Amplifier Circuit Description
This is a fairly standard amplifier circuit that provides excellent audio quality, due to its simplicity, low power requirements and use of modern, high-quality semiconductors.
The input stage (T1, T2) was built with ultra high-gain, low-noise MPSA18 transistors; the current mirror (T5, T6) further increases the open loop gain and forces the input stage to the fully balanced state, thus increasing linearity and drastically reducing the offset voltage on the output.
The voltage amplification stage (T9) was built with the medium-gain BC556 transistor, though theoretically better performance can be achieved with the low-noise versions (BC557, BC560).
The Miller-capacitor (C3) is 100pF foil-type. The amplifier stops oscillating with a 33pF Miller-cap, and with 100pF, it's still able to deliver 5Vpp 100kHz sharp square wave signal on its output, full of upper harmonics.
The Vbe multiplier (T10) can be built with any type of transistor (either NPN or PNP), it should be able to provide constant 1.2V voltage between its collector and emitter. (NOTE: the resistor (R7) must always go between B and C, and the trimmer (R19) should go between E and B)
Physically, the Vbe multiplier should be mounted near to the driver transistors (T3, T11) but far away from the output transistors (T12, T4), in order to ensure the best thermal stability.
The output stage (T3, T12, T11, T4) was built with compound (Sziklai) pairs, this arrangement has several practical advantages over the more commonly used Darlington pair (e.g. flexible mounting options for the power transistors due to superior thermal stability, lower saturation voltage, etc..). The driver transistors (T3, T11) are of special types (MPS650 and MPS750) - they amplify with relatively high gain at high collector current (hfe = 75 @ 0.5A min), thus able to drive the power transistors (T12, T4) even under heavy load.
With a 2x9V 30VA toroid transformer, the idle voltage is approximately 2x13V at the output of the rectifier and buffer caps.
The input impedance is relatively low (2k ohm), the amplifier works very well with the headphone output of any sound source (smart TV, phone, mp3 player, laptop). The voltage amplification is approximately 22x which gives a 22Vpp at 1Vpp nominal input.
The amplifier was designed to drive 6-8 ohm speakers, but is able to drive 4 ohm speakers at a somewhat reduced performance.
Using 1kHz sine wave and 6.8ohm dummy loads, I measured 2 x 20Vpp without any visible distortion (using an oscilloscope) at the speaker terminals, which corresponds to approximately 2 x 7.3W true (RMS) power.
Output offset: less than 4mV
Power-on spike: 300mV, the output voltage stabilizes within 50ms
Noise: The amplifier noise including hum (60Hz) is probably less than 2mVpp. There is no audible noise on the speakers, even if listening closely.
C1, C3 and C6 must be foil-type caps, C3 and C6 should be rated at min 40V. Cheap ceramic caps may be used for C2 and C7, as these parts are non-critical. C15 and C19 are aluminum electrolytic caps, rated at min 25V. Tantalum capacitors should be avoided.
R8 and R9 have to withstand low-mid loads, ideally these parts should be rated at 0.5W minimum. R10 and R11 are high-power wirewound resistors, rated at 3W; R18 is a wirewound resistor, rated at 1W. Ideally R14, R15, R16 and R17 should be 1% metal-film resistors, as these parts are directly responsible for the accuracy of the amplifier. All other low power resistors are non-critical, they can be plain 0.25W carbon-film ones.
MPSA18 -> 2N5089
BC556 -> BC557, BC560
BD139 -> BD137, MJE180, MJE243
BD140 -> BD138, MJE170, MJE253
MPS650 -> MPS651, BC337-40
MPS750 -> MPS751, BC327-40
The amplifier with the rectifier and buffer caps fits on a small (3" x 2") perforated PCB. The high-current wires and the legs of associated parts (e.g. output transistor collector and power supply wire) are soldered closely together, otherwise the board was laid out in a way that the majority of the components could be connected by soldering the adjacent legs together. Where that was impossible, I used thin wrap-wire.
The wonderfully simple idea of using thick cables to connect the chassis-mounted power transistors to the circuit instead of soldering them directly onto the board came from the inglorious Videoton Hi-Fi amps.
These steps can be used as a general guide to bring up any similar class-AB audio amplifier by an average builder, without any special tools or instruments. The main goal of this process is to ensure that the amp will work according to the specs, and to eliminate any adverse behaviour.
First of all, all the connections should be double-checked. The Vbe multiplier pot (R19) should be trimmed to the highest resistance in order to completely close the output transistors during initial testing. The capacitance of the Miller-cap (C3) is to be determined yet; a relatively large (100nF) temporary Miller-cap (C3) should be connected to the circuit, in order to ensure stability for static testing and alignments. The Zobel-network (R18 and C6) components should not be built in for now, due to possible high-frequency destruction (see reasons in detail below).
Once all the parts are in place and the soldering job is complete, the idle voltages should be checked; it's a good idea to protect the amplifier and the power supply (which can be as simple as two 9V batteries) with resistors (47ohm) at this initial testing stage.
The voltage at the junction of R3 and R4 should be -5.6V, the voltage at the speaker terminal should be max +/-20mV. The amplifier should not draw more than 10-15mA current from the power supply in this setting (NOTE: the current draw on the negative (-) side is approximately 3-4mA higher compared to the positive (+) side, due to the extra current consumption of D1-R4)
Once the static testing is complete, the amplifier can be connected to its main power supply. The next step is to eliminate the two major issues that such an audio amplifier can have: crossover distortion and self-oscillation.
Crossover distortion is the result of the non-zero (approx. 0.6V) PN-junction voltage between the B and E of the driver transistors (T3, T11), the usual way of eliminating it is by slightly opening the output stage transistors; this is called the class-AB operating mode.
The quiescent current of a class-AB amplifier is optimal when the emitter impedance of the output transistor matches the emitter resistance (0.22ohm). This condition occurs when the thermal voltage (26mV at 25C) appears on each of the emitter resistors; the optimal quiescent current corresponding to 0.22ohm emitter resistors is approximately 118mA.
Quiescent current increase can be measured either by measuring the voltage increase on the output resistors (R10, R11), or by directly measuring the current draw increase from the power supply; it can be set by slowly trimming the Vbe multiplier trimmer pot (R19) until the desired quiescent current is reached (NOTE: once the voltage on the Vbe multiplier reaches approximately 1.2V, the output stage transistors abruptly open. Trimming hence should be a slow and delicate process).
Self-oscillation is typically the result of negative feedback (see R16 and T2) turning into positive feedback at high frequencies. Oscillation frequency is typically in the 2-3MHz range. At his frequency, the Zobel-network capacitor (C6) behaves as a short-circuit, meaning that all the power is dumped onto the Zobel-resistor (R18), leading to its destruction; hence the Zobel-network should not be connected unless the amplifier is stable and free of self-oscillations.
The simplest way of detecting self-oscillation is by using an oscilloscope, however a simple, fast peak-detector (1n4148 diode and a 100nF cap) with a multimeter should work as well (NOTE: the temporary large-capacitance Miller-cap should be disconnected at this point).
The standard way of eliminating self-oscillation in audio amplifiers is by limiting their bandwidth, by increasing the Miller-capacitance of the voltage amplification stage transistor (T9) with an external Miller capacitor (C3).
The easiest way of finding a suitable Miller-cap is by consecutively attaching capacitors of increasing capacitances until the self-oscillation stops. The final Miller-capacitance value should be 2x - 3x the capacitance of the cap that stopped the oscillation; this ensures safe operation, but still gives a good slew-rate and decent (> 100kHz) bandwidth. The Zobel-network components can be finally built in, once the amplifier has been made stable.