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Old 26th February 2009, 05:55 PM   #11
Cobra2 is offline Cobra2  Norway
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My friend "Jocko Homo" has tested/measured them...
He also says that anything other than 1:1 ratio is useless.
See: diyhifi.com ...

Arne K
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Old 26th February 2009, 09:02 PM   #12
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From what I've read Jocko's objections seemed to be based on his experience with 2:1 shielded trafos from SC, most likely the SC944-05. I'm not quite sure how that can be extended to their entire range which includes unshielded 1:1 transformers.
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Old 14th November 2009, 10:15 AM   #13
jkeny is offline jkeny  Ireland
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I was about to start a new thread on "whether SPDIF transformers were needed" & I saw this thread. I'm prompted to ask the question based on this post Music Server Computer transport - Hop On! where he replaced the SPDIF transformer with a resitor network as per the op with similar good results.

Given that so many pulse transformers are compromised (only Neweva is recommended by Jocko & he says that they are still not ideal) would it not be better to find another way?

So what are SPDIF transformers supposed to do ideally - remove potential ground loops? This is probably not a problem in a lot of situations & in DIY it can be addressed anyway.

Can anybody highlight the flaws in this argument?

Edit: It would also be good to see some scope shots of a Neweva S22083 transformer output versus resistor network output

Last edited by jkeny; 14th November 2009 at 10:31 AM.
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Old 14th November 2009, 10:30 AM   #14
jkeny is offline jkeny  Ireland
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Here's a scope shot of a Neweva (taken from Jim Hagerman) which still shows ringing. Top pic is a bad trafo, bottom is neweva.
Quote:
Both transformers were terminated in 75 ohms and then run through a 2nd order low pass filter, which slowed down the edges and added overshoot. The important thing to notice is the zero crossings
Edit: Oh & another problem raised about this whole area - RF experts over on Diyhifi say that SPDIF cable should be longer than 2 meters on a properly terminated 75 ohm line (I think), shorter gives all sorts of cable reflection problems! I don't know if this is related to the trafo or not?
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File Type: jpg spdif trafo scope.jpg (20.8 KB, 698 views)

Last edited by jkeny; 14th November 2009 at 10:38 AM.
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Old 14th November 2009, 04:50 PM   #15
jkeny is offline jkeny  Ireland
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Hey Kurt, any dirty picture? (scope shots )
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Old 29th January 2014, 03:49 PM   #16
VS is offline VS  Portugal
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Quote:
Originally Posted by Kurt von Kubik View Post
Most CD drives and players with SPDIF out do employ pulse transformers.
The PT is used of two reasons.
First of all it is often used as a step down transformer from the standardised ~3V to ~1V out.
Next it provides insulation and protection.

The downside of using PT´s is mainly hysteresis and thus jitter.
Hi All,

What kind of jitter are you talking about? Cycle-to-Cycle, Period, Phase?

Regards,
VS
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Old 14th March 2014, 09:40 AM   #17
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Hi Guys, I am a newbie in this forum and also newbie in electrical/electronic knowledge. So some of you may find these questionnaire dummies compare to what you all have been debating about; but would be glad if you are patience enough to guide this fool

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).
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Old 14th March 2014, 10:50 AM   #18
DF96 is online now DF96  England
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SPDIF is 75 ohm. That is reasonably important.

I fear you are trying to 'improve' something you don't understand; that is what your questions indicate. You need to start with something much simpler than 're-engineering' a digital interface.
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Old 15th March 2014, 07:19 AM   #19
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Hi DF96, u r right, I don't fully understand. So help me out here.
I do understand the reflection when impedance isn't properly match. But I appreciate u can answer each of my question. That will be great.
The coaxial cable n bnc plug is 75 impedance. Now by shorting 350 ohm instead of 75 ohm. Is it acceptable?
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Old 15th March 2014, 02:01 PM   #20
Eldam is offline Eldam  France
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Hi,

We can read at the beginning of the post than a cap would be better sometimes for sound quality than a pulse transformer for the DC stopping function.

What is the best according to you if the length is less than 20 cm between the spidf source and receiver with a BNC connectors and 75 ohms cable?

1) - Stopping the ground loop with a pulse transformer. Is there a minimum 1:1 pulse transformer quality to avoid the problems seen in the first posts ?

2) - Allow the ground loop with sharing the Gnd of the two pcbs (when it's possible) and only use a cap for security as DC blocker, because we can hear it (more details or bass resolution - see the first posts- ) against pulse transformer ?

If 2), I read than the maximum pulse fhz of a spidf signal is 6M hz : what sort of cap would you use ? The just enough band pass width you need for it or the fastest and stable like COG-NPO or polystyren?

A) smd or bulk ?
B) bulk X serie (military) COG ceramic 1uF - 5 mm between legs, plastic case
C) Polystyren bulk like 100 nf MIT for speaker crossover
D) Film : MKT,PEN, MKC, MKP
E) bypassing using A) to D)

Sorry if two cents, thanks
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