CS4397 dac

[yiannis]

Hi everybody. I have a CS4397 dac with the following mods: All electrolytics replaced with oscons at the digital section. Bypass the analog stage by removing C27,28,30,31 and connecting two scr mkp caps directly from the cs4397 (AOUTR-, AOUTL-) to the output RCAs. It was working great for weeks, but suddenly the following problem appeared: I get strong noise (like an impulse signal) when changing to the next track of a cd. During a track's playback everything is ok. Changing to the next track gives a strong noise, and the woofer gets displaced out to its limits, like dc is passing to the speakers for a very short period of time, which does not make much sense since i have dc blocking capacitors at the outputs.

While this noise is heard, i measure the chip's output dc falling from 2.5V to approx 2.0V dc and then it gets back to normal 2.5V. This is when measuring directly the output pins (AOUTR- for example). When i measure AFTER the dc blocking capacitors that i have added, the voltage is 0V dc (as expected) but when the noise happens the analog voltmeter indicates a negative voltage, like a negative impulse signal. Check the following video to see what i mean. I measure the voltage on my rca outputs (= after the dc blocking capacitor) and the needle is on 0V. I press the "next track" button and the needle moves to negative voltage for a while.

http://www.youtube.com/watch?v=Be4j2biZPgU

I measure the voltage on C3 and it remains stable when the noise happens. I measure on C25 and the voltage drops when i change track. If i change tracks many times continuously (pressing the next track button 2-3 times per second) the voltage drops more each time i change the track (like something sucks much current instantly) and when i stop changing tracks the voltage gets back to normal. I increased the C25 to 150uF and the voltage drop is quite smaller but the noise is still quite strong and the woofers pop out to their Xmax

I re soldered most of the connections (input rca, power supplies etc) with no improvement. The same happens when i connect the dac to my computer and play a sound file. At the beginning and the end of the file the "tsak" noise happens and the woofers stick out. It seems like when the spdif is initiated or stopped the noise is heard.


Any ideas what could be wrong?