DIY iPod digital out

[joneeboi]

For the boutiques, I was thinking of airwiring them since they make everyone's life so difficult and deserve it as punishment. =/ Making board room for them would be a huge no-no, surely.

And by going with USB power, it makes the project a bit more portable. If I went with the external power supply, I'd pretty much be redesigning the Opus. My proposed design would somewhat be a niche product, I suppose, as I don't imagine most people wanting to grab the power from USB and going with the digital source of the iPod over ASIO over USB. I guess that really defeats the purpose of portable. Shoot. I guess we could go with something like a 1455J12 and throw in some batteries with leftover room. Is that portable? Transportable. Going with an integrated amp seems to be the smartest thing, as nobody really wants to carry three things around like that. PPAS? I was hoping to control the volume on the external amp side of things, but if we throw it all in one case, it would still be viable with the PPAS. Mini^3? 3ch CMoy? We'd have to keep things high quality in the small footprint to make good use of the high quality DAC.

It feels like I'm trying to force something that's not going to fit (easily). Can someone talk some sense into me here? Should I just go with a USB-powered Opus? Is that even an option?

 

[FS2]

Um, USB power? Thought this was meant to be a portable amp that took I2S from the iPod? But if it's a desktop amp, is there any reason to limit it to USB power instead of a wall wart? I wouldn't have thought USB power was that clean..

Quote:

Originally Posted by error401 /img/forum/go_quote.gif I can't really think of an easy way to switch between balanced and SE output stages. I guess it could be done by substituting some resistors around the positive side of the bal->SE and taking the output directly. How are you going to do volume control here? Maybe you can come up with something, but with the space considerations, it's probably better to just choose an output topology and go with it.

Tap off the balanced output from the I/V converter op-amps before they go to the balanced to single ended converter?

 

[error401]

Quote:

Originally Posted by FS2 /img/forum/go_quote.gif Um, USB power? Thought this was meant to be a portable amp that took I2S from the iPod? But if it's a desktop amp, is there any reason to limit it to USB power instead of a wall wart? I wouldn't have thought USB power was that clean..

If you use USB power you need to use a switcher onboard anyway. IME USB power isn't all that bad anyway. But yea, I don't really get that either.

Quote:

Tap off the balanced output from the I/V converter op-amps before they go to the balanced to single ended converter?

It's a voltage-out DAC so there is no I/V, I'm just thinking that routing the board so that you can easily pass either a balanced or unbalanced signal to a balanced or unbalanced (onboard) amp might be difficult. I haven't tried though...

 

[FS2]

I was wondering about that. I've only skimmed through the datasheet and didn't see it explicitly stated whether it was voltage or current out.

Powerwise, I was thinking I'd need a switcher anyhow, since I'd be obtaining a boosted and a negative voltage of my batteries. I'll, have to look at filtering for that.

 

[joneeboi]

I was thinking USB power because I was planning on making this my transportable, laptop, doing homework at school rig, though I can see the redundancy in such a design. Having an external DAC/amp for iPods would mean double battery charging, less space on board for whatever, simpler circuits due to space restriction unless we're really good. Perhaps there can be a charging circuit that will charge both the DAC's and iPod's batteries. The iPod dock connector would have to be either hardwired into the DAC/amp casing or use something like a D-Sub connector. I fear the project will inevitably become some sort of portable Opus, basically redesigned to fit into a Hammond or other case. I can live with that, I guess. Battery powered DAC...

 

[error401]

Quote:

Originally Posted by joneeboi /img/forum/go_quote.gif I was thinking USB power because I was planning on making this my transportable, laptop, doing homework at school rig, though I can see the redundancy in such a design.

Design what you need, not what you think others will need (unless you're trying to create a salable product). Do that first, and if you can, make it flexible so others can use the design. Don't concede your needs because of our comments

. It's your project, do it the way that's best for your needs.

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Having an external DAC/amp for iPods would mean double battery charging, less space on board for whatever, simpler circuits due to space restriction unless we're really good.

If you get creative with routing I think it can probably be done without much compromise. Put the battery pack and any tall parts ('lytic caps, i/o connectors) on one side of the board and the majority of the stuff on the other side.
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Perhaps there can be a charging circuit that will charge both the DAC's and iPod's batteries.

Wouldn't be too hard. You can just pass the USB power lines directly to the iPod. Fitting a charger in the DAC might be possible, or it might not.
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The iPod dock connector would have to be either hardwired into the DAC/amp casing or use something like a D-Sub connector.

I'd probably use mini-DIN or 1394 (or even HDMI or eSata). You get 6 conductors with each and a much nicer and smaller connector than D-Sub.

Quote:

I fear the project will inevitably become some sort of portable Opus, basically redesigned to fit into a Hammond or other case. I can live with that, I guess. Battery powered DAC...

Opus is basically a datasheet implementation of the DAC, it doesn't do anything else. Ending up with a similar circuit isn't a bad thing. Besides, you're adding the bal->SE stage and an amplifier onboard, there's still room for something new.

 

[joneeboi]

Quote:

Originally Posted by error401 /img/forum/go_quote.gif Design what you need, not what you think others will need (unless you're trying to create a salable product). Do that first, and if you can, make it flexible so others can use the design. Don't concede your needs because of our comments . It's your project, do it the way that's best for your needs. Opus is basically a datasheet implementation of the DAC, it doesn't do anything else. Ending up with a similar circuit isn't a bad thing. Besides, you're adding the bal->SE stage and an amplifier onboard, there's still room for something new.

I needed to hear that. I didn't realize what I was doing till I thought about it today. *shakes head*

Okay, onto MY needs.

Thinking out loud here, USB power, boosted and regulated for linear power, iPod charging ftw, PPAS if possible or SMD PIMETA since I've recently fallen in love with the AD8610/AD8620 combo I have. I talked to my professor just now and he referred me to one of his employees who works at the University and is very busy, but I'm hoping he can give me some of his expert time. How so very exciting. BUF634s are out of stock forever, so I'll probably go with one of Sijosae's simpler discrete buffers. I'm thinking top side for amp, bottom side for DAC. Slip the board into the bottom slot of the 1455C8, making some kind of room for the PIMETA's power 220uF/470uF 16V rails. Maybe I'll get two boosters, one for 7VDC on the DAC side and one for 12VDC for the amp section. I may be pushing the USB's current capabilities here. Maybe I can shoot for a 9V compromise to power both sections, though heat may become an issue.

What have we got here? 3.3VDC for DVDD, 5VDC for AVDD, 9VDC for amp section, 9VDC for iPod charging. Yikes. If the WM8740's DVDD is 5V instead of 3.3V, its output current will be a third more, though it would only sit at 19mA. Looking at the TPS793* line, input is limited to 6V and output is max 4.75V. REG101 looks like our candidate, and it seems it's in stock but only in SO-8 and not SOT-23. One can safely input up to 12V, so that works out. Summing things up, REG101 for 5V, TPS61040 for boosting DC to 9V, and I'm unsure for 3.3V regulation. Using Digikey's fine search function, a whole bunch are suitable for the application, but sticking with the brand names, I clicked on Fairchild Semi's FAN2500. From TI, we have both the TPS76033 and TPS76033 at 50mA. We have a total power consumption of 32mA max from the Wolfson, so 50mA should be enough I think. Is there any preference for any of the above regulators or am I just being absurd?

More head-scratching to ensue.

 

[00940]

Quote:

Originally Posted by joneeboi /img/forum/go_quote.gif Maybe I'll get two boosters, one for 7VDC on the DAC side

Why 7V ? You're wasting power in linear regulation. 5.5V is more than enough with modern LDOs. You could even not use a booster for the DAC, as the WM8740 will very happily run with both supplies at 3.3V (and it eats less power that way too).

Quote:

Originally Posted by joneeboi /img/forum/go_quote.gif Looking at the TPS793* line, input is limited to 6V and output is max 4.75V.

Wrong I'm affraid... max output is 5.5V (variable output) TPS79301DBVR. And even if you insist to use the wm8740 with 5V supplies, 4.75V is close enough. Noone forces you to use either 3.3V or 5V. It must be in between those two values and the analog supply equal or higher than the digital. That's it.

 

[Jambo]

Quote:

Originally Posted by joneeboi /img/forum/go_quote.gif Is there any preference for any of the above regulators or am I just being absurd? More head-scratching to ensue.

I haven't looked at the ones that you suggested, but something you should consider is staying away from LDO regulators if you can. Because of the way their feedback works their stability depends on the load and you need to be careful what you load them with (output caps need to have their ESR within a given range, lower is not always best). Not getting at anything in particular here, just food for thought.

 

[joneeboi]

Quote:

Originally Posted by 00940 /img/forum/go_quote.gif Why 7V ? You're wasting power in linear regulation. 5.5V is more than enough with modern LDOs. You could even not use a booster for the DAC, as the WM8740 will very happily run with both supplies at 3.3V (and it eats less power that way too).

The DC boosting would be for the amp section, though your point is well taken. I'm just assuming that as input voltage increases for the DAC, the better the efficiency and performance. Does my assumption correspond to reality? Then I could just run both supplies off of a REG101's 4.75V output, could I not? It'll run a little hotter and draw more power, but if the USB can take it, I'm down. If it doesn't improve performance in any significant way, going all the way down to 3.3V is cool too. Thanks for that.

Jambo:

If I followed the datasheet recommendations, wouldn't that make your point moot? I don't know, I'm just wondering. TI seem to have compensated for that in the REG101 because they say in the first couple paragraphs of the datasheet that an output cap is unnecessary for stability.

 

[Jambo]

Quote:

Originally Posted by joneeboi /img/forum/go_quote.gif Jambo: If I followed the datasheet recommendations, wouldn't that make your point moot? I don't know, I'm just wondering. TI seem to have compensated for that in the REG101 because they say in the first couple paragraphs of the datasheet that an output cap is unnecessary for stability.

Absolutely... But I don't read every word of datasheets and I don't know anyone who does :d

While an output cap may not be necessary for stability it still helps for load regulation and an unwise choice can *cause* instability.

 

[error401]

Try your layout only on the topside first. It might not be that hard to route since there are few signals that cross stages and their path is very linear. I think you can probably fit it all on one side.

For analog, especially if you're driving low impedance phones, I'd keep the voltage relatively low. This will make it easier to ensure that the necessary transient current is available on a small board. 9V seems a good compromise to me. So I'd boost to about 10V and use a good linear reg to get 9V. A ~4.75V linear reg for AVDD (directly from USB power) and 3.3V for DVDD.

Use REG101 for both analog supplies if you can get it. I've had a hard time finding anything with similar specs. LP2985 is about as good, but it's fixed voltage only. If you really wanted to use a 'quality' reg for the digital supply you could use one. Otherwise I'd choose the TI from the ones you linked, it's specs are a lot better and it's not much pricier. Plus I like TI more than Fairchild

Read the datasheets carefully. Jambo is right, LDO regs can be finicky, so make sure you're loading them properly. If they recommend particular part numbers for capacitors, use those parts.

Since the Wolfson is spec'd only at 5V, I'd hazard a guess that performance does degrade with AVDD at <5V. Whether it's worth wasting the current regulating your high voltage rail down to 5V is up to you. Personally I'd probably go with the 4.8V solution.

 

[00940]

Quote:

Originally Posted by joneeboi /img/forum/go_quote.gif The DC boosting would be for the amp section, though your point is well taken. I'm just assuming that as input voltage increases for the DAC, the better the efficiency and performance. Does my assumption correspond to reality? Then I could just run both supplies off of a REG101's 4.75V output, could I not? It'll run a little hotter and draw more power, but if the USB can take it, I'm down. If it doesn't improve performance in any significant way, going all the way down to 3.3V is cool too. Thanks for that.

According to the datasheet the performance indeed increases with voltage but efficiency decreases. At 5V (both AVDD and DVDD), you have a 32ma current draw. At 3.3V, a 24ma draw. It's worth noting that the digital section is accounting for most of that change. So, having a first regulator at 3.3V for the DVDD and a second at 4.75V for the AVDD seems the best compromise between efficiency and performance. And it can run directly from USB power. I thus echo error401 on this.

I don't think one should do without LDO in such a project. The 2 or 3V of headroom a conventionnal regulator needs are a luxury. Just follow the layout and part indications of the datasheet. To avoid problems with the reg101, it is often good practice to have after the regulator: a "not too low esr" electrolytic cap (like 10uF), followed by a ferrite and a ceramic cap (10-100nF) right at the supply pin of the powered device.

To layout your digital section (the whole wm8740), as well as the associated regulators, the best way to proceed on a two sided board is to have your chips sitting on a groundplane, with all (if possible) the signal traces running on the other side. It usually provides for the shortest ground loops. For the amp, scrap the groundplane and go for star ground.

Good luck

edit: and don't forget the thermal relief option on the groundplane.

 

[WindowsX]

ok guys, how could iMod get better than squeezebox, transporter or olive music station? could something like this really worths a shot? squeezebox3 is around the same price of iPod.

 

[FallenAngel]

Quote:

Originally Posted by WindowsX /img/forum/go_quote.gif ok guys, how could iMod get better than squeezebox, transporter or olive music station? could something like this really worths a shot? squeezebox3 is around the same price of iPod.

The Squeezebox uses the BB PCM1748, iPod/iMod (best case scenario - 4G iPod Photo) uses the WM8975. Just compare the specs. It looks like the Squeezebos DAC performs better than the iMod one, but that's DAC specs alone. Best tests would be to actually listen to both and see which one sounds better using the same files.

There's an idea, how about listening

Not trying to be a jerk, but way too many times people recommend things they never heard themselves that it's just a good idea to get your hands on both of the things you're trying to compare and have a listen yourself.