Power Amplifier 50W [Osu IEEE]

PA_50W_0SU_1.gif (19172 bytes)

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 amplifiers have impressive performance, 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.

Note: The TIP33C and TIP34C have been discontinued and are generally not available anywhere. I replace the Q11=TIP33C and Q12=TIP34C with BD249C and BD250C


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 counter clockwise (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 +V is at 24 volts, the side of this resistor connected to the BC550C transistor ought to be at about 22 volts.
  5. Measure the DC voltage on the output line. It should be appox zero volts. +/-0.05 volts is probably fine.
  6. Turn the variable resistor slowly until the amplifierís current consumption is approx 50 mA. Turn slowly and be careful... if you turn too far you could damage the output transistors.
  7. Connect 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 C1=330pF capacitor installed. The amp should output a 20 kHz square wave with very little "ringing". It should not oscillate.
  8. Solder the C1 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.


Part List

R1=4.7 ohm 1/4W R23-25=0.33 ohm / 4W Q8=BD240C [on small Heatsink]
R2-16=47 Kohm 1/4W TR1=5 Kohm Top adjust cermet trim pot Q10=2N3906
R3=10 Kohm 1/4W C1=330pF NPO Capacitor Q11=BD249C [on Heatsink]
R4-5=47 ohm 1/4W C2-3-6-7-12-13=100nF 100V MKT Q12=BD250C [on Heatsink]
R6=3.3 Kohm 1/4W C4-5-8-14=100uF 63V D1-2=12V 0.5W ZENER
R7-8=2.2 Kohm 0.5W C9=47pF NPO Capacitor D3-4-5=1N4002
R9-15=1 Kohm 1/4W C10-11=100uF 16V D6-7-8-9-10=1N4148
R10=6.8 Kohm 1/4W Q1-2-3=BC550C J1=3pin connector with 2.54mm step
R11-17=1.1 Kohm 1/4W Q4=MPSA56 J2=5pin connector with screws
R12-18-19-20-21-22=220 ohm 1/4W Q5-9=2N3904 F1-2=Fuse2A Fast
R13=330 ohm  1/4W Q6=MPSA06
R14=22 Kohm 1/4W Q7=BD239C [on small Heatsink]

Power Supply

Power_supply.gif (20965 bytes)

Part List

TR1=220VAC/2X28V  6A C1=33nF 630V C9-10=15000uF 63V
F1=Fuse 1A Slow C2-3=22nF 100V  MKT C8-11=100nF 100V  MKT
S1=2X10A SW C4-5=22nF 100V  MKT D1-4= rectifier bridge  400V 35A
JF1=3P Connector 3.96mm step C6-7=15000uF 63V


Sam Electronic Circuits 02/04