The principle of KMW-306 dual-channel wireless microphone and how to change its use

KMW-306 wireless microphone is a cost-effective electro-acoustic equipment used in karaoke and other dance halls. Its biggest features are extremely low background noise and excellent anti-howling function. The author once turned up the main volume and microphone volume of a 2×100W amplifier to the maximum, and held the microphone 2 meters away from the speaker, and no howling occurred. The author analyzed and studied its receiver and drew the analysis circuit, as shown in the figure below. Due to constraints, handheld microphones were not analyzed. 1. In the working principle diagram, channel A is taken as an example. The 109~120MHz FM signal output by the transmitter is received by TX-A, filtered by the four-level LC network and sent to the pin of IC11. IC11 is an AM/FM single-chip radio integrated block produced in South Korea. Only the FM high and high amplification part and the frequency identification and detection circuit are used here, and its non-station squelch function is specially omitted. The detected audio signal is transmitted by the pin of IC11. output. After comparison by the author, I found that KA22425D is completely equivalent to CXA1191M in terms of packaging and circuit usage, indicating that the latter can completely replace the former and provide convenience for future maintenance. It is worth noting that when this product is produced, the handheld microphone will not be adjusted as long as there is no fault, and all adjustments will be made on the receiver. Therefore, the four-stage filter circuit and the FM RF and OSC circuits are sealed with high-frequency wax, and we generally cannot move these components casually. The audio signal from IC11 is divided into two channels through R105 and C124: one channel is used for the VCVS (voltage controlled voltage source) low-pass filter composed of IC12D; the other channel is used for the VCVS high-pass filter composed of IC12C. In the low-pass filter, the audio signal is sent to the non-inverting end of the IC12 pin, and is output to the IC3 pin in the form of a transmitter follower, controlled by the IC3 pin. The characteristics of using VCVS low pass are: it can obtain larger positive phase gain with fewer components, low output impedance, easy adjustment and QP<10. After calculation, the low-pass cutoff frequency is 852Hz. Readers may feel strange: It is said that the high frequency of human voice is at least 8kHz. Isn't it too low to choose such a cutoff frequency? In fact, the low Q value of VCVS just solves this problem. The amplitude-frequency characteristics after the cut-off point do not drop very steeply, but extend downwards relatively slowly. The remaining features are just convenient for manufacturers to reduce costs and control product quality. In the high-pass filter, C132 picks up the background noise signal in the audio output by the FM detector. As we all know, the FM band will produce a strong "hiss" sound when there is no station. If it is not filtered, this signal can extend to hundreds of kHz. The VCVS high-pass is used to filter out this noise, and the calculated cut-off frequency is approximately 124kHz. IC12B amplifies this signal by 23dB and doubles the voltage to form a rectified wave, forming a DC voltage drop across both ends of C131. IC12A is a voltage comparator that judges the DC voltage drop output by IC12B. When the handheld microphone is turned off, the pin outputs low voltage, causing the SW-D of IC3 to close: the pin outputs high level, SW-D is turned on, and the audio signal output by IC12D passes through R127, × (the manufacturer uses solder to connect) and R134 , C140 is sent to the CZ output, and then sent to the power amplifier to drive the speaker. 2. Effect of use Although the manufacturer's instructions stipulate that the audio output is >25mV, that is the value when the microphone is placed near your mouth. When the purpose is changed, such as for conference amplification, the output is too small, and the working range is ideal. The author placed this system in an auditorium that is 25m long and 15m wide. The handheld microphone can still work normally if it is more than ten meters away from the auditorium. However, the range of the two microphones is different. The B channel has a smaller range than the A channel. But the manufacturer’s parameters are completely trustworthy. 3. Changing the use In response to the need for conference amplification, the author used three methods: 1. Replace the capsule in a handheld microphone with a high-sensitivity core. 2. To process IC12D, connect a resistor from the pin to the ground, cut off the connection between the pin and the pin, and insert a resistor in series. The resistance values ​​are all 62kΩ, giving the low pass an amplification of 6dB. 3. Repair the component by welding it at the position of the discrete component emitter follower left by the manufacturer, and transform it into a voltage amplifier with collector output. After weighing the pros and cons, the author finally adopted the third method. The reasons are: high-sensitivity microphone cores are not that easy to buy, and even if they are purchased, they are not cheap; it is difficult to operate on the printing plate, and it is very inconvenient to adjust the magnification. For specific values, see the dotted line "Additional circuit" in the figure. Specific method: open the solder at "X", connect the jumper at "Y", and install various components, among which C141 and C142 are soldered on the copper foil surface. 4. Debugging: Use a 100kΩ potentiometer and a 51kΩ resistor in series to replace the upper bias resistor in the "Additional Circuit". Adjust the "Microphone Volume" knob on the power amplifier to the maximum, the main volume to 2/3, and the high and bass knobs. Adjust both to the maximum, debug according to the actual wiring at the venue, rotate the 100kΩ potentiometer until the whistling is just heard, and replace it with a fixed resistor. Through the above transformation, good results have been achieved in practice.