[Alf]

Quote:

Originally Posted by doobooloo I think this is more like, transportable or just "small enough on a desk" concept... no?

Agree. You need USB for it and therefore a laptop or a computer.

Quote:

Originally Posted by doobooloo I honestly think someone should start an integrated usb dac + amp project soon.

We need to get the DAC part right first. Then we will do an integrated revision with an amp. It should be easy comparing to what we are going through now.

The integrated revision is not really required. Right now there is still some space left even if you put the DAC in 1455C802. It should be enough add a simple amp on a separate board. You can build an integrated solution with an amp of your choice in a small enclosure.

 

[Alf]

Quote:

Originally Posted by Sinbios One thing, I think the original DAC wasn't bit-perfect -- is it possible to make this version bit-perfect?

Sorry, missed your question.

The DAC is capable of "bit-perfection". The problem is that the standard Windows USB driver resamples the audio stream to 48kHz. This is a limitation of the driver, not the DAC. There is a commercial USB driver that solves this problem but you have to pay for it. I don't have a link for it right now. Ask in the computer source forum.

 

[jhawk22]

edit: will keep eyes out for a gb thread

 

[MASantos]

Quote:

Originally Posted by Alf The size of Rev A board is 1922 sq mm. The size of the new board is 2725 sq mm. The component areas are about the same size. The increase is caused by additional mounting holes and empty areas at the top and the bottom. The board is designed to fit the smallest Hammond and the empty areas are required to slide it in. It is still very portable and will be much easier to mount. Now we have more components on the board and will have even more than that soon (RCL filters). However we will change more components to SMD and the final board may be even shorter than it is now. The pads are 2.15mm in diameter and the holes are 1mm.

I need new lenses in my glasses!!!!

 

[Alf]

Quote:

Originally Posted by Francis_Vaughan The option might be available to create an external clock - the 2702 will accept one. Then build a low jitter clock - for instance Elso Kwak's designs. They are not large. Once that was done one could think about a different DC-DC converter - one that can take an external sync, and then run the converter at a divided down DAC clock rate, avoiding the possibility of heterodyned image products appearing. And so it goes.

Honestly, I think it is overkill for this project. This will push the price up while giving not so much value. Also the board will become too big. The target enclosure is the smallest Hammond. And an amp should fit in it too!

Why don’t we keep this idea for a high(er)-end DAC design? The size and the price will not be a problem then! A lot of people are asking for this too.

Quote:

Originally Posted by Francis_Vaughan I think where I was trying to get to with the layout ideas is to look at the IC not as a monolithic thing, but as a system of connected units. The designers give you this ability since they bring out all the power pins. So if you refer to the block diagram in the data sheet it becomes clear what the goals are. Treat each of these units separately, give each its own well bypassed power so they can't interfere with one another, but also be cogniscent of the currents that must flow between the units, and thus the consequent return currents. Return currents are trivially dealt with with the solid ground-plane. It is hard to do better. So we worry about power. The digital power is in two places, clock and main digital logic. Each should be separately fed, and in this case this really means adding an appropriate ferrite bead isolator before each of the bypass capacitors. Thus split the pad C18 and C19 attach to, and feed each of the splits with an SMD ferrite bead. Something similar might be appropriate for the analog side, although the lower frequencies involved might suggest some R in series with a bead as well, and perhaps more C. You are building a mini LRC power supply filter for each unit - one designed to keep the crud floating about the board out of each unit. Since the units each have different functions we can design the filter to match the unit.

Despite my complex relationships with beads and inductors, I think this is a good idea. Definitely it is worth a try. If things go terribly wrong and the beads cause problems, we can easily fall back by jumpering the beads and corresponding resistors.

I updated the schematic and added new components as per your post. I plan to use Wurth 1206 package ferrite beads. Anything I missed there?

Schematic v3


Francis, I leave the joy of calculating the values to you if you don’t mind.

 

[Sinbios]

Quote:

Originally Posted by Alf Sorry, missed your question. The DAC is capable of "bit-perfection". The problem is that the standard Windows USB driver resamples the audio stream to 48kHz. This is a limitation of the driver, not the DAC. There is a commercial USB driver that solves this problem but you have to pay for it. I don't have a link for it right now. Ask in the computer source forum.

Aha, that removes all my reserves about this project. Sign me up for beta testing once it's in that stage or something

 

[Botch]

Quote:

Originally Posted by Alf Honestly, I think it is overkill for this project. This will push the price up while giving not so much value. Also the board will become too big. The target enclosure is the smallest Hammond. And an amp should fit in it too! Why don’t we keep this idea for a high(er)-end DAC design? The size and the price will not be a problem then! A lot of people are asking for this too.

Agree with you Alf. I've read that the SpAct is rather good anyway and let's try to keep it simple.

Quote:

Originally Posted by Alf Despite my complex relationships with beads and inductors, I think this is a good idea. Definitely it is worth a try. If things go terribly wrong and the beads cause problems, we can easily fall back by jumpering the beads and corresponding resistors.

Nice, schematic is looking good and I think this will bring a win for us. Just make sure the caps are as close to the ground pins as possilbe on the layout.

Francis, nice post above.

Looking forward to the next update Alf.

Botch...

 

[00940]

Some times ago, someone suggested to take the inputs of both regulators from the V+ rather than derivating the digital supply from the analog supply.

As far as an external clock is concerned, here's what I was toldhere :

Quote:

No point to do it. You 'll not improve nothing. SPACT system has own VCO, which determine phase noise level of the output clock. XTAL clock only provide functionality of SPACT system, but it is not reference signal of SPACT. Phase random walks, which depend by jitter of XTAL clock, has enough high frequency spectrum to be eliminated by analog PLL, which followed after SPACT.

 

[Francis_Vaughan]

I totally agree. The quip about an external clock and bits was really to put in context what is trying to be achieved here. One can always argue that no matter how good the internal PLL is, it can never totally remove all jitter, and any improvement on an upstream clock will have some effect. But it is very likey to to be as close to a waste of time as one could get. I'm certainly not seriously suggesting it, and indeed would actually argue against it. (One could - and probably should - also argue that unless one actually tries the experiment no-one actually knows.)

I'm not sure why the particular choice of Wurth ferrites, although they do rather nicely provide Eagle files for download.

 

[Alf]

Quote:

Originally Posted by Francis_Vaughan I'm not sure why the particular choice of Wurth ferrites, although they do rather nicely provide Eagle files for download.

That's why.

In fact, I have no preference.

 

[Alf]

I replaced MAX1722 with TPS6734 and LM317A. Both ICs are cheap and easy to get. This gives us 9V from USB to power an amp. It will not be as good as TREAD but should do for portable use.

Schematic v4

 

[cetoole]

Alf, looks good, but any reason you chose to have 9v final? I think it would probably be a better choice to have about 12v after regulation, so maybe use a 15v DC-DC converter, and possibly an LDO regulator instead of the LM317. There are a couple 5v to +/-5v or so DC-DC converters available, which would probably take care of our issue completely right? Only problem would be the voltage being a little low on each rail.

 

[diablo9]

just curious, I heard Headroom Micro DAC use Cirrus CS4398, supposely a flagship part. Is there any reason not to use that?

 

[Nisbeth]

Quote:

Originally Posted by cetoole Alf, looks good, but any reason you chose to have 9v final?

Max. input voltage for the REG102xx is 10V

diablo9: The CS4398 is "only" a DAC, it doesn't have a USB-interface.


/U.

 

[diablo9]

what about giving BB PCM2902, which feed to CS chip? just like what they did on MicroDAC,
nevermind, that's becoming a clone. against the policy...