DIY iPod digital out

[error401]

Quote:

Originally Posted by joneeboi /img/forum/go_quote.gif Now, I'm not a design expert of any kind just yet, but perhaps we can design our own portable, full on DAC/amp solution using some of Wolfson's better DACs. I realize there are already solutions available along these lines, but none fitting the I2S input requirement. stevenkelby, you mentioned using the Headamp Pico for its I2S input, but that DAC/amp converts USB to I2s. Unless we open it up and route some wires, we can't do it without voiding the warranty. I'm not sure how we'd go about doing this, but is it a viable solution having a portable DAC/amp running off a 9V? That's quite a huge energy loss converting down to 3.3V, so maybe running off some AAs would be wiser. Before we even go there, we should probably pick out a DAC that'll work. I'm thinking something along the lines of a portable full-on DAC, like a portable TPA Opus DAC (okay, maybe not that far). I'm looking at some Wolfson DACs over at Mouser, and a couple of the models jump out immediately. They all seem to be designed for DVD players or digital set-top boxes, but if somehow we can plow through with the design, I think the head-fi community would be better for it. I'm no Alf, cetoole, Jambo, justin w., Ray Samuels, etc., but I can offer my limited knowledge and unbridled passion and enthusiasm to this kind of project. Please, someone with actual knowledge help me out here. I've had a class in digital logic design, but I'm not sure how much of it actually applies here. Some of the DACs that I think would work here. http://www.wolfsonmicro.com/uploads/.../en/WM8761.pdf http://www.wolfsonmicro.com/uploads/.../en/WM8521.pdf http://www.wolfsonmicro.com/uploads/.../en/WM8956.pdf http://www.wolfsonmicro.com/uploads/...728_Rev4.4.pdf http://www.wolfsonmicro.com/uploads/.../en/WM8761.pdf

I too would choose WM8740. It uses about 24mA at 3.3V, which isn't too bad if you're using AA cells (typically 2500mAh+), whatever amp topology you choose, it will likely use a lot more. Choose opamps carefully and you should get good battery life (I'd guess about 16h is possible).

It's probably possible to fit a WM8740 DAC and (SMD) opamp-based amp into the smallest Hammond with two AAs, if you're very careful with your layout. I don't think you'd manage cramming a charger in there too though. If you choose a larger case, it should be easy.

Be warned that I might get bored and do a layout for you

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[joneeboi]

Actually, error, I thought of listing you in that "group of people that I am not," but I already submitted the post.

How did I miss the WM8741? It's listed in Mouser under "Stereo DACs, High End." *rolls eyes*

Part of the rub becomes whether or not we should include the amp, much like what justin w. is doing with his Pico. The initial amp that comes to mind is the PPAS. In fact, they're so small we could probably fit two for ultimate in balanced configuration. I'll bookmark the WM8741 and read endlessly. I'll also try to grab one of the professors at school and see if they can guide me a little.

How does USB power sound? I want to regulate it, but it would have to be cut down a bit. I vaguely recall a similar problem in the Alien DAC. But fear not, even in the tiny Hammond 1445C801(BK) we can fit some sort of DC-DC converter.

Power supply solutions. The Fairchild Semi seems quasi-viable as it can supply up to Vin. I wonder what Team Alien DAC pored over before deciding on the REG101s.
http://focus.ti.com/lit/ds/symlink/tl2575-05.pdf
http://focus.ti.com/lit/ds/symlink/dcr011203.pdf
http://www.fairchildsemi.com/ds/FA/FAN6520A.pdf

Brain...power...leaving...

 

[error401]

Quote:

Originally Posted by joneeboi /img/forum/go_quote.gif Part of the rub becomes whether or not we should include the amp, much like what justin w. is doing with his Pico. The initial amp that comes to mind is the PPAS. In fact, they're so small we could probably fit two for ultimate in balanced configuration. I'll bookmark the WM8741 and read endlessly. I'll also try to grab one of the professors at school and see if they can guide me a little.

I think you should be able to fit a decent amp in a small case without making any major compromises. It's your project, but I don't see a reason to exclude it. The other thing is that you need either a) output caps or b) 4 amp channels (balanced) - if you include the amp, you can do a balanced solution, otherwise you probably need to include the caps since not many portable amps are balanced.

Quote:

How does USB power sound? I want to regulate it, but it would have to be cut down a bit. I vaguely recall a similar problem in the Alien DAC. But fear not, even in the tiny Hammond 1445C801(BK) we can fit some sort of DC-DC converter. Power supply solutions. The Fairchild Semi seems quasi-viable as it can supply up to Vin. I wonder what Team Alien DAC pored over before deciding on the REG101s. http://focus.ti.com/lit/ds/symlink/tl2575-05.pdf http://focus.ti.com/lit/ds/symlink/dcr011203.pdf http://www.fairchildsemi.com/ds/FA/FAN6520A.pdf

So you're planning on stealing power from the iPod, or are you talking about having it plugged to a computer all the time? I think you should include batteries for your DAC/amp myself, the iPod batteries are small and you'll drain the whole shebang quickly. A couple hours of use doesn't seem enough for anyone, and then you can't even use it unamped!

You'll definitely need a DC-DC of some kind or you'll have no overhead whatsoever to drive your cans - or you have to use small and inappropriate 9Vs. I'd say 2AAs is about optimal - battery life should be similar to an iPod and it's small enough. You'll need quite a few regulators for an 'audiophile' approach though, I count a minimum of 4 (step up, linear for amp, linear for avdd, linear for dvdd), 5 would be better for battery life (extra dc-dc to 5.5V or so for the low voltage lines). Check out how dsavitsk did his HPDAC supply, you could probably poach most of that design, just respec it for 3V input.

I'm really not sure why the AlienDAC chose the REG101 in SO8 package. The regulator itself is quite impressive; few match its specifications - but very few low noise regs are made in SO8 package so you cant' substitute anything! Plus it's a larger package than the alternative SOT23 that has lots of similar regulators with the same pinout. Neither has great availability right now though, so I wouldn't recommend them for a new project since TI can't seem to keep them on the shelves. There are tons of good switching chips out there, but I'd probably use the TPS61040 dsavitsk chose. I would probably use LP2985s for the Vdds. Not sure what would be good for the amp, would have to do some looking.

That Fairchild device is a switching step-down converter - not sure why you'd need one of those - you should probably be using linear regulators for all your final regulation, and you need a boost converter from the batteries. Everything you listed is also physically rather large. Look for regulators in SOT23 or SO8 - you don't need to handle a lot of power dissipation, and you need as much space as you can get. Since you've got the DAC to deal with, no point in trying to do a non-SMD board or anything

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[joneeboi]

After doing a bit of looking around, TPS61040 looks good for DC boosting, 3.3V regulation can be done by the in-stock-at-Digikey Diodes Inc. AP131, with the LM7805 covering the 5V bit. I'm thinking of powering the DAC with USB power, and if possible we can stuff the thing in the Hammond 1445C801. That way it would operate like the Alien DAC, where we could stack the iPod on the DAC. Further, unlike the HPDAC, we could choose our own amplifiers. Even with this theoretical design, we could make a portable and full-on version. But before we go on, I need to do some more learning.

=T

 

[00940]

We can probably explain the reg101 thing as an heritage of the "Guzzler USB DAC" on which the alien dac is originally based. I'm pretty much responsible for it

When we (Guzzler and I) designed a battery powered USB dac in February 2004 (!), the reg101 was readily available from digikey. When I started the layout, I was a real newbie when it came to smd parts. The reg101 was available, easy to solder and probably the "best" integrated LDO regulator out there. Why not pick it ?

When Guzzler later designed a small usb-to-spdif converter, the reg101 was considered too expensive and was also getting harder to source. He then switched to the TI TPS793* ldo regulators. Those are still available by the thousands at digikey. You need to get the adjustable version to go up to 5V though.

The so8 reg101 was still the regulator we picked for our tentative "all-out" transportable USB dac with Ble0t.

 

[dsavitsk]

Quote:

Originally Posted by error401 /img/forum/go_quote.gif Check out how dsavitsk did his HPDAC supply, you could probably poach most of that design, just respec it for 3V input.

I believe that it is happy with input DC as low as 1.8V -- look at the datasheet though.

Quote:

There are tons of good switching chips out there, but I'd probably use the TPS61040 dsavitsk chose.

I don't know that it is anything special, other than being cheap and easy to find. I does need that ferrite after it as otherwise it is pretty noisy. But, I found it by calling TI. If you get the right person on the phone, they are pretty helpful.


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Originally Posted by joneeboi /img/forum/go_quote.gif Further, unlike the HPDAC, we could choose our own amplifiers.

Why couldn't you do that there? You were welcome to choose anything you wanted, it is just that most of those choices, other than the LM6172, were poor ones.

Quote:

Originally Posted by 00940 /img/forum/go_quote.gif The so8 reg101 was still the regulator we picked for our tentative "all-out" transportable USB dac with Ble0t.

What ever became of that?

 

[FS2]

How about volume control? The I2S data isn't affected by the volume setting from the scrollwheel. It'll either be a matter of using the DACs on-board volume control using a microcontroller, or doing it on the analog output. Unless there are DACs that allow easy hardware control of the volume settings.

Powerwise, I was looking at an inverting switcher to generate a negative voltage for the op-amp. Any advantage to simply using a rail splitter and a higher boosted positive rail?

And a general question for you guys who have better ears than I do: Is there any difference in the "stock" iPod's sound when charging it?

 

[error401]

Quote:

Originally Posted by FS2 /img/forum/go_quote.gif Powerwise, I was looking at an inverting switcher to generate a negative voltage for the op-amp. Any advantage to simply using a rail splitter and a higher boosted positive rail?

I don't think that's necessary if you do a balanced configuration. There will be DC offset on each output node, but the net offset will always be 0V (or very close to it). So since the DAC operates from 0-5V, your opamps can be single ended as well.

 

[FS2]

I thought it was... If, say, L+ and L- both have the same DC offset, and one is applied to the non-inverting op-amp terminal and the other to the inverting, the DC offset cancels out. Unless I've missed something...

 

[error401]

Quote:

Originally Posted by FS2 /img/forum/go_quote.gif I thought it was... If, say, L+ and L- both have the same DC offset, and one is applied to the non-inverting op-amp terminal and the other to the inverting, the DC offset cancels out. Unless I've missed something...

I'm talking about going balanced all the way to the headphones. In essence you have two amplified lines that are 180 deg out of phase for each side, instead of a single one and ground. The DAC itself provides these inverted outputs, and you simply need to use four opamps instead of two for your output stage.

To make things a bit more clear:

The DAC can only output a voltage between 0-AVdd. Since this single ended signal needs to represent an audio signal, it needs to be biased to exactly 0.5AVdd, which will be the output voltage of both the + and - outputs when outputting silence. If you follow both + and - with a non-inverting amplifier, they will both output Gain x 0.5AVdd - exactly the same voltage. Each output goes to the + or - input of a headphone driver. Since there's no ground reference fed to the headphones, the ground-referenced voltage is meaningless at the headphones - only the difference matters. They see 0V since both + and - are exactly equal.

Problem is that normal headphone connectors don't allow this (it requires 4 wires instead of the usual 3), and the convention is to use XLR connectors which are rather massive. You could rewire your headphones yourself and use a less insane connector like a mini-DIN or 4-pin 3.5mm.

Alternative options are to only use DC blocking caps with only one DAC output, or a proper balanced to SE converter (conventional designs usually need 3 opamps/channel). Or a DC servo I guess, but the converter usually makes more sense.

 

[FS2]

Right, I'd thought that you were outputing to a common 3 terminal headphone, not going balanced all the way.

Been going over the design I have, trying to flesh it out. It'll need a DAC, micro for volume control, 6 op-amps for the output, a +/- voltage for the op-amps and a lower +ve voltage for the DAC.

 

[joneeboi]

I just ran through the Opus DAC thread, and I like the jumpering idea. My great idea is to use tiny switches instead, but progress is super slow. I was looking at the "Recommended External Components" section in the WM8740 datasheet and I wonder if we could open a discussion on the output stage in between the chip and the amplifier. Trying to fit this bad boy into the 1455C8 really limits coupling caps, as film and tants I've seen are usually well over 100V. If we slip the DAC board in the bottom slot, we can fit those giant caps lengthwise above the board. That still limits cap choices, but it still accommodates most popular boutique caps. We could also leave the option of opamp balanced-to-SE DC-eliminating output onboard too. Was there another option? DC servo, IIRC. I actually don't understand what that means, so maybe you guys can enlighten me.

 

[error401]

Quote:

Originally Posted by joneeboi /img/forum/go_quote.gif I just ran through the Opus DAC thread, and I like the jumpering idea. My great idea is to use tiny switches instead, but progress is super slow. I was looking at the "Recommended External Components" section in the WM8740 datasheet and I wonder if we could open a discussion on the output stage in between the chip and the amplifier. Trying to fit this bad boy into the 1455C8 really limits coupling caps, as film and tants I've seen are usually well over 100V. If we slip the DAC board in the bottom slot, we can fit those giant caps lengthwise above the board. That still limits cap choices, but it still accommodates most popular boutique caps. We could also leave the option of opamp balanced-to-SE DC-eliminating output onboard too. Was there another option? DC servo, IIRC. I actually don't understand what that means, so maybe you guys can enlighten me.

A DC servo is basically a circuit connected with a very slow low-pass filter. It ignores all the audio data and essentially only reacts to DC. It is configured to 'want' 0VDC into the output. Properly connected so it can control the output circuit, it will bias the output to 0V, thanks to the slow low pass, even if there is an intrinsic offset. If you wanted to use one, I believe you'd need two opamps: the servo and an output stage after the DAC. Since you can't inject feedback into the DAC, I don't believe you could connect a servo directly to the output (though I might be wrong...).

If you do bal->SE with a separate output stage, you need 6 opamps for the bal->SE and another 2 (or some other type of amp) for output. If you can fit it, that's what I would do. Use low-Iq opamps as much as possible and you can probably keep the Iq fairly low. It's going to hurt battery life though. You'll also need a negative-side DC-DC as well I believe, if you want to avoid coupling caps. You *might* get away without one by creating a buffered 1/2Vpos and using it as ground for the DAC IC. Not sure if it'd work though, I'd have to think about it more than I have.

Using one side of the DAC and amplifying it in the conventional manner only requires 2 opamps, so uses less space and power. If you have a battery life budget you might be able to use 'nicer' opamps as a result. But you need coupling caps, and boutiques aren't going to fit. If you're one of those that believe that boutique caps make a difference, that might be a showstopper.

 

[joneeboi]

Thanks for the help. It sounds like we might have to copy Twisted Pear more than I'd prefer. IIRC, they have the option for all three, right? I'm thinking of regulating USB and forgoing the whole battery mess.

With the boutique caps issue, I understand you can block DC with any appropriately spec'ed cap; the boutique bit would only be useful for preventing the use of poorer spec caps. Their sound difference is doubtless overemphasized and almost deified/idolized in some sense, and I can understand wanting to shy away from them. Nevertheless, I'd still argue they make a difference when placed directly in the signal path just like switching in lower-quality parts, audible or not. Is there something I'm missing? I'm sure the layout can be implemented such that their inclusion doesn't interfere with conventional non-boutique options.

I merely suggested the boutique option for those who want a simpler output stage as opposed to using six (expensive?) surface mount opamps. One lead to the positive leg, other lead to output jack. Six opamps in that casing along with switches/jumpers for hardware control on that tiny board is going to be tricky for this EAGLE-newbie.

 

[error401]

Quote:

Originally Posted by joneeboi /img/forum/go_quote.gif Thanks for the help. It sounds like we might have to copy Twisted Pear more than I'd prefer. IIRC, they have the option for all three, right? I'm thinking of regulating USB and forgoing the whole battery mess.

That makes things easier. You should still watch your current consumption though, the iPod will suck some and really you shouldn't draw more than 500mA from USB. Since you'll need a boost converter, 50mA on a dual-opamp probably converts to 100mA or more on the USB supply.

If you're willing to make this non-portable (by depending on USB), why not use a higher voltage external supply. Then you don't need the onboard switcher and don't have to worry so much about current.

Quote:

With the boutique caps issue, I understand you can block DC with any appropriately spec'ed cap; the boutique bit would only be useful for preventing the use of poorer spec caps. Their sound difference is doubtless overemphasized and almost deified/idolized in some sense, and I can understand wanting to shy away from them. Nevertheless, I'd still argue they make a difference when placed directly in the signal path just like switching in lower-quality parts, audible or not. Is there something I'm missing? I'm sure the layout can be implemented such that their inclusion doesn't interfere with conventional non-boutique options.

Well, no reason to say 'NO BOUTIQUE CAPS ALLOWED', but it's not going to be easy to do a layout to accommodate them, since they're so long. I think you're pretty much going to have to put pads for a small poly film cap and if you want to use boutiques, you'll need to airwire them. Twisted pear uses OSCONs for output; not exactly boutique

.

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I merely suggested the boutique option for those who want a simpler output stage as opposed to using six (expensive?) surface mount opamps. One lead to the positive leg, other lead to output jack. Six opamps in that casing along with switches/jumpers for hardware control on that tiny board is going to be tricky for this EAGLE-newbie.

Good opamps don't have to be expensive. OPA134 is fairly cheap, and sounds good. There are lots of others out there that aren't prohibitively expensive, but sound good. And you don't need 6 parts, you need 6 amps. You could use a quad amp in SO-14 and a dual amp in SO-8 for all the amps you need. Just two packages to route

. If you go this route you'll probably still need a separate output stage though, so volume control can be done.

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.