I needed an audio amplifier (AF amp) for a regenerative receiver that I am building. In 2015, Texas Instruments announced that they would discontinue the LM386 and the LM380. There are some alternate manufacturers, such as New Japan Radio Company and Unisonic Technologies, but New Japan Radio’s NJM386 is also marked as obsolete now.
The LM386 was one of the most common amps used in DIY guitar preamplifiers and sustainers due to its ability to run on a single 9V battery. It was also used in many homebrew Amateur Radio designs. In 2021, surprisingly you can still buy the TI LM386 in a DIP package, from Mouser but who knows how long that will last. You can also find them on Amazon and eBay, but these parts are often rejected or poorly performing copies.
For a one-off DIY project, it is perfectly fine to use an obsolete part, or a part you have on hand, or can get on eBay, for example. But if you have to make more than a couple, you will have to choose your parts carefully to avoid supply issues.
I wanted to design and build an audio amplifier that would perform like the LM386, use a minimal amount of parts, and use parts that will not be obsoleted very soon. I initially considered building the discrete AF amp that is in the book Experimental Methods in RD Design. However, this amplifier was designed to drive a set of walkman-like earphones, which are generally 32 ohms. It is really not suitable for driving an 8-ohm speaker. Instead, I came up with the simplest design that I could think of, which was an amp that I built way back in the late 1980s.
The amplifier consists of an op-amp voltage gain stage and an NPN/PNP output pair of transistors to drive an 8-ohm speaker. Feedback from the NPN/PNP transistor stage allows the op-amp to linearize the AF amp and overcome cross-over distortion. I used a 4558 op-amp which I had laying around and a 2N3904/2N3906 for the NPN/PNP output stage. You can pretty much use any op amp with varying performance depending on the particular op-amp you use. The best op-amp would be a low noise, rail to rail, high slew rate op-amp, but even the lowly LM358 op-amp gives satisfactory results.
In the schematic, I have the gain of the op-amp set at 10x. The second op-amp gives a gain of 2x for an overall gain of approximately 20x. You can change R1 and/or R2 to achieve different gains.
In testing, I found that using 100 ohms for both R4 and R5 gave the best slew rate performance from the 4558 op-amp and also minimum cross-over distortion. If you are using an op-amp such as an LM358 then a 330-ohm resistor at R10 gives the least crossover distortion performance. For other op-amps, you may or may not need a different value for R10 including not populating R10 in the case of the 4558 op-amp.
This circuit works well with supply voltages from 9 V up to 24 Vdc. I built the circuit up using manhattan construction. Generally, when building up a circuit manhattan style, I use a combination of round pads created from some double-sided circuit board with a hand punch, and pieces or strips of Vero stripboard. I have these pads and strips as well as various components modeled in Autodesk Fusion 360 where I can arrange the parts beforehand to optimize the layout when doing manhattan type construction. Once I have a board layout that I can visualize in 3D, I then go ahead and build it up in manhattan style.
The main advantage of this circuit is that it can be thrown together from parts that you have on hand and none of the values are critical. It can be used anywhere you would need a power amp to drive a 4 to 32-ohm speaker. It works well as a small guitar practice amp as another example.
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Manhattan-style construction:
www.sdmakersguild.org/the-art-...
aa7ee.wordpress.com/tag/manha...
www.unixnut.net/files/manart.pdf
Materials used in this video:
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Негізгі бет Ғылым және технология A Small General Purpose AF Power Amplifier
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