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Old 30th March 2009, 08:25 AM   #1
ivant is offline ivant  Singapore
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Default lm3886 oscillates with preamp

Hope someone can help with my problem. The chipamp.com lm3886 kit works fine when connected to an ipod or 0404usb soundcard. However, when i connect my preamp to it, the lm3886 oscillates. It only happens when I have the volume knob either at fully clockwise or anticlockwise. Anything inbetween is fine. The oscillation happens regardless of load being connected. I get around 1.5vdc and 4.5vac at the outputs during the ocsillation. I can't find anything on the preamp out wih my multimeter.
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Old 30th March 2009, 06:49 PM   #2
AndrewT is offline AndrewT  Scotland
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does the chipamp.com implementation have an RF filter on the input and a Zobel or Thiele network on the output?
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Old 31st March 2009, 02:30 AM   #3
ivant is offline ivant  Singapore
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No filter on the input. Zobel is present on the output. Feedback cap is used in the build. If there's no problem when connected with other sources, I assume the problem to be with the preamp instead?
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Old 31st March 2009, 07:22 AM   #4
AndrewT is offline AndrewT  Scotland
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No filters at all on the input?
RF filter attenuates the very high frequencies that may instigate instability in the amplifier.
LF filter blocks the DC from entering and attenuates the very low frequency that can cause LF instability.
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Old 31st March 2009, 03:35 PM   #5
ivant is offline ivant  Singapore
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The kit only has a 1k resistor in series and a 22k resistor tied to ground on the input. I'll see if the filters solve the problem. Thanks
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