SPDIF impedance correction - diyAudio
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Old 19th January 2012, 03:45 PM   #1
Koenjer is offline Koenjer  Netherlands
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Join Date: Oct 2009
Default SPDIF impedance correction

I'm using a HiFace USB->SPDIF as a source. And a modified Rotel receiver as DAC.

As you may allready know, the SPDIF output of the HiFace is rather high (1,5V pp)

I have read a lot of threads of Jenky about attenuating the SPDIF signal and the benefits it has. But that thread also showed the importantness of a correct spdif transmission.

I found the schematics of both devices, and found out that there are 2 transformers in the signal path. I assume that this won't be of any positive influance.

As i'm allready modifying the rotel, I would like to make this transmission as well optimised as possible.

The SPDIF receiver in the rotel is the AK4114.
http://www.asahi-kasei.co.jp/akm/en/...k4114_f04e.pdf

This is an image that show's the current signal flow.
Click the image to open in full size.

The transformer on the Hiface side is acutally a PULSE ratio 1:2.

Can someone help me with optimising this? I do have some knowlege of electronics but not enough to optimise this myself.
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Old 19th January 2012, 03:56 PM   #2
DF96 is offline DF96  England
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The receive end looks wrong. Check the circuit again. Where is the 75 ohm resistor - series or shunt?
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Old 19th January 2012, 04:47 PM   #3
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I am against using transformers on SPDIF inputs. One 75 ohm resistor to ground, one 0.1uF capacitor to the cip input pin is all that's needed.
If you really want to play with the transformer, one optimized input should have some kind of snubber circuit, calculated for that specific transformer parameters, like the one below (just an example, values might differ on your case). Ignore the differential connection of the secondary.

Click the image to open in full size.

Last edited by SoNic_real_one; 19th January 2012 at 04:50 PM.
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Old 19th January 2012, 05:12 PM   #4
gk7 is offline gk7
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Why are you against using transformers on SPDIF inputs ? Almost all professional equipment uses them, no ? On cheap consumer stuff they are omitted for cost reasons.
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Old 19th January 2012, 06:11 PM   #5
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Because the professional equipment needs them for insulation reasons. They can be plugged in different phases of a three phase power distribution typical in a commericial building.
At home you don't have that, you have all plugged in the same receptacle.
In USA is even better - most of the new houses have dedicated grounding conductor and polarized power plugs (phase/neutral) are standard.
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Old 19th January 2012, 06:24 PM   #6
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Look at Cirrus Logic website; there is a nice overview of the consumer and professional SP/DIF interface to be found on the datasheets of their input receivers.
First to note is that the single ended (consumer) SP/DIF output found on most CDP's does not have to use a transformer, whereas the AES/EBU professional interface is transformer based for obvious reasons.
Nevertheless in the majority of CDP's the SP/DIF output is transformer coupled; sometimes the quality of the transformer is crap (as in my Rotel RCD 955) completely destroying the quality of the signal.
At the other hand the signal coming out of the last chip must often be attenuated to the 0.5 Vpp norm, and a good quality step down pulse transformer might have benefits over a resistive divider (must still try that).
For a good interface one should also look at the receiver side.
My DAC has input coupling capacitor, 75 ohm resistor to ground, then another pulse transformer, then another coupling capacitor (pfff) before we reach the input receiver.
So actually more than double the amount of devices which are needed when we take the original output of the CDP into account.
As long as the signal at the end is OK, it is OK, but I removed all surplus parts, checked the signal on the scope, and think it sounds better now.

Last edited by pieter t; 19th January 2012 at 06:27 PM.
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Old 19th January 2012, 07:27 PM   #7
macboy is offline macboy  Canada
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Quote:
Originally Posted by gk7 View Post
Why are you against using transformers on SPDIF inputs ? Almost all professional equipment uses them, no ? On cheap consumer stuff they are omitted for cost reasons.
Professional doesn't use S/PDIF, it uses AES/EBU (AES3) which uses differential transmission. The transformer in those is used for common mode rejection and balanced-to-SE conversion. S/PDIF uses shielded coax instead of twisted pair and doesn't need the common mode rejection.
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Old 20th January 2012, 06:16 AM   #8
gk7 is offline gk7
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I do know the difference between AES/EBU and S/PDIF. Nevertheless a galvanic isolation is beneficial for S/PDIF too. My RME ADI-2 and RME Digi96 for example has it.
Further reading (more to be found on the net):
http://www.cirrus.com/en/pubs/appNote/AN134REV5.pdf
http://www.rane.com/pdf/ranenotes/In...S3_&_SPDIF.pdf
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Old 20th January 2012, 06:59 AM   #9
Koenjer is offline Koenjer  Netherlands
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Default SPDIF isolation

In this case were are talking about a USB-> SPDIF, so you do want to have galvanic isolation! Second is that my PC is on a different 230v Phase then the rotel processor, so in my case I absolutly need isolation.

After some new investigation i found that the Hiface has a 1k resistor from the isolated ground to the USB ground...

See the new schematic.
Click the image to open in full size.

Official statement from m2tech:

the 1K resistor is there because of certification needs. A totaly floating connector is very dangerous when it comes to ESD tests.

As DF96 allready stated, it looks that the receiver end is not designed properly. In my first thought I would remove the transformer and resistors on the receiver side, and make a connection as stated in the datasheet of the AK4114.

Click the image to open in full size.

But with the 1k resistor on the Hiface side, i'm not sure if that's right.

Any suggestions?

Last edited by Koenjer; 20th January 2012 at 07:02 AM.
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Old 20th January 2012, 07:29 AM   #10
Koenjer is offline Koenjer  Netherlands
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Default Suggestion

I just thought of a suggestion, am I thinking in the right direction?

Click the image to open in full size.
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