Please can someone tell what this cap is for? - diyAudio
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Old 3rd October 2003, 10:48 AM   #1
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Default Please can someone tell what this cap is for?

In my Rotel 965 cd player there is a 47uf cap per channel directly in the signal path coming out of the DAC (SAA7323) This leads to the output stage followed by 2 more caps 100uf (all are blackgate type F). I know the 100uf are for DC blocking. What are the 47uf for? The same? (guess nothing more). Here is the daatsheet for the SAA7323:

http://www.docethifi.com/saa7322-7323.pdf

At the end is a app note that features the same 47uf on the output of each channel. Even though these are blackgate (not wishing to start any arguments about blackgate merits or not) no cap is better than any cap if not required. So are these required? Please could someone with more knowledge comment on this for me?
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Old 3rd October 2003, 02:14 PM   #2
A 8 is offline A 8  Sweden
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The 47uF are also dc blocking, you probably have 2.5v dc on the dac output.
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Old 3rd October 2003, 02:17 PM   #3
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hmm interesting. I thought it might be. Although the datasheet says the dacs output is maximally 1v - hence the output stage to make it 2v. Or so i assumed. I wonder what would happen if i removed.
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Old 3rd October 2003, 03:02 PM   #4
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1 volt is probably correct but refers to ac level, you can still have a 2.5v dc offset.

I would not remove them without understanding how the rest of the analog curcuit looks.
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Old 3rd October 2003, 03:13 PM   #5
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Actually thats the RMS figure. Doesnt seem to be that much data in the datasheet. from what i can gather the output can be -0.5v DC to 5.5v DC (0.5 above supply voltage at 5) Thnk it best to leave em there then - or upgrade em with soething better if they have to be there.
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Old 3rd October 2003, 03:17 PM   #6
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Default DC blocking cap

The cap is for blocking DC at the output. If you switch to a high quality opamp AD or BB part you should be able to get rid of the offset and you would not need the 47uf cap. That cap will only degrade the sound. Another option is to get a quality replacement cap and get rid of the crummy electrolytic cap.

Check this out on quality opamps

New high quality opamps...
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Old 3rd October 2003, 03:19 PM   #7
A 8 is offline A 8  Sweden
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You could try a small film cap in parallel with the 47uF.
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Old 3rd October 2003, 03:22 PM   #8
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No sorry (jewilson) think u misunderstood - the opamp is on the DAC chip itself in this instance - bitstream philips part saa7323. The output from the DAC chip is not for IV conversion it has already been thru one opamp - probably an onboard 5532/34 at a guess. Nice. The cap as it happens is not that crummy and could only be improved upon using a better blackgate. (space wise) I was only wondering if the cap was needed before the second opamp (which is then followed by another Dc blokcing cap)
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Old 3rd October 2003, 04:31 PM   #9
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Default That for the de-emphasis circuit

Rotellian

Well it must be my sinus meds. I Look up pin 8 and thats the de-emphasis circuit. You can remove it as it work as a high cut filter. I've removed mine in my DAC. The circuit is only activated when the bit from SPIF is on. The emphasis is add 6.5dB of loss at 800Hz, who needs that.

Still I would consider replacing a 5532 or a 5534 opamp with somthing better and a Black Gate cap to replace the 47uf would improve the sound.
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Old 3rd October 2003, 04:33 PM   #10
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Smile nah...

Get rid of it.
And you'll have two spare BGs for other projects.
Or send them to me.
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