Hello,
Thanks for the quick reaction.
I have used the HEF40106, a 100nF and a 100uF capacitor over the supply. The input pins of the non-used buffers were indeed floating. I have connected them to the ground now but the tone is still very present. The tone isn't introduced by the schmitt triggers anyway since connecting ports 9 and 10 directly to the resistor network still causes the tone to appear on the output.
With the most simple piece of code the tone still appears:
#include <SimpleSDAudio.h>
void setup()
{
if (!SdPlay.init(SSDA_MODE_FULLRATE | SSDA_MODE_STEREO | SSDA_MODE_AUTOWORKER)) {
while(1); // Error while initialization of SD card -> stop.
}
void loop(void) {
}
I'm using this amplifier: http://www.ebay.com/itm/PAM8403-Mini-Digital-Power-Amplifier-Board-Class-D-2-3W-/221293221137?pt=LH_DefaultDomain_0&hash=item33861a9511
Could that be of any influence?
The PWM frequency is 62 kHz, but can a minor random shift in the duty cycle cause a tone that is hearable?
Marcel