Easy TTL to S/PDIF coax converter - diyAudio
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Old 27th January 2009, 12:54 PM   #1
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Default Easy TTL to S/PDIF coax converter

I want to connect a cdrom drive to a DAC using the dacs coaxial input but assume I need a converter (TTL --> coax).

I've seen a few examples some using the 74HC04 and some just using a couple of resistors and a capacitor.

Can anyone recommend a circuit that they have tried and works. Any diagrams / pictures will be greatly appreciated.

Has anyone using the circuit below succesfully with a cdrom?

TTL ----||-----330ohm--+--------- coax S/PDIF signal out
in |
Ground -----------------+--------- coax signal ground

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Old 27th January 2009, 06:29 PM   #2
jitter is offline jitter  Netherlands
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I know the circuit around a 74HCT04 well, I built it more than once.

I recently built a S/PDIF-switch which I loosely based on these schematics. In it you'll also find the TTL to coaxial S/PDIF output. Based on a design I saw a long time ago I changed the 150nF C to 3x 47nF in parallel. Works fine, very near the S/PDIF specs of 75 ohms output impedance and 0.5 Vpp signal (when terminated with 75 ohms or an input).
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Old 28th January 2009, 05:00 AM   #3
jitter is offline jitter  Netherlands
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Come to think of it, for my Shigaclone project, I just used the 374R and 93R1 to GND to divide the TTL-signal coming straight from the digital output of the signal processing IC, no 74HCT04. Most other people did the same, so the output seems to be able to cope with the current.
If your source will be able to supply enough current then you could do the same, otherwise you'd need the 74HCT04.
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Old 8th December 2010, 02:06 AM   #4
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Your diagram misses a 100-ohm resistor, which should connect across the two "+" sign on your diagram.

I had built it before. It works fine.
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Old 9th December 2010, 01:35 AM   #5
macboy is offline macboy  Canada
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I've never understood why people try to use digital logic gates to drive S/PDIF cables. A proper analog line driver is what you should want. Don't forget that a digital signal is fundamentally just special kind of analog signal. An opamp (appropriate to the task) would work nicely, or even a video line driver. Coax S/PDIF has the same basic specs as composite video (75 ohm impedance, 0.7 VRMS, >6 MHz BW, etc.) so using a video line driver makes sense.
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Old 18th February 2012, 02:33 PM   #6
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I i connect th spdif out of a cdrom to a coax in of an AV reicever and don't work. I have a multi input selector (usb, toshlink, coax) that can lock with this simple circuit but reicever don't. I think that need biger resistor. Any opinion?
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Old 20th February 2012, 01:49 PM   #7
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Try to connect directly the digital output signal from the CD to the DAC Spdif input .....
I do it and it's work with a CS8416 .....
It's not the best solution but if you have no oscillo !!!!!!!it's not easy to debug

Best regards
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Old 3rd March 2012, 06:53 AM   #8
jitter is offline jitter  Netherlands
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Agreed, the output of the CDROM drive probably already conforms to SPDIF standards (i.e. no TTL-levels but SPDIF-levels) and connecting it directly to the DAC/AV-receiver should work.
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