Build the EM -10/+4 Level Converter

Oct 1, 1999 12:00 PM, Peter Mosher

           
 

The circuit itself is very simple.

In the audio world, "professional" studio gear generally features line-level audio inputs and outputs on balanced, three-conductor XLR connectors, operating at a signal level of +4 dBu. In contrast, "consumer" gear uses unbalanced, two-conductor RCA or 11/44-inch connectors, and it sends and receives audio signals at -10 dBV. "Semipro" gear (including most synthesizers and low-cost signal processors) adheres to the consumer standard, except that unbalanced, 11/44-inch connectors are more prevalent than RCAs. (For more on the various types of line-level signals, see "Square One: The Wizard of dBs" in the January 1996 EM.)

Mechanically and electrically, the +4 and -10 audio standards bear little resemblance to each other and rarely coexist without a fight. With some creative wiring, you can often get around some of the major gremlins that crop up when you mingle them in your studio, but curing one problem sometimes creates another. These problems can include hum or buzz, RF noise, impedance mismatches, phase cancellation, and overloaded or underdriven inputs. Such glitches often appear at the most inconvenient times, and they can cause you to pull your hair out in bafflement. You end up scrambling for an extra ground wire, a preamp to boost a signal, an attenuator to reduce another, or one more weirdly wired cable that makes little sense electrically, but hey, it clears up the problem-more or less.

Of course, the simple solution is not to mix different audio standards in the first place. But we must live with the facts that we need both types of gear, that interconnection differences exist, and that we need a reliable method of interfacing different types of equipment.

CIRCUIT OPERATION

In order to go back and forth between the two audio standards, you need two separate circuits: one that translates an unbalanced -10 dBV signal into balanced +4 dBu, and one that does the same thing in the opposite direction. For this project, both circuits have been kept quite simple.

FIG.1: The top portion of this schematic converts -10 dBV to +4 dBu, and the bottom portion converts +4 dBu to -10 dBV.
Click on image to enlarge

The top half of the schematic that is shown in Figure 1 is the consumer-to-professional level converter. An unbalanced signal enters the circuit at capacitors C1 and C2 and is passed to the inverting input of op amp U1A through resistor R1, which sets the input impedance of the circuit to 10 kz. U1A amplifies the signal slightly and feeds it to the noninverting input of U2A. The signal that appears at the right side of R5 is a boosted and inverted version of the original input signal.

U2B inverts the signal from U2A again and feeds it to the output through R8. As a result, the output appearing at pin 2 is an amplified, in-phase version of the input. The signal at pin 3 is a phase-inverted version of this output. Together, they make up the balanced signal output.

The bottom half of the circuit is the pro-to-consumer converter. A balanced input signal is fed through an RF filter made up of R9, C3, R10, and C4. The signal then passes through resistors R11 and R13 to the inputs of U1B, which is wired in differential mode. As a result, this op amp passes the difference between the signals that appear at its inputs.

The strength of balanced circuitry is that any induced signal common to both signal wires is canceled out when it passes through the op amp. The engineering term for this is common mode rejection ratio (CMRR), which is a measurement of a differential circuit's ability to reject unwanted common-mode electromagnetic interference (that is, signals that are in phase in both signal wires). This brings us to the importance of using precision resistors in balanced circuitry.

You'll notice that all resistors specified in the circuits are 1 percent tolerance, metal-film type (see the sidebar "Parts List"). If you're used to building projects with lower-tolerance, carbon-film resistors, you might wonder why precision types are necessary. Strictly speaking, they're not, but if you don't use them at least for critical components (R1, R2, R11, R12, R13, and R14), you're going to have level problems and poor CMRR. The nature of balanced circuitry demands the use of tight-tolerance resistors; if you have access to a 411/42-digit ohmmeter, matching the critical ones to within 0.01 percent or better would not be going too far.

Both circuits were designed using the lowest possible number of parts to minimize signal coloration. In the unlikely event that you can't find the necessary parts at your local electronics shop, you can easily get them all by mail order. I've found that Digi-Key usually has everything I need in stock, and it delivers promptly (tel. 800/344-4539 or 218/681-6674; Web www.digi-key.com).

FIG. 2: Scan this full-size PCB layout into your computer, and use a TTS kit to make your own printed circuit board.

BREADBOARD OR CIRCUIT BOARD?

If you prefer, you can breadboard the circuit, but if you're planning to make more than two converter channels, I recommend using a printed circuit board. PCBs are extremely reliable and rugged, and they are the most time-efficient way to go, even in small quantities. In addition, you'd really have to work hard to make wiring errors, and it's much easier to troubleshoot a well-designed circuit board than to find a problem in a rat's nest of wires and parts sticking out at all angles.

If you're still not convinced, let me tell you about a dead-easy method of making PCBs at home. It's called the Toner Transfer System (TTS), which is made by DynaArt (tel. 813/524-1500; e-mail mail@dynaart.com; Web www .dynaart.com). To do this, you'll need a blank circuit board (copper-clad on one side only), the TTS kit, a suitable etchant (ferric chloride or ammonium/sodium persulphate), a clothing iron, and a hobby drill with a tiny drill bit. You'll also need some way of printing the image; you could use a photocopier, but I prefer to print a scanned image of the circuit board on a laser printer.

The procedure is simple: just cut out the actual-size PCB template shown in Figure 2 and scan it into your computer. Using a graphics program, copy and paste the template as many times as you need for the number of circuits you plan to build and line them up squarely. Print the templates onto a sheet of TTS paper using a laser printer. Cut the templates into pairs, side by side, so they measure 4 by 4 inches.