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Old 15th July 2005, 05:56 AM   #1
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Default 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
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Old 15th July 2005, 07:33 AM   #2
dddac is offline dddac  Germany
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when asking questions like this, it is good to publish the circuit....

On my Site you can read a lot of background how to implement a 8 dac parallel set up including all DC bias stuff...

doede
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Old 15th July 2005, 07:39 AM   #3
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Thanks for the link. The circuit is just the 1543 pcb from diyparadise.com.

As a side note, I plugged it into a preamp, and when I measured from signal to ground dc offset was down to 0.1mV. It is playing, and all seems to be well.
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Old 15th July 2005, 07:45 AM   #4
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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
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Old 15th July 2005, 07:47 AM   #5
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Probably when you measured there was no resistor from output to gnd so the output cap couldn't get charged, thus showing the same voltage on both sides.

Insert a 50-100k from the capacitor to gnd (output side) and you'll have no DC on output even without connecting it to anything.

Cheers

Andrea
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Old 15th July 2005, 07:50 AM   #6
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Quote:
Originally posted by Andypairo
Insert a 50-100k from the capacitor to gnd (output side) and you'll have no DC on output even without connecting it to anything.
Other than changing what the meter says, is there a practical reason for doing this?
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Old 15th July 2005, 07:59 AM   #7
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Because the open side of a coupling cap needs to be referenced to something. Otherwise it can float up to anything , just plug it into your pre then measure.

Cheers george
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Old 15th July 2005, 08:17 AM   #8
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Quote:
Originally posted by dsavitsk


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
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