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by Jay Rose       Published September, 1998

Leonardo mixed his own paints. The least you can do is build your own simple audio tools.

I am not a brilliant audio hardware designer. I've worked with a few of them, and don't consider myself worthy of carrying their voltmeters. But when I started in this business, audio was done in circuits instead of CPUs and DSP, and any respectable recording engineer had to understand basic electronics. Back then, it was expected that we'd build our own little circuits to solve basic studio problems. This article describes two of the simplest: they're cheap, ridiculously easy to build (and to understand), and still highly useful for the computer-based digital editor or animator.

Most of these instructions include Radio Shack part numbers. Their products aren't necessarily the best or the most economical, but there's one of their stores in every shopping mall in the known universe. If you're scared of soldering -- and there's no reason you should be -- just hand these designs to any high-school hacker: they probably already have most of the parts on hand.


Real-time ISDN audio codecs are great if you can afford them, but sometimes you just need to play a library cut or dialog edit to a distant client for content approval. This $10 coupler lets you plug a computer or mixer directly into a phone line, with far better results than holding the handset up to a speaker. It's based on a circuit the phone company used to provide, and under ideal circumstances can sound as good as the connections at radio call-in shows.

The coupler doesn't work into digital phone lines. But chances are your phones are analog, even if your sophisticated office system includes voice mail and call-forwarding. To find out, turn a phone over and look for a "Ringer Equivalent Number" or REN value: this means the phone wiring is analog. (Sometimes the REN label is hidden under a snap-on mounting bracket, so look carefully.) If you don't see a REN, you can still use the coupler with one of the $50 handset modem adapters sold for business travel... but some of those already have equivalent audio jacks built-in.

The circuit (figure 1) is laughably simple, with only two components. A 1:1 600 Ω transformer isolates the computer and phone system from each other, while still letting audio pass through. Radio Shack's tiny version (part #273-1374) isn't exactly high-fidelity, but its quality matches the telephone system perfectly. A better-sounding transformer might not work as well. A .1 μF capacitor (#272-1069) protects the transformer from the constant voltage that's always on a phone line, and keeps the coupler from interfering with normal dialing and hanging up. If you're getting parts from a more comprehensive supplier, a capacitor with a 100 volt rating will provide a safety margin.

fig01 Figure 1: Schematic for a simple phone coupler.

It's also so easy to build that I threw one together on a piece of scrap cardboard (figure 2), using cellophane tape and paper clips. The thick black wire on the left comes from the computer or mixer's line output. Find a cable with an appropriate plug, cut off one end, and use a knife or cutting pliers to pull off the outer covering. You'll see a metal shield -- braided or twisted wires, or a foil wrapper -- around a single insulated conductor. Separate the shield from the inner wire and twist it together, and then strip back some of the insulation from the end of the inner wire (or buy a #42-2370 cable, which is already prepared for use). The gray telephone wire on the right is a #279-310, which has a standard modular plug on the other end. Follow the color codes in the photo and schematic, and connect the wires by twisting them together (soldering isn't necessary). Make sure the connections are secured so they don't touch each other, and be sure to insulate the wires on the telephone side of the circuit so nobody accidentally touches them: when the phone rings, these carry as much as 125 volts. It's low current, but can give you quite a tickle.

Figure 2: The coupler, built on a scrap from a cardboard box.

To use the coupler, just plug it into a sound source and a telephone jack. You'll need a regular phone on the same line to dial and monitor the call; the easiest way to hook them both up is a Y-connector (#279-357). Raise the source's volume until it just starts to distort in your phone's handset, then back it off a little. It's okay to leave the coupler connected between calls. You can also use this coupler to record telephone calls through a line-level input, but it's less than ideal: the local sound can be as much as 20 dB louder than the distant caller. To even them out, you need a totally different circuit.


Even though most tv mixes are stereo, mono compatibility is important. There are still a lot of mono sets out there, and stereo sets revert to mono when the signal is weak. You can't take this compatibility for granted: bad wiring, asymmetrical processors, and some "three-dimensional" effects can make a good stereo track sound awful in mono. A few stereo music synthesizers completely disappear when their channels are combined.

This gadget lets you check mono compatibility quickly, by just flipping a switch and listening. Both channels get sent to one speaker, and the other speaker is silenced: if it sounds good, it's compatible. The circuit also lets you simultaneously lower the volume to get a better idea of how a mix will sound on the air: your ears reject very high and low frequencies when a sound gets softer.

You can see how it works by following the schematic in figure 3: The left and right outputs of a computer or mixer are combined through the two 4.7kΩ resistors (#271-1330). These resistors keep the individual outputs from drawing current from each other. They also cause a tiny loss of stereo separation, but with most program material it won't be noticeable. The resistance is correct for most computer and semi-pro audio gear, but not for tubed or transformer-coupled equipment... if you've got that stuff, you probably already have sophisticated monitor controls. The "5kΩ audio-taper potentiometer (#271-1720) lets you preset a lower volume for mono listening. With the switch up, the amplifier sees the stereo outputs of the computer; with it down, one side of the amp sees the mono sum and the other is grounded to stay silent.

Figure 3: Schematic for a compatibility tester.

This circuit isn't complicated to build, but it does require soldering. There aren't any hazardous voltages so it's a good project for the beginner (Radio Shack can sell you a soldering kit with instructions, #64-2802). The hatched lines in the schematic represent the cable shields; it's important that these all be connected to each other as well as to the volume control and switch. The ground connection is optional, but may reduce noise if you tie it to a grounding screw on the amplifier. The red letters on the schematic refer to specific terminals on the switch and control, and should match figure 4. Any light gauge hookup wire can be used. For best results, build the circuit in a metal box (#270-238) and also connect the ground point to the box: this provides shielding as well as a solid mounting surface for the switch and volume control.

Figure 4: The red letters in figure 3 refer to specific terminals on the components.

Plug one side into your computer or mixer's line outputs, and the other into a monitor amplifier. With the switch in one position, you'll hear stereo. To check compatibility, flip the switch to the other side and make sure every element still sounds right. To check the mix balance, turn down the volume control until you barely hear the track: in a well-mixed track, you should still be able to follow the dialog even when it's too soft to catch all the music and sound effects.

Both these circuits are simple, cheap, and useful. You'll find similar ones in most professional studios... some even built by old-timers like me.


Jay Rose is a Clio- and Emmy-winning sound designer specializing in broadcast projects. He built his first studio (a pirate radio station) while in high school, too many years ago. Write to him at, or admire his current facility at