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tbj 29th October 2012 12:51 PM

Newly built ESP P03 (not 3A) - rail to rail oscillation - suggestions?

I built Rod Elliott's P03 (not P3A) 60 watt power amplifier.

I have built it on stripboard, which I know is not ideal, but I have checked and re-checked my board layout, and yet I have a weird problem. (I have only built one channel so far, in case of problems like this!)

I used the second schematic here:

High Quality 60 Watt Power Amplifier

At first the amplifier appeared to be playing music, but after a while would latch up to one of the rails with a loud 'hum' and stop working. I decided to change out the two "long-tailed pair" transistors on the input. Note that, although I'm not sure if this makes a big difference or not, I am using BC556s here instead of BC559s - however, until I did this the amp was indeed working, so I don't know if this is the cause of the following problem in itself.

Now, whatever I've done, the amp goes into full rail-to-rail oscillation at about 100khz as soon as it's switched on.

Photo from scope taken at speaker output: (10v/div, 5us/div)

I have made a post over at the ESP forums and one of the guys there has given me some helpful suggestions which I will try tonight, but I just thought I'd put it to you guys too as I know some of you have built this amp and might have some ideas about what could be causing this to happen!



lineup 29th October 2012 01:40 PM

Check those two 100pF caps.
Those around BD139 and BD140.
They normally protects for oscillations.

Also check the output terminal, the 10 Ohm and 100nF cap.

jaycee 29th October 2012 02:44 PM

Do you have any decoupling capacitors on your board's power supply rails? The schematic doesnt show any, but they are absolutely essential.

Incidentally, why did you build this schematic over P3A ?

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