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#1 |
diyAudio Member
Join Date: Aug 2016
Location: Athens, Greece
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EDIT: Sep 15 2019. It is advised to read the whole thread before start doing anything with your soundcard. There are revisions that make previous posts invalid!
Honestly, this Test & Measurement interface for Soundcard together with ARTA was more than enough for my humble tube projects. But it couldn't do impedance measurements with ARTA limp. It has been explained that those cheap soundcards don't have adequate input channel separation for this job. Understood... So, I spent the double ![]() So, at the end the feelings are mixed... I could spend a little more, or maybe a little much more ![]() I talk about Behringer, ARTA and Pete Millett's PC interface. I do not have any relationship with these other than I use them as a low cost measurement rig. In case anybody would be interested on the modifications, I can provide all the details. Thanks for reading! Last edited by MagicBus; 15th September 2019 at 08:42 AM. |
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#2 |
diyAudio Member
Join Date: Aug 2016
Location: Athens, Greece
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Almost good... I can't find what's causing this high pitched noise. I'm pretty much clueless about it. I'm posting three observations that might tell you something...
First is that although the frequency will be stable at each session, it is moving from day to day. Before I start modifying it this was between 8 and 11kHz. Now I see it happening between 6 and 8kHz. Second is that the ADC input alone -chip pins floating- is silent (see first pic). If it is connected to anything, the peak appears. By anything I mean buffers, the one on board as well as a number of external circuits, all silent and with low THD. However, everything was sharing the same ground for what matters. The third observation has to do with what you see in the second pic. It is the same channel recorded two times -note the orange line under the green- and above this is the difference of the two measurements. Zero for everything except for the peak! ![]() |
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#3 |
diyAudio Member
Join Date: Feb 2009
Location: Melbourne, Oz
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In the EMU0404 USB, internally generated noise was coupled in on one channel a lot more than the other - and the fix for the noisy channel was to divert signal from a long trace that went past a noisy circuit section by going via a shielded cable. So that issue was very much board layout and trace layout dependent.
E-MU 0404 usb 2.0 and some "artifacts peaks" on left channel Last edited by trobbins; 19th August 2019 at 01:58 PM. |
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#4 |
diyAudio Member
Join Date: Aug 2016
Location: Athens, Greece
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Thanks for reply!
Although I've seen this thread in the past, I forgot about it. I'll read it again. I'm sure it is a layout problem. Perhaps I was looking in the wrong place. The board is multilayer with ground planes on top and bottom so hard to see a ground problem. The ADC/input and the USB interface sections also seem very straightforward. But the DAC/output section is a mess. I thought that it could not affect the input but you never know with digital circuits. After all, this is the only thing left to check. I'll be back. |
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#5 |
diyAudio Member
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Does the choice of the sampling frequency affect the "high frequency peak" in any way?
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#6 |
diyAudio Member
Join Date: Aug 2016
Location: Athens, Greece
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Hi,
About software settings: I tried Behringer ASIO, WDM and ASIO4ALL with all possible sampling frequencies but nothing affects the peak. In particular, with WDM it is possible to set sampling rates as low as 8kHz and what I noticed is that when the sampling rate is not a multiply of 8kHz (11kHz - 22kHz) there are more artifacts. With 8, 16 and 32kHz I get only the "peak". And with 44,1kHz too, even though it's not relevant to 8. |
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#7 |
diyAudio Member
Join Date: Aug 2016
Location: Athens, Greece
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Bingo!
![]() While working on this, I tried the ADC without DC bias at all. Distortion rises dramatically and it is not about clipping. It seems that the DC bias at half the rail voltage keeps the balance internally in the digital domain. At least this is how I understand it, maybe it's something else. Anyway, all this gave me the idea to replace the fixed voltage divider with a pot to allow trimming "on the fly". I'll check this and also optimize the input capacitors of the SE to BAL converter because this has to do with 50Hz peak. |
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#8 |
diyAudio Member
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Good stuff! Why did I switch from the 202HD to an RTX6001?
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#9 |
diyAudio Member
Join Date: Aug 2016
Location: Athens, Greece
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I guess because RTX6001 is better! OTOH, this one is much cheaper...
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#10 |
diyAudio Member
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And the RTX has proper switches to set the signal levels, not pots. And yes, it is better. But your tweaks make the Behringer really a lot more useful!
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