by Harry Lythall - SM0VPO

Here are some Audio Frequency power amplifiers for up to 100 watts. You may have to sprinkle 100pf (or more) capacitors between the base/emitters of the lower level transistors in order to retard the HF response and improve the HF stability. These amplifiers are also suitable for driving modest powers at 74KHz or even 140KHz so they are quite useful for VLF transmitters (at a low impedance of course).

AF Power - Quarter Watt (RMS)

This is about the simplest AF power amplifier. Almost all the components are contained in a single 14 pin package and requiring few external components.

The supply voltage can be from about 8 volts to not more than 15 volts. The device is supposed to have internal current limiting and be almost indestructable. I find that the output is a little distorted and, with the small output power, is of little more use than just driving headphones so I prefer to use an AF power amplifier with a little more "grunt". I have included it for reference just in case you need something really small and does not need a heatsink.

AF Power - One Watt (RMS)

This just another extension of the three transistor amplifier shown in the pre- amplifier section, but a much more "meaty" output current amplifier has been added. I have a PCB foil pattern for this amplifier which I will include as soon as I have it in electronic format. This circuit is probably my most used AF power amplifier circuit in my own homebrew equipment.

ComponentTypical valueComponentTypical value
R1100RR2, R3, R447K
R71K0*R9, R101R0
C21ufC1, C3, C42200uf
C510000ufTR1PNP BD557
TR4PNP BFX88Diodes1N914

* The voltage across D1 and D2 is necessary to prevent "crossover distorsion" caused by TR3 and TR4 each requiring about 3/4 volt before they will conduct. The voltage across D1 and D2 is governed by the current through them (governed by R6 and R7). If you look at the output of the amplifier and R6 and R7 are too high in value then you will see bad crossover distorsion:

Crossover distortion is particularly noticeable at low volumes. If the TR3 and TR4 get hot without a signal then increase the values of R6 and R7, keeping the ratio about 1:2 - 1:3. If the amplifier is overdriven or has too high a gain then the signal through the amplifier will be load and have flat peaks at high volumes:

Adjust R8 if necessary. The correct output waveform should look something like this at full volume:

This circuit will deliver the full 1 watt output with about 400mV RMS of drive voltage, but for other input levels you can set the gain by adjusting the value of R8. The gain is equal to 1 + R8/R5. For full output power the output voltage is 4volts RMS (12v Pk-Pk) with a 13.8 volt supply. With a 4 ohm speaker and heavier output transistors you should be able to get up to four watts of AF from this amplifier.

Supply voltage for this amplifier is 13.8 volts. You can replace the output transistors TR3 and TR4 with transistors having a higher current rating and drive speakers of just a few ohms to get more power out. This circuit is a very suitable circuit for homebrew receivers. The usual homebrew "standard" LM380 will only deliver a few milliwatts and I find it distorts quite easily. One watt is a much more respectable output power and can make your homebrew RX useful when the kids have got the telly on next-door.

AF Power (up to) One Hundred Watts RMS

Now this is an interesting amplifier and can be built for almost any AF power level you want from 10 watts to 100 watts or more. I will give you suitable examples for the 100 watt version.

ComponentTypical valueComponentTypical value
R3, R447KR568R
R65K6R7, R80R1
C2, C32200u 3vWKGC4100n
PNP TR12N2904PNP TR2, TR62N4036
NPN TR3, TR42N3725PNP TR5, TR7NPN 2N6099
+VE+45 volts-VE-45 volts

RV must be set to minimum resistance before application of power. To set up the amplifier, advance RV SLOWLY whilst monitoring the DC current through R7. Set RV for about 40mA. If you have a digital voltmeter then you can adjust for 8mV accross R7 and R8. If this standing current becomes too high (more than 80mA without a signal) when the transistors become warm then add a 1K0 thermistor accross RV, increase the value of RV to 470R and readjust when cold. When warm the current should remain stable. The thermistor should be mounted in direct contact with the heatsing upon which TR5 and TR7 are mounted on. If after adding the thermistor the standing current falls below 20mA then you can place a fixed resistor of 470R in series with the thermistor to reduce it's negative temperature effect (When I say warm I mean after a good blast of music has been applied to "cook" the amplifer a bit). All current measurements should be made WITHOUT the speaker connected.

All transistors except TR1 MUST be mounted on some form of heatsink which must be adequate for the job. You can replace TR4/TR5 with a single NPN power darlington and replace TR6/TR7 with a single PNP power darlingdon transistor. The power supply must be capable of handling more than 10 amperes as well as being quite well filtered. The speaker impedance should be about 8 ohms.

The low frequency response of this amplifier is not restricted by an output capacitor. There remains the possibility of a standing DC through the speaker. R3, D1 and C3 is included to prevent this from happening. You could replace C2 with a dead short but I prefered to leave it in circuit so that the negative feedback rises to 100% at DC.

This amplifier should not be attempted by those lacking experience since high voltages and powers are available. When DC-coupled high power amplifers decide to "go" then the results can be very spectacular. You therefore cannot afford to make any mistakes during the building of this project.

You can of course reduce the supply voltage and use cheaper transistors to make a smaller amplifier. If you reduce the speaker impedance to 4 ohms then there is the possibility to get yet more power out of it, but you will need to increase the current rating of the output and driver transistors as well as the standing current through TR2 and TR3. Do NOT use the 2N3055 or similar devices since their Hfe reduces to as low as 5 when pulling more than 10 amperes. This means that they will require over two amperes of drive from TR4 and TR6.

Have fun, de HARRY, Lunda, Sweden.

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