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Old 16th January 2006, 08:35 PM   #1
jmar is offline jmar  United States
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Default NO low pass filter?

Has anyone had any luck removing ALL of the audible digital noise
on a PCM2702/2902 chip without using a low pass passive or active filter?

Just with good construction technique and filtering? (Impossible?)

Mine is very low level....but it's there.
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Old 17th January 2006, 06:22 PM   #2
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Why not use the manufacturer's recommended opa2353-based LPF?
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Old 17th January 2006, 06:52 PM   #3
jmar is offline jmar  United States
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Just wondering (for my own education)...if it's possible to live without it and whether the noise can, in fact, be made inaudible.

Like I say, the noise level I hear on my board is quite low and I probably will use the filter.

Just learning.

I don't have that much experience with digital audio.

It may be impossible? to remove it all?
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Old 19th January 2006, 10:02 AM   #4
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I use it without filter and without output op amp( i use dc blocking capacitor).
This way it sounds best. I do not hear any noise.
This is true for 2702. 2902 not sounds that good.

good luck,

borisov57
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Old 19th January 2006, 11:09 AM   #5
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Default Re: NO low pass filter?

Quote:
Originally posted by jmar
Has anyone had any luck removing ALL of the audible digital noise
on a PCM2702/2902 chip without using a low pass passive or active filter?

Just with good construction technique and filtering? (Impossible?)

Mine is very low level....but it's there.

This noise is part of the signal and results from the DA conversion itself. Therefor, no amount of shielding or sophisticated construction will decrease it.

Depending on the sample frequency, the noise may actually reside above the audio frequency and as such is not audible directly. But it may upset the amplifier it feeds and/or cause intermodulation and/or slew rate limiting: the amp will now get hi freq, hi speed signal components it was not designed for. Some will be relatively unaffected, some will audibly degrade. It really depends on your particular situation.
Using at least some form of filtering will protect you to some extend.

Jan Didden
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Old 19th January 2006, 11:37 AM   #6
SY is offline SY  United States
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Without an antiimaging filter, do you end up with a correct analog reconstruction?
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Old 19th January 2006, 12:02 PM   #7
jmar is offline jmar  United States
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Ok...thanks Jan.

I thought it had been more noise coming from the USB connection.

I know when you disconnect the USB cable, it turns the chip off?
(SSPND?)

Maybe I'll toggle the SSPND pin with the USB cable disconnected and listen.

If it's all just the DAC....it's pretty low and I have to turn the amp up to it's maximum to hear it....you can hear a high frequency "switching squeal."

Hey, it's alot better than tape hiss and I'm old enough to compare the two.
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Old 19th January 2006, 12:53 PM   #8
Werner is offline Werner  Europe
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Quote:
Originally posted by SY
Without an antiimaging filter, do you end up with a correct analog reconstruction?

Ask the question like this:

at a sample f above 40kHz, do we conceptually need anti-imaging
filtering?
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