Introduction

Below are three Power Amplifier designs for 2, 3, and 4 watts. The Power Amplifer is usually the last stage of a audio system, just before the speaker(s). Rather than Voltage gain, the Power Amplifier has current gain.

The schematics list specific transistor types, however, that doesn't mean that other transistors will not do a good job.

Power Amplifier, 2 Watt

This is the circuit diagram of 2 Watt audio amplifier. This circuit is inexpensive, easy, and quite simple. This is general audio amplifier which can be used for computer, CD player or other devices which have headphone output.

Technical Data
Output power:	1.5 W RMS @ 8 Ω, 2.5 W @ 4 Ω, 3.5 W @ 2 Ω (1KHz sinewave)
Sensitivity:	100mV input for 1.5W output @ 8 Ω
Response:	30Hz to 20KHz -1dB
Harmonic Dist. @ 1KHz & 10KHz:	Below 0.2% @ 8 Ω 1W, below 0.3% @ 4 Ω 2W, below 0.5% @ 2 Ω 2W.

The amplifier(s) can be supplied by a 12V wall plug-in transformer. Closing SW1 provides a Bass-Boost. At the same time, volume control (VR1) must be increased, to compensate for power loss at higher frequencies.

In use, VR2 should be adjusted to provide minimal audible signal cross-over distortion, consistent with minimal measured quiescent current consumption. A good compromise is to set the quiescent current at about 10-15 mA. To measure this current, wire a DC current meter temporarily in series with the collector of Q3.

Power Amplifier, 3 Watt

This is the small audio amplifier circuit, with decent power output of 3 Watts RMS into 4Ω load. The circuit is only a single channel. For full stereo production, two complete amplifiers would be required. The original drawings contained two 150 Ω resistors that were in series with the left and right stereo input channels and combined at the input to C1. If all your interested in is a single channel, this works out fine. What follows is a brief description of the circuit.

R1 and R2 form a voltage divider that sets the bias for Q1.

For a V_C = 9.4 Volts, measured with respect to ground, and R3 = 560Ω, The collector current of Q1 is:

R4, R5 are the emitter resistors connected to Q1, since the Q1 is the input transistor of the amplifier circuit, so small ac signal appears across the emitter resistors, therefore low impedance path must be created to bypass the ac signal using capacitor C4 the value of C4 can be calculated by taking two things into account.

One is that the audio amplifier must cover the frequency range from 20Hz to 20KHz. The other is the reactance of the capacitor should be 1/10 of the emitter resistance, or less than the emitter resistance. So for 20Hz, the following is true:

f is in Hz and X_C is in Ω. While 120µF is standard, a 470µF capacitor was used for C4.

Q2, 2N2905A, is the driver transistor (class A). This transistor is little old, but you can use a BD140 or a 2N3906 in it's place. R7 is the base resistor and R8 is the collector resistor, keep the Q2 in the active region (not a big gain at this stage).

R9, R11, VR (5K pot) and Q3 (2N2222A, but can be a 2N3904) form the V_BE multiplier circuit. To eliminate the cross over distortion, adjust VR to 1.7K up in series to the 1K (R9) so it forms (1.7K+1K). R11, and the V_BE multiplier circuit, control the quiescent current for thermal control over output the Q3. Q3 must be mounted to the main heatsink. This V_BE multiplier is effectively a biasing diode, with adjustable voltage drop.

Q4/Q5 are the output transistors. The drawing shows a 2SD882/2SB772 pair, but a BD139/BD140 pair can be used

C8 is the output capacitor which is 1,000µF, if you want to have good low frequency response, 3,300µF is good.

R6, 2.2KΩ, in parallel with C5, 0.001µF, creates a pole frequency of about 1/(2×PI×R×C) = 72.343 KHz. This feedback keeps the amplifier linear and stable in operation.

R13, C9 are to remove the high frequency noise.

C2, C3, C6, C7 are decoupling capacitors intended to remove high frequency, mid frequency and low frequency noise in the power supply lines. These capacitors should be mounted as close to their associated parts as possible.

C1 is the input capacitor C1 and R2 together form a high pass filter cutoff-frequency is 1/(2×PI×10uF×10K) = 1.59Hz.

The original drawing has two 150 Ω resistors in series with the left and right inputs, to combine both channels. The other end of the resistors are tied together and to the input of C5. These can still be used or two circuits can be built for good stereo production.

The amp sounds really good, from the scope results I did calculated THD, it is 2.5% at 3 watts RMS output. Frequency response is flat from 30Hz to 100KHz

Power Amplifier, 4 Watt (SM0VPO)

The circuit is very simple and incorporates darlington output transistors that will provide more than enough output current than is needed to drive a 3-ohm speaker. The gain may be pre-set for a variety of input levels, making it suitable for amplifying computer and cassette-deck Line-Output levels.

The completed unit is 60mm x 75mm and only 30mm deep. The depth could be reduced to 10mm if the output capacitor is mounted at the speaker and the on-board electrolytics are mounted horizontally. Here are the typical performance figures that may be expected from the finished amplifer using a 3-ohm load with a 13.8-volt supply. I see no reason why the supply voltage cannot be increased a little to obtain more output power:

Parameter	Min	Max	Units
Supply voltage	8	15	V
Output power	-	5.4	W
I/P for full O/P	30	4,000	mV (RMS)
Noise O/P no I/P	-	0.0005	V (RMS)
Supply current (no-signal)	-	50	mA
Supply Current (Full O/P)	-	1.9	A
3dB Frequency Response	42	34,000	Hz
6dB Frequency Response	21	62,000	Hz
Distortion at 2-watts	-	0.01	% (Vgain=10)

No heatsinking is required for the output transistors when running at a modest output level with either speech or music. A small heatsink should be fitted to the two TIP41 transistors if running a constant tone level. The heatsink could be bolted directly to the TIP41s without electrical isolation if the heatsinks are not going to touch anything. A heatsink with a 15-square surface area is all that is required. Here is the circuit of the amplifer.

The gain of the amplifer is set by selecting the value of the feedback resistor (Rf) on the PCB. The value of Rf is equal to 4700 / (Vgain -1) where Vgain is the voltage gain required. 4-volts RMS is the full-output level. Here is a guide for selecting the resistor.

Value	Vgain	Typical use
2K2	3	1300mV RMS - High levels (Small radio speaker)
470R	11	400mV RMS - e.g. computer/tape-deck Line-out
100R	48	80mV RMS - e.g. from TDA7000 RX
33R	143	30mV RMS (e.g. from microphones)

The distortion and noise levels may increase at higher gain levels, but the test board was measured with Vgain = 10 and was driving a 3-ohm car Hi-Fi speaker. With higher-impedance speakers the frequency response will become wider but the output power will reduce a little. There was no trace of instability throughout the AF range and up to 150KHz so I thought it unncecssary to include a Zobel network.