Help!! Oscillation on - input of CD Player Filter Stage - diyAudio
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Old 30th January 2005, 02:36 PM   #1
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Default Help!! Oscillation on - input of CD Player Filter Stage

I have a rather humble Cambridge Audio D500SE. I have noticed that on the last filter stage which uses NE5534's there is 1Mhz oscillation on one of the frequency compansation pins and also oscillation of a few hundred KHz on the - input pin.

I cant work out whats causing it, I suspect it may be noise from one of the previous stages setting it off.

The filter looks like a unbalanced input, multiple feedback type. The password to get into Cambridge Audios service area on their website is service2000 if anyone wants to take a look at the circuit and give some help""
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Old 31st January 2005, 11:25 AM   #2
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Default Re: Help!! Oscillation on - input of CD Player Filter Stage

Quote:
Originally posted by primalsea
I have a rather humble Cambridge Audio D500SE. I have noticed that on the last filter stage which uses NE5534's there is 1Mhz oscillation on one of the frequency compansation pins and also oscillation of a few hundred KHz on the - input pin.

Hi

Are you sure that this oscillation is not induced by the scope probe?

There is oscillation at the output ,when the circuit is not disturbed?
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Old 31st January 2005, 12:55 PM   #3
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I never thought of that!! However the oscillations are not on the output pins only on the - input of the opamp and the frequency compansation pin (pin 8).

I cannot see any oscillations on the output pin of the opamp but I suspect this may be hidden by the general noise from the digital circuits that seems to be polluting the audio output stage.

I might try connecting a 1K resistor to my scope probe maybe this would stop the probe causing the oscillation if that is the problem. I dunno any advice????????????

I might just bypass the last filter stage. The first stage is a 2nd order XO set at about 50Khz. At least this seems we designed as a 50Khz filter will smooth the stepped output of the DAC and according to simulation (my simulation) this is the optimum filter for the first stage of filtering. The 2nd stage only increases the slope abit more the compound f3 of both filters is the same as the 1st stage.
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