problems on TDA1541A

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Excuse me for the broadcast, probably it's a very silly question but I would like to know the expert opinion.

I'm building a DAC based on CS8412 and TDA1541A and I have a little problem.

The implementation is mainly based on the datasheets information and on some useful topics I've seen on this site.
The DAC is fed using MOSFET regulators followed by local regulators based on TL431.
Currently I'm using an I/V converter based on OPamp just for initial evaluation.
The output of the OPamp feeds a very silent audio mixer I use for experiments in order to have variable gain.

The problem is an audio tone I can hear at the output of the DAC when there is no digital signal coming from the SPDIF interface.
I've also tried to verify if the problem is due to the I/V converter but I'm sure it's due to the TDA1541A.
Unfortunately I don't have an oscilloscope and I cannot evaluate frequency and level of this disturbing tone but, initially it was more or less located at midrange frequencies.
Later I've replaced the capacitor connected to pin 13 and 18 of the TDA1541A (100nF as suggested from Philips) with 2.2uF capacitors as indicated by Pedja Rogic. After this modification the frequency of the tone has been shifted higher and the level is lower than previously but still present.

I'm sure about no problems in the audio output chain therefore there must be something wrong on the TDA1541A layout.
Any suggestion is kindly welcomed.

Thank you in advance

Best Regards

Maurizio
 
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