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/product/ak4114/ak4114_f04e.pdf
This is an image that show's the current signal flow.
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.
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/product/ak4114/ak4114_f04e.pdf
This is an image that show's the current signal flow.
An externally hosted image should be here but it was not working when we last tested it.
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.
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.
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.
An externally hosted image should be here but it was not working when we last tested it.
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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.
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.
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.
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.
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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/Interfacing_AES3_&_SPDIF.pdf
Further reading (more to be found on the net):
http://www.cirrus.com/en/pubs/appNote/AN134REV5.pdf
http://www.rane.com/pdf/ranenotes/Interfacing_AES3_&_SPDIF.pdf
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.
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.
But with the 1k resistor on the Hiface side, i'm not sure if that's right.
Any suggestions?
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.
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.
But with the 1k resistor on the Hiface side, i'm not sure if that's right.
Any suggestions?
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I just thought of a suggestion, am I thinking in the right direction?
Why do you want to reduce the signal level ?
I think there is no problem to input with 1.5v on the AK4114 !!!!
Serge
True, in this case 104c would be nonsense.
But should I remove the 1K resistor in the hiface?
You made me doubt
I found some postings on head-fi and they all speak about a 2:1 so allready a stepdown.
the used transformer is the 78604/1C
http://www.murata-ps.com/data/magnetics/kmp_786.pdf
.
I checked a bit (datasheet DIT4192).
That pulse transformer should be 2:1.
When I am right output level of DIT4192 is Vdd - 0.4 = 4.6Vpp.
Stepped down by T1 to 2.3Vpp.
100/150 ohm resistive divider attenuates to 0.92Vpp (we are approaching our target...).
It would be best if you could check levels using a scope.
That pulse transformer should be 2:1.
When I am right output level of DIT4192 is Vdd - 0.4 = 4.6Vpp.
Stepped down by T1 to 2.3Vpp.
100/150 ohm resistive divider attenuates to 0.92Vpp (we are approaching our target...).
It would be best if you could check levels using a scope.
Transformer
As the manufacturer claims, the output would be around 1.7v pp.
(they use a different resistor)
I will connect my scope to it tomorrow and see what happens.
The datasheet of the AK4114 states that it needs a minimum of 0.2v pp to function but i couldn't find the max value.
I read stuff about using attenuators to reduce reflections that develop during transmission, so it's still wise to attenuate to +/- 0.3/0.5v pp?
As the manufacturer claims, the output would be around 1.7v pp.
(they use a different resistor)
I will connect my scope to it tomorrow and see what happens.
The datasheet of the AK4114 states that it needs a minimum of 0.2v pp to function but i couldn't find the max value.
I read stuff about using attenuators to reduce reflections that develop during transmission, so it's still wise to attenuate to +/- 0.3/0.5v pp?
......
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.
I always find it respectful towards the originator of schematics, ideas and other "intellectual work" to
reference him when posting part of his work.
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