audio production powered by DC-DC??

[bluelaser]

Hi,all:
I make an audio production powered by DC-DC circuit. the specication of dc-dc is : input:8vDC---12VDC ,output:13.5VDC. the max current is 1A.
the chip is Max1771.
now I meet the problem:
a)when the speaker use the same ground with this unit. I can hear the noise. the frequency of noise is the same as the frequency of DC-DC.
b)when the unit boot up, I can hear the noise come from the motor of hard
disk .
Do you think whether it is suitable to use dc-dc power in the audio productio. or these problems are caused by the wrong pcb layout?

I hope your help !!!


Thank you!
bluelaser

 

[Marky]

Use additional filter on output voltage (coil+capacitor). If you can, show schematic you use (only part of DC-DC). In application note, Maxim suggest inductor about 22uH and low ESR capacitor. And working freq is about 300kHz (you can't hear it 8O at normal operation)

 

[bluelaser]

this is the schematic.

 

[shanren]

hi,i have some experience:
1:13.5v output need a big capacitor,such a 2200u-10000u elec capacitor;
2:do not use a low ESR capacitor of c2,it is easy to cause unstabiliable;
3:use more large elec capacitor parallel a low ESR,such as a 2200u-10000u+1u MKT,it is good for audio;
4:separate the power amp ground from motor,it will reduce the noise from motor;
5arallel a low esr cap in the motor,can also reduce the noise from motor.
hope this can help you.

 

[bluelaser]

Thank you!
Now ,I try your methods at once.

 

[Marky]

DC-DC

check working freq of DC-DC also. If is below 20000 Hz, you have to change it. Are you using 22uH coil ?? (rather bigger size for this current with ferrite core - not power iron ).