spdif input circuit help

hi
has anyone got a circuit 75 R spdif consumer to ttl levels, i have been using this one but having problems with oscillation (elekor design i think) tried using different versions of the ic.
regards
bob
 

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Hi !

1st make sure that it is 74hc_U_04
The U stands for unbuffered. each inverter is just a komplimentary fet-pair. with R2 X5 is driven to his working point (yes it's analog !)
2nd the circuit should work fine :)

i use the same circuit without R18 and without C12.
Some people vary R18 between 0 - 10K, some people use a 10k resistor from output X5 to ground ...

telefunken use in his DA1000 a transformer with a transistor after instead.

There are many ways :)


Regards

Jobstens
 
I've built this and it's stable...

http://www.diyaudio.com/forums/showthread.php?s=&threadid=54742

Warning though, it doesn't get full Jocko approval. ;)

I have no idea how much jitter the circuit adds - I'm thinking/hoping that hysteresis-induced phase noise is up in the MHz region, which a CS841x loop filter can eliminate and an AD189x ASRC can completely destroy. One of these days I'll feed the output of it back into the AP System Two and measure it.
 
I don't know why you had trouble with it. An 'U04 inverter is only a pair of CMOSFETs tied together. It doesn't get much simpler. Did you use the right one...........i.e., unbuffered?

I have other circuits, but probably more than a noob wants to futz with.

Yes, Elso's circuit does work. But the diodes start to conduct, and that throws off the actual impedance that the signal sees.

We won't discuss hysteresis. Been done before. Although we would like to see if someone can measure its effect.

Jocko
 

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Hey guys.

I've built jocko's new circuit with the LMH6550MA chip and it works quite well. I've also built elso's spdif input comparator. but I've found that it's not as good. (sorry elso, no offense man)

I can't post the schematic now, because i'm in class at the moment learning SQL :)

When i do post the schematic, it didn't work with the 100 ohm resistors on the outputs of the LMH6550MA, so just don't use them.

but so far, i've concluded that jocko's circuit is better because:
1) it's more dynamic
2) the treble is way clearer (slight treble roll off with elso's)
3) more 3d like
4) more transparent
5) tighter and better defined bass

Yes, so basically jocko's circuit is a lot better.

I'm powering the circuit with dual supplies as opposed to a single supply and i'm guessing that it's much better. As a result, you don't need to power Vcm with Vcc/2 volts. (but according to the datasheet, you need to still bypass the pin)

On another note, I've also biased the LMH6550MA chip by installing two resistors in series. One end to Vcc and the other to ground and the midpoint being Vcm. Jocko also recommends that a filter cap be placed at Vcm pin to ground.

Anyways, it's worth the effort!

Take care,
300_baud
 
Since I haven't actually buit it.......

I am guessing that it may work better with Vcm biased up to 2.5 V even with dual supplies. That should allow DC coupling to the '841x RX chip.

But most guys will not have split supplies in their DACs, so running it on +5 only should work fine. Getting a nice linear circuit to work on only +5 V is tricky. I have used similar circuits made with discrete parts, and I am not crazy about how they act on +5V. You have to be careful that the input stage does not exhibit any compression, as that lowers the input impedance if the input starts to go into saturation.

Just like Elso's diodes.

BTW........there are ways around that...........and they have other benefits. But some other time for that.


Jocko