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Old 29th April 2010, 04:30 AM   #1
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Default S/PDIF input transformer

Question: How to choose an input transformer for S/PDIF ?

What's the difference between a 1$ transformer (I am now using the DA103C from Murata Power solution available on Mouser) and a 75 $ transformer from Silk (SACThailand)

What are the parameters that matters ? Does this transformer have a real impact on the sound ?

Thanks,

Davide
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Old 30th April 2010, 01:47 AM   #2
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search for high interwinding capacitance and low leakage inductance transformer.
Lowest interwinding capacitance is BAD, it is a nightmare to cancel inductive kick.
If you want to stick with Murata DA100 series for Audio Digital Transmission, use DA101C.

Yes, digital transformers can have a real impact on sound. If you just stick transformer into 75 ohm line, you will find that
You should make TX side 300 - 400 Ohm, then put transformer between and make right termination on RX side to have 75 Ohm specific impedance.
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Old 30th April 2010, 09:56 PM   #3
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Thanks,

Unfortunately the DA101C is not available. The choice of Murata was kind of random, so if you have better recommendation, let me know. What about the 75$ Silk trafo ? Is it worth ?

I notices that the capacitance is often not reported in the data.


Thanks,

Davide
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Old 1st May 2010, 12:31 AM   #4
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funny, I just ordered some 103C. It was a bit of a whim, since I was placing a mouser order and that's all they have ;-)

specs on these little pulse transformers are a bit hit and miss. I highly doubt a $75 transformer is worth the money. Even if the Murata parts are junk, there are plenty of other good options. (just not on mouser

Everything I've read say low interwinding capacitance is good. They do need to be pretty high bandwidth & transformers are pretty tough to get right, but it shouldn't be very difficult to get a decent SPDIF signal. I figured once I have the parts, and have some time, I'll hook it up and take a look on a scope.

another discussion of these here

Cirrus Logic AN134 says (and not much else other than more resources):
"The transformer used in the professional interface should be capable of operation from 3.6 to 24.6 MHz, which is the audio data rate of 28 kHz to 192 kHz after biphase-mark encoding. Transformers used for digital audio should also have low inter-winding capacitance. Inter-winding capacitance couples the primary and secondary windings, degrading the isolation between the windings with increasing frequency. This energy appears in the form of common mode noise on the receive side ground and has the potential to degrade analog performance."
http://www.cirrus.com/en/pubs/appNote/AN134REV5.pdf
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Old 1st May 2010, 12:41 AM   #5
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Quote:
Originally Posted by Nikon1975 View Post
What about the 75$ Silk trafo ? Is it worth ?
BTW, I don't see any SPDIF pulse transformers on the SACT website. I just see analog & power transformers of all types. ??
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Old 1st May 2010, 11:21 AM   #6
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@Nikon1975
any datasheet from Silk trafo? I don't see any suitable trafo on SACT site.
I tested Murata DA101C. It is good, not junk. Murata 76601/3C can be used too, need more work around to get it right. Another good transformer is Newava S22083.

@eclectic2k
datasheet from Cirrus is wrong.
Low interwinding capacitance = high leakage inductance.
High leakage inductance =

Last edited by stormsonic; 1st May 2010 at 11:35 AM.
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Old 1st May 2010, 02:51 PM   #7
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Thanks,

You can find the Silk in the input transformers section, it's the last one

"Pulse Transformer for D/A convertor Digital Audio Data transfer via Supermalloy core pulse transformer"

I have now the Murata DA103C, because it's the only one I found :-(

D.
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Old 1st May 2010, 07:09 PM   #8
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Quote:
Originally Posted by Nikon1975 View Post
Thanks,
You can find the Silk in the input transformers section, it's the last one
"Pulse Transformer for D/A convertor Digital Audio Data transfer via Supermalloy core pulse transformer"
D.
Ah, see it now. Doesn't look very good. First off, it's 50 ohms impedance. (I suppose the right choice of resistors could help this) Strange though, since SPDIF is 75 ohm. The 50MHz bandwidth should be OK for 48kHz audio, not likely for 96k or higher. It's tough to tell anything more from the minimal specs. I'd certainly not think it's worth more than a few $. I won't be taking any risks to test it at $75

Stormsonic - Thanks for the advice.
I'll need to be researching a bit more. I haven't played much at all with these yet. I have noticed the newava parts are popular. I think they're available (when in stock) from Digi-Key. Maybe I'll pick up a couple one of these days...
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