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Test & Measurement interface for Soundcard
Test & Measurement interface for Soundcard
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Old 13th January 2019, 02:24 PM   #811
dch53 is offline dch53  Australia
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Default Better shielding did the trick!

Quote:
Originally Posted by phofman View Post
Interesting that the 50Hz peak was not eliminated by the balanced input. Any reason for that?
It must have been picking up the mains from the environment.

I have thin sheets of mu metal stuck to the inside of the top, bottom and sides of the case. I linked the side pieces to the top and bottom sheets with blobs of solder and earthed them which I hadn't done before.

The mains hump has disappeared altogether.

This is without the higher performance components. I included these when I ordered all the parts so I might as well fit them.
Loopback spectrum -30dB 230mV earthed mu metal shield.png
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Old 13th January 2019, 02:54 PM   #812
phofman is offline phofman  Czech Republic
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Well done. I did not know your enclosure was not fully shielded.

For shielding smaller non-conducting enclosures or compartments a solderable copper tape has been of great help to me 50mm x 3m EMI Copper Foil Shielding Tape Conductive Self Adhesive Barrier Guitar | eBay
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Old 14th January 2019, 07:52 PM   #813
MagicBus is offline MagicBus  Greece
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Here is how I built mine. I used a generic cheap metal box modified to the rear to fit the pcb. I built it with the THAT chips and the light bulbs but sans the LCD. I put connectors for my DMM for this job. I attach loop measurements with a behringer UCA 202 usb soundcard. The performance is primarily compromised by the soundcard. I'm making some thoughts to transplant it into the interface box, bypass its analog in/output and connect the chip directly to to the interface making it a stand alone device.
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Old 15th January 2019, 01:36 AM   #814
dch53 is offline dch53  Australia
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Quote:
Originally Posted by MagicBus View Post
Here is how I built mine. I used a generic cheap metal box modified to the rear to fit the pcb. I built it with the THAT chips and the light bulbs but sans the LCD. I put connectors for my DMM for this job. I attach loop measurements with a behringer UCA 202 usb soundcard. The performance is primarily compromised by the soundcard. I'm making some thoughts to transplant it into the interface box, bypass its analog in/output and connect the chip directly to to the interface making it a stand alone device.
Thanks Magic. I included the THAT parts and lamps in my component buy so I can go down that route.

I was confused by your first trace with the fundamental at -1.5dBFS. I realised there must be something wrong with my calibration because I can never get anything like that. Even -3dB produces serious clipping according to Time Record.

Paragraph 1.5.2 Calibration of Soundcard Input Channels has never made sense to me. Step 3 says "Press the button 'Generate sine (400Hz)' and monitor the input level at bottom peak-meters..." When I do that, the peak-metres don't register anything.

I've just left input calibration unchanged from the value resulting from the output calibration. If I set the Generator to -30dB (240mV) and use the 200mV scale on the soundcard interface I get -28.3dBFS.

I'll have to learn how to use STEPS too.
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Old 15th January 2019, 07:11 AM   #815
MagicBus is offline MagicBus  Greece
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I started from the same level as you about a year ago and still learning so, I can only describe what I've found hoping it will help. It seems you have digital clipping, you will know how ugly that is when you feed real music to your speakers under this condition. Anyway, you can go to your soundcard control panel and see if you can adjust line in levels - you should. Set out level to max and adjust input until you get the best results for calibration and measurements. To give you an example, in my soundcard, the input level slider has a range from 0 to 100 and I had to take it down to 3. Once you get the calibration right, STEPS is easy to use.
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Old 15th January 2019, 07:46 AM   #816
jan.didden is offline jan.didden  Europe
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Quote:
Originally Posted by phofman View Post
Interesting that the 50Hz peak was not eliminated by the balanced input. Any reason for that?
Not to beat my own drum, but a SilentSwitcher from the diyaudio store, fed by a Powerbank, will completely cure that.

Jan
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Old 15th January 2019, 08:53 AM   #817
phofman is offline phofman  Czech Republic
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Jan: Does that mean the peak is caused by 50Hz noise in power supply? Or 50Hz noise in the ground loop caused by the way the device is powered?

Or perhaps induced into the tool from the outside?

IMO the easiest way would be trying to power the tool from the soundcard itself. Power lines are easily available in Juli (pinout e.g. hifi.slovanet.sk :: Zobrazi? t?mu - Anal?gov? ?as? ESI Juli@ + I2S )
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