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PostPosted: May 18th, 2017, 11:10 am 
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I mentioned the possibility of a cold solder joint within the Burson product. ???

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PostPosted: May 18th, 2017, 11:18 am 
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Check the power supply voltages at the Burson.

The Burson is spec'd for a max PS voltage of +/-15VDC. The LME49990 is spec'd at +/-18VDC max.

If it is happening in both channels, since it is a single opamps, not likely to be a solder problem.

Just a thought.


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PostPosted: May 18th, 2017, 11:50 am 
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Incidentally, I have a virtually identical board (with dual AKM 4490 DACs) and noticed the same thing using Burson V5 with the regulator heatsinks on the board becoming unusually warm. I had other problems with the board (excessive noise and low output) and moved on but in your case I want to speak to something I noticed based on the transformer picture you attached.

The power transformer is a 115VAC input. It is obviously location dependent, but I find my line voltage tends to runs a bit high, at least 120V. If you have something similar to this, the voltage is around 4% higher than the nominal design voltage transformer. The 15V windings are actually putting out 15.65 VAC, when rectified through a bridge (2 diode drops) would be 20.66VDC with silicon rectifiers and 21.5 VDC with Schottky rectifiers (which I believe are used on the board). It could be even higher than this if the transformer output voltage is designed to output 15VAC at full load. Running at a lower load would increase the output voltage due to reduced copper losses in the transformer. Therefore, the regulator could be dropping 6.5 to 7 volts across it (assuming 15V regulators). If the Burson modules run at a significantly higher current, a 10mA increase in current per device would result in 0.14W of additional heat dissipation on the regulator. This does not seem to be alot, but if the Bursons are running closer to 20 to 30mA hotter, then this can be 0.4W of increased dissipation. The heatsinks on the DAC board are tiny as I remember, and really cannot handle much heat dissipation. Because the heatsink cannot dissipate the heat fast enough, the regulator temperature increases over time and eventually malfunctions. At least that is my hypothesis.

David


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PostPosted: May 18th, 2017, 11:59 am 
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Wait for the 15 minutes until the distortion starts then put a fan on it.


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PostPosted: May 18th, 2017, 12:30 pm 
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Location: Highland, MD
It's also interesting that the trouble occurs in both channels, assuming at about the same time? No doubt they share the same analog supplies, so it may be enlightening to replace just one op amp with a Burson and see if the trouble occurs after more than 15 minutes, or whatever the threshold is for using two Bursons. :think:

If the trouble occurs 20 or more minutes after turn-on, then it supports David's suggestion of a heat problem. :confusion-confused:

DON'T use your thumb to test the heatsink temperature - I did that years ago: the only time I got a square blister! :oops: This could be one place that surface-mount regulators may be a good choice (if that's what they used.)

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PostPosted: May 18th, 2017, 2:10 pm 
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Thanks for the great suggestions. I will try all of them tonight and report.


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PostPosted: May 18th, 2017, 3:46 pm 
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David,

The LM317 regulators should be getting D.C. Or close to it. If the regulator is malfunctioning due to overheating, why would the distortion sound like clipping at 60Hz?


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PostPosted: May 18th, 2017, 5:13 pm 
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I cannot answer that, except if one power supply rail is out because of a malfunctioning regulator, you will have an imbalanced and may throw off the power supply rejection in the op amp. You should test and troubleshoot, there is only so much we can do via email suggestions.

David


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PostPosted: May 18th, 2017, 6:43 pm 
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I wonder if the Bursons aren't getting warm/hot and oscillating. And, there are no dual LME49990 chips. You can make a dual by putting two on a carrier. LME49990 are high performance and pretty darn stable, I think they sound great. It would be interesting to see how the DAC sounds with known real LME49990s.

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PostPosted: May 18th, 2017, 10:48 pm 
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DAC with SS V5 opamps started distorting at 13 minutes. Heat sinks on both the regulators, LM137 and LM337 are at room temperature.

Input voltage 120.5 V and the power transformer output voltage is 15.7V as David calculated. Rail voltage at the input of the regulators is +/- 20.2V and the output is set to about +/- 12V.

Adjusted the output of the regulators to +/- 14.7V and the DAC went completely silent on both Burson and LME49990 opamps.

Input voltage of LME49990 is +/-18V and Burson is +/-15V.

Any suggestions.?


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