cannot disagree. The lower drawing is the solder side, mirrored - the side you actually work on.
The finished board on a dark desk, wired but not boxed. A small rectangle of orange stripboard sits in the middle with the LM386, four black electrolytic capacitors and a few small parts. A black speaker lies to its left on white wires, its cone lit red by the LED of a purple pedal behind it lettered duet, tempo and tone, whose purple cable runs down to a quarter-inch jack on flying leads beside the board. A small blue toggle switch hangs off three leads to the right, a green 9V battery sits further right on red and black leads, and a lit LED on its own two leads lies at the bottom of the frame. A pink keyboard and jars of paint sit behind.
Built and working, September 2026. Everything off the board is still on flying leads: the speaker, the LED, a power toggle, the 9V and the input jack, with the Duet playing into it. The volume pot went on after this was taken.

What you'll need

tap to expand / collapse
  • 1× LM386 audio power amplifier, 8-pin DIP - spot it: LM386 printed on top, with a notch or a dot marking pin 1. The N-1, N-3 and N-4 grades all work here
  • 1× 220µF electrolytic (C4) - spot it: the biggest can in the pile. Stripe marks the minus leg, and that leg is the shorter one
  • 1× 100µF electrolytic (C6) - spot it: the next size down. Stripe = minus, same as C4
  • 2× 10µF electrolytic (C2, C3) - spot it: small cans, both polarised
  • 2× 100nF ceramic (C1, C7) - spot it: little disc marked 104. Not fussy which way round
  • 1× 47nF ceramic (C5) - spot it: disc marked 473. Half of the Zobel network
  • 1× 10R resistor (R1) - spot it: brown – black – black, then gold. The other half of the Zobel
  • 1× 1k resistor (R2) - spot it: brown – black – red, then gold. It feeds the power LED and nothing else
  • Stripboard, at least 14 columns by 12 strips, plus solid-core wire for the links - spot it: five links on this layout

Off the board

  • 1× LED, any color - spot it: the panel indicator. It is optional - leave R2 and the LED out and the amp is unchanged
  • 1× volume pot, 10k audio - spot it: wired as a divider, all three lugs used
  • 1× 8Ω speaker - spot it: anything small. This is the whole point of the build
  • 4V to 12V supply - spot it: a 9V battery is the usual answer. The LM386 will not take more than 12V
  • 1× switched input jack, 1/4 in - spot it: the kind with a third contact that rests on the tip when nothing is plugged in. Wire that contact to the sleeve, so an empty jack shorts the input to ground. Not optional. With a plain jack the open tip is an antenna straight into pin 3, and every one of these I have built hummed and picked up AM radio until the jack was switched. The tip feeds the volume pot, and the pot's wiper goes to IN. Building it into a box with the input soldered straight to the pads needs no jack at all, and has no hum at all
  • 1× DC jack or a 9V battery snap
  • Bug Bus
  • Bug Mixer

The layout file is the guide

Every coordinate lives in the layout file rather than being retyped here, which is the point: open it in Copper Bottom and it will walk you through the build hole by hole, check the copper against the netlist, and print a bench sheet. Nothing on this page can drift from it because nothing on this page repeats it.

The build document. ↓ Download the Hornet layout (.json) and import it into Copper Bottom.
Twelve strips by fourteen columns. Four cuts, all of them under the chip, and five wire links.

The netlist

Quoted from the layout file exactly as it appears there, because it is the contract: if your board matches this, it is right. Copper Bottom checks it for you.

The off-board wiring

Seven pads leave the board. The volume pot is the only one that needs explaining.

Pad → where it goes
PadGoes to
9VSupply positive, 4V to 12V
GNDSupply negative. The speaker return and the LED cathode share it
INThe volume pot's wiper
SPK+Speaker, hot leg - it comes off the 220µF output capacitor
SPK−Speaker return. It sits on the ground net
LED+Panel LED anode, fed through R2
LED−Panel LED cathode, on the ground net

The volume pot is a divider, not a variable resistor. All three lugs are used: one outer to the input jack tip, the other outer to GND, and the wiper to IN. Wired that way it reaches silence at one end, which is what a volume control is for. Two lugs would give you a level trim that never quite shuts up. The input jack is switched, its shorting contact tied to its sleeve, so with nothing plugged in the pot's top lug sits at ground and the amp is silent. The drawing shows that tie as the dashed wire on the jack.

Don't eyeball the holes. There is a 1:1 drilling template for the 125B - the box drawn unfolded, so you wrap the sheet around it and every hole lands by folding instead of by measuring. Five holes: the LED, the volume knob, the input jack, the supply and an output jack.
↓ Same box, jacks on top (PDF)
The second sheet keeps every control where this one has it and moves the jacks - power and audio - to the top wall, so the cables leave upward. Two ways to drill one box; pick one.
Print at 100% / “Actual size”, never “Fit to page” - check the bar measures exactly 4″ before you touch a drill. The face is 66 by 121 mm but only 42 mm of it is drillable once the lid lip is clear.
Nobody has drilled this one yet. The hole sizes come from the layout file; where they sit is this format applied to it. Offer the sheet up to the box and check it against your own parts before you make a hole.

Built, and heard

Built on the bench in September 2026, straight off the file above, and it works. It is loud. In the photo it is amplifying the Duet into a bare speaker off a 9V battery, with the LED, a power toggle and the input jack still on flying leads. The volume pot went on after the photo, wired as the pad table says, and works fine. The toggle is the power switch and is not in the layout file. Not boxed yet; the drill template above is still a proposal.

Radio and hum with nothing plugged in

Every one I have built does this: leave the input empty, turn it up, and it hums and picks up AM radio. It is the design, not the build, and three things stack up to do it. With nothing in the jack, the tip, the wire to the pot and the pot are an open wire feeding straight into pin 3 through C1, with nothing at the jack to shunt it and nothing to filter it - an antenna. The LM386's input stage is a pair of PNP transistors, and a transistor junction rectifies radio frequency, which is exactly what an AM detector does, so the chip demodulates whatever the wire picks up and then amplifies it. And gain 200 is the chip's maximum, put there so a microphone can drive a speaker; a CMOS oscillator putting out full-rail squares is thousands of times hotter than a microphone, so the amp is set to hear the antenna as loudly as it can while the signal you feed it is already clipping at gain 20. The hum is the same antenna at 60 Hz, worse on flying leads, on a bare board with no box around it, and on a wall wart.

The build above already carries the fix: the input jack is a switched one, its shorting contact tied to the sleeve, so an empty jack grounds the input. Silence when idle and no other change, confirmed on my board, which is why it is in the parts list and on the drawing rather than down here as a suggestion. Three more, untested here, if you used a plain jack or the hum is still there. Leave C2 out: no cap across pins 1 and 8 is gain 20, still plenty for these oscillators, and a tenth of the pickup - the Bridged Amp runs at 50 for the same reason. An RF stop at pin 3: 1K to 4.7K in series into the pin and 100pF to 1nF from pin 3 to ground, both at the chip, not at the jack, so audio passes and radio never reaches the detector. And short, twisted input leads kept away from the speaker wires, inside a grounded metal box, which is a shield. And the case where none of this applies: a Hornet built into something, with its input soldered straight to the pads - in line with a Dragonfly inside one box, say - has no jack to stand open, and the hum is gone entirely. That is how it sits in the Bug Box and in my mini bug box.

How it works, in one breath

The LM386 is a power op-amp with its feedback already inside it. On its own it gives a gain of 20. C2 across pins 1 and 8 takes the internal feedback resistor out of circuit and the gain goes to 200, which is the setting this layout uses and the reason it is loud enough to be worth building.

Everything else is housekeeping. C1 couples the input in and blocks DC from the pot. C3 on pin 7 bypasses the internal bias divider, which quietens the hiss. C4, the big one, couples the output to the speaker and keeps the chip's DC offset out of the coil. R1 and C5 are a Zobel network: a speaker is not a resistor and gets more inductive as the frequency rises, and that pair gives the amplifier something resistive to look at up there so it stays stable instead of oscillating. C6 and C7 hold the supply still, which an amplifier drawing current in bursts very much needs.

R2 and the LED are optional. They are a power indicator hung off the supply and nothing else in the circuit depends on them. Leave both out and the amplifier behaves identically.

LM386 application circuit from the datasheet. Layout drawn in Copper Bottom by Onion Madder.