This is a hi-fi 10 W stereo audio amplifier built with general purpose and medium power BD237/BD238 transistors. Complete schematic, PCB design and LTspice simulation provided.

Most audio amplifiers nowadays are built with special integrated circuits. These ICs are getting smaller, but the power they can deliver and efficiency are increasing. The following amplifier is a classic design built with common parts and some power transistors in output stages. It can deliver a maximum of 10 W into 4 ohms speakers on each channel when it is fed with a 0.5 V peak-to-peak signal. Input impedance is above 100 kilo-ohms. The amplifier should be powered from 24 V regulated supply.

The output transistors must be able to handle a collector current of at least 2 A 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 (pay attention to pin order). The drivers of the power transistors, just like them, must be complementary and with similar current gain. So, if you have a hFE meter, it's recommended to test the transistors and match them based on hFE. If you can't measure them, follow the hFE markings. Do not match a BC547B with a BC557C because they have different gains.

Built channel amplifier (without heatsink)

The schematic for a channel is below. You need to build two of that if you want a stereo amplifier.

Amplifier schematic (one channel shown)

And this is the component outline on the PCB. Silkscreen markings are made for BC547/57/48/58 transistors so if you use others be sure to check their pinout and fit them accordingly. I used at my prototype S9014/S9015 transistors (that's why they are rotated 180 degrees). The PCB dimensions for one channel are 65 by 70 millimeters.

Component outline for the amplifier PCB

Pay attention to feedback resistor R8 (33 k). It should be measured and matched with the one from the other channel. You can replace it with 24 k, 27 k or 30 k resistor (use the same for both channels). It has an important role in modifying output power. If a lower resistance is used, the output power increases a bit. All resistors can be even 0.125 W type with the exception of the 3 W emitter resistors and R10 (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 1.2 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. Turn RV2 to maximum towards R11 (to the right) to get the minimum collector current. Just in case, before powering the circuit for the first time, mount the heatsink!

You will need to make some measurements and adjustments. RV1 adjustment is mandatory, while RV2 is optional for decent operation of amplifier. RV2 must be turned to the right.

Do the following adjustments for each channel. There are two preset resistors that control the DC collector voltage (RV1) and the quiescent collector current (RV2) of the power transistors. Put a jumper across the input, connect the 4 ohms speaker and apply power to the circuit. Using a voltmeter connected between the transistors collectors and ground, tweak RV1 (200...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 total current drawn by the amplifier (put it in series with ammeter). Tweak RV2 (2.2 k) until your ammeter reads about 15 - 40 mA. Be careful because the current swings rapidly between microamps and amps when turning the preset (that's why you need the heatsink at this point). The amplifier works pretty good with minimal quiescent current (class B operation), so you may as well leave RV2 turned to the right. The collector current gets to a few tens of mA when RV2 cursor is close to the middle. Recheck the collector voltage and readjust it with RV1 (200...250 k) if necessary. If you can't reach half of the supply voltage, replace R2 with a 100 - 220 k resistor. 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 (Q3, Q7, Q8) and there is no need to isolate them electrically. Just don't place both channels transistors on the same heatsink! The heatsink may carry the collector voltage of the power transistors.

SPICE simulation proves the 10 W medium output power with 500 mVp-p input signal.

Simulation of amplifier output power (green trace) vs. input signal voltage level (blue trace)

If you looked carefully at the photos of my built PCB, you can see some misplaced transistors. That is because I used S9014 and S9015 transistors which have reversed pinout to BC547/BC557, for which the PCB is designed. Only Q3 is BC547 in my build.

Circuit adapted from Romanian electronics literature (Ing. Emil Marian in Tehnium Almanah 1984, pg. 114-116 and in Scheme și montaje de audiofrecvență, Editura Tehnică 1992, ISBN 973-31-0437-X, pg. 108-109).

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