TDA1543 to replace saa7321 - diyAudio
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Old 7th July 2011, 12:07 AM   #1
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Default TDA1543 to replace saa7321

Hi All,

I've been tweaking my Philips cd634 wich had saa7321 and saa7310. Now I made a simplest tda1543 DAC wich I feed directly with the saa7310 with 2k3//150nF as Vref resistor and like in the datasheet an opamp stage with opa2134. The tda has it's own power supply with oscon and bypass cap. It sounds great and very open with superb dynamics but produces an awefull amound of background noise in my headphones and thru the speakers. The original opamp stage is still in the player but replaced by again an opa2134 and also the headphones opamp is 2134.
How to get rid of the noise? Any thoughts? Drop me a line!


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Old 7th July 2011, 12:47 AM   #2
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The noise is almost certainly grounding related. Have you removed the SAA7321? It has a typical digital draw of 40mA and its predecessor the SAA7220 is well known for being a noisy brute.
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Old 7th July 2011, 08:44 AM   #3
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Hi Abraxalito,

No I haven't removed the 7321, it's still in place. I could lift the power supply and some other pins to make shure there is no 40mA's lost.
You say grounding is the problem but I can't figure out where. The tda and opamp ground comes from the groundplane on top of the board........
If I remove the tda board completely there is even more background noise........any tricks I could try?



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Old 7th July 2011, 09:10 AM   #4
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Just had a look at the datasheet for the SAA7323 (upgrade part to the '21) and it seems you can't remove it completely because its the clock master for the whole player. But they have separated out the digital supply to the clock (pin 23) from the rest of the digital stuff (pin 22). So you could try just gently lifting pin 22.

The grounding is a problem primarily because there's a groundplane shared between digital and analog ground. So noisy digital power supply currents get coupled into supposedly clean analog grounds.
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Old 7th July 2011, 10:01 AM   #5
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Any chance by decoupling grounds with inductors or is this thought too simple?
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Old 7th July 2011, 10:13 AM   #6
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Problem is that there's no isolation of grounds within the IC - I think all the noise will be coupled into the substrate. Lifting the ground for the digital supply with an inductor probably just forces the noise to take another route through the IC substrate. So I don't think that's worth trying.
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Old 7th July 2011, 10:25 AM   #7
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Lifting p22 is not working, player doesn't start up normally, cd spinning rapidly without a reading. Trying lifting p23 (VDD2)? SM is here:

PHILIPS CD634 CD834 Service Manual free download, schematics, eeprom, repair info for electronics
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Old 7th July 2011, 12:42 PM   #8
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Sounds like then that some other circuits depend on that digital power. So reconnect pin 22 and we'll try some better filtering on that supply.

Remove resistors 3661 (4R7) and 3662 (33R) and capacitor 2758 (56p). In place of the 33R fit a ferrite bead with two or three holes through it, loop some bare wire through the holes with at least 360 degrees of looping. We need something lossy at HF, a ferrite bead is better than an inductor for that. Then see if the noise you're getting changes when you short out that ferrite bead. If it doesn't then my original hunch wasn't correct and the noise is arising some other way.
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Old 7th July 2011, 04:30 PM   #9
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Why do you think it's from that PS connection? You think it's digital noise or analog? I could just add a ferrite bead with some wire thru instead of the inductor for the test.
Let you know what the outcome is when I have the time to mod.
TNX!
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Old 7th July 2011, 05:50 PM   #10
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No succes! I have removed the components and fitted in a choke but it seems to become worse in the new situation.
I'm used to silence in the 600 series from Philips. I've already tweaked 600, 614, 502, 710. Some fitted a new clock in, some made NOS. I have a 634 spare and can compare to the original. The sound of the tweaked 634 is so much better but the original is much more quiet in noise. The tweaked 634 is the best player I have at the moment, not in noise but in sound.
By the way, I did more to it. Muting transistors out, output caps replaced, power supply caps replaced by low ESR ones and bypassed by the orange Philips 150n's, molded the X-tal in Shin Etsu glue, damped the lid etc.
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