Digital Input circuit

[jkeny]

I'm looking to optimise the performance of a Panasonic SA-XR57 Digital (Pure digital Equibit) amplifier & I am beginning at the front end - the digital input circuits.

The SPDIF input doesn't use a transformer so is not galvanically isolated, which I don't think is too much of a problem, as the 0.1uF caps block any DC. But there is an inductor before this cap on the line - is this a good idea? - what benefit does it have? Any suggestions for improving this circuit?

[jkeny]

I found this thread just now - didn't find it in the search before spdif input circuit help
and found the attached galvanically isolated spdif circuit from Jocko Homo.

Would this be better than the above circuit up to 192KHz?

Another, thing I came accross was not to use shielded cable to connect this circuit onwards to the digital receiver board - it introduces capacitance and this circuit works best as pure resistive ! Currently a flat cable is connecting it to the next board - is this OK?

[cuibono]

I recently did something similar - upgrading the circuits/parts of my spdif In and Out circuits.

Here is the most relevant thread:
moding P Daniel's NOS DAC?

My equipment is a delta 1010lt soundcard and peter daniels nos dac - so the circuits are probably not going to be the same for you, but should be similar.

Doing the upgrade made a positive difference, IMO. Use a good quality transformer - PE65612 or Newava SS22083 (better, more expensive).

I think the main thing is to make sure you're transmitting at the proper levels (5Vpp). I know very little about how to design these circuits, so I suggest you read Jocko Homos posts carefully, he knows what he is doing (although doesn't always explain...). You will have to understand what is going on to make sure it is applicable to your use - especially, you need the data sheets of the transmit and receive chips, which will tell you what the chips want to see. JH posted a number of partial/generic circuits, where he was trying to show some particular point (like the circuit you posted of his). The thing you need to do is interpret his designs to your needs, as they won't be automatic drop-ins.

In my circuit, C1 and 2 are there to block dc from the transformer, which is important. R1 and 2 set the output levels to 5Vpp. I don't know what the rest of the circuit is doing, some sort of balanced conversion...

Jon Risch has some relevant postings on his website and audioasylum with regards to spdif cable. He has some specific recomendations, and he seems to know his stuff. I use Belden 1506A, which he recommends.

[kevinkr]

All of the inductors I see are in the power supply circuits, and you would not want to remove those.. Have I missed something?

A spdif transformer would be a worthwhile improvement not just for galvanic isolation but because it removes common mode signals from the spdif input as well. (Think external RFI and common mode EMI from the source it is connected to.)

[jkeny]

No Kevin,
Most of the inductors are on the Spdif line in & ground, although there are inductors on the PS line which I'm keeping - for the moment I've added some OS-Con bypass 2.2uF on the PS lines

This spdif feeds a Digital Audio Interface Transceiver and AKM AK4114 & looking at its datasheet, this is the Spdif input circuit in there, so maybe I should leave the existing circuit as-is?

Should I use a shielded coax between boards? - there is about 6 inches of flat ribbon cable at the moment (looks like high freq cable as it's also used from the HDMI board - 65MHz?)

[rossl]

jkeny, The inductors are there just to keep stray RF and EMI out of the circuitry.

Leave them alone, they probably don't hurt the SPDIF. I wouldn't take the effort to remove them.

I don't know what value they are, but I think those should be ferrites that only block RF above 50-100MHz

They are not required for the SPDIF circuit operation, and you could remove them.

They probably were put in to pass a government EMI regulatory test.