Rotel RCD-971: Anyone revised the PCM63 Current Output Config-Source of DC Offset??? - diyAudio
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Old 15th September 2006, 11:26 PM   #1
dtm1962 is offline dtm1962  Canada
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Join Date: Apr 2003
Location: Sarnia, Ontario, Canada
Default Rotel RCD-971: Anyone revised the PCM63 Current Output Config-Source of DC Offset???

Has anyone jumpered Pin 5 (Bpo) to Pin 6 (Iout) of the PCM63P like the Burr-Brown datasheet?

I was curious to know if it eliminated the 400 mV of DC Offset that is present at the output of the PCM63P DAC chip (input of the 1st opamp)

Rotel's unorthodox method has 100 ohm resistor in series (@ input of opamp) with Pin 5 ( instead of being jumpered at Pin 6) is terminated downstream of this 100 ohm resistor.

Is there any reason for Rotel using this configuration?

I was going to revise to see if it eliminates the DC offset.

Any help would be appreciated.

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Old 16th September 2006, 05:40 PM   #2
dtm1962 is offline dtm1962  Canada
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Join Date: Apr 2003
Location: Sarnia, Ontario, Canada
Here 's where I first seen this possible cure for the DC Offset.
Any comments for the following quote from Mirek.

Other possible changes. DC offset on the output is about -0.4V which comes from PCM63P DAC. I decided finally to remove Black Gate caps from the signal path completely as my amp doesn't pass DC to the speakers, so it is done. However I wonder how to make DC compensation. Rotel's interface to PCM63P is nonstandard and at least strange. It takes Iout (pin 6) with RF shunted down by two caps 8.2+10nF, then R703 100ohm resistor links to the inverting input of the opamp which has its own feedback resistor (internal reference resistor pins 9&10 is not used). Bipolar Offset Current (pin 5) links also to the inverting pin of the opamp (instead of wiring pin 5 to pin 6 together on the DAC). It is possible that this 100ohm resistor in the current loop is responsive for imbalance. If so, one extra 100ohm resistor serial with pin 5 should solve problem. I must try it later, but it needs the same type of resistor as R703 (black, marked JAJ 100R). Otherwise, isn't pin 25 responsive for the offset adjustment (currently NC)? [I]
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Old 22nd August 2009, 01:08 AM   #3
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Join Date: Apr 2008
Location: Newcastle, Australia.
Default interesting old thread

Yes, I am modding the same rotel cdp and have been wondering the same thing...does seem strange...I will bridge out the 100 ohm resistors tomoro and see what it sound like...
" Digital is only ac anyway "
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Old 25th November 2010, 08:39 PM   #4
Iliya is offline Iliya  Russian Federation
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Join Date: Oct 2005
It seems that many people are wondering about the purpose of those resistors but so far I have not seen any answers or even any opinions regarding this matter. Another Rotel player with PCM63 RCD991 also has somewhat similar design with two 47 Ohm resistors connected in series to the current output of the PCM63. My understanding is that these resistors do increase DAC output impedance. But the reason for doing that still remains a mistery.
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