Bypass of the internal opamp of a dac : missed

[00940]

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 :

An externally hosted image should be here but it was not working when we last tested it.

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]

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]

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]

the dac is a tda1549t.

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

 

[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.

 

[00940]

... 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]

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.