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Old 4th July 2004, 08:34 PM   #1
00940 is offline 00940  Belgium
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Default Bypass of the internal opamp of a dac : missed

I saw that thing in an earlier thread, that was supposed to work with a philips cd753 :

Click the image to open in full size.

I tried to implement it in mine, this way :

Click the image to open in full size.

And it's a failure. I've sound, actually it's even good sound, but the noise floor is absolutly huge, about as strong as the music.

I've double checked everything, it's correctly wired and I can't see any solder bridge.

Any idea on what I should check ?
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Old 5th July 2004, 05:12 AM   #2
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Now you have 2 I/V converters - the first in the DAC and the second is the NJM4560. You could be happy that they don't oscillate (or does them ?)
You must switch off the internal I/V ... I think it's impossible :-(

Maybe it helps to re-equip the 470pf.

Another idea: short pin 4 to GND (without 470pf) ;-)


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Jobstens
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Old 5th July 2004, 05:31 AM   #3
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Stop !

Don't try the 'Another idea' - because it shouldn't be ground. It must be Vref (2.5V) - I think. (then you could take a great cap to ground)
I can't see the hole DAC.

Maybe you could burn the output of the internal OP. Don't ask how. Do this at your own risk.

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Jobstens
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Old 5th July 2004, 09:24 AM   #4
00940 is offline 00940  Belgium
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the dac is a tda1549t.

Its datasheet is there : http://www-us.semiconductors.philips.../TDA1549T.html
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Old 5th July 2004, 09:43 AM   #5
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... hm.. excuse me but can you explain why you have taken the inverting input to the reference voltage? If you are going to modify a circuit you must know how it works in order to improve it.
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Old 5th July 2004, 09:54 AM   #6
00940 is offline 00940  Belgium
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Quote:
Originally posted by peranders
... hm.. excuse me but can you explain why you have taken the inverting input to the reference voltage? If you are going to modify a circuit you must know how it works in order to improve it.
Vref is connected to the non-inverting input. Filtcl (and the output of the dac) is connected to the inverting one.
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Old 5th July 2004, 06:36 PM   #7
00940 is offline 00940  Belgium
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Doesn't matter anymore, I made exactly the contrary. I scrapped the external buffer and use only the internal opamps.

IC are short so the buffer wasn't strictly needed.
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