10W Stereo Audio Amplifier with Transistors

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This is another simple to build audio amplifier using common parts and medium power transistors. It outputs 10 W into 4 ohms speakers on each channel when it is fed with a 0.5 V peak-to-peak signal.

It is powered from 24 ± 1 V and has an input impedance greater than 100 k.

The output transistors must be able to handle 2 A of collector current and dissipate at least 20 W. Something like BD237 & BD238 or BD 437 & BD 438 pairs will do. The rest are general purpose transistors such as BC547, BC171, 2N2222, S8050, 2N3904 and their complementary BC557, BC177, 2N2907, S8550, 2N3906. The drivers of the power transistors, just like them, must be complementary and with similar current gain. So, if you have a hFE meter, do test the transistors and match them based on their hFE. If you can't measure them, follow the hFE markings. Do not match a BC547B with a BC557C because they have different gains.

10 W Stereo audio amplifier with transistors PCB photo
Amplifier PCB showing both channels
Below is the complete schematic.

10 W stereo audio amplifier with transistors schematic
Amplifier schematic
And this is the component outline on the PCB. The markings are made for BC547/57/48/58 transistors so if you use others be sure to check their pinout and fit them accordingly.

10 W stereo transistor audio amplifier component outline on PCB
Component outline
Except common ground, the two amplifiers are completely separated on the PCB so if you want to separate them just cut the PCB to the middle line. Neither the power bus connects one to the other. You'll have to do that with a wire when you power them up.

Pay attention to feedback resistor R12 (33 k). It should be measured and matched with the one from the other channel. It has an important role in modifying output power. All resistors can be even 0.125 W type with the exception of the 3 W emitter resistors and R5 (680) which can be 0.25 W, although I would recommend it to be 0.5 W.

When you build the PCB there are a few tracks that need attention. Those are the power and signal lines coming to and from the power transistors and need to handle high currents properly. They are 1.5 mm wide but if by any means you get them thinner place some solder on them. They will be handling a peak current of over 2 A! Also the jumpers need to be made of copper wire of 0.7 - 1 mm diameter.

Higher current tracks
Tracks and jumpers that need to withstand higher currents
You will need to make some measurements and adjustments. Do the following adjustments for each channel. There are two preset resistors that control the DC output voltage before the output capacitor and the quiescent collector current of the power transistors. When tweaking the collector current I highly recommend hooking those transistors to the heatsink. As you tweak the preset, the dissipated heat can become significant depending on actual transistors bias. Put a jumper across the input and connect the 4 ohms speaker. Using a voltmeter connected between the transistors collectors and ground, tweak R4 (250 k) until you read the half of the supply voltage on your meter (that is 12 V for a 24 V supply). Now, measure the quiescent current drawn by the amplifier. Tweak R9 (2.2 k) until your ammeter reads about 30 - 40 mA. Be careful because the current swings rapidly between microamps and amps when turning the preset. Recheck the collector voltage and readjust it with R4 (250 k) if necessary. Repeat for the other channel. That's it.

The heatsink should have an area of at least 80 square cm. Q3 (BC547) transistors aren't there just because there wasn't a better place (the small transistors on the edge of PCB between the power ones). They must be thermally connected to the heatsink to ensure a proper negative feedback when temperature rises. Each channel's heatsink should be common to all three transistors (Q1, Q3, Q7) and there is no need to isolate them electrically. Just don't place both channels transistors on the same heatsink! The heatsink actually carries the output voltage of the amplifier.

It's also recommended that you place a ground connected metal sheet screen between the channels on the PCB. Just remember to avoid at all costs ground loops.

If you looked carefully at the photos of my built PCB, let me explain you the strange things. Refer to the below photo. You can see a misplaced transistor (a). Well it is actually placed correctly. I used a S8550 that has the mirrored pinout of BC557. And two misplaced power transistors (b). That is the BD535 and BD536 pair. Both have mirrored pinouts when compared to BD23x or BD43x. When placing them on the heatsink I will turn them around and cross their base with the emitter. It's not nice but that is what I had available. There are also some strange looking coils (c). Well, those are my homemade 0.47 ohms 3 W resistors.

10 W Audio amplifier with transistors and DIY resistors
Misplaced parts and strange resistors
I used some heater wire (that is probably a nickel alloy - nichrome - and is common among electric devices that generate heat - mine came from a hairdryer). Using Ohm's law I determined that 7 cm of this wire has 0.5 ohms resistance. I couldn't measure it directly because my crappy multimeter shows 0.8 ohms when I touch its probes... So I placed a known resistor in series with a variable length of nichrome wire, powered them from a known voltage and measured the voltage drop on my wire.

DIY resistors with nichrome wire
DIY resistors
If you go for the DIY there are some things to know. You must use a wire that can withstand the current through it without becoming very hot (no more than 70 - 80 degrees Celsius). I hope it's not necessarily to say that it shouldn't get incandescent. Another thing is that you cannot solder this wire. Solder just stays away from it. So you will have to use a copper wire to make the transition from it to the PCB. As long as you use at least the same thickness copper wire all is fine. Make 3 - 5 turns of nichrome wire around the copper one, then bend the copper wire over the nichrome one. To reduce the size of the resistor I shaped it into an air coil. You can also use any other kind of mechanical holders to fix the nichrome wire.

If you can only get very thin nichrome wire, remember you can twist multiple wires together. But that decreases their total resistance so you will make a... longer resistor. The thing with nichrome is that it has a small variation of resistance when temperature changes when compared to other conductors. You can use pretty much any other electrical conductor to make resistors as long as the current flowing through it does not cause significant increase of temperature (which will increase resistance) and as long as you don't turn the resistor into a coil!

Since I had them around I also used those ceramic rings at the pins of my resistor. You can also place two 1 ohm resistors in parallel. There are plenty of pads in there. Here are some construction photos.

10W stereo audio amplifier with transistors - PCB with heatsinks

10W stereo audio amplifier with transistors - PCB with heatsinks

I'm providing a personal PCB design (there was none in the sources) that I tried to make as "universal" as possible with extra pads for presets and the big output capacitor. Also you can download a LTspice simulation file so you can analyze and tweak this circuit as you want.

Downloads: PCB as PDF | LTspice and ExpressPCB files.

Source(s): I actually found the same schematic in more than one Romanian electronics books and magazines:
  1. Ing. Emil Marian, Amplificator de 10 W, Tehnium 1984 Almanah, pg. 114-116.
  2. I. C. Boghițoiu, Electronica peste tot, Albatros 1985, pg. 159-165.
  3. Ing. Emil Marian, Scheme și montaje de audiofrecvență, Editura Tehnică 1992, ISBN 973-31-0437-X, pg. 108-109.

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