Bypass of the internal opamp of a dac : missed

00940 · 4th July 2004, 08:34 PM

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

I tried to implement it in mine, this way :

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 ?

jobstens · 5th July 2004, 05:12 AM

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) ;-)

Regards

Jobstens

jobstens · 5th July 2004, 05:31 AM

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.

Regards

Jobstens

00940 · 5th July 2004, 09:24 AM

the dac is a tda1549t.

Its datasheet is there : http://www-us.semiconductors.philips.../TDA1549T.html

peranders · 5th July 2004, 09:43 AM

... 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.

00940 · 5th July 2004, 09:54 AM

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.

00940 · 5th July 2004, 06:36 PM

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.

4th July 2004, 08:34 PM	#1
00940 diyAudio Member Join Date: Sep 2003 Location: Paris	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 : I tried to implement it in mine, this way : 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 ?

5th July 2004, 09:43 AM	#5
peranders Electrons are yellow and more is better! diyAudio Member Join Date: Apr 2002 Location: Göteborg, Sweden Blog Entries: 4	... 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. __________________ /Per-Anders (my first name) or P-A as my friends call me Tube Buffered Gainclone in work \|Thread

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5th July 2004, 05:12 AM	#2
jobstens diyAudio Member Join Date: Jun 2004 Location: near Hannover	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) ;-) Regards Jobstens

5th July 2004, 05:31 AM	#3
jobstens diyAudio Member Join Date: Jun 2004 Location: near Hannover	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. Regards Jobstens

5th July 2004, 09:24 AM	#4
00940 diyAudio Member Join Date: Sep 2003 Location: Paris	the dac is a tda1549t. Its datasheet is there : http://www-us.semiconductors.philips.../TDA1549T.html

5th July 2004, 06:36 PM	#7
00940 diyAudio Member Join Date: Sep 2003 Location: Paris	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.