Simple Ring Modulator

Four diodes and two transformers, drawn in Copper Bottom · By Onion Madder · Mess O' Pedals

A ring modulator multiplies two signals together and gives you the sum and the difference, with neither of the originals getting through. It is the sound of the Daleks, of a bell that is not quite a bell, of a guitar turning into a machine. The classic circuit is passive: four diodes in a ring between two transformers, and not a volt of supply anywhere. No chip, no battery, no power pads. Feed it two things and it works.

The Simple Ring Modulator layout drawn as a stripboard diagram, titled Simple Ring Modulator, 24 by 8 stripboard. Eight orange copper strips lettered a to h run left to right, with twenty-four columns numbered above. Two pale square transformers sit toward each end, T1 at columns 3 to 7 and T2 at columns 17 to 21, each labeled 10kCT to 10kCT and showing five legs: P1, PCT and P2 down one side, S1 and S2 on the other. Between them four small outlined diodes, D1 to D4, stand in the middle columns and form the ring; two of them are drawn upright rather than lying flat. Three film capacitors labeled C1, C2 and C3 sit at the left edge, the right edge and the top right. Three dark blue link wires drop to the ground strip along the bottom. Dark blue dots mark the holes where wires leave the board, labeled in the margins: MOD_IN, CAR_IN and GND on the left, OUT on the right. A parts list runs underneath, and a credit line reads Made with Copper Bottom. — The board as the layout file draws it. Every part, link, cut and off-board pad in one picture. The file itself is below.

What you'll need

tap to expand / collapse

4× 1N4148 diode - spot it: a small glass body with a black band at one end, and that band is the cathode. All four go in the ring, and their direction is the circuit: two point one way, two the other. Get one backwards and you have a rectifier instead of a modulator
2× audio transformer, center-tapped - the reference calls for a Triad TY-141P (10k CT to 10k CT), and its sister the TY-145P (600 CT to 600 CT) is the same bobbin if that is what you can get. Spot it: six pins in two rows of three, with a dot on the flange above pin 1. Pins 1-2-3 are one winding and 4-5-6 the other, the center tap being the middle pin of each. These are the expensive part and the one to buy first, because the layout is drawn around them. Their pins do not land on a 0.1in grid: the real spacing is 0.187in between pins and 0.420in between the windings, so each pin takes a small bend to reach 0.2in and 0.4in. Plan for that before you solder
3× 2u2 film capacitor - the coupling caps. No polarity, so they cannot go in backwards. They keep any DC on your inputs and outputs out of the transformers
1× stripboard, 8 strips × 24 holes or larger
Three wire links and hookup wire for the four off-board leads. The links only carry the two windings and one center tap down to the ground strip
Six track cuts. Three of them split a strip between a transformer's two windings; the other three give a coupling cap something to bridge, which is the whole reason a cap on a single strip does nothing

The layout file is the guide

Like the 555 theremin and the Opto Duet, the build document here is the file itself: ↓ Download Simple_Ring_Modulator.json

Open Copper Bottom and import it. The board is 8 strips of 24 holes, with 6 track cuts, 3 links and 12 parts, and nothing spare around the edges. The editor draws it, walks you through placing it in a sensible order, checks the copper against the netlist, and prints the whole thing at 1:1 for the bench. None of those coordinates are typed out on this page on purpose: the file is the one copy, so there is nothing here to fall out of step with it.

The netlist

This is the netlist embedded in the file, verbatim. It was written from the drawn board, so checking that board against it can only agree with itself. What it is good for is carrying the circuit somewhere else, and giving your meter something to walk when you are hunting for why it is silent:

# Simple Ring Modulator - written from the board # 8 strips of 24 holes, 12 parts, 0 chips, 6 cuts # # This is the circuit as the copper currently has it. Checking the board # against it can only pass - its use is to carry the circuit somewhere else. GND: @GND T1.S2 T2.PCT T2.S2 MOD_IN: @MOD_IN C1.A T1_S1: T1.S1 C1.B CAR_IN: @CAR_IN C3.A T1_PCT: T1.PCT C3.B T1_P1: T1.P1 D1.A D4.B T2_P1: T2.P1 D1.B D2.A T1_P2: T1.P2 D2.B D3.A T2_P2: T2.P2 D3.B D4.A T2_S1: T2.S1 C2.A OUT: @OUT C2.B

The off-board wiring

Four pads leave the board, and there is no supply among them:

Pad	Goes to
CAR_IN	The carrier, usually the higher and duller of the two: an oscillator, a square wave, one of the theremins on this shelf. It goes through C3 to T1's center tap and nowhere else, which is the whole trick - it reaches the diode ring only through that tap
MOD_IN	The program input, the one you care about hearing: a guitar, a mic preamp, a drum machine. Through C1 into T1's untapped winding
OUT	The output, taken through C2. It is a passive circuit, so this is quieter than what you put in and it wants a high-impedance input after it. An amp, a pedal, a buffer - not a pair of headphones
GND	Ground, shared by both inputs and the output. Everything that is not a signal ends up here

Why this one has a banner on it

Everything the editor can check about this board passes. The copper matches the netlist, nothing is shorted, nothing floats, and no part hangs off the edge. That is worth something, and it is more than most published layouts get. It is not the same as working.

What a checker cannot know is whether the circuit is right. This layout has already been redrawn once for exactly that reason: the first version put the carrier on a winding end and hung the ring between two different windings, which passed every check and was not the circuit. It matches the reference now, node for node, and the editor agrees the copper matches - but agreeing with a drawing is not the same as working. What a checker still cannot know is whether the levels land anywhere useful, or whether it is loud enough to be worth boxing. A ring modulator is also fussy about balance, and the classic ones use matched diodes for exactly that reason. Four out of one bag is the usual starting point and often fine, but if yours bleeds carrier into the output when nothing is plugged into the program input, that is the first thing to chase.

So: this is a drawing, honestly labeled. When it is built and heard, the banner comes off and this paragraph gets replaced by a photo.

How it works, in one breath

The program signal comes in on T1's untapped winding and appears across the tapped one, whose two ends are two opposite corners of the diode ring. T2 hangs on the other two corners the same way, and its untapped winding is the output. The carrier does not go through the ring at all: it arrives at T1's center tap, which sits at the electrical middle of that winding, so it pushes both ends the same way at once while the program pushes them in opposite directions.

That is what makes the thing multiply. When the carrier swings one way, one pair of diodes conducts and the program reaches the output one way up; when it swings the other, the other pair conducts and the program arrives inverted. The program gets flipped in step with the carrier, and flipping a signal in step with another is multiplying by it. Out comes the sum and the difference and neither original, because a balanced tap feeds equal and opposite halves that cancel what they have in common.

That is the principle. Whether this arrangement of taps and windings delivers it is the thing the bench build will settle, and until then the banner at the top of the page means what it says.

Things to try

Feed the carrier from a 555 theremin and wave at it while you play through the program input
Swap which input is which. There is nothing sacred about the labels, and the two sides sound different because the transformers see different things
Put a volume pot on the carrier. Backing it off takes you from full ring modulation toward the dry signal, which is the tremolo end of the same circuit
Try germanium diodes in the ring instead of the 1N4148s. Lower forward voltage, so quieter inputs get through, and the character changes

Drawn in Copper Bottom by Onion Madder. The ring modulator is one of the oldest circuits in electronic music and this is the plainest version of it. Only the layout and the words here are mine.