Ahh yes.. Those mysterious dB's.. They are everywhere. On meter faces, on back panels of electronics devices, on warning labels.. And then there are the suffixes, giving us dBm, dBu, dBc, etc.. Not to mention balanced and unbalanced connections such as XLR and RCA, respectively. What to do, what to do...
This document considers beginning electrical dB nomenclature and measurment of audio signals, and does not address acoustic levels. This document focuses upon the electrical signal levels which are often described in dB form. These designations can be confusing to those who have become used to the typical home-stereo type 'RCA' connections, and it is the purpose of this document to make clear the meanings of the designations. First, some definitions will be given, followed by some simple measurement techniques which will serve to illustrate the relationships between the concepts. If you want to jump into the math and the serious knowledge, check out a couple pages straight out of The Electronic Warfare And Radar Systems Engineering Handbook.
The dB expression is a mathematical shorthand, steeped in cruft and venerable history, that uses logarithms (a shortcut using the powers of 10 to represent the actual number) to reduce the size of the expression. For example, instead of saying the dynamic range is 32,000 to 1, we say it is 90 dB [the answer in dB equals 20 log x/y, where x and y are the different signal levels]. Being a ratio, decibels have no units. Everything is relative. Since it is relative, then it must be relative to some 0 dB reference point. To distinguish between reference points and measurment methods, a suffix letter is added as follows:
1 --- Open_____________________1 - Shield
2 - Signal wire 1_____________________2 - Signal wire 1
3 - Signal wire 2_____________________3 - Signal wire 2
Balanced source to Balanced input.
Dual conductor shielded cable.
Less expensive audio gear uses unbalanced circuits and RCA connectors. The unbalanced circuit has a definite 'hot' and 'ground' scheme, generally being composed of a woven wire shield through which an insulated center conductor is passed. The shield serves as the 'ground' or reference for the signal present in the center conductor.
Since most modern audio equipment uses the dBu specification, it is simple to convert between balanced and unbalanced levels. A few rules must be followed to insure proper operation and avoid possibly grounding or shorting one side of a balanced output amplifier circuit.
Many people make the mistake, when connecting an unbalanced measurement device such as an AC voltmeter which has a definite 'hot' and 'ground' connection scheme to a balanced output connector, of connecting the ground lead of the test instrument to one of the signal wires, and the input probe to the other signal wire. The problem with this is that it creates a ground loop and also shorts out half of the balanced output stage, which can cause an improper measurement and damage the audio equipment's output amplifier.
The proper way to connect an unbalanced meter input to a balanced signal output is to terminate the output circuit properly, and connect the ground or common lead of the meter to the common signal lead, and then check either of the balanced signal wires for voltage.
Remember that in a balanced circuit, both signal wires carry the same voltage level with reference to common, and they are 180 degrees out of phase. In this case only half the voltage will appear on the meter, so the engineer must mentally double the voltage level when making a reading.
If the meter is calibrated in dB, it will show a level which is actually 6dB less than the actual level present. For instance, if a VTR is playing back a tone at +4dBu, and a single-ended (unbalanced) dB meter is connected to the deck, it will read -2dBu. A meter with a balanced input will of course read +4dBu.
Termination and measurement of a balanced audio output is easily accomplished by connecting the circuit shown below. The voltmeter is assumed to have a high input impedance (1 megohm or more, although a 10K input Z will be acceptable for most purposes).
Modern solid state circuitry has a much lower source impedance, and will generally deliver the same output voltage into either a high impedance (10K) load or a 600 ohm load. When measurements are made, it is always good practice to properly terminate the equipment's output as called for in the alignment instructions.
As can be seen, no matter what kind of equipment is being checked, the unbalanced dB meter or single-ended AC voltmeter is always connected as shown, and the equipment is terminated properly. If this is done, the following two statements will always be true:
1.) Whatever voltage is shown on the meter is exactly half the true output voltage.
2.) Whatever dB level shown on the meter is always 6dB less than the dB which the meter would show if it were able to be connected across the entire output.
You can confirm this by connecting the test setup to an audio signal generator which has a balanced output such as an XLR connection.
Example: You wish to set the output of the tape deck to +4dBu. This corresponds to 1.23 volts RMS across the output. Since the output measurement is in dBu, you do not use the termination resistors. Connecting the dB/volt meter as shown, you would look for 0.615 volts RMS on the meter. If the voltmeter happens to read dBm and is calibrated for 600 ohms (most are), it would read '-2' instead of '+4'.
Example: You wish to set the output of the tape deck to 0dBm. The deck calls for a 600 ohm load on the output. You terminate the output as shown, and connect the voltmeter. The voltage corresponding to 0dBm (1 milliwatt) into 600 ohms is 0.775 volts RMS. The voltmeter would read 0.3875 volts. If the voltmeter has a dBm scale, it would read '-6' instead of '0'.
Where to get a decent dB meter? Of course, one can buy a new meter for $300 and up, but for almost all audio measurements, a good used meter will be just fine.
For most people who do not wish to maintain and calibrate their own test equipment, seek out a used Leader or Tenma meter from a surplus equipment vendor. Such items cost about $200-300.
For hobby use, and if you can deal with it, try a classic Hewlett-Packard 400LR, which is a rackmounted AC VTVM which reads directly in volts and dBm (@600 ohms). It has a 1 megohm input impedance and is accurate to 0.1dB over a range of 10Hz to 1MHz, and since it uses vacuum tubes and is American made, it's built like a tank and will still be around after the pulse. The cost? $40 and a bit of TLC every now and then.