A brief discussion on using a multimeter to test the quality of an electret microphone

Taking the MF50 pointer multimeter as an example, this article introduces the specific method of using the multimeter under amateur conditions to quickly determine the polarity of the electret microphone and detect the quality and performance of the electret microphone.

Figure 1 Electret microphone detection

(a) Determine polarity and good or bad

(b) Detecting the sensitivity of two-terminal microphones

(c) Detecting the sensitivity of a three-terminal microphone

Determine polarity

Since 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 as long as the drain D and source S are determined. As shown in Figure 1 (a), set the multimeter to the "R×100" or "R×1k" resistance range, connect the black probe to any 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 a 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 judgment: If the metal shell of the electret microphone is connected to the detected source S electrode, 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 is 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 is 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".

Detection of good or bad

In the above measurement, the resistance value of 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 has been 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 has been open. Since the electret microphone is a one-time compression seal, it is generally not repairable when an internal fault occurs, and the old one can be replaced with a new one.

Detection sensitivity

Set the multimeter to the "R×100" or "R×1k" resistance range. 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 terminal (or the red test lead to the source S, and the black test lead to the ground terminal). At this time, the multimeter pointer indicates a certain scale. Then blow a breath at the sound inlet on the front of the microphone. The multimeter pointer should swing greatly. The larger the pointer swing range, 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 deteriorated. 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 terminal (metal shell) at the same time, and then blow air to test in the same way.

The above test method is for machine-mounted electret microphones. For external electret microphones with lead plugs, the test can be performed directly on the plug as shown in Figure 2. However, please note that some microphones are equipped with switches. When testing, the switch should be turned to the "ON" position, and the switch should not be turned to the "OFF" position. Otherwise, normal testing will not be possible.

Figure 2 Detecting an electret microphone through a plug

Use common sense

1. Although there are many varieties and models of electret microphones, their main characteristics are generally not very different, and the difference often lies in the difference in sensitivity. In particular, the dimensions of most commonly used machine-mounted electret microphones are also very close, so their general interchangeability is good. During electronic production or maintenance, if you can't find the required model, you can use any electret microphone with similar size and characteristics to replace it. But it should be noted that some models of electret microphones use color dots to mark their sensitivity. For example, the sensitivity of the British brand CM-18W electret microphone is divided into 5 gears, each with a difference of about 4dB, which are: red is -66dB, small yellow is -62dB, large yellow is -58dB, blue is -54dB, and white>-52dB. When replacing, even if the model is the same, it is not enough. It is necessary to require the same color point or close sensitivity. If it is a non-color-coded product, it is best to check the product manual or instruction manual to find out 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 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 selected. In this way, the background noise of the recorded program is smaller, the signal-to-noise ratio is higher, and the sound sounds 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-amplifier circuit. In addition, it should be noted that ordinary electret microphones have a large discreteness, and even microphones of the same model 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, you must 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 connected to an electronic device with high input impedance is allowed. 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 selected 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 amplifier 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 4 different wiring methods, and the specific circuit is shown in Figure 3. 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 state and performance 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 amplifier 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 range is relatively small. Three-terminal microphones are relatively rare in the market. When used, they are mostly connected to the source S output mode shown in Figure 3 (c), which is similar to the emitter output circuit of a crystal triode . Its characteristics are small output impedance (generally ≤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 it should be noted that no matter which connection method is used, the electret microphone must meet certain DC bias conditions to work properly. This is actually to ensure that the built-in field effect tube is always in a good amplification state.

5. The working state 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 range 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 post-stage amplifier through the external load resistor R, the value of the resistor R is crucial to the actual use effect 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 in the circuit it is connected in parallel with the input resistance of the subsequent amplifier circuit to form the load resistance RL of the microphone. The resistance value of RL should always be 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 often causes the input impedance of the amplifier circuit to decrease, thereby destroying the impedance matching between the front and rear stages and reducing the efficiency of the amplifier. Since the output impedance of the microphone is about 2kΩ, RL must be at least 10kΩ to meet the requirements.