Mr White's "Opus", designing a simple balanced DAC

[Russ White]

Originally posted by hayenc
Are you looking to do the AGND to VGND connection like on the eval board? It is a zero ohm resistor under the chip on the reverse side of the board.

Yeah, AGND to DGND. The only way to insure they have the same potential is to join them. I see now how the eval board did it, I wonder if that is the best/good enough, or if there is a better way...

[Russ White]

Originally posted by hayenc
Russ,

Do you expect any problems with the variable input impedance of the JT on Opus?

Not unless it drops below the minimum load for the DAC. I would expect it to be fine. One can also alter the JT so that it has a constant input impedance, but thats for another thread.

[hayenc]

Probably worth putting the pads there like on the eval board. That leaves the options of bridging or leaving off. I notice the eval board has two other sets of pads, but the board is huge compared with yours and you may not need other sets along the ground interface.

[Russ White]

What I did was joined the two planes at a single spot with a very thin trace.. That way people can simply cut the trace if they like, or connect a ferrite or something between an AGND pad and a DGND pad.

[BrianDonegan]

I was actually thinking of using a balanced TS stage immeadiately after the DAC as a sort of buffer/line driver, probably at unity gain, in the same case, then going off to JT and TS or X-BoSoZ in my preamp.

[Russ White]

Originally posted by BrianDonegan
I was actually thinking of using a balanced TS stage immeadiately after the DAC as a sort of buffer/line driver, probably at unity gain, in the same case, then going off to JT and TS or X-BoSoZ in my preamp.

You could do that, but I doubt the buffer stage would be necessary.

[Russ White]

Here it is:

notice I added jumpers so you can DC or AC couple the output even with the output caps in place.

Also note the SMD caps around the VREGS (only kind we could find that were a reasonable size).

Cheers!
Russ

[Russ White]

Hopefully complete...

Now notice I don't make any direct provision for optical input, but it would be very easy to P2P one from the chassis and then just use existing pads.

Cheers!
Russ

[jean-paul]

Originally posted by Russ White
What I did was joined the two planes at a single spot with a very thin trace.. That way people can simply cut the trace if they like, or connect a ferrite or something between an AGND pad and a DGND pad.

That very probably is a good solution and won't need a change at all. The PCB looks OK and complete, certainly for a beta version. I do not see errors although I do not have enough time to reread datasheets of both the chips so I might not see mistakes. I hope rfbrw is around to check things over. Series and/or termination resistors for I2S use would have been nice but the board would have to change too much.

A power PCB with separated supplies of a 2 x 6V 250 to 500 mA transformer, Schottky diodes and caps + extra filtering ( common mode coils ) and the same dimensions as the DAC board would be handy for testing. Or am I asking too much now ?

Those small regs will be a challenge for my hands and eyes...

[hayenc]

One last question. It would seem we could do differential digital in with some resistor changes. I am not too sure about grounding for differential digital input. Would we only ground at the XLR connector or would we want a third pin on the SPDIF for ground?