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Old 15th October 2013, 12:25 PM   #1
breez is offline breez  Finland
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Default Delta 1010LT external power for analog section

Hi all,

I am the lucky (?) user of 3 Delta 1010LT PCI sound cards. With various motherboard+graphics card+PSU combinations there have been noise issues when using the cards' analog inputs. The question is how to improve the situation with external noise (tonal components in spectrum) while not making other aspects worse (inherent broadband noise of ADC).

I took a look at the cards power supply. It is powered from PCI bus +5V (to digital section), -12V and +12V. That +12V feeds a 7805 regulator giving cleaner +5V for the DAC/ADC chips (AK4524) to use as VA and VREF.

+12V and -12V are further filtered with 10 ohm + 470uF RC-filter to power the opamps (NE5532). After resistor loss there is about +/- 10 volts left, well enough for the input/output range of the DAC/ADC.

+12V is heavily loaded by CPU and graphics cards in a modern PC. I would hazard a guess that in the motherboard the various places where +12V is available have lots of shared common traces, i.e. the sound card doesn't get a private supply line back to the PSU. Same for the ground. Thus there may be some noise coupling through the shared supply lines.

So, I took a knife and soldering iron to one card and cut off -12V and +12V out and replaced them with an external Meanwell T-50B (+5V, -12V, +12V) SMPS supply that I had lying around.

I can't say if the external noises are gone or not, but in circumstances where there is only white noise from the ADC with unmodded card I had a worse result. I recorded 10 second samples of silence from the inputs of modded and unmodded cards and found out that the unmodded card could fulfill the SNR specification of ~101 dB (A-weighted) while the modded card had some 5-6 dB more noise. I tested this with two different PCs with similar results. One PC did have some very low level of high frequency tonal components in the spectrum with the unmodded card. While using the external supply seemed to get rid of those the noise level was higher.

Where to go next? Maybe the Meanwell supply just has a high level of broadband noise. I quickly set up extra regulators for the external power lines using 7808 and 7908, giving +/- 8 volts. Enough for the 7805 to do its job and no problem with the NE5532. To my surprise the noise worsened yet another 1 or 2 dB

So any power supply/mod gurus around that could give an explanation of what has happened? And how to really improve (or at least not make any worse) the noise performance? Even if I got rid of the external noises (from CPU/GPU usage...), this 5-6 dB penalty in broadband noise performance is not acceptable, I'm using the PC as a digital crossover after the system's volume control (AV receiver with preouts). Already I'm struggling with good enough dynamic range without audible hiss at the listening position.
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Old 3rd November 2013, 12:10 PM   #2
Wungun is offline Wungun  Canada
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Join Date: Sep 2013
Default Delta 1010LT external power for analog section

Trying to follow what you did...
You didn't mention if the card used 7812/7912 regs...I'm assuming those are what you desoldered...?
I'd leave those reg's in place, lift the left input pins and connect your external PSU directly to it and the center ground.
Is the external PSU grounded to the PC chassis as well, other than at the regulator?

The T-50B is switching and linear...
Which is the 12V rail? You definitely want linear feeding your soundcard.

I'm in a similar situation trying to decide how to better regulate/power my Xonar D2.

Feeding 12V to a 7812/7912 isn't really a good design. I'm thinking a dual 15V transformer of some sort for this, maybe replacing the reg's with quieter ones and smoothing them with better caps. Also a 7808 in between the 7812 and 7805 with caps to cleanup the feed to the 5V rail...?



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