Diagram of measuring buzzer and microphone with multimeter

This article uses the MF50 pointer multimeter as an example to introduce the detailed method of using the multimeter under professional conditions to quickly distinguish the polarity of the electret microphone and check the quality and function of the electret microphone.

Figure 1 View of the electret microphone

(a) Distinguishing between polarity and good and bad

(b) Check the sensitivity of the two-end microphones

(c) Check the sensitivity of the three-terminal microphone

Distinguish polarity

Because the drain D and source S of the field effect tube inside the electret microphone are directly used as the lead-out electrodes of the microphone, it is not difficult to determine the electrodes of the electret microphone by simply distinguishing the drain D and source S. As shown in Figure 1 (a), set the multimeter to "R×+0" or "R×1k" resistance, connect the black probe to one pole and the red probe to the other pole, and read the resistance value; after swapping the two probes, read the resistance value again and compare the two measurement results. In the one with the smaller resistance value, the black probe should be connected to the source S and the red probe should be connected to the drain D. Further distinction: If the metal shell of the electret microphone is connected to the source S electrode being checked, the microphone under test should be a two-terminal electret microphone, and its drain D electrode should be the "positive power supply/signal output pin", and the source S electrode should be the "ground pin"; if the metal shell of the microphone is connected to the drain D, the source S electrode should be the "negative power supply/signal output pin", and the drain D electrode should be the "ground pin". If the metal shell of the microphone under test is not connected to the source S and drain D electrodes, it is a three-terminal electret microphone, and its drain D and source S electrodes can be used as the "positive power supply pin" and "signal output pin" (or "signal output pin" and "negative power supply pin"), respectively, and the metal shell is the "ground pin".

Check out the good and bad

In the above measurement, the resistance value measured by the electret microphone should be one large and one small. If the forward and reverse resistance values ​​are both ∞, it means that the field effect tube inside the microphone under test is open; if the forward and reverse resistance values ​​are close to or equal to 0Ω, it means that the field effect tube inside the microphone under test has been broken down or short-circuited; if the forward and reverse resistance values ​​are equal, it means that the crystal diode between the gate G and the source S of the field effect tube inside the microphone under test is open. Because the electret microphone is a one-time compression seal, it is usually impossible to repair when a defect occurs inside, and the old one can be replaced with a new one.

Check the activity

Set the multimeter to "R×+0" or "R×1k" resistance, as shown in Figure 1 (b), connect the black test lead (the multimeter is connected to the positive battery) to the drain D of the two-terminal electret microphone under test, and the red test lead to the ground (or the red test lead to the source S, the black test lead to the ground). At this time, the multimeter pointer indicates a certain scale, and then blow a breath at the sound inlet on the front of the microphone. The multimeter pointer should swing greatly. The larger the pointer swings, the higher the sensitivity of the microphone under test. If there is no response or the response is not obvious, it means that the microphone under test has been damaged or its performance has declined. For the three-terminal electret microphone, as shown in Figure 1 (c), the black test lead is still connected to the drain D of the microphone under test, and the red test lead is connected to the source S and the ground (metal shell) at the same time, and then blow air in the same way to check.

The above inspection method is for machine-mounted electret microphones. For external electret microphones with lead plugs, you can measure directly on the plug as shown in Figure 2. But be careful, some microphones are equipped with switches, and when testing, turn the switch to the "ON" position, not the "OFF" position. Otherwise, the test will not be normal.

Figure 2 Viewing the electret microphone through the plug

Apply knowledge

1. Although there are many varieties and types of electret microphones, their main characteristics are usually not too different. The difference is often in the difference in sensitivity. In particular, the appearance specifications of commonly used machine-mounted electret microphones are mostly very close, so their universal compatibility is good. In electronic manufacturing or repair, if you can't find the required type, you can use any electret microphone with similar specifications and characteristics to replace it. But it should be noted that some types of electret microphones use color point symbols to divide their sensitivity. For example, the sensitivity of the British brand CM-18W electret microphone is divided into 5 gears, each gear is about 4dB different, in order: red is -66dB, small yellow is -62dB, large yellow is -58dB, blue is -54dB, white>-52dB. When replacing, even if the type is the same, it is not enough. It is necessary to require that the color points of the two are the same or the sensitivity is close. If the product has the wrong color code, it is best to check the product manual or instructions to clarify the specific characteristics and main parameters before determining whether it can be replaced.

2. The sensitivity of the electret microphone is a key issue in use. Whether it is better to choose a high or low sensitivity should be determined according to the actual situation. In situations where a large dynamic range is required, a product with a lower sensitivity should be used. In this way, the background noise of the recorded program is smaller, the signal-to-noise ratio is higher, and the dynamics sound cleaner and clearer, but the gain of the circuit is relatively higher. In a simple system, a product with a higher sensitivity can be selected to reduce the gain requirements of the post-stage expansion circuit. Another thing to note is that the discreteness of electret microphones is generally large, and even microphones of the same type and color point sometimes have large differences in sensitivity.

3. When connecting an electret microphone to an electronic device, pay special attention to the impedance matching of the two. No matter what kind of microphone is used, it is necessary to always keep in mind this principle: a high-impedance microphone cannot be directly connected to an electronic device with low input impedance, but a low-impedance microphone can be connected to an electronic device with high input impedance. In addition, the high-impedance microphone lead should not be too long, otherwise it will easily cause various noises and increase frequency distortion. When a longer microphone cable is required, a microphone with lower impedance should be used as much as possible. Regardless of whether the microphone lead is long or short, a shielded cable should be used to prevent external clutter signals from being induced into the lead and interfering with the subsequent expansion circuit.

Figure 3 Four ways to connect an electret microphone

(a) Negative ground, D-pole output

(b) Positive grounding, S pole output

(c) Negative ground, S pole output

(d) Positive grounding, D-pole output

4. When the electret microphone is connected to the circuit, there are four different wiring methods, and the detailed circuit is shown in Figure 3. The R in the figure is both the external load resistor of the field effect tube inside the microphone and the DC bias resistor of the microphone, which has a great influence on the working condition and function of the microphone. C is the microphone output signal coupling capacitor. Figures 3 (a) and 3 (b) show the wiring method of the two-terminal microphone, and Figures 3 (c) and 3 (d) show the wiring method of the three-terminal electret microphone. Most of the electret microphones currently on the market are two-terminal, and almost all of them use the connection method shown in Figure 3 (a). This connection method is to connect the field effect tube into a drain D output circuit, which is similar to the common emitter expansion circuit of a crystal triode. Its characteristic is that the output signal has a certain voltage gain, which makes the microphone more sensitive, but the dynamic solution is relatively small. Three-terminal microphones are relatively rare in the market today. When used, they are mostly connected to the source S output method shown in Figure 3 (c), which is similar to the emitter output circuit of a crystal triode. Its characteristics are small output impedance (usually ≤2kΩ), relatively stable circuit, large dynamic range, but relatively small output signal. Of course, the three-terminal microphone can also be connected to the circuit shown in Figure 3 (a) or Figure 3 (b) and used directly as a two-terminal microphone. But please note that no matter which connection method is used, the electret microphone must meet certain DC bias conditions to work properly. In practice, this is to ensure that the built-in field effect tube is always in a good expansion state.

5. The working condition of the built-in field effect tube of the electret microphone not only determines whether the microphone can work normally, but also directly affects the sensitivity, dynamic design and distortion of the microphone. Since the DC working voltage UDS and working current IDS of the field effect tube are obtained from the power supply circuit of the rear stage expander through the external load resistor R, the value of the resistor R is crucial to the practical application of the microphone. The size of R can be determined by the formula: R = (U-UDS)/IDS, where U is the power supply voltage. R is not only the load resistance of the field effect tube, but also forms the load resistance RL of the microphone after being connected in parallel with the input resistance of the subsequent expansion circuit in the circuit. It should be ensured that the resistance value of RL is always greater than 3 to 5 times the output impedance of the microphone, so that the microphone can be in a good matching state. Too small R resistance value often leads to a decrease in the input impedance of the expansion circuit, thereby damaging the impedance matching between the front and rear stages and reducing the power of the expander. Since the output impedance of the microphone is around 2kΩ, RL must be at least more than ten kΩ to meet the requirements.