Topic: Coax-in mod for JB3 (?)

[dklein]

This thread was just crosslinked and it got me thinking...if you run a jumper wire from two of the 1/8" analog tabs over to the leads where the optical input connects, you could then use a standard 1/8" connection plug to tap into the JB3s s/pdif input electrically, bypassing the optical front end.

A mod like this would allow you to maintain everything - the analog in, electrical s/pdif and optical s/pdif.  Then, if there were any voltage level issues (depending on the a>d in use) you could deal with those outside the JB3 with an appropriate box.

Yes, you'd be jamming some electrical signal into the analog inputs at the same time but as long as you select optical in it would presumably be ignored.  If anyone thinks this might blow the analog in, then I suppose you could lift the analog tabs or perhaps choose the L+ and R+ connections as the s/pdif carrier.

Whaddya think?

[jk labs]

This thread was just crosslinked and it got me thinking...if you run a jumper wire from two of the 1/8" analog tabs over to the leads where the optical input connects, you could then use a standard 1/8" connection plug to tap into the JB3s s/pdif input electrically, bypassing the optical front end.

A mod like this would allow you to maintain everything - the analog in, electrical s/pdif and optical s/pdif.  Then, if there were any voltage level issues (depending on the a>d in use) you could deal with those outside the JB3 with an appropriate box.

Yes, you'd be jamming some electrical signal into the analog inputs at the same time but as long as you select optical in it would presumably be ignored.  If anyone thinks this might blow the analog in, then I suppose you could lift the analog tabs or perhaps choose the L+ and R+ connections as the s/pdif carrier.

Whaddya think?

Passing DC has some drawbacks. The S/PDIF signal is RF (MHz) so I suggest you try using 560pF or so caps instead of wire links. You'd need to experiment with the exact values of the caps. The aim would be to get sufficient RF signal level across the cap at 1 MHz and above in order to enable secure detection of the S/PDIF transitions. But at the same time small enough to effectively block alternative paths for the analog signal in it's search for ground. Probably you'll need more signal voltage on the feedline than the standard 0.5 Volts peak to peak specified for S/PDIF but thats a minor challenge here.   

I imagine the analog inputs have some overvoltage protection. If not they can be protected. If they are protected they will clip the signal received at some level related to the voltage on the power rail available. Anybody actually measured the logic voltages inside? Is it 3.3 or 2.5 Volts?

There is a ground issue here as well. Are the analog and digital grounds within the jb3 separated (as they ougth to be)?  One needs to think carefully about how to connect the coax shield to proper ground. Using L+ & R+ as you suggest needs to be evaluated. Connecting say R+ to a noisy digital ground could have some real ugly consequences wrt the analog capability (already suffering from noise according to user feedback.

So I think it is possible to add coax input capability to the jb3 analog input port without sacrificing analog performance.

However, based on a brief look at the pictures posted by Lee, it's not clear to me if you can just dump RF energy onto the output of the optical receiver. A photodiode f.ex. will not like that. Neither will a low impedance output buffer (it will kill the signal). Removing the optical in capability is an option. If you insist on keeping it you might be forced to adding a little bufferstage to protect the optical sensor. 

So I say go right ahead and keep us updated !!

Jon

[dklein]

So I say go right ahead and keep us updated !!

ha ha - I'm not hacking into my JB3 for a mod I don't need!