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Old 14th July 2009, 02:53 PM   #1
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Default OPA2134 Oscillating?

I have tried to follow the ESP P06 phono stage schematic using stripboard and have run into some problems which I feel sure are of my own making.

When it was first tested, the output volume was very low with weak treble and poor sound quality on both channels. About 5 minutes later one channel stopped working entirely, not good.

I re read the article and realised I had neglected to include the 10uf decoupling capacitors to the supply rails mentioned in the text, using only the 100nf from supply pins to ground. Further head scratching revealed I had also misread my own hand writing and used 27nf caps for C1L and C1R rather than 22nf as shown. Embarrassing.

I am assuming I have oscillation problems going on and wonder if the lack of decoupling is to blame, or whether I have grounding issues also. I ended up with 5 strips serving as grounds jumpered together, connected by one wire to the power supply. I had wondered whether these would be better returned with an individual wire for each strip.

I would appreciate any input on this before I go for version 2 with the correct values used this time and hopefully a better outcome.

John
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Old 14th July 2009, 03:01 PM   #2
lineup is offline lineup  Sweden
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Both things could possibly be the reason of oscillation.
Correct decoupling of supply pins is so important, that it is standard for OP-Amps.
Improper design of PCB-layout with a less good grounding route can give problem.

Of course the main reason for your oscillations is most probably those values.
If you try with correct values now, I think it will work good.
OPA2134 is not known for oscillations. It is a very stable op-amp.
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Old 14th July 2009, 03:35 PM   #3
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Hi lineup, thanks for your comments.

I am using stripboard rather than a pcb, I need to learn Diptrace or similar but it is one thing at a time. I will order some more capacitors and try again.

If nothing else I have a chance to learn from the problems.

John
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Old 14th July 2009, 03:44 PM   #4
nonoise is offline nonoise  South Africa
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Also try to keep the physical implementation of the feedback loop as short as possible
Sometimes just cleaning the board thoroughly after all the soldering have being done could also help
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Old 14th July 2009, 04:15 PM   #5
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Hi nonoise

I had the feedback loop down as small as I could, in fact everything was as compact as I could make it. I scrubbed the board in isopropyl with a toothbrush before looking for shorts etc.

Maybe version 2 will be better if I learn to read my writing and buy some spectacles!

Many thanks.

John
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Old 18th July 2009, 10:08 PM   #6
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Version 2 is built using the right values, the right opamp decoupling and more care with the grounding. It has been playing for a few hours now and all is well.

It sounds good even using an old JVC turntable I had kicking around and less than perfect vinyl. Once it gets to my mates Garrard 401 it should be splendid!

Thanks for the help chaps, there has been good learning had from the project for me.

John
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Old 19th July 2009, 01:45 PM   #7
nonoise is offline nonoise  South Africa
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Congratulations
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