Low-power circuit and headphone amplifier designed by NE5532

       NE5532 has been around for several years, and everyone is very familiar with its circuit and has a variety of ways to play with it. The headphone amplifier introduced here is simple, low-power, and focuses on the production process.

　　Speaking of low-power headphone amplifiers, we have to mention the NE5532, the king of op amps in the 20th century. It has appeared in countless excellent preamplifiers and tuning circuits. Its mid-frequency is warm, delicate and thick, and it has a strong tube flavor. It is very cost-effective! Until today, we can still easily find it in some mid-to-low-end audio products. Due to its small size and simple circuit, it is the first choice for practical, low-investment hands-on people.

　　1. Principle Analysis

　　NE5532 is a typical bipolar input operational amplifier. There are many versions of low-power circuits composed of a single NE5532. I have modified those various circuit diagrams through constant comparison and thinking, and finally determined the schematic diagram (Figure 1). The amplification factor is controlled by R3 (R4) and R5 (R6). Theoretically, if R3 (R4) is 1kΩ and R5 (R6) is 100KΩ, the amplification factor is 100 times, but for the headphone amplifier, this will cause self-excitation. Besides, even if it can really reach 100 times, the effect cannot be good, so it is best not to adjust this circuit to 100 times when used in the pre-stage. Of course, for the headphone amplifier, setting it to 2 to 3 times can make the negative feedback appropriate, the sound quality softer, and the clarity more transparent, but the amplification factor cannot be too small, otherwise it will also affect the sound quality. You can repeatedly debug to achieve the effect you are satisfied with. The author set R3 (R4) to 1kΩ and R5 (R6) to 20 kΩ, that is, 2 times. C5 (C6) is the ground path of the input circuit and should be increased when used in a headphone amplifier circuit. The value in the schematic diagram is 22 uF, but it should be increased to 100 uF for this headphone amplifier.

　　It is worth mentioning the power supply issue here. If you are using a regulated power supply, you should pay attention to the filtering of the regulated power supply. Because this circuit itself is very simple, the selection of components is more picky. It is recommended to choose components of better quality when selecting materials.

　　2. PCB drawing

　　I used Protel 99 SE for wiring design. The PCB diagram you see (Figure 2) is the third version I drew, and it is also the one I am most satisfied with. The previous versions all had flying wires, but this version does not. Many versions on the Internet have the problem of flying wires, which is intolerable for picky hands-on people. Due to the small area, try to get a little grounding in terms of grounding. The input and output terminals can also be changed according to the actual situation. Leave space for all components, arrange them neatly, and look very harmonious.

　　3. Material Selection

　　As for the selection of materials, I believe friends from all over the world have different channels. Since the author is in the main urban area of ​​Chongqing, components are relatively easy to purchase. This time, in addition to buying new components, I also bought some old supplements, which cost less than 10 yuan.

　　1. Polar capacitor: Choose the brand ELNA, C1 (C2) 10 uF/50V, C5 (C6) 100 uF/50V, C9 (C10) can be 47 uF ~ 220 uF. I use 47 uF in all three versions, it has no impact, you can change it by yourself.

　　2. Non-polar capacitors: It is best to buy them in pairs. I chose second-hand German WIMA and Swedish EVOX white square MMK film capacitors. C3 (C4) is 200pF (or 220pF), and C7 (C8) is 0.1 uF.

　　3. NE5532: The NE5532 produced by the American Signetics Company is used. It is commonly known as the Big S5532. It is the best-sounding model among many manufacturers of 5532 (it has been discontinued for many years) and the top of the four ace op amps at that time. The author only found a second-hand one. If you can't find it, you can choose PHILIP. Of course, the ceramic sealed one is also good.

　　4. All resistors are five-ring metal film resistors. The domestic Da Hong Pao resistors in my collection are also used, so I will not go into details.

　　5. The copper clad board used in the circuit is a scrap material (I heard that someone used it to make TV circuit boards before, so I believe it will not be very bad and the thickness is enough).

　　4. Processing

　　After sufficient preparation, it is time to make PCB. There are many ways to make PCB by hand, such as special engraving machine (only schools with conditions have it, the cost is high, but the quality of the PCB is also the highest), commonly used methods are photosensitive PCB making and thermal transfer. Since the author has limited materials at hand, the thermal transfer method is used. Here is a suggestion: after the PCB is corroded and dried, it is best to punch holes first, then polish it with sandpaper, and then apply a solution of rosin and alcohol, which can both help soldering and prevent oxidation.

　　After the PCB is done, you can start soldering. The principle is to solder the resistor first, then the capacitor, and it will be done in a few seconds. It is worth reminding that my PCB input and output are selected as this plug-in, but in practice, due to the shell, I did not solder the plug-in directly on the PCB. Friends who have materials and strong hands-on ability can make a shell by themselves.

　　5. Beautify the appearance

　　After welding, debug first. After debugging according to the previous method, it is time to pack. After searching through boxes and cabinets, I finally found a box. It was a metal packaging box when I bought a USB flash drive before. There was a transparent window on it for viewing. After arranging the position, I took it to punch holes. Many friends like to listen to pure sound and do not use the volume potentiometer, so I did not draw it on the board. If there are friends who want to add a volume potentiometer like me, the potentiometer is a dual 50kΩ, and the connection method is very simple.

　　In terms of power supply, friends who like to use a regulated power supply should adjust the voltage to about dual 15V for a good effect. The author uses two 9V dry batteries, mainly for convenience.

　　After closing the box, this small headphone amplifier can be put into use. Its effect is much better than that of previous products, with a wider range and softer sound quality. Interested friends may wish to try this simple circuit.