[MWP]

Hi all,

Im just about to start desiging the PCB for this USB->DAC->Headphone-AMP design of mine.

The aim was to build a decent small headphone USB "soundcard" which is only USB powered (its for laptop use).

Any comments/sugestions?

 

[Porksoda]

A slightly more detailed description of what you are doing would be helpful. I'm not familiar the various power supply ICs. Overall, my biggest concern would be if a USB port can supply that kind of power. It looks to me like you are taking the USB's voltage and stepping it up to +/- 12V, which is going to draw a LOT of current. If I were doing it, I would add a small NiMH battery pack, and add a wall wart powered fast charger. The power would be much cleaner that way.

I dunno a lot about actual DAC chips, perhaps someone can shed some light on the options there.

 

[MWP]

The switching power "blocks" are from TI (as is just about everything else in this project).
I actually have no idea how well suited they will be to audio, but its the only ready made compact +12v and -12v switchers i could find so i thought i would give them a try.
Both switch at 800kHz, can supply 500mA, and are around 75% efficient.

The PCM2707 is a USB sound chip which outputs pure 16bit I2S.

The DAC is the PCM1798 which has decent specs of "Dynamic Range: 123 dB, THD+N: 0.0005%".

The OPA4224 is the best quad opamp TI make... i would have used OPA627's but i only have them in PDIP, so they were too big (and power hungry) for this.

The headphone amp has been discussed quite a bit on these forums recently.

The whole DAC->OPA->TPA setup is pretty much a copy of the schematic in the TPA6120 data sheet.

 

[guzzler]

Direct USB:
PCM2707: 30mA

From switcher:

Positive: secondary:24mA - primary: 58mA
PCM1793, 5V: 8mA
PCM1793, 3.3V: 16mA

Positive/negative: secondary: 62mA - primary: 300mA
OPA224: say, 4x8mA
TPA6120: 2x15mA

Total: 86mA from switcher
Total to switcher: 360mA

Total from USB: 390mA

That's not taking into account the efficiency, so you'll be running VERY close to the maximum supply current from USB. This isn't really a problem for a desktop, as the USB supply is often overspecced, but on a laptop you'll be running very close. Best idea is to remove the TPA6120 and replace it with a traditional opamp/buffer. Also, the efficiency of the linear power sections (REG102s) will be low as well, switching supplies would help there, but might not sound the best. The PCM1793 is good as it's very low current.

 

[MWP]

Hrm, youre right, it will be close.

Well ill build it, see what happens.

 

[guzzler]

I'm sure it will work fine, but on marginal computers it could cause instability. USB crashing is a pest. The other thing is for laptop use is you have to consider the battery life of the laptop as well. An extra half amp is quite a bit extra and you'll certainly notice a difference in battery life.

Look forward to seeing your design

 

[Nisbeth]

Maybe use the AD8397 for the output stage? It's rail-to-rail and 170 mA output current. Typical supply current is 7 mA

/U.

 

[MWP]

Well i just finished building this DAC/AMP and it sounds friggin incredible

I made a few changes to the schematic above, but nothing too drastic, mainly around the PCM2707 (wasnt setup correctly) and in some of the power supply.

I managed to build it on a double-sided PCB thats 45x75mm (!!!) so itll fit in a hammond credit-card sized extruded case.
PCB was home made and uses down to 15mil tracks.

Not sure about power usage... works fine off the desktop PC, i havnt tried it on the laptop as yet.

 

[guzzler]

Nice, any pictures of it?

 

[Nisbeth]

Quote:

Originally Posted by MWP Well i just finished building this DAC/AMP and it sounds friggin incredible I made a few changes to the schematic above, but nothing too drastic, mainly around the PCM2707 (wasnt setup correctly) and in some of the power supply. I managed to build it on a double-sided PCB thats 45x75mm (!!!) so itll fit in a hammond credit-card sized extruded case. PCB was home made and uses down to 15mil tracks. Not sure about power usage... works fine off the desktop PC, i havnt tried it on the laptop as yet.

C'mon you can't post something like this without including pictures

/U.

 

[MWP]

Yup, gimme 5mins.

Just tested power usage... it sucks ~500mA at idle (not playing, no headphones connected), and ~600mA when playing music through 32ohm headphones at a decent (above normal) volume.

The laptop's power meter says it uses an extra 2W compared to when playing music into the headphones using the internal soundcard.

So its a little power hungry, but from how it sounds so far, very worth it.

 

[Jazper]

Quote:

Originally Posted by MWP Yup, gimme 5mins. Just tested power usage... it sucks ~500mA at idle (not playing, no headphones connected), and ~600mA when playing music through 32ohm headphones at a decent (above normal) volume. The laptop's power meter says it uses an extra 2W compared to when playing music into the headphones using the internal soundcard. So its a little power hungry, but from how it sounds so far, very worth it.

well 5v/600mA = 3W, though it will be idling sometimes so it might drop below this periodically.

 

[MWP]

Im hindsight, i probably should have built in power control to turn off all the ICs except the USB recv when idle.
Although i think i would have run out of PCB space then

 

[MWP]

Ok, photos:

During building:

Working:

Building it into the hammond case atm.

 

[Nisbeth]

That looks good! Would you mind showing us pics of the PCB-layout? (or better still, eagle files

)


/U.