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Old 15th March 2014, 02:06 PM   #21
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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Old 15th March 2014, 02:17 PM   #22
DF96 is offline DF96  England
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Quote:
Originally Posted by adrichia
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?
No. The fact that you have asked this question shows that you do not understand reflection.

You need to do a lot more reading and experimenting before you will be ready to 'improve' a digital output interface. You will also need some serious test equipment and the knowledge to correctly use it.

I would be cautious about 'improving' digital circuits, as I only have 45 years experience of RF and audio circuits and only have a fairly standard 20MHz analogue oscilloscope.
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Old 16th March 2014, 01:10 AM   #23
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Hi Eldam, thanks a lot for some input. Giving more questions instead
The best approach that I am using now is the improvise spdif I circled in the picture. I am relying on the Dac end cap to do the blocking.

Yes the ground is carry over through the coaxial shield to the dac, which has a proper ground. Strangely, the improvise spdif will not work if shield is not grounded to the CD player.

If u could answer my questions on my initial post, it will be great. Rest assure, I read the beginning of this thread couple of time already. Like I said, my knowledge is lacking, so I need u guys to help me out.

The transformerless spdif is meant to take a better square out of standard digital out the player can offer. There has been graph in the web showing how bad the transformer is doing to the signal.
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Old 16th March 2014, 10:17 AM   #24
Eldam is offline Eldam  France
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hi Adrichia,

I'm just a noob, sorry I can't help. My question is near yours because I will use some old dac chips I have with a spidf board using the Wolfson receiver ship. As it will work with two pcb I suppose myself than the question of ground loop is quite important.

But I'm not planning to improve anything but just to understand where i have to go both the money and my understanging as it seems there are two solutions : with and without pulse transformers. Some very good DAC dont use pulse transformer. I go out with a simple coax spidf from my source

The few things I believe to understand are (but don't know really) :

- a very good pulse transformer is better than without, but are expensive if you want a good enough quality to avoid the problems seen in the first posts here.
- I'm not sure that they are able to stop the DC and at the end a DC bloking cap have to be use. In this case the benefice is just to have not a big ground loop as the GND is stopped by the pulse transformer.

- as the spidf fhz wave is between 100 K hz and 6 M hz (or 19 Mhz with 24 bits/192 hz ?) and the impedance line is 75 ohms : the DC blocking cap have to be > at 100 nf (because a serie cap is a high pass filter and 100 nf give 106 K hz for the lower fhz !). So at least 220 nf seems to be better but I really don't know !

- parasitic inductance seems to be important for a cap in serie for signal coupling and in this case a smd cap should be better than a bulk one... at the end the choice of dielectric for loss is important and the polystyren in theory is the best for that !

- piezzo electric effect (noise with stress and micro moves of the caps pn its pcb) of smd can be a problem and maybe bulk capacitor is better ?

It is just the state of my understanding but not the truth, not sure it can help you ! Sorry if you think I high jack your question, I am like you, lost between question and practical applications. I think DF96 is right : firsdt you have to try to understand what you do before going to practice. I am at this first point.

regards....
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Old 16th March 2014, 02:16 PM   #25
DF96 is offline DF96  England
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Quote:
Originally Posted by adrichia
There has been graph in the web showing how bad the transformer is doing to the signal.
A transformer does not go down to DC, so the waveform looks different. Fortunately, this does not matter at all - it has no effect on sound quality. Transformers have advantages, such as galvanic isolation, which is why they are used. As a general rule, nobody adds wound components to a circuit unless they are necessary so if they are present you can be reasonably sure they are doing something useful.

Note that this is for commercial equipment; some DIYers sprinkle chokes and transformers randomly.
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Old 17th March 2014, 03:54 AM   #26
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Hi Eldam,
Thanks for sharing. My main concern here is whether I've short too much power to the ground. If Leg 2 is outputing 5volt, and voltage divider is 550 in series and 350 to ground.. mean 1.9volt in series, while 3 volt short to ground? May sound silly, but appreciate you can help out.

Hi DF96,
Without a doubt, the original circuit serve a very important purpose as you have explained. Just like a combustion engine car, where a very fine restrictive air filter is a must because the car is meant to operate in various kind of imaginable condition; that's the design principle every car manufacturer must adhere.

Now just consider, the environment has been best controlled, and the variable has been made singular, only 1.

What is the best method moving forward? Am I shorting too much voltage to ground?

Thanks for being patience with me and I am looking forward on your knowledgable inputs.
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Old 18th March 2014, 03:19 PM   #27
Eldam is offline Eldam  France
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Adrichia,

I know the the cd player philips C710 & C723 use this chip before TDA1545 and spidf output also (with trafo) : if you found the shematic... you can maybe copy it ?
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Old 20th March 2014, 06:08 AM   #28
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Hi Eldam,
Good day, I am sure Philips player use similar method to do the digital output. Where it goes through cap, voltage divider, transformer, and couple of noise shutting cap to ground.
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Old 20th March 2014, 05:25 PM   #29
Eldam is offline Eldam  France
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Hi,

i make a mistake : CD723 use SAA7378GP before the dac chip

But : Philips CD710/721/722/740/750/751 use the SAA7345GP, some of those use the pin 2 as EBU output and resistors for adaptation impedance and most of time a little shelded trafo who can seting up by screw on the top of it. HP filter or LP filter (I don't know) are used maybe with a 1 uf caps if my memory is good...

Maybe you will find some shematic on lampizator site : it talks about modding the spidf output of some Philips & Sony players : check it.

this one can help you like the vasily list : The complete d/a DAC converter list - DutchAudioClassics.nl

good luck,

Last edited by Eldam; 20th March 2014 at 05:27 PM.
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Old 20th March 2014, 08:06 PM   #30
bappe is offline bappe  Sweden
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Hi,

I design RF and high speed data lines for a living and I would never put a transformer on such a line if it wasn't absolutely necessary for isolation reasons. Careful PCB layout, a good line driver, NP0/C0G coupling caps is a good place to start. If it is only for diy I would also skip the low level of spdif and keep the signal at whatever level it is and then take it down to whatever level is required by the receiver right at its input. If not using BNC I Woulrd recommend a cheap RCA rather then an expensive one, the reason being that the cheap ones are shorter and hence, presents a very short impedance discontinuity that will have a very limited influence on the transmitted signal.

BR,
Anders
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