[dsavitsk]

Quote:

Originally Posted by Garbz I'm not sure how the USB dac work because I haven't been following it. But if it shares GND with the computer then I suggest offer optical as well. Call me paranoid but I'd rather put up with the slightly higher jitter of toslink adapters then contaminate the DAC on the otherside.

This gets mentioned as a theoretical problem with computer soundcards, but I've never actually seen it cause a problem. Perhaps others have?

The point of this project for me is to make a little board that can be dropped into an existing DAC. That it might fit a hammond and might have other outputs is gravy. We'll see what 00940 comes up with

-d

 

[dsavitsk]

Quote:

Originally Posted by 00940 Ok. Here's a digital only version. It has a better (buffered) coaxial output, using a 74HC04. To be short, the TTL output goes into a first buffer, is tapped afterwards for the TOTX, goes into a pair of paralled buffer and then is finally sent to the RCA. Everything is powered from usb bus of course, the 74HC04 and the optical transmitter have their regulators. Following the evaluation board, I just put everything on a groundplane.

This looks great. If one wants to skip the optical, can it be left unpopulated, or does it need a jumper, or a resistor. or ...?

 

[00940]

Quote:

Originally Posted by dsavitsk This looks great. If one wants to skip the optical, can it be left unpopulated, or does it need a jumper, or a resistor. or ...?

You just let everything related to the optical section empty. You could also cut some traces. I'll post a pic.

 

[dsavitsk]

Just to be difficult (really I apologize. I don't mean to be indecisive), but the more i think about it, maybe there is a point to putting an analog out ... ?

I hate to compete with the guzzler dac, but perhaps there is a reason to include the option. This would make it a more universal device, but it also might raise cost and difficulty. Any thoughts?

 

[00940]

If I skip the switch, it could be possible without too much problems. I'm gonna see that later in the evening.

 

[Nisbeth]

Quote:

Originally Posted by dsavitsk Just to be difficult (really I apologize. I don't mean to be indecisive), but the more i think about it, maybe there is a point to putting an analog out ... ? I hate to compete with the guzzler dac, but perhaps there is a reason to include the option. This would make it a more universal device, but it also might raise cost and difficulty. Any thoughts?

FWIW, I think there should be an analog out unless the point was to create the absolute smallest USB -> SPDIF-converter possible (in which case the pcb should be a lot smaller than it is now)

/U.

 

[Magsy]

Quote:

Originally Posted by Nisbeth FWIW, I think there should be an analog out unless the point was to create the absolute smallest USB -> SPDIF-converter possible (in which case the pcb should be a lot smaller than it is now) /U.

Seconded! Maybe just a header for analog and not a pcb mount jack, but provision to use the analog out would be a nice feature

 

[Nisbeth]

Quote:

Originally Posted by Magsy Seconded! Maybe just a header for analog and not a pcb mount jack <snip>

That would be even better! However it would be nice if it could still fit in the small Hammond, which means that there must be some space on the PCB on the part where the jack is located

/U.

 

[00940]

Ok, ok guys. Here it is, with analog out but without mute switch. Price should stay the same considering the price of a switch. It has one more regulator since we have to regulate the analog stage of the pcm2902.

 

[dsavitsk]

Quote:

Originally Posted by Nisbeth FWIW, I think there should be an analog out unless the point was to create the absolute smallest USB -> SPDIF-converter possible (in which case the pcb should be a lot smaller than it is now)

This was my original idea -- to make a board that was tiny and simply converted from usb to spdif. I thought we could go with all surface mount stuff and keep it down to like a square inch, or something. It seems to me, though, that if the other options (toslink and analog) can simply be left unpopulated, then including them all saves making two different boards. This comes at the expense of a few extra square inches, but that's probably okay.

Perhaps the biggest problem is that the analog is really just a convenience out. The 2902 is said to be inferior in quality to the 2702 (used in the guzzler usb dac.)

Really, I am pretty torn and could go either way. I think if I were intending to make an analog out usb dac, I'd go with the guzzler option. I I just wanted digital out I'd want this one. What to do if one needs both is a toss up.

Anyone have any thoughts?

-d

-d

 

[dsavitsk]

The pcm2902 has spdif input. Is this worth using?

Never mind as that would involve a power supply ...

 

[guzzler]

Thought I should chime in, as I had a little look at doing this a couple of months ago, then ran out of time... I really like the idea, personally. But, I'm going to make some picky comments on the layout

1. Tombstone resistors are horrible, to my mind. Burnt fingers, bad joints, and a fragile final board are all the come from them. I'd go 1206/0805 surface mount if space is paramount and use vias right at the pins where necessary.

2. More ceramic decoupling right near the PCM2902, and especially next to the logic chip (I presume it is...). tkerby suggested this, and I have to agree with him; he knows what he is doing

3. Remove any angles <90*, they're called acid traps in PCB manufacturing terms, and do exactly what they say on the tin. Basically, you can over etch into the corner of the joint as the acid doesn't flow out when washed. Put a shot straight section in, or rework the traces.

Could be made smaller, as is obvious, but since we're going for a certain case size, that's not an issue. So, that's about it from me

best regards

g

PS, I'll do the group buy again if people would like it, and 00940 doesn't want to do it. I'm certainly in for a few...

 

[Voodoochile]

Don't you want pins 5 and 2 on that jack? It looks to me like you have the switch contacts in the layout.

Nice looking board and project, I'm interested.

 

[00940]

board above updated to reflect comments. Here's a clearer view of the bottom plane to show the 1206 ceramic caps added. Resistors are also in 1206 size.

 

[guzzler]

Another wee thing, I would turn the thermals off in the ground plane. I didn't do it on the original as it would have cost more to have the board house make a complete new tooling set. Basically, every little cross at a pin acts as an inductor. It's a last 2% type thing, but might as well. Just use the change command in Eagle and you're set. From the picture, it looks like to Isolate could be turned up a little as well, as the ground plane looks a bit close to things; could just be an artefact of the image. If the DRC says it's okay, it probably is as it's fairly conservative. Looking very nice

g