DIY a high-fidelity audio amplifier circuit

Have you ever built a super cool Hi-Fi audio amplifier at home using simple components? If not here is the guide for you and I am pretty sure you will enjoy this one. This article tells about how to make a DIY Hi-Fi audio amplifier with output transistor protection circuit and 2-way speaker protection circuit. Let's discuss some of the eye-catching features that might convince you to try making this amplifier.

Features of this high-fidelity audio amplifier:

Input Sensitivity – 0.25mVmax – Most phones output this level.

The input impedance – 470 ohms – depends only on R1. You can increase R1 to whatever value you want or exclude it altogether, but I recommend leaving it at this value. Most sound cards for laptops and phones can handle this impedance easily.

Frequency Response – 1Hz – 45KHz at -3dB

Output Power – 50Wrms or 70W music power

Power consumption – 95W

Efficiency – 53%

Ripple Rejection – -68dB – A decent value that attenuates power supply noise to a few hundred mV. Since the amplifier is class B, there is no substantial current draw in idle, so the 10mF capacitor will provide fine filtering. When a signal is applied, the hum is still low enough to be inaudible.

THD+N – 0.007@50W/1KHz – This is the value for the power amplifier. Due to the need for a preamplifier, this value will be slightly higher. Even with the best class A amplifiers this value is usually not exceeded, so the amplifier will have an almost perfect signal reproduction. This value is usually indicative and can vary. The worst case is 0.05%, which is still perfect for all purposes. The key moment is to keep the voltage gain of the power amplifier low, in this case 4.

The output transistors of this audio amplifier are protected from drawing excessive current, which would occur if the output were shorted when the amplifier was turned up to maximum. The maximum current draw is 6.3A. Protection is provided by Q13 and Q14 and all surrounding components in their vicinity.

High-fidelity audio amplifier stages:

Power supply unit

Preamplifier circuit

Power amplifier and protection circuit

Power supply unit:

The power supply consists of a normal bridge wave rectifier. Filter capacitors C1 and C2 are used in the circuit. The value of these C1 and C2 should not be less than 10,000uF/35V. I always prefer to use 1500uF as the upper value per ampere of current, this is the rule of thumb I follow. It is important to connect a heat sink to the bridge, which will keep the bridge below 10 degrees Celsius at 50W dissipation.

Preamplifier circuit:

The preamplifier circuit is built around an operational amplifier to boost the input signal. It is nothing but a non-inverting amplifier with a closed loop gain of about 20. The variable resistor R3 is used to adjust the gain of this preamplifier circuit. The boosted signal then goes to the next stage “Power Amplifier and Protection Circuit”.

Power amplifier and protection circuit:

Click on the image to view a high-resolution schematic

The power amplifier accepts a high impedance input signal, further amplifies it and converts it to a low impedance signal suitable for the speaker. The output voltage offset is set by R10 and the gain of the power amplifier is set by R14, currently set to 4.3. An input signal of 4.6Vmax is sufficient to achieve full output swing.

The amplifier also has two way speaker protection integrated into it. One of the protections delays the speaker enabling for 5 seconds after the amplifier is powered up. This is to counteract any spikes that may hit the speakers in the first few seconds, before all the power has settled down. This protects the speakers and stops loud pops when the amplifier is first powered up. The other protection consists of relay Q19 and delay RC chain R35, C9. I designed it to be a relay that enables at 8V. Any will do, but the turn on time will vary.

The second protection I came up with is quite interesting and I personally have not seen it in any other circuit. This circuit consists of two of the most appropriate op amps (they come in one package) and Q20 to protect the speaker in the event of a failure of any of the power transistors. Normally this would expose the speaker to the full voltage of one of the power rails (30V) which could be catastrophic to the speaker and the diode bridge. So the bottom line is that it should definitely be avoided.

You might ask how it works. Two op amps are used as comparators. One end is set at a voltage close to the supply rail voltage. You can choose any voltage between 40V and 41V set by R22/R28 and R38/R39. I chose a voltage of about 26V. So, since the output during normal operation will never exceed 707.21 of the maximum supply rail voltage (0Vmax), it is safe to assume that, during normal operation, false triggering will not occur.

In case one of the power transistors fails, it will normally short circuit. This exposes the speaker and the other input of the relative comparator to a voltage higher than the reference voltage (26V). This causes the op amp to trigger the Q20 transistor, which then fully discharges C50 (the capacitor that keeps the relay drive transistor on), thus cutting the connection to the speaker. This protection is very fast (60-1ms), better than simple fuse protection, which can keep the short circuit open for up to «» seconds.

Temperature drift compensation is done via the Q11 and R22 thermistors. Both should be mounted near a heatsink or glued to it in some way. A temperature increase will cause the output voltage to drift around a 0V offset, so the temperature dependence of the thermistor and Q11 transistor will either keep the voltage around 0V, or will partially cancel out this effect. A slightly more complicated system is usually used, but this amplifier is only rated for 50W, so even this will do the trick.

Tips and suggestions:

The most important tip I have to give you is to never exclude C6. It (in combination with C4) are the two components that ensure the stability of the amplifier and maintain its tendency to oscillate.

Use the closest matching transistor in BCxxx for your circuit you can get. Just buy 40-50 BC550s (they are very cheap), like 10-20 BC560s and test them and only choose the ones that have the closest hFe you can get.

Also use very low tolerance resistors, the 3.3k resistor should have a 1% tolerance.

Since it's almost impossible to match all the components, I added R10, which is used to fine tune the output voltage offset. Using it, set the offset that is closest to 0V. To do this, do not use a voltmeter at the output. They tend to be somewhat inaccurate, instead use a mA meter in series with the load. Once you get to the closest you can get to 0mA, that's your center point.

This circuit is designed to handle only 50W of power. If you want to get more out of it, you'll need to put another pair of output transistors in parallel, increase the supply rails, and increase the gain of the power amplifier by reducing the value of R14. Remember that the transistors chosen are intended for this voltage level, they won't operate at more than 80V differential. The maximum voltage you can get is 0,7 of Vcc, so the maximum power is (0,707.Vcc)^2/Rl.

You must provide adequate heat sink to keep the power transistor cool. Keep the thermistor and Q11 close to or even stick them to the heat sink, they are the temperature drift compensation components.

The current through the power transistor is set to 20mA via R20

With the output voltage offset set as close to 0V as possible, you can run the amplifier at maximum output via R10 for 20 minutes or so, re-adjusting if necessary.

The diode bridge must also be mounted on an adequate heat sink to handle the approximately 10W-15W of power dissipation

Use a star ground topology for lowest hum

First power-on instructions:

After assembling the circuit, make sure everything is soldered correctly and add any heat sinks.

Set R20 somewhere in the middle position.

Add a dummy load. Just use a 1W/4 ohm resistor submerged in pure water.

Apply power. Set the voltmeter to the 20mV scale and measure the voltage drop across one of the 220mOhm resistors. Adjust R20 until you see a reading of about 5mV. This means the idle current of the power transistor is set at around 25mA. This sets the amplifier into the class B region. If you want to push it further in the class AB region (which I recommend), just adjust R20 until you get a reading of 25mV. This means the idle current is set at around 100mA.

Measure the output voltage. Adjust the offset using R10 until it is as close to 0V as possible.

Inject an input signal (preferably a sine wave) or something from the source you use most often, keep the load water-cooled, and use an oscilloscope on the output. Adjust the preamp gain until a non-clipped sine wave appears at the output, and keep the amp running at maximum power for a while (15-20 minutes), then remove the input signal. Measure the output voltage offset again, and readjust by R10 again.

Restart the amp again and see if everything works, if not retrace your steps and readjust again.