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50 Watts Simple Audio Power Amplifier from OSU IEEE Student Group

This simple audio power amplifier was originally designed for a circuit board workshop, conducted by the OSU IEEE Student Group. At the workshop, 20 participants each constructed this amplifier, by etching and drilling the single sided circuit board, soldering all components, and attaching a pre-built heatsink assembly with the output transistors. Three workshops were held between 1995 to 1996. Though the design is simple, these amplifers have impressive preformance, with a frequency response to approx 40 kHz, very low noise, reasonably fast slew rate, and approx 50 watts (true "RMS" power) with the proper +/- 40 volt unregulated power supply.

Someday, I'll do some substantial testing to determine exactly what the power output is, and create some more detailed pages about how to build this amplifier.


Update: the input transistor are 2N5210, not 2N2510 as shown above

board layout

part placement step 1

part placement step 2

part placement step 3

Transistor Color
2N5210 Blue
MPSA56 Pink
MPSA06 Yellow
2N3904 Green
2N3906 White

part placement step 4
These color parts placement diagrams are also available in as postscript files in a ZIP archive.

This parts list is under construction... I'm gathering part info for several lists, so pleast don't assume this list is totally correct or complete.

Qty Vendor Part # Description
1 Newark 58F508 Wakefield 421k Heatsink
2 Mouser 567-7-373-BA Low-Power TO-220 heatsink
3 Mouser 592-2N5210 Low Noise NPN, TO-92
1 Mouser 161-4215 Phono Jack, 90 deg PCB mount
1 Mouser 592-MPSA06 Medium Power NPN, TO-92
1 Mouser 592-MPSA56 Medium Power PNP, TO-92
1 Mouser 511-TIP29C Power NPN, TO-220
1 Mouser 511-TIP30C Power PNP, TO-220
1 Mouser 511-TIP33C High Power NPN, TO-218
1 Mouser 511-TIP34C High Power PNP, TO-218
2 ?? 2N3904 General Purpose NPN
1 ?? 2N3906 General Purpose PNP
2 Mouser 583-1N4742A 12V Zener Diode
5 Mouser 592-1N4148 Small Signal Diode
3 Mouser 583-1N4001 1A (slow) rectifier diode
6 Mouser 140-XLR16V100 16V 100uF Capacitor (radial)
1 Mouser 140-CD50N6-331K 330pF NPO Capacitor
1 Mouser 141-100N5-051J 51 pF NPO Capacitor
6 Mouser 140-PF2A104K 0.1uF Mylar film capacitor
2 Mouser 28PR002-0.3 3 Watt 0.3 Ohm Power resistor
1 Mouser 594-63P502 5K Top adjust cermet trim pot
2 Mouser 29SJ500-2.2K 2.2K 1/2 Watt Carbon Resistor
1/2 Injectorall PC18P 4x6 board

1/2 inch 4-40 machine screw

4-40 nut

4-40 lockwasher

Shoulder Washer

Insulator, TO-218 size

Cable Clamp

Red Wire, 18 AWG

Yellow Wire, 18 AWG

Orange Wire, 22 AWG

Blue Wire, 18 AWG

Purple Wire, 22 AWG

Green Wire, 18 AWG

Black Wire, 18 AWG

Black Wire, 22 AWG

White Wire, 22 AWG

Gray Wire, 22 AWG

Resistor, 4.7 Ohm, 5%

Resistor, 47 Ohm, 5%

Resistor, 220 Ohm, 5%

Resistor, 330 Ohm, 5%

Resistor, 1k, 5%

Resistor, 1.1k, 5%

Resistor, 3k, 5%

Resistor, 6.8k, 5%

Resistor, 22k, 5%

Resistor, 47k, 5%

Resistor, 10k, 1%, metal film

Resistor, 47k, 1%, metal film

Note: The TIP33C and TIP34C have been discontinued and are generally not available anywhere. A wide range of power transistors will work, but they should be rated for at least 100V, 8A, and 80W power dissipation. Safe area operating curves and good thermal dissipation data are rarely available, so it's a guessing game. The more expensive TO-3 package parts, such as the MJ15003 & MJ15004 will certainly be more than sufficient for replacing the TIP33C & TIP34C. The only really compelling reason to use the TIP33C & TIP34C are because they cost less and come in a TO-218 package, which requires only one mounting hole.


  Diode assembly:         Gray 22 AWG (cathode)
White 22 AWG (annode)
NPN Power Transistor: Red 18 AWG (collector)
Orange 22 AWG (base)
Yellow 18 AWG (emitter)
PNP Power Transistor: Green 18 AWG (collector)
Violet 22 AWG (base)
Blue 18 AWG (emitter)
Input Signal: No wires, PCB mount jack
Output Signal: Blue 18 AWG (from PC board)
Black 18 AWG (from power supply)
PC Board Power: Red 18 AWG (to +35V on supply)
Black 22 Awg (to ground on supply)
Green 18 AWG (to -35V on supply)


Mouser - 800-346-6873, 619-449-2222
Newark - 800-463-9275, 503-297-1984
Injectorall - 800-878-7227, 516-563-3388


If any of these tests fail, the amp is not constructed properly... the easiest and best way to find the problem is visual inspection.
  1. Turn variable resistor fully counterclockwise (max resistance)
  2. Connect to +/- 24 volt supply with 200mA current limit. No input and no output connected. Monitor current from power supply with a current meter.
  3. Apply power... if current is above about 25 mA, shut off immediately!
  4. Measure voltage across the 1k resistor connected to the input stage and Vcc. The DC voltage should be about 2 volt, or 2 mA of current through this resistor. Eg, if Vcc is at 24 volts, the side of this resistor connected to the 2N5210 transisor ought to be at about 22 volts.
  5. Measure the DC voltage on the output line. It should be appox zero volts. -0.2 volts is probably fine.
  6. Turn the variable resistor slowly until the amplifer's current consumption is approx 50 mA. Turn slowly and be careful... if you turn too far you could damage the output transistors.
  7. Conect an oscilloscope to the output and apply a low amplitude 20 kHz square wave to the input. DO NOT connect any speakers during this test. This test should be done without the 330 pF capacitor installed. The amp should output a 20 kHz square wave with very little "ringing". It should not oscillate.
  8. Solder the 330 pF capacitor into the circuit.
  9. Shut off the power, connect audio input and a speaker. Make sure the volume is turned all the way down. Apply power... watch current meter again and shut off the power immediately if the current jumps to something much higher than 50 mA.
  10. Slowly turn up the volume and see if the amp works. DO NOT turn it up very much... the amplifier should not be operated with a supply less than +/- 30 volts. It should never be used for high volume output without a power supply rated for at least 2 amps of current (8 ohm load). After this initial test with +/- 24V at 200 mA (current limited) only a proper power supply should be used which can provide enough current.


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