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DIY soundcard intended for measuring amplifiers
DIY soundcard intended for measuring amplifiers
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Old 4th December 2020, 01:55 PM   #31
MagicBus is offline MagicBus  Greece
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Partially answered in post #28. I don't have an explanation for this. I'll try it with another PC.
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Old 4th December 2020, 01:58 PM   #32
popeman is offline popeman
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OK I see, thank you
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Old 4th December 2020, 09:33 PM   #33
Vovk Z is offline Vovk Z  Ukraine
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Quote:
Originally Posted by MagicBus View Post
Also attached a earlier measurement taken with the opa1632 connected directly to the attenuator without the bulbs in place.
This spectrum looks fine. Possibly bulbs change something?
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Old 5th December 2020, 01:07 PM   #34
MagicBus is offline MagicBus  Greece
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I'll spend some time with this in the weekend. Of course, you are all right suggesting psu induced noise. And it seems I'm also right thinking it's emissive interference. I guess, high impedance input is not an easy task.
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Old 13th December 2020, 03:51 PM   #35
MagicBus is offline MagicBus  Greece
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I think I found it... First, it's not the lamps! Against the odds, they are doing remarkably well! And apparently, it's not EMI at all. Everything is packed between ground planes and this pays off. The problem seems to be the high value potentiometer. 100k may be too much even for fet input op amps. We may get away with the 1/f of opa1656 but there must be something else. I was looking at some loop measurements of my UMC202 and Pete Millett's interface. First pic is the sound card alone and the second is with the interface. So, should anyone needs the 100k input impedance, the best THD+N is what posted earlier. But for better figures, it may have to drop down to 10k. I need to order new parts -and wait for them to arrive- to verify this. Anyway, keep in mind that the true input impedance should be the double for balanced mode which can be used for SE DUTs as well. Only output should be set to unbal to drive a SE DUT.



More work to be done...
Attached Images
File Type: jpg Loop FFT.JPG (174.8 KB, 204 views)
File Type: jpg 100k.JPG (174.2 KB, 207 views)
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Old 13th December 2020, 04:02 PM   #36
MagicBus is offline MagicBus  Greece
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Default About bandwidth

I found a plausible explanation on the internet. The usb speed - theoretically 480 Mbit/s but usually much lower in real life- is shared between multiple channels processed by the microcontroller. And my usb interface runs 8 channels regardless how many are used. It's a matter of firmware. I now can see that it could be ordered for higher sampling rates and 4 channels, but I can't tell if that would made a difference.
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Old 13th December 2020, 04:15 PM   #37
phofman is offline phofman  Czech Republic
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Isochronous transfer in highspeed reserves 1024 bytes in every frame. If the DAC requests data every microframe (bInterval = 1) that makes 1024 x 8000 = 8,192,000 bytes per second. IIUC you are using something like XMOS Multichannel high-quality USB to/from I2S/DSD SPDIF PCB - DIYINHK which runs at 192/8/24or32.
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Old 13th December 2020, 04:42 PM   #38
MagicBus is offline MagicBus  Greece
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Yes, this is the module I use. So, you suggest the speed should suffice?
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Old 13th December 2020, 04:50 PM   #39
phofman is offline phofman  Czech Republic
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The vendor speficies 8ch@192kHz which even for 32bit sample size is within a single isochronous packet (1024 bytes max) per microframe (125us).

I have asked before - are you sure your link runs at USB highspeed 480Mbps? Because the limits you get are typical for fullspeed 12Mbps.
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Old 13th December 2020, 04:59 PM   #40
MagicBus is offline MagicBus  Greece
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I could use some help how to check this...
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