Cross talk problem connected to the transformer

[mingyee]

hi, I'm having this cross talk problem in my design where there r 8 audio inputs with 8 isolating audio transformer going to a 8 in 2 out mixer chip SSM2163 then to a LMC1982 stereo & audio tuniing chip...& there r 2 output channel going out throught another 2 isolating audio transformer. However the cross talk problem lies at the transformer. It seems to cross each other electromagnetically. Are there any way to resolve this problem? I'm tried to shunt the output with capasitor.. but it will also shunt the actualy audio output... please help me...

 

[IanP]

Re: Cross talk problem

Audio transformers transform very low power levels so it is hard to believe that your problem is the magnetic inductance.
It has to be something else ..
Is it possible that you post the circuit diagram and a sketch of PCB layout?

 

[mingyee]

Re: Cross talk problem

hi.. thanks for your reply.. I've the pcb layout.. hope it could be useful.. hope there is a way to resolve this... i've also tried metal shielding the transformer...but it doesn't seem to be effective enough..

 

[IanP]

Re: Cross talk problem

Try this:
add a 1kΩ (or lower) resistor to each input of the IC after the transformers. What you have there are zener diodes and capacitors .. Maybe here lays the problem ..

 

[mingyee]

Cross talk problem

hi...r u saying a 1k or less resistor across the transformer toward the chip? or in series to the chip input?

Added after 30 minutes:

hi.. i just tried with 1k & 470 resistor in series to the chip input... it does decresse the cross talk.. but there r still a little cross talk..
by doing this will lower the volume too?

 

[chinara]

Re: Cross talk problem

I do not have much experience on Audio circuit. But, from you pcb, I have several questions:
First, it seems that the pcb do not have a dedicated layer for gnd, and is this ok for the current circle?
Second, the power filter capcitors is a little far from the chip, is this OK?
Third, it seems that the pcb is layouted by auto router, all the trace is in right directiong and no cross?

 

[mingyee]

Re: Cross talk problem

hi.. for the pcb.. there are digital ground & audio ground... for digital ground... u use the entire background polygon as digital ground.. & a 10uF connected from the audio ground to digital ground/. the audio ground is the thick line at the transformer output.. to wut i knwo that.. the capasitor's distance should be ok... coz if it's too far away.. it will only cause humming & electrical noise... it shuldn't cause any cross talk... am i rite? please advise on this...

 

[homefree]

Cross talk problem

1. Make a magnetic shield cover, material M6 lamination sheet, to cover each transformer, if it does reduce the cross talk, then you will find which transformer makes trouble.
2. When you located the trouble transformer, you may oriented the transformers, so that the magnetic field will perpendicular to each other. Or you may ask your transformer supplier to put cross-talk test on the transformers, and if necessary, change the structure of transformer, for example, a shielded type of transformer.