Newly built ESP P03 (not 3A) - rail to rail oscillation - suggestions? - diyAudio
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Old 29th October 2012, 12:51 PM   #1
tbj is offline tbj  United Kingdom
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Default 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!


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Old 29th October 2012, 01:40 PM   #2
lineup is offline lineup  Sweden
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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.
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Old 29th October 2012, 02:44 PM   #3
jaycee is offline jaycee  United Kingdom
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Location: Norwich, UK
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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