Build the EM -10/+4 Level Converter

Oct 1, 1999 12:00 PM, Peter Mosher

Clean the copper side of a 4-by-4-inch circuit board with a powder-type sink cleanser, rinse it, and dry it with a clean cloth. Place the template face up on a hard, flat surface that you won't mind scorching, and orient the circuit board, copper side down, on top of the template. Heat the iron to the "cotton" setting, and place it on the back of the circuit board, applying light but steady and evenly distributed pressure for about three minutes. Lift the iron, let the board cool a little, and place it face up in a bath of room-temperature water. Let it sit there until the paper literally slips off on its own; don't "help" it off, or you'll risk damaging the imprint.

Rinse the board in room-temperature water, place it in the etchant, and leave it there until all the unwanted copper is gone. Using acetone or nail-polish remover, remove the printer toner from the copper traces. You're left with a nice pattern of copper lines that look exactly like the template. Drill the board and mount the parts on the opposite side of the board from the copper lines, following the placement diagram (see Fig. 3).

This might sound complicated, and it does take a few tries to learn, but it's easy once you've made a couple of PCBs. If you've ever made circuit boards using the old photographic method, you will especially welcome this new technique. Give it a try. Be very careful with the hot iron and circuit board, though, and use caution with the etchant, which is toxic and very corrosive. If these things make you nervous, get someone with experience to help you.

CONSTRUCTION HINTS

There are a few golden rules regarding the construction of the level converter that will save you a lot of time and frustration when you get to the point of actually turning the thing on. For example, there are many reasons not to build your own power supply these days. As you've probably noticed, I didn't even include a schematic for one in this project. Ready-made, regulated 12-volt bipolar supplies are widely available, which means that you don't have to mess around with potentially lethal line voltages. (I've used up most of my nine lives on this one; don't tempt fate.)

These power supplies can often be found at your local thrift store for a couple bucks. Look for ones that were originally used for old home PCs, modems, and game computers; they not only have the voltages you need for analog projects, but they have a regulated 5-volt supply, as well, which you will need should you branch out into digital projects. Take me seriously on this; once you've salvaged a used commercial power supply, you'll never consider building another one from scratch for small projects.

Be careful to get the capacitor polarities correct, especially C9 and C10, which will turn into firecrackers if you wire them backward. Make sure the op amps are oriented correctly, too.

Never use bargain jacks; get the best ones you can afford. Gold-plated jacks are ideal if you can mate them with gold-plated connectors.

TESTING

This must be one of the all-time easiest projects to test, because there are no adjustments to be made. First, establish a reference point: put a test CD with a 1 kHz, 0 dB test tone into a consumer-grade CD player. Using a recording device with -10 dBV inputs and a high-resolution bar-graph level meter, adjust the CD player's output level and/or the recorder's input level so that the meter reads exactly 0 dB.

Next, connect the line outputs of the CD player to the -10 inputs on the converter, route the converter's +4 outputs directly to its +4 inputs, and connect the converter's -10 outputs into the recorder's inputs. If you read 0 dB on both channels, everything is working as it should. If the levels are too high, too low, not there, or they sound like a broken analog synth, go back and check for connector-wiring errors, poor solder joints, or hairline cracks in the circuit board. Make sure you have properly identified all the resistor values, as well; the color coding on precision resistors can be tricky to decipher.

APPLICATIONS

In my line of work (forensic audio enhancement), you never know the format of a tape that someone is going to submit for examination. Most of the processing and playback equipment I use is -10 dBV unbalanced (cassette, microcassette, or VHS). When someone came to me one day with a Betacam tape and machine, I had a problem: +4 dBu balanced outputs. I had no way to quickly and properly connect this device to my setup. So I built a level converter with two channels each of -10 to +4 and +4 to -10 conversion, and it deals with most of my interconnection problems very nicely.

Of course, many other applications are possible, especially in a music studio. Suppose you have keyboards strewn around your studio, and the low-level, unbalanced cables running from their outputs to your mixer pick up noise. You can build a 2-channel converter for each one and run a high-level, balanced signal to the mixer inputs instead. This will increase noise immunity dramatically. You can use this trick with any -10 dBV equipment that's located any distance from the mixer.

Say you have a bunch of gear, both balanced and unbalanced, mounted in a rack. The patch bay includes modules that handle -10 and +4 signals, which means you're constantly faced with level problems. Building two channels of two-way level conversion and connecting them to the patch bay will let you deal with many of the problems you encounter on a regular basis.

For the ultimate luxury, build enough converters for all of your semipro and consumer equipment and make all the signals coming to the patch bay balanced. Only one standard signal level and format will appear at all the jacks of the patch bay, and many of your interconnection hassles will disappear entirely. In most personal studios, however, it is more practical to convert a few pro-standard items to the consumer standard. Although this is not as elegant as running +4 dBu balanced lines exclusively, at least everything will be interfaced properly.

If you've ever run into any of the headaches I've mentioned, you owe it to yourself to take the time to build a level converter. Once you've discovered how many problems it can solve, you'll wonder how you ever got along without it. They're so cheap and easy to build, you'll be heating up your soldering iron to make more of these useful gizmos before you know it.

Forensic work can be downright weird at times. To unwind, Peter Mosher likes to create his own "individualistic" electronic music using homemade analog synthesis equipment.