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Old 22nd July 2013, 01:29 AM   #71
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Quote:
Originally Posted by Davey View Post
Ken,

The audio signals on the expansion connector are already downstream of the output filter networks. (They're wired directly in parallel with the RCA jacks.)

Dave.
Thanks Dave. That's a pity and you'd think they'd give you direct access to the DAC outputs. Oh well, just means you'd have to take the outputs on the + side of the output coupling caps (and possibly remove them). I still think it's a worthwhile thing to do.
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Old 22nd July 2013, 02:00 AM   #72
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Here is another measurement with the board powered by a 317+337 power supply. The noise floor is definitely lower, about 100dB or more below the signal level give or take.
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File Type: gif 10x gain measurement 317 337 PS.GIF (33.4 KB, 213 views)
File Type: gif no signal noise floor 317 337 PS.GIF (29.2 KB, 90 views)
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Old 22nd July 2013, 04:35 AM   #73
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Almost forgot to post the frequency response measurement!

No issues here...

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File Type: jpg 10x gain stage frequency response.jpg (51.9 KB, 206 views)
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Old 22nd July 2013, 05:39 AM   #74
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...and yet more tweaking. I inserted 10R resistors between the shield of the cables leading to and from my measurement system and the circuit board's ground. I've seen this recommended by several prominent authors for reducing shield currents, and it seems to have reduced the noise floor in this case by a couple of additional dB. I went back and checked, and these numbers are getting close to the noise and THD limits of my measurement system.

What remains is a peak at the mains frequency (60Hz) and I would guess that this can be reduced by shielding (e.g. putting the circuit board in a metal enclosure. Comments on this are welcome.

Also of note, the distortion no longer rises above 1kHz, remaining at or below 0.002%. I'd say that this is looking pretty good.

See attached.


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Old 29th July 2013, 12:21 PM   #75
DrDyna is offline DrDyna  United States
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How's the circuit coming along Charlie?
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Old 29th July 2013, 06:21 PM   #76
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If people want to buy boards with parts please contact me. This would be for 2 channels per board, using my Universal Filter Board. There is no DC servo (it's not needed). Performance plots are posted in this thread.

-Charlie
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