Distortion from Burson Audio Opamp

[Cogito]

Recently I replaced the dual LME49990 opamps in the DAC with Burson Audio SS V5-OPD-S opamps which I had for a while. First day, they worked great, but from the second day onwards they are causing several distortion after about 10-15 mins of playing. The distortion is very severe. it sounds like clipping lower half of the wave at constant frequency (60Hz?). But, when I put back the stock LME49990 opamps,the DAC works without an issue.

I have a 15 second audio clip of the distortion, but the forum is now accepting mp3 attachments. I can send it by email.

Any insights of the problem might be and how to solve it?

opamp url:
https://www.bursonaudio.com/products/su ... -opamp-v5/

dac url:
http://www.ebay.com/itm/Hi-end-Dual-AK4 ... 2170139472

Shashi

[David McGown]

The Burson discrete opamps draw more current, so if the power supply is marginal on the DAC, there could be an issue with power supply rails.

David

[David McGown]

On the other hand, from your photo, it appears your DAC may have a robust power supply (though depends on the regulators). Burson does say it is compatible with the LME49990, so there may be something in the way the opamp is used in the DAC. The LME 49990 is a pretty good OpAMP by itself.

David

[Cogito]

David,

The power supply transformer seems to be 15V 1A and the filter caps are 6800 mu f.

Just came across TNT-Audio review which seems to agree with you.
http://www.tnt-audio.com/accessories/burson_supreme_e.html

Should I try increasing the voltage or the capacitance? I dont have an oscilloscope.

BTW, the difference between LME49990 and SS V5 is significant to my ears.

[David McGown]

Really depends on the regulators. A +/-15 volt transformer should provide about +/-20V rails after the rectifier, which should be enough voltage headroom for regulator drop if using +/-15VDC to the opamp. If the design regulates to +/-18VDC (doubtful), then there could be a regulator dropout issue. I would think a 1A supply would be sufficient, but I think the DAC analog section may draw additional current in addition to the opamps.

David

[SoundMods]

My view? Somewhere there is a cold solder joint. With differential amplifiers you have only to screw up one-half of one part and you have a problem. Based on the picture provided -- the back-to-back assembly will give you a work-out if you choose to repair it yourself. And besides, the LME49990 on paper is smokin' hot in terms of slew rate, power bandwidth, and settling speed (perfect for a DAC). I know how the LME49990 performs as the front-end of a phono stage -- very good is a gross under statement.

Maybe the problem with the LME49990 is possibly gain related. Too much of a good thing. The stepped output of a DAC chip can give any amplifier a run for its money. I just looked up the LME49990 -- it is a single device in 8-pin mini-DIP. Should I assume that you had two of them built in to a module to fit an 8-pin mini-DIP position for a dual op-amp?

Anyhow, a trick I developed (based on advice from the late Walter Jung) many moons ago with unity-gain stable devices was to close down the loop at 2-mHz. It allows the chip to breathe a litter easier and get more in line with a vacuum tube experience.

[tomp]

The supply current spec on that amp is between 9 and 11 ma. You would have to have an awful lot of them to stress the supply. The output current is rated at 26MA and the short circuit current is limited to 75ma. I don't see the operation of the amps being a problem with the power supply unless it has a problem.

Tom

[Cogito]

Soundmods,

DAC works perfectly with LME49990s installed, so the loose solder is unlikely the cause. Problem comes only when I install SS V5 opamps, that too after 10-15mins of operation.

Last night I saw a video ps clipping on YouTube due to small filter capacitor. My dac sounds exactly same with Burson opamps.

There are dual LME49990s and DAC chips. The two channels are complately isolated from each other, and the analog and digital stages have their own power supplied.

The short experience with Burson opamp made so much difference to me, don't feel like listening to LME49990s anymore, even though thr DAC sounds good with them.

[David McGown]

The fact that it is happening after 10 to 15 minutes of operation makes be wonder if there is a thermal issue coming into play. Do you notice if the regulator heatsink(s) are getting hot? Sometimes the heatsinks are undersized (or barely adequate) for the load, and if the Burson modules are pulling more current, it could increase the heat dissipation of the regulators.

David

[Cogito]

David, I will check tonight.