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Originally Posted by RFL  No good. I would bet it's oscillating; but perhaps of a different kind of oscillation that doesn't case the amp's opamps to oscillate too.
I suspect that those filtering capacitors around the opamp in the DAC board have nonlinearities at high frequency that are causing the faster opamps to oscillate. In fact they're just normal polyester capacitors - not the best choice but I guess it works with the stock opamp and similar ones.
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It's also my guess. I'm working on it. I have ordered WIMA caps and will try direct decoupling of the DAC opamp.
I have done some "reverse engineering" on this DAC LPF circuit. In fact it's almost based on the typical application circuit described in the AD1852 datasheet. Two modifications have been made to this standard circuit.(plus the fact that the resistors have been rounded to the nearest standard value)
The first is the use of four electrolytic 47µF caps to isolate the AD1852 from the LPF op amp(2 per channel). It's good from a electrical point of view (no DC offset transmitted by the DAC) but not that good musically speaking (I will try later on to bypass them).
The second one is a different choice of 2 caps and a resistor in each channel of the filter (C9, C10 and R20 for the left channel on datasheet). This leads to a non linear group delay response in the audio frequency range (C9<>C10) and a reduction of the frequencies above 6khz.
With R20=10ohms, C9 and C10 should be 470pF to have the same response than the LP filter in the AD1852 datasheet. I will try it too when I receive my caps order.(of course on both channels
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The funny thing is that I found the schematic of the zhaolu
here (the one with the AD1852) and even if the circuits are different, they have exactly the same behaviour in frequency and in group delay, plus the same 47µF caps mod... cousins or brothers?
really poor to my ears (hd600, never tried it with my 325i), but I guess is the DAC you have to look at.....
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