Carrie USB-Powered Headphone Amplifier - Page 9

I don't know about offering a kit because it's a mostly straightforward build. The best I feel like providing is a Mouser and Digikey BOM. I can even provide it in EZ Buy format although I recognize that you live across the Atlantic pond. I'll see if Jeff will be interested, although I don't know if there would be enough demand. It won't hurt too much to ask, I suppose.

I was pleasantly surprised by the gold finish. This will be one slick-looking amp.

edit: Okay, maybe Farnell too.

edit2: Maybe a group buy of kits would work.

I just sent out the proto boards today. I wouldn't say they'd be arriving "soon," but they'll get there. My projects have been slowed to a crawl because my place is being renovated, and I don't have a place to do any work as all my belongings have been thrown on top of my bed and pushed to the centre of the room. I have all the parts on me and it's a pretty quick build, but I just don't have enough time to devote to it now.

As for GB and other options, it all depends on demand. I don't know how many people want a device I'll classify as laptop-fi, but if enough people get on board, I don't mind running a few more boards. Last resort would be to hold onto a few boards and sell it to whoever's interested. We'll see.

I have some more proto boards if you're interested in one. Only 3 or 4 to go.

I was asked why I chose a single-rail DC/DC converter, and I thought it would be useful if I posted the answer.

I'm happy to report that I've completed my first build of Carrie. It took me a grueling 5 hours to complete this morning (started at 6 am), and it looks nice. I've learned a few things about design and procurement just from building the thing, so I had a pretty good morning.

http://files.getdropbox.com/u/886264...e/DSC01060.JPG

http://files.getdropbox.com/u/886264...e/DSC01061.JPG

http://files.getdropbox.com/u/886264...e/DSC01062.JPG

http://files.getdropbox.com/u/886264...e/DSC01063.JPG

http://files.getdropbox.com/u/886264...e/DSC01064.JPG

http://files.getdropbox.com/u/886264...e/DSC01065.JPG

The BantamDAC that I built from before wasn't putting out any music, so I'm not quite ready to write my final report. Right now, signal input is from my iPhone.

Here are some things I learned when building this amplifer:

- Buy the right parts. R5L/R are physically too large. I neglected to get the micro-sized resistors. Are there stability issues involved here?

- Make sure the edgeside components don't bump into the enclosure. Notice the capacitors near the ground channel that come close. This is being addressed in the next board version.

- Make sure you carefully choose the components that involve a plane. I soldered in the wrong filter capacitors (voltage rating too low), so I had to do some rework. Pulling out the caps was easy enough, but cleaning up the holes was the hard part. One of the pads fell off, and it was impossible to get the solder cleaned out of the holes. Now I only have the inductor doing the filtering, and the switching noise is still getting through. This should not be.

- A small project like this can take a long time. Wow, I totally underestimated the time it took to build it.

- The space allotted to the filter capacitors was not enough. I had to tombstone the capacitor first time through. Everything fits the way it is in EAGLE, so I have to do some major wiggling.

- The Molex connectors are too long; without wires, it fits to the top of the case. I had to dremel off the female, cable-side connector so I can slide it into the case. Hardwiring underneath the boards may be preferable to Molices at all. I think I'll leave it in though.

- Panel-mounting the output connector can open up some board space. I don't want to change too much, but I might have to.

- L1G caught on the enclosure on my first fitting. This also happened with my Mini^3 builds.

- The knob I chose is so sexy. Goodness, I like it so much better than the ones with the two rubber O-rings. The knurls make for better grip when controlling the volume anyway. It doesn't fit with the look of the Hammond, but it still looks good.

I've noticed a peculiar problem of the output signal clipping. I don't have any sine wave tones or an oscilloscope, but it doesn't sound right when I turn up the volume to a comfortable listening level. Sounds clippy. I only used a gain of 2 and the voltage at the opamp pins is still +/-4.72V. The full output voltage of the iPhone's Wolfson is AVDD/3.3 Vrms, and running off the battery, I can't see it being higher than 1.212 Vrms, or 3.17 Vpk-pk. With a gain of 2, the max signal I would see at the output is 6.34 Vpk-pk, right? I'm using JVC Marshmallows which have an impedance of 14 Ohms, and it even happened on my 32 Ohm Yuin PK3s. Is too much current being drawn from the AD8397? It should be able to output 250mA into a 12 Ohm load when the opamp pins are given 10V or +/-5V, so I can't see it drawing too much from the opamp. This didn't happen with the Mini^3 (gain 5) with modified PS. I'm thoroughly perplexed and annoyed.

Does anyone have any insights? I have an extra DCP01 which I can parallel with the current one, so I can double the available power if it's currently insufficient. I will try disconnecting the current Bantam from the USB by taking out L3, so we'll see if it's a power draw issue. For now, enjoy the photos.

Thanks, Bonthouse. I hope to have time to work on my second Carrie, so there should be more juicy photos and, I hope, a working amplifier.

If you didn't see the other thread, the problem with the amp is that I was shipped the wrong resistor for R5LR by Mouser, and I installed those wrong resistors. I received the resistors yesterday, but I won't be back in town until after the weekend. I will have time on Monday to work on it, and I'll report back. It's all so exciting.