Transformerless Digital out

I am sure some of you will laugh your heart out over the questions I am posting here, but while you are doing that, pls help me out of this endless loop in my amateur mind to find an answer:

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This is part of a circuit from Marantz CD63Ki as I have been toying with the direct tap into SPDIF circuit to get the purest signal out to DAC.
SPDIF Questions:
- SAA7345 datasheet showing output of leg 2 DOBM is 4.5 to 5v. But measure using digital multimeter through DC is 2.5v. Should I be measuring using AC instead?
- Consider all 5v of DOBM short to ground without any resistor or cap, regardless of equipment failure also, why the SAA7345 will not malfunction?
- Base on voltage divider calculation, I should be getting 1.944v from my Improvised SPDIF. Correct?
- If 1.944v achieved, does it mean it is slowly bleeding 3.1v to ground through No 4?
- If it is not 1.944v achieved, does it mean the rest of the circuit from No 5 onwards influence the changes?
- Is getting the 75ohm impedance important? Where value of No 4 is set to 75ohm? (The receiving chip on the DAC is DIR9001 can take a max 6.5v if not mistaken, also the coaxial lead from No 3 to )
- Is having the cap in series to block DC before No 3 important?
- Why is there no current leakage that I can feel from the Marantz player consider the main power input still uses the original 2 pin plug? Is there a resistive circuit in the player that “consume” all those voltage? (all the noise filter caps are short to ground).

The signal is oscillating at 2.8224 MHz (assuming sampling at 44.1KHz). A normal multimeter will not able to 'capture' such a high frequency. You really need a oscilloscope for the job.  
 
Getting the impedance matched is important...Remember that a mismatch in impedance will causes reflection and high frequency wave bouncing back to the source...This is not good. The aim here is really to get a good clean clear S/PDIF signal.
 
Not knowing how much current the DOBM pin puts out, I would recommend a series capacitor to decouple the voltage divider.
 
No leakage current 'felt' on the player?...This is becaase the chassis is fully isolated from the main supply...This player is using a linear power supply that uses a normal power transformer.
 
Hope my answers help... 

Hi vt4c, thanks for responding. But it's not helping
I do understand the frequency travel high speed, but it carry by mean of 5v to 0? 
I am not from electronic background, so those theory isn't my thing n I look into it through common sense n logic instead, so bear with me if the question is a bit weird.
I am concern about the amount of voltage shorting to ground. Yes the player is certainly isolated. But as I mention, it is a 2 pin power cord n no grounding to our common earth. So in the circuit there are so many caps n resistors shorting to ground. The chassis need to take time to dissipate/neutralize this parasitic leak..?

75 impedance is spec in the coaxial cable n bnc plug, is it sufficient? Knowing the bypass I did, do not offer 75ohm to ground.

Help...

Hi vt4c,
     Here is the current output of the DOBM, I think  it is 5 to 10mA.

Appreciate your feedback.

[img][/img]

adrian4454,
   There might be differences between supply ground (where the SAA7345 is powered from) and the chassis ground that you're refering to when you mentioned " current leakage that I can feel from the Marantz player" and the "common earth" you've mentioned in the previous posting.
 
If the power supply ground (on the board) is connected to the chassis, then both ground are essentially the same. If otherwise, then, the chassis is not connected to the power supply ground. If it's the later, then shorting pin 2 to the chassis is fine. This can be verified by checking continuity between the chassis ground and power supply ground.
 
  75 ohms impedance must applies from the source all the way to the load; for your case, from the SAA7345's pin2 to the DIR9001 S/PDIF input pin. The cable plays part of the equation. Though the DIR9001 pin is able to survive 6V peak; this is the absolute maximum voltage the pin is able to survive. Based on the standard, the voltage swing is no more than 0.6V p-p. Going above this voltage risk some other problem like saturating the S/PDIF transformer (if you use one in the circuitry). The other issue is noise generated by the large voltage swing from the high frequency signal of the S/PDIF. This might interfere with the electronics.
 
  For a person that is not from electronic background, I applaud you on your effort. I would recomend that you do some additional reading on electronics before embarking on your work above as it needs certain level of understanding on electronics. Without this, there is more damage done rather than improvement after all the effort. If you need to pursue further with more of this type of electronic works, you might want to look into acquiring the correct tools as well.

Hi vt4c,
    Thanks a lot for the knowledgable feedback. The reading part is easy, but the understanding part is hard.

Do you manage to see illustration in 1st and 4th threads from me?

Hi Adrian,
Let me see if I can shed some light...


- SAA7345 datasheet showing output of leg 2 DOBM is 4.5 to 5v. But measure using digital multimeter through DC is 2.5v. Should I be measuring using AC instead?

---> 4.5-5v is the digital waveform output strength (where the digital music is). What you have measured is just the DC offset (the music is not here).


- Consider all 5v of DOBM short to ground without any resistor or cap, regardless of equipment failure also, why the SAA7345 will not malfunction?
---> Some pins are referenced to ground to function properly. It's nothing to do about shorting stuffs.


- Base on voltage divider calculation, I should be getting 1.944v from my Improvised SPDIF. Correct?
---> About there la...


- If 1.944v achieved, does it mean it is slowly bleeding 3.1v to ground through No 4?

---> something like that

- If it is not 1.944v achieved, does it mean the rest of the circuit from No 5 onwards influence the changes?
---> something like that

- Is getting the 75ohm impedance important? Where value of No 4 is set to 75ohm? (The receiving chip on the DAC is DIR9001 can take a max 6.5v if not mistaken, also the coaxial lead from No 3 to )
---> 75 ohm is important. From my experience setting up 75 ohm via input resistor in the DAC is good enough.

- Is having the cap in series to block DC before No 3 important?
---> Not necessary if your DAC input stage has a cap.

- Why is there no current leakage that I can feel from the Marantz player consider the main power input still uses the original 2 pin plug? Is there a resistive circuit in the player that “consume” all those voltage? (all the noise filter caps are short to ground).
---> If you're referring to the slight shorting experience when your tender skin touches some electronic chassis, that is not the effect of current leakage. It's got to do with difference in earth reference between 2 equipment.


cheers... too lazy nowadays to post

HI Wikin,
    Thanks alot! Clear as a bell now. Answers a lot neither nor in my brain.

Last one about the current leak, not sure what you mean by difference in earth reference.

Consider only 1 equipment,  those usual 2 pin plug power cord equipment, which does not have a true earth grounding pin.. why some have "shocking sensation" while some doesn't? Is it because it has good dissipative property that slowly neutralized the standing voltage?

adrian4454 wrote:Consider only 1 equipment,  those usual 2 pin plug power cord equipment, which does not have a true earth grounding pin.. why some have "shocking sensation" while some doesn't? Is it because it has good dissipative property that slowly neutralized the standing voltage?

Hi Adrian,
Brand new equipment will usually not have the shocking sensation. It's when they age that sometimes, depending on the quality and design of the circuit, may cause the shocking sensation.
Biggest culprit comes from the power transformer where some of the mains voltage may leak into the chassis. So depending on the severity of the damage, the entire chassis may have a charge of a few volts to 10s of volts. When you touch the chassis, your body with your foot to the ground is zero volts, so you are actually a conductor hence 'shock'. This is where earthing is important. If the equipment is faulty i.e. it has 10s of volts on the chassis, the earth wire will actually cause your ELCB to trip and shutting down that plug point rather than letting it run and give you more 'syok' feeling....

cheers.