After a century searching the web, finally I decide to build the “Simple Class-A Amplifier”. This amplifier circuit was designed by John Linsley Hood in year 1969 and then updated in year 1996. Later on, it have been improved by a group of forumer at diyaudio.
All the information of the amp can be found at Geoff website. After reading all the contents at the website, I choose the “Capacitance Multiplier(CM)”(Fig.3 Simple CM) as my power supply unit because it can produce a very low voltage ripple. For the amplifier circuit I take the “Fig2.The Final Circuit” which the feedback capacitor have been removed. I have a casing/housing which is (16 x 8 x 3) inches, because of this I targeted my amp to be 15watt and using dual power supply to reduce the size of the transformer.
I buy the transformer(Input : 240v; Output :18v-0-18v; 50Hz 3A) at local electronic market well know as Pasar Road, but I’m not satisfy with it even thought I already ask for an over rated(5A). Anyway it’s fine for one channel. Building the CM is quite easy but be careful with the big filter capacitor. Make sure it’s discharged before trouble shooting. When I turn it ON, the output voltage of the CM is +/- 20VDC. Is this correct? Then tested with a 12R 50watt dummy load, the reading is +/-18VDC. Yes, It is correct right now. So I continue to build the amplifier circuit.
The nightmare begin here, after 7 years I did’t work with electronic project, I make every thing screw up. I confuse myself when defining the Base(B), Collector(C), Emmiter(E) of the transistor. I’m very lazy to build the PCB which need a lot of process like etching, drilling and sticking, because of this I blow up some units of 2sa970, capacitor and burn my MJ15003. At the end of the day, my pocket money was depleted. The project freeze for couple of month.
Finally, I raised some fund and restart the project again. This time, I’m not that lazy but I still use the PCB with hole but with a drawing plan of every components. After all the components have been soldered and the circuit have been created, I check it again and again for 100% correct. To double up the safety, I only use a single channel and some more I do consult the expert 1st before I do anything wrong again. Mr. Geoff explain to me on how to test the whole things, complete like A-Z guide.
I turn ON the compact disk(CD) player, make sure my speaker is protected by insert a axial capacitor in serial. Now, turn ON my amplifier, HOLA ……, the CD is spinning but nothing happen to my amplifier, it didn’t blow up, I hear no sound at all. Then I do some measurement on the voltage. It is not normal as what Mr.Geoff said. Turn it OFF, I hear some sound but it become weaker and weaker. Then I recheck the circuit. It is fine for me, nothing wrong too. Repeat again, the same thing happen. So, what was going wrong?
I read back all the article and also the email Mr.Geoff send to me. Go back to Geoff website. Inspecting the PCB picture of the Class-A amplifier for Quad ESL57. No clue at all. Go search the web again, I found diyaudio. It seem that they are a lot of people building this amp. Diy forumer is great, they have all the thing, I downloaded the PCB picture posted by Mr.Paulb, print it out and compare it with mine. Oh My God, Do you know that we have two type of transistor? one is P-N-P and the others is N-P-N. I never work with pnp before, therefore never notice that the collector and base is in the opposite side of the npn in a schematic drawing.
I point out all the error, repaired the pcb with some jumper. Check all the transistor and the pcb again for double confirmation. Connect all the thing, Turn it On, I hear some distorted music. Adjust the DC offset, set the Iq to 1A. Suddenly, the clouded sky now become clear, the gear stared to sound and the music is great!.
Assemble the 2nd channel, put all the wires nicely. Turn the amp ON again, adjust the Iq and DC offset. After listened to three song, I put my hand on the head sink. It is very hot, measure it with Digital multimeter, 70′C(should be around 50′C). I calculate the power of the amplifier it is 25watt. Then I reduce it back to 20watt. It’s still very hot. Study the heat sink calculation, recalculate every thing and finally I found out that I need a bigger heat sink and a bigger casing.