NOS DAC DC offset issue

I built a TDA1543 based NOS-DAC (w/ 8 chips in parallel) and when I measure the DC offset I get something like 5V. If I connect an interconnect and measure at the end of it, I get about 140mV, which, while better, is still not so good.

What seems particularly odd is that there are 4.7uF caps in the output signal path. It was my understanding that this should block DC. Even stranger, because I am a glutton for punnishment, I plugged it into a headphone amp (that does block DC on the output) and used some $5 phones just in case, and it played fine.

Is this human error, multimeter error, or is it possible something is going on? Any thoughts?

-d
 
You need a reference to ground after the 4.7uf caps otherwise the cap will not block the dc offset as far as your seeing on your meter, try puting a 10k resistor to ground after the cap and then measure it, it should be zero offset. or you can try to plug it in to your pre amp is will also reference it to earth and should again be zero dc.

Cheers George
 
dsavitsk said:


Other than changing what the meter says, is there a practical reason for doing this?

There are safety reasons for doing so. The current needed to charge the capacitor must have a path to gnd. If you plug the output of the DAC to a pre its input stage has to provide this return path to gnd; most times it will do without problems but in some occasions (very high impedance and DC coupling for example) it migth not like, giving loud turn-on thumps.

A fairly high resistor (50 to 100k) won't hurt the sound and will prevent such problems.

Cheers

Andrea