Active speaker troubleshooting_Active speaker production steps

Active speaker troubleshooting

Whether it is the end of its service life or the active amplifier speaker is not properly maintained (for specific audio maintenance, please refer to "How to maintain audio? 2.0 Audio Maintenance Methods"), it will cause a component of the speaker to be damaged and need repair. So what are the problems with active speakers? Let's take a look at some common troubleshooting methods for speaker repair.

1. Repair method for active speakers with noise

Fault phenomenon: The active speaker makes a "beep" sound when it is powered on

Analysis and repair: According to experience, this fault is mostly caused by the breakdown of the power amplifier tube of one channel, resulting in a DC short circuit. After disassembling the machine, it was found that both power amplifier tubes D1499 were broken down, and the current limiting resistor was also black and charred. Replace the current limiting resistor with a new one, because there is no D1499 on hand. Take two D1266 power amplifier tubes and replace them. After a long test, everything is normal.

2. Repair method for active speakers without sound

Fault phenomenon: The active speaker screen is normal, but there is no sound from the input audio signal and the volume control is invalid.

Analysis and repair: The volume adjustment knob of the active speaker is a scroll type, equipped with a 16-pin integrated block PT2253A. The external 12V power supply is normal, but the negative end of the voltage regulator ZD is short-circuited to the ground. Disconnect the PT2253A (16) pin and measure that the pin has broken down to the ground, indicating that the PT2253A is damaged. Because the chip is not easy to buy, it is decided to cancel the volume control motherboard. Take a Bl00 kΩ sliding carbon film potentiometer, as shown in the figure, one end is connected to LIN, RIN, the other end is grounded, and the middle is connected to LOUT, ROUT. After checking that it is correct, it is still fixed with the original installation hole and adjusted with the original knob. Power on, the volume and control are normal.

3. How to repair the speaker volume problem of being loud and soft

Fault phenomenon: There is a crackling sound when adjusting the volume, and the volume is sometimes louder and sometimes softer.

Analysis and repair: In fact, as long as this happens, it can be judged that there is a problem with the phaser that adjusts the volume. Most speakers use potentiometers to change the strength of the signal (except digital tuning potentiometers) to adjust the volume and subwoofer. The potentiometer changes the position on the carbon baffle through a movable contact, thereby changing the value of the resistance. As the use time increases, dust or impurities will fall into the potentiometer, and the contacts of the potentiometer may also oxidize and rust, resulting in loose contact. At this time, there will be a "crackling" noise when adjusting the volume.

The solution is relatively simple. You only need to replace a new potentiometer, which costs no more than 2 yuan. However, the simplest solution is to open the speaker, open the four crimping pieces behind the potentiometer, and expose the active contacts of the potentiometer. Then, clean the carbon resistor with anhydrous alcohol, drop a drop of oil on the carbon resistor, and finally install the potentiometer in its original position to solve the noise problem.

Of course, the above is the situation that most people will encounter. But there is another reason that can also cause the above failure: the quality of the potentiometer is unstable. When in use, the reeds of the left and right channels are originally separated, but now they are misaligned, causing them to be intermittent during use, which produces the "crackling" noise. It is also very simple to deal with this. We just need to use pointed tweezers to gently pull it straight and then put it back in place.

Active speaker production steps

A friend asked me to buy a small active speaker for him. Looking at the products on the market, although there are many, the quality and effect are not satisfactory. I had no choice but to do it myself.

1. Circuit selection and cabinet design

The circuit (see Figure 2) adopts the working mode of (NE5532+TDA1521)×2, that is, two sets of identical power amplifier circuits are made with two NE5532 and two TDA1521, one set for each speaker. In the circuit, half of the NE5532 op amp is used to make an impedance converter (the function of the impedance converter is to solve the matching between the microwave transmission line and the microwave device. Under normal circumstances, the impedance of the coaxial transmission line is 75Ω, and the input and output impedance of the polarization separator and channel filter connected to the feeder is 50Ω), and the other half is used to amplify the signal by about 5 times. The signal output by the line amplifier is first divided by the resistor-capacitor network and then sent to the TDA1521 to amplify the high and low frequencies separately. In this way, the power divider in the box that is prone to cause the frequency characteristics to deteriorate is discarded, which is very effective in improving the sound quality.

The cabinet of this active speaker is made of 15mm thick medium-density fiberboard, and the joints of each board are processed at a 45° angle. The external dimensions of the cabinet (front, side, and back) are shown in Figure 1. After the boards are cut, the remaining boards can be assembled together first, except for the back board. When assembling, first apply glue to the joints and then tighten them with 2.5cm long self-tapping screws every 5cm. Then brush a layer of asphalt about 0.5cm thick inside the box to enhance the rigidity of the box and reduce vibration when playing at a high volume, and finally evenly spread a layer of sound-absorbing cotton.

2. Component Selection and Amplifier Production

Because there are very few components in this power amplifier circuit, the quality of each component will have a great impact on the overall performance, so the selection of components should be given full attention.

Except for those noted, all resistors in the power amplifier circuit are 1/4 watt five-color ring metal film resistors (colorless metal film resistors have small temperature coefficient, small current noise, moisture resistance, oxidation resistance, strong overload capacity, long-term stable operation, wide temperature range, and the packaging methods include tape and bulk). Small capacity or signal coupling capacitors use tantalum capacitors or CBB type, and large capacity decoupling and filter capacitors use "rubies". TDA1521 is produced in Taiwan Province, and NE5532 uses American products marked with a large "S". Except for the signal input line, the rest of the wiring in the box uses high-quality speaker wires. Because the circuit of this power amplifier is simple, when making the printed board, you can choose a copper-clad board and use a utility knife to carve it as required, which saves time and effort. Each component should be strictly tested and screened by an instrument before being put on the board. Two "L"-shaped irons can be used to connect the circuit board and the radiator that dissipates heat for TDA1521 with small bolts in an "L" shape (note that the circuit board and the radiator are insulated). The heat sink removed from the STK6153 amplifier board is used to dissipate heat for the amplifier block in this amplifier, which is very suitable for use here. Then, evenly apply a layer of thermal conductive silicone grease on the back of the amplifier block and fix it to the radiator with holes drilled using small bolts. The power supply used in this active speaker is external, and the circuit is shown in Figure 3. The transformer, rectifier filter components, and fuses in the power supply are all placed in an iron chassis, and are connected to the speaker with a high-quality three-core small cable when in use. Although this approach is troublesome, the benefit is that it greatly reduces the interference of the power transformer on the components on the amplifier board (especially the front stage), which is beneficial to improving the playback sound quality. The speakers used in this speaker are products of Shanghai Yindi Factory, with the tweeter being YDQG4-7 and the bass unit being YD120-19. In actual use, these two speakers work well together.

3. Installation and fixation of power amplifier board and connectors

When assembling, first open a rectangular hole slightly wider than the amplifier circuit board and slightly higher than the highest component on the board in the middle and lower part of the speaker back panel, stick double-sided tape of appropriate thickness and width around it, then place the amplifier board horizontally and push it in from the outside; the radiator is exposed outside the box and close to the outside of the back panel, and then fasten the radiator to the back panel with self-tapping screws. Open a 2cm×5cm hole in the appropriate part of the back panel (above the radiator), and use a 3cm×6cm copper-clad plate to fix the small lotus seat here as a signal input board. Then drill holes in the back panel (below the radiator) and fix three different colored binding posts as the DC input terminal of the active speaker. After connecting the internal circuits and connecting the speakers, you can apply glue to the back panel and then assemble it. When using, be sure to connect the DC power supply line before turning on the AC power supply. At this point, the production of this active speaker is completed.

This active speaker is easy to use with CD and cassette players. Low cost, good sound quality and high production success rate are the remarkable features of this speaker. As for the listening effect, there is no need to use fancy words to describe it. I believe you will not be disappointed.