12 Volt 5 Watt Amplifier
Circuit :   Julius Chen, China
Email  :  juliussw@gmail.com

Description:
This solid state amplifier uses 10 transistors and 5 diodes. It can deliver 5 Watts into a 4 ohm loudspeaker and about 3 Watts into an 8 ohm loudspeaker. It is made using Chineese transistors from the 1970's and 1980's.


To the Reader
I am a student in electrical engineering. I would like to hear any comment or suggestion on the circuit from you. Please feel free to drop me a line about the circuit or other issue about electrical engineering. Please do NOT send any political or religious message.

General Description
This is a general purpose amplifier designed for listening to radio and MP3s. It can be battery power and therefore it can prevent the noise from the grid to disrupt the shortwave reception and can be a portable device. It features purely discrete design, nostalgic transistors, low distortion and noise, DC coupling, and sufficient power for general applications.

Input Stage
The input stage consists of Q1 through Q5 and 3 diodes. The differential pair is biased by Q1, R3, R4, D3, D4, and D5. The biased current is set to 1 mA. Q4 and Q5 form a current mirror as the active load for the differential pair, and the output is taken out single-endedly from the collector of Q2. The open loop gain of the input stage is more than 60dB.

Second Stage
Q8 act as a common-emitter amplifier driving the output stage. Current mirror formed by Q6 and Q7 serve both biasing current source and active load for Q8. The open loop gain of the second stage is about 40dB. Q6 and Q7 are mounted together inside the blue tube to provide thermal compensation to each other.
Output Stage
The complimentary pair BD139/140 is biased by D1, D2, and R10. Adjusting the value of R10 can yield the desired quiescent current. R13 and R14 prevent thermal run-away of the complementary pair. DC voltage at the output is always half the power supply.

Feedback
The feedback network consists of R6, R5, C3 and C4. The overall gain is about the ratio of R6 to R5. C4 provides lead compensation; C3 provides DC-blocking.

Frequency Response
The cut-off frequency is determined at the upper end by C2, R5, and C4, and at the lower end by C1, C3, and C5.

Power
Power supply voltage 12V, 5W for a 4 ohm speaker, 3W for an 8 ohm speaker.

Simulations Acknowledgement
I thank Andy for his work of running the simulation and putting this circuit on line. I did all the calculation on paper. But for his work, it would be more difficult for me to obtain a Bode plot and know the harmonic distortion. Such data provide guidance for other circuit builder and me to further improve the circuit.

I have made some simulations on Julius's amplifier using LTspice. Unfortunately I do not have exact models for the transistors so all these figures should be taken as an approximation.

Bode Plot

A very flat bandwidth. Mid band gain approximately 33dB.

Fast Fourier Transform @ 1 KHz


A FFT was applied to a transient waveform at mid-band, 1KHz only the 2nd harmonic is visible.

Harmonic Distortion

Harmonic	Frequency	 Fourier 	Normalized	 Phase  	Normalized
 Number 	  [Hz]   	Component	 Component	[degree]	Phase [deg]
    1   	1.000e+03	1.191e+00	1.000e+00	   -0.36°	    0.00°
    2   	2.000e+03	9.793e-03	8.221e-03	  111.16°	  111.52°
    3   	3.000e+03	1.202e-03	1.009e-03	 -177.60°	 -177.24°
    4   	4.000e+03	7.744e-04	6.501e-04	 -179.48°	 -179.12°
    5   	5.000e+03	6.412e-04	5.383e-04	 -178.04°	 -177.68°
    6   	6.000e+03	4.949e-04	4.155e-04	  177.97°	  178.33°
    7   	7.000e+03	4.281e-04	3.594e-04	 -178.52°	 -178.16°
    8   	8.000e+03	3.752e-04	3.150e-04	 -176.32°	 -175.96°
    9   	9.000e+03	3.589e-04	3.013e-04	 -179.24°	 -178.88°
    
Total Harmonic Distortion: 0.835506%

Overall low harmonic distortion measured at 3 Watts into 8 ohms, rising to 1.5% at 5 Watts into 4R load.

Total Noise

Total noise measured at the output into a 4 ohm resistor load.



Prototype
Below is prototype amplifier made by Julius himself. TBlack heatsinks are attached to the output stage BD139 and BD140.

Nostalgic Transistors
Q1 through Q8 are all metal can transistors made in 1970s and 1980s in China. Those transistors are no longer produced today. In those days, few people care about cost in China, and those transistors are far superior to their modern counterpart manufactured in China today. Some of them were even with their pins gold-plated. I have measured many of them with a transistor tester. Though their β values are usually no more than 130, their leakage currents are usually less than 1µA.

Nostalgic Transistor Click Images to Zoom


Alternative Transistors
All the nostalgic transistors can be replaced by their contemporary counterparts, as long as it does not exceed the maximum ratings of the transistor. Q1~5 and Q8 can be replaced by TO-92 or TO-18 transistors; Q6, Q7 can be replaced by TO-220 or TO-39 transistors or BD139.

The amplifier is a feedback system that will try to stabilize itself, deviation from the recommended value will not have a significant impact on the performance for this particular circuit, at least it is not detectable by ear. In other words, it is not important that what types of transistors are used.

For those who insists that I should recommend contemporary types transistor, I would prefer 2N2222, 2N2907 or BC550, BC560 for Q1~Q5 and Q8, and BD139 for Q6, Q7. Be aware of the matched pairs.

One thing being very important: Q6 and Q7 must be in close thermal contact with each other!

The quiescent current is 50~100 mA, and there is very little distortion at this value. You may use a larger quiescent current, provided there is a large heat sink. Adjusting R11 will yield the desired quiescent current and the value of R11 must be determined by experiment. I would suggest using a potential meter to adjust the quiescent and then replace with a resistor.

Julius did all the calculations for this amplifier on paper, but the amplifier still sounds good. Julius would also like to hear from anyone building his amplifier.

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