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Old 28th April 2009, 12:55 AM   #1
Tenson is offline Tenson  United Kingdom
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Default I want a bigger S/PDIF !

I am using a modified DCX2496 as my DAC, and I have found for some time that every now and again it will drop the high frequencies. It is exactly the symptom described in the CS8420 datasheet as follows:

Quote:
Occasionally the CS8420 SRC will enter an invalid state. This can happen after the RUN bit has been set
when an AES3 stream is first plugged into the part or when a source device interrupts the SRC input stream.
When this happens, two symptoms may be noticeable - notches occurring in the frequency response, and
spurious tones being generated in response to some input frequencies.
After some messing about with my new pocket scope, I found that the digital output levels from both my CD player (Cyrus DAD7) and soundcard (M-Audio Fast Track Ultra) are below the minimum input for the CS8420! The minimum is 200mVpp. The CDP outputs about 100mV (under 75R load) and the soundcard around 170mV which still seems to play up.

I read that the spec for S/PDIF is 0.5Vpp - 1Vpp so why the feck are these supposedly professional business' making kit that doesn't meet spec?

Anyway, my main question is how do I up the voltage of each S/PDIF output? I have an old CA DiscMagic and that has a ~1V output on the AES/EBU connection and that never seems to 'loose lock' and drop the high frequencies. My thought is perhaps to replace the 1:1 ratio pulse transformer in the DCX2496 DAC with a higher ratio. OEP make some with 3:1 or even 8:1 ratio, though I'm not sure I can get hold of them. Spec in the File Here, are they even suitable for digital audio?

My other thought is that I have a digital switch box that uses an HD74HC04 gate device and I wonder if this can be configured to give 4x gain or something? I have traced the circuit and uploaded it below for you. I don't really know much about digital circuiets so please be gentle!

Click the image to open in full size.
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Old 28th April 2009, 12:31 PM   #2
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They are using those 5 parallel gates as a linear amplifier, whose gain is set by R8/R7, i.e., theoretically about 3.5, but in practice not as high.
You can increase R8 to increase the gain - maybe even put a trimpot in there so you can 'tune' it on the fly.
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Old 28th April 2009, 11:40 PM   #3
rossl is offline rossl  United States
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Try pulling out one end of R11. You don't need that resistor if the receiving end is terminated.
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Old 28th April 2009, 11:57 PM   #4
rossl is offline rossl  United States
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Most likely, the CD player and the soundcard also have resistive dividers that you don't need. Pull out one end of the resistor that goes to ground and see if it works better.

If it doesn't work better, you can just solder it back in

They put those in to pass emitted electrical noise tests.
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Old 29th April 2009, 12:46 AM   #5
Tenson is offline Tenson  United Kingdom
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Thanks for the replies.

You mention pulling R11. This co-insides with a question I had anyway - Is it only the reciving end of the line that needs to be terminated at 75R, or should the transmitting end also be done like this? I have seen digital outputs that have a 75R resistor between the pins of the pulse transformer, but I'd have thought this is only needed on the reciving end and the transmitting end should be low impedance?

If I pull R11, should I not also bypass R10? And should I put a 75R in the gap of R5, which is currently left blank.

I did actually remove a resistve diveider in my soundcard and it boosted the level a little. I can't really do this with my CD player as it has a strange digital output design that I don't really understand.

I'll try putting a pot in the feedback loop of the digital switch.

Thanks!!
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Old 29th April 2009, 03:37 AM   #6
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The idea is to have a source impedance of 75 ohms. This might consist of a 5 ohm real world source in series with a 70 ohm resistor.

This source then feeds a line with a 75 ohm characteristic impedance (i.e. 75 ohm coax).

This is then terminated on the receiving end with another 75 ohm impedance. So, there are basically two 75 ohm resistors, one on each end of the 75 ohm "transmission line". The theory is that, at very high frequencies, the line transmits the most power to the load, and has the least amount of reflection, with identical 75 ohm resistors on each end.
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Old 29th April 2009, 10:44 PM   #7
rossl is offline rossl  United States
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Quote:
Originally posted by Tenson

If I pull R11, should I not also bypass R10? And should I put a 75R in the gap of R5, which is currently left blank.
I would change one thing at a time and see how it sounds.

Pull out R11 and see what happens.

R10 should be in the circuit, and it should be 75R minus the output impedance of the 6 inverters.

As zigzagflux said, a 70R resistor in at R10 would be a good choice for starters.
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Old 1st May 2009, 10:38 PM   #8
Tenson is offline Tenson  United Kingdom
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Hi,

I removed R11 and the output is indeed higher, it has a gain of about 3x.

However, it doesn't work quite right like this. When connecting my CD player as the digital source, there is no sound, unless I touch the meter probe on the input terminal. I'm thinking this is maybe because the input impedance of the circuit is too high, (since they seem to have left out R5) and touching the probe on the input offers it a little input impedance to load the source? For some reason with R11 in place it never had this problem. My soundcard works fine though.

Also, with nothing connected there is about 2.3V on the input of the circuit, which I guess is generated by the output of the optical receiver. This drops to about 40mV if a coax source is connected.

Any suggestions? My thoughts are firstly to disconnect the optical receivers since I don't use them, and then add a 75R resistor in gap R5. Apparently the input impedance of the gate is very high so it can basically be ignored, although I don't know what happens to that when it has that 100K resistor in the feedback?

I think R10 should also be replaced with 75R because the output impedance of the parallel gates is also negligible.

Does all that sound sensible? Cheers!

P.S. this datasheet mentions input and output Z of the gates - http://www.datasheetcatalog.org/data...or/mXvyusq.pdf
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Old 4th May 2009, 05:47 AM   #9
EC8010 is offline EC8010  United Kingdom
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I recently made an AES3 driver for my Marantz CD5400 to drive my DCX2496 for exactly the reasons you describe. Electrically, AES3 is balanced and 110 Ohm, rather than the unbalanced 75 Ohm of S/PDIF. There were some spare inverters in the player, so I used the paralleled inverter technique shown earlier but without feedback resistors. I used a transformer coupled by a 1uF capacitor and 95 Ohm (determined by experiment) of series resistance to get the output impedance right. Here's an eye pattern of the output data:
Attached Images
File Type: png aesebu00.png (18.2 KB, 554 views)
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Old 4th May 2009, 05:57 AM   #10
EC8010 is offline EC8010  United Kingdom
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And here's a trace to show what the edges of the waveform look like:
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File Type: png aesebu02.png (10.3 KB, 560 views)
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