Carrie USB-Powered Headphone Amplifier - Page 7

I really appreciate all the discussion that's going on. It's good.

Allow me to give some context. I'm boosting the voltage because I'm trying to replicate the Mini^3. In some of my feeble attempts at producing a board for a CMoy, I kept coming back to what the Mini^3 had and was. I talked to amb about it, so now I'm more worried about doing the name justice. I was thinking of trying a chip that accepts 3.3V to 4.75V, but might come after this project.

Boosting to +/- 12V doesn't give me enough current, plus the 5V-in, 12V-out chip that this paper says exists no longer is listed on the DCP02 product page. Having 2W would be a lot easier, but neither Mouser nor Digikey supply the product which seems to no longer exist. Plus, the OPA690 doesn't accept that high of a power supply voltage.

Regarding the LED, I put it in at the end as an afterthought because, as rds mentioned, I don't really care about the light indicator, plus I could easily move the switch. If it plays music, it's good enough for me. I merely put it in as an afterthought for people who might want to build this thing as well, but personally, I wouldn't put in an LED. I was thinking of just using the planes to power it, thereby removing any need to route anything, but I didn't want to waste current from the TLE. I put it in the back just to satisfy the mere fact of having an LED, but I clearly didn't think it through. That all being said, it's just an LED. Let's worry about something more important for now.

I'm quite tired of looking at the board editor in EAGLE, so let's discuss schematic and layout changes all at once to reduce my going back and forth to switch little bits here and there. I figured it would be good to put a capacitor between the TLE and TL750 as an input/output capacitor. They're so close I didn't think it would matter, but that seems to not really make sense. I'll throw C3 between the two and make it some ceramic/low ESR cap. I was also thinking of rotating the ground channel opamp just like how it is in the Mini^3. I don't know why I didn't start off like that. Anyway, that's something that will help with the parts placement, and it'll be more familiar and easier to build. Reducing the size of the test points will also help with the board space. Something else I was wondering about was the meaning of something in the TL750 datasheet. It says the output regulation voltage is 50mV to 100mV. Surely, that doesn't mean its output ripple is 50mV to 100mV, right?

Another big question is about the wires. If I'm connecting both ground planes to the case, will I even need to connect the audio or power supply ground wire from the BantamDAC to the board? The ground plane will lie underneath the board, and it will connect to the case. There should be lots of current through those exposed planes, so it will save a little room and wiring if I don't have to connect that. The USB connector will sit in the back and will presumably connect to the case. Either way, it might be easier to connect the wires anyway, but I just wanted to feel out the possibility.

Also, I was wondering what the full power supply draw will be from the AD8397. It says the supply current is maximum 12mA/Amplifier, but it also says the highest output current is 250mA at 3Vpp and Rload 12 Ohm. I was wondering what the power draw will be in total from the AD8397. Does 250mA * 12 Ohm ~= 0.75W make sense?

Also, could anyone who understands USB weigh in here? I've read a little about it, but there's still a lot I don't get. I get that there's a handshake that occurs at the beginning, but I don't think the PCM2702 would request for full power output. It could very well demand 100mA and be done with it since it does have such a relatively low current draw at about 55mA, at least with the voltages cetoole chose. I get that it limits inrush current at plug-in and then gives the full power after enumeration, but do I have to get that enumeration to take place? How would I do that?

In summation, my questions are:

1) What does output regulation voltage mean?
2) Would it be worthwhile connecting two ground wires between the boards?
3) What is the max power/current draw of the AD8397?
4) Do I need USB enumeration?

More out loud thinking to come later.

Just a quick mention on this. Even the iPod Wolfson DACs work on a low supply and can feed headphones directly, so maybe one can skip the amp altogether. Plus there are plenty of MP3 players that work off a mere AAA battery. Who really knows what's going on in there, but there is some merit to what you're saying though we won't get into the "audiophile" side of the discussion.

I've updated post 3 with the current layout, but I'll add it here for convenience. I'm going to stick with the two ground wires for audio and bus power. Maybe I'll add another one for kicks.







My head is going to explode if I look at this layout editor any more. I wonder how bad it would really be if I just sent what I have right now to a board house. I haven't picked one yet, so that research will come within the next few days. Anyone have any recommendations for the layout? One of the big differences from the Mini^3 board is that there is a V- plane on top of the board. I wonder if that will cool the chips any better, but I also wonder about the instantaneous power requirements of the opamps and whether the V- plane will be good for that. There are a few extra capacitors next to the opamps, so we'll see how well that plays out.

I also read on this thread that power will be readily available from the USB regardless of enumeration. I hope that that's true.
diyAudio Forums - USB powered HIFI headphone amp/DAC - Page 1

Right, how abouts do I do that?

Thanks a bunch, fellas. I'll have to get even more creative with the layout. That's fine. I'll probably just cut out the V- plane near the opamps entirely. And thanks, error401, for suggesting APCircuits. I moved to Calgary in January, so that ought to save me a little on shipping.