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Old 3rd December 2008, 01:10 PM   #1
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Default Zero DC-offset TDA1543

Found Jim Hagermans old Jimdac in another thread:

http://www.hagtech.com/pdf/jimdac.pdf

With a clever arrangement the 1543s outputs are without offset and fineadjustable to zero. Suits me fine as I want to use (nonDC amorphous core) transformers directly at the DACS output.

Any comments on the zero offset arrangement?
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Old 4th December 2008, 06:44 AM   #2
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In other words you think it´s so lousy it isn´t even worth considering ?!

Personally I have the uttermost respect for Hagerman, that has the ability to "think outside the box".
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Old 4th December 2008, 07:00 AM   #3
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I see no offset cancelling in the Hagtech's scheme.
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Old 4th December 2008, 07:34 AM   #4
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Take a closer look!

The TDA1543 is fed with +/-2,5VDC making the output 0V.

Zero DC-offset adjustment is with Vref resistor.

The digital input is therefor "level translated", see schematic!

My idea was to feed it with +/-4V and skip the output filters and OPs. As my output transformers can take no DC at all this must be a possible way to fix it.

My concern is how to look at the output loading as there will be no DC-current through the I/V-resistor, only AC?
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Old 4th December 2008, 08:13 AM   #5
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I'm sure you can attach a DC-servo from the output and to the regulator. The servo tweaks the +-2.5 voltage. It should be possible.

If you'll have a floating 5 volt supply and to that you'll add a virtual ground (with an opamp) which is controlled from the output of the dac.
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Old 4th December 2008, 08:40 AM   #6
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Hej Per-Anders!

Think I am OK with the "manual" zero adjustment as proposed in the original schematic.

More concerned about how one should look at the passive I/V loading.

As you know, this is not my territory !
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Old 4th December 2008, 10:23 AM   #7
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Check this DC-servo. It's the same idea.

http://www.ibiblio.org/tkan/audio/dynahi.html

This servo is tweaking the two current generators.

http://www.tangentsoft.net/audio/ppa/

The PPA has a "ground channel". You'll need almost the same.
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Old 4th December 2008, 12:40 PM   #8
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Per-Anders!

Think you will have be more specific about "tweaking the two current generators".

Can´t see any servos in the original schematic. There is just I/V resistors to ground followed by two OPs.
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Old 4th December 2008, 01:50 PM   #9
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The dac design has no servos. Check the schematics of the headphone amps for inspiration.
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Old 4th December 2008, 02:06 PM   #10
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Per-Anders!

Remember I am total novice in the digital/sand-department.

If it works without servos, why then add them?

My idea was that the RDC of the transformers should be the passive I/V-load. So in my ignorance I thought it was just to have the right value of RDC.....
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