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Rotel RCD971 PCM63 output filter question
Rotel RCD971 PCM63 output filter question
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Old 28th January 2018, 10:06 PM   #1
erin is offline erin  Australia
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Default Rotel RCD971 PCM63 output filter question

Hello,

I own a Rotel RCD-971. I was looking at the schematic which I downloaded from here Rotel RCD-971 - Manual - Stereo Compact Disc Player - HiFi Engine

I see that it has some capacitors from Iout to ground before the op amp.

I've noticed that not many DAC output stages seem to do this.

Is this a good way to design an output filter or not?

What are the disadvantages and advantages?

Any comments welcomed!

Thanks
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Last edited by erin; 28th January 2018 at 10:41 PM.
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Old 31st January 2018, 02:08 PM   #2
Mooly is offline Mooly  United Kingdom
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Rotel RCD971 PCM63 output filter question
Rotel obviously think it necessary, not only that, they use two in parallel to get a specific value.

Many DAC's use a small cap across the feedback resistor of the opamp, this one does not. Its just a different way of addressing the problem of HF noise from the DAC, either roll it off at source or roll off the response of the following stage... bit over simplified but that would be my take on it.
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Old 31st January 2018, 10:15 PM   #3
erin is offline erin  Australia
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Thanks Mooly.
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Old 1st February 2018, 05:52 AM   #4
erin is offline erin  Australia
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I was really wondering, is that this at first glance looks like a good way of preventing the opamp from receiving too much hf which might upset the opamp. I read some people say that opamps need to have a high bandwidth to sound good as an IV stage to deal with the hf pulses.
So this should ameliorate this problem? ( If we assume it's a problem?), But this cap to ground is not frequently used. Why not?
What's the disadvantage?
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Old 1st February 2018, 07:27 AM   #5
Keruskerfuerst is offline Keruskerfuerst  Germany
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You can also have a look at the datasheet of PCM63P here:
PCM63P Datasheet pdf - Colinear 20-Bit Monolithic Audio DIGITAL-TO-ANALOG CONVERTER - Burr Brown
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Old 1st February 2018, 07:38 AM   #6
Mooly is offline Mooly  United Kingdom
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Rotel RCD971 PCM63 output filter question
I've long suspected that the main reason for the apparently big sonic differences that you get when swapping opamps in the output stages of CD players is down to HF affecting the opamp input stages. I read somewhere that the 5532/5534 for example was particularly prone to this, caused in part by its bjt input stage and also input diodes that could partly 'demodulate' the HF and cause non linearities and distortion.

FET opamps are much more immune to this in the first place.
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Old 1st February 2018, 10:18 AM   #7
Keruskerfuerst is offline Keruskerfuerst  Germany
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I donīt think it is a good idea to connect a 18.2nF capacitor from Iout to ground.
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Old 1st February 2018, 02:12 PM   #8
Mooly is offline Mooly  United Kingdom
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Rotel RCD971 PCM63 output filter question
You have to remember that there is virtually no measurable signal voltage at that point because the opamp is configured as an I/V convertor. Even if you placed a 10 ohm across the cap, it wouldn't alter the signal level at the opamp output at lower frequencies, although it would at HF as the opamp ran out of loop gain.

So 18.2nF has no effect at audio frequencies.

Another way of looking at it is that it increases the noise gain of the opamp at HF, which has a similar effect to shunting the feedback resistor with a cap.
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Old 1st February 2018, 04:17 PM   #9
Keruskerfuerst is offline Keruskerfuerst  Germany
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Can you post the complete low pass section of the CD Player?
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Old 1st February 2018, 05:05 PM   #10
Mooly is offline Mooly  United Kingdom
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Rotel RCD971 PCM63 output filter question
Its fairly conventional apart from the muting.
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