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). With the BC556, there is no audible noise, even if listening close to the speakers.

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 kind 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 (R17) should go between E and B)

Physically, the Vbe multiplier should be mounted near to the driver transistors (T3, T11), in order to ensure thermal coupling.


The output stage (T3, T12, T11, T4) was built with compound (Sziklai) pairs. 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.


Power Supply


I chose a 2x9V 30VA toroid transformer. With this setup, the idle voltage is approximately 2x13V at the output of the rectifier and buffer caps.


Sound Source


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.


Speaker Output


The amplifier was designed to drive 6-8 ohm speakers, but is able to drive 4 ohm speakers at a somewhat reduced performance.


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: 4.5mV

Power-on spike: 300mV, the output voltage stabilizes within 50ms

Noise: unable to measure due to noisy environment and instrument sensitivity limits. The amplifier noise including hum (60Hz) is probably less than 2mVpp.


Components


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.


Both 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.


Transistor Alternatives


MPSA18 -> 2N5089
BC556 -> BC557, BC560
BD139 -> BD437, BD137, MJE200G, KSD1691
BD140 -> BD438, BD138, MJE210G, KSB1151
MPS650 -> MPS651, BC337-40
MPS750 -> MPS751, BC327-40


Construction


The amplifier with the rectifier and buffer caps fits on a small (3" x 2") perforated PCB. The high-current wires (power supply + power transistor emitter, power transistor collector + 0R22 resistors, 0R22 resistors + speaker cables) are soldered together in groups, otherwise the board was laid...

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