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Old 20th July 2019, 07:41 PM   #201
ishizeno is offline ishizeno
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Join Date: Mar 2015
Default USB Board

Here is a very crude sketch of the USB board. It's 35mm 35mm so that we could put two of them alongside the 70mm 35mm PSU board within a 1 1 brick (aka "unit brick"). It is designed around the XUF232-1024-FB374, which is currently the most powerful chip offered by XMOS (32 cores). It also includes three Si866x low power six-channel digital isolators in order to get up to 18 isolated channels. The red line on the chip outlines the boundary between the two isolated sides of the board. In fact, there will be three isolated sections: one for the XMOS chip, one for the ADAT and S/PDIF outputs, and one for the oscillators (currently shown on the right hand side).

At the top, we have 14 pins (if we stick to exactly 35mm, we'll be limited to 12), and 28 at the bottom, for a total of 42. As a point of comparison, the MCHStreamer Kit has 44 of them, while the DIYINHK has 66, but these do not include the USB input. Therefore, we'll probably need more pins, but we should have space for them on the top left and right sides of the board.

Of course, this sketch does not show any of the capacitors and resistors that we will need. That being said, it's hard to say how many of them are really necessary, because if you take a close look at the PCBs for the DIYINHK and MCHStreamer kits, you'll see that they are quite different on that front: the DIYINHK has very few capacitors and resistors, while the MCHStreamer has many of them. That being said, we're not too worried about these, because we should be able to mount most of them on the other side of the PCB.

Quite clearly, we're really pushing the envelope there, for I don't think there is any such small XMOS board on the market today, and certainly not one with 32 cores. But this is all part of the fun, so we'll give it a serious shot.

Last but not least, the BoM for this board should be well under $75.

Now on to the PSU board...
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Old 20th July 2019, 10:26 PM   #202
ishizeno is offline ishizeno
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Default PSU Board

Here is another crude sketch, this one for the PSU board. It is inspired from the OSVA AAPSU01, with an extra LT1965 for the +3.3V supply that we need for the DAC brick. That being said, we'll make sure that our PSU board could be used for many other bricks, including the ADC brick. The board is 70mm 35mm and has a 35mm 35mm section similar to the one of the AAPSU01 that is covered by the WE-SHC shield. We also included 6 pins on the input side and 14 pins on the output side. This board is probably the easiest of the three that we have to design, and I am confident that we can fit everything we need on its allocated real estate.
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Old 20th July 2019, 10:54 PM   #203
Markw4 is offline Markw4  United States
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Quote:
Originally Posted by ishizeno View Post
...with an extra LT1965 for the +3.3V supply that we need for the DAC brick...
Why regulate down to 3.3v for a whole dac brick? There may be a good reason, don't know. However, might make more sense to pre-regulate down to somewhere in the range of 5v to 8v, then do the final regulation locally where it will be needed. At the lower end of that voltage range there will be less power dissipation at secondary regulators. At the higher end of that range they may have a little better performance characteristics.
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Old 20th July 2019, 11:23 PM   #204
chris719 is offline chris719  United States
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Quote:
Originally Posted by Markw4 View Post
Agree, at least for the digital supplies. For analog supplies, I would say perform listening tests and measurements before finalizing an alteration to the eval board design. If this were a low cost project I wouldn't worry too much, but an ultra-high end part like AK4499 needs some care to enable its intended performance level. An old part like 7805 doesn't actually sound too bad when used for analog audio. Don't know so much about all the new regulators on the market, but haven't had much success so far getting the best sound out of LDOs for analog circuits. That's not for having failed to try either. At the same time, I have not done an extensive effort. There is a paper at Linear Audio you might want to study: https://linearaudio.nl/sites/lineara...s/v4%20jdw.pdf

Regarding capacitor choices, ESR may not be the only consideration. I don't have the desire to scour the eval board schematic looking for each capacitor you might ask about from the BOM. At a minimum, I would want to know where on the schematic it is used.
There is no real external difference between an LDO and a 7805. They are both series regulators, except the 7805 is a design older than I am. The 7805 is not remotely high end and does not belong in a high end product. It is low performance and has a very low bandwidth control loop.

I've read that LA article years ago, it's not very useful.

Edit: I will say, the NJM part appears to be better than an old 7805 according to the datasheet. It's adequate, I am sure. I'd use something else, but it's not terrible.

Last edited by chris719; 20th July 2019 at 11:28 PM.
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Old 21st July 2019, 06:25 AM   #205
ishizeno is offline ishizeno
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Quote:
Originally Posted by Markw4 View Post
Why regulate down to 3.3v for a whole dac brick? There may be a good reason, don't know. However, might make more sense to pre-regulate down to somewhere in the range of 5v to 8v, then do the final regulation locally where it will be needed. At the lower end of that voltage range there will be less power dissipation at secondary regulators. At the higher end of that range they may have a little better performance characteristics.
The AK4499EQ needs +3V (or +3.3V) for TVDD and the T102 LT1963AEST-1.8 producing the +1.8V DVDD. Are you suggesting that we get the required +3V from a +5V to +8V supply then regulate it down to +3V with a regulator mounted on the DAC board?

I so, this might be possible, and it would make the design of the PSU board even simpler.
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Old 21st July 2019, 08:01 AM   #206
chris719 is offline chris719  United States
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Quote:
Originally Posted by ishizeno View Post
The AK4499EQ needs +3V (or +3.3V) for TVDD and the T102 LT1963AEST-1.8 producing the +1.8V DVDD. Are you suggesting that we get the required +3V from a +5V to +8V supply then regulate it down to +3V with a regulator mounted on the DAC board?

I so, this might be possible, and it would make the design of the PSU board even simpler.
I might have missed it, but what is your input DC power supply? Is it a switcher or a linear PSU?

Both can be made to work well, but there are different concerns for each. You might want to look into adding a common-mode choke on the input of the PSU board anyway.
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Old 21st July 2019, 08:08 AM   #207
ishizeno is offline ishizeno
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Quote:
Originally Posted by chris719 View Post
I might have missed it, but what is your input DC power supply? Is it a switcher or a linear PSU?

Both can be made to work well, but there are different concerns for each. You might want to look into adding a common-mode choke on the input of the PSU board anyway.
For the PSU, I was thinking of building something very similar to the OSVA AAPSU01, with a 20V input coming from a USB-C port. Should I add something else?
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Old 21st July 2019, 08:41 AM   #208
chris719 is offline chris719  United States
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Quote:
Originally Posted by ishizeno View Post
For the PSU, I was thinking of building something very similar to the OSVA AAPSU01, with a 20V input coming from a USB-C port. Should I add something else?
That's probably fine, if you clone that or use it directly, then add your 3.3V regulator after the 5V output. If you put a USB-C port, you should make sure it's compliant with the USB PD spec as a sink (IMO anyway), which adds extra complexity. Frex is very experienced and knowledgeable, so you can learn a lot from his projects.

Not to dissuade you, but even the power supply alone is a lot of work .
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Old 21st July 2019, 08:56 AM   #209
ishizeno is offline ishizeno
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Quote:
Originally Posted by chris719 View Post
That's probably fine, if you clone that or use it directly, then add your 3.3V regulator after the 5V output. If you put a USB-C port, you should make sure it's compliant with the USB PD spec as a sink (IMO anyway), which adds extra complexity. Frex is very experienced and knowledgeable, so you can learn a lot from his projects.

Not to dissuade you, but even the power supply alone is a lot of work .
Frex seems to be really good at what he does indeed. I've already learned a ton by following his projects, and I intend to continue doing so. As far as the power supply is concerned, there is no way that I could build one from scratch myself, therefore the closer we stick to what Frex has already designed, the better, and so far, I am just planning to make some slight adjustments to the board's dimensions.
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Old 21st July 2019, 09:34 AM   #210
ishizeno is offline ishizeno
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Default Pins

We have started to put together an analysis of the input and output pins we need for the PSU, USB, and DAC boards (input only for DAC board so far). This is a very instructive exercise.

PSU
All we need are three input pins (+20V, GND, ENA). The ENA pin will be used to program or shutdown the PSU board. This is by far the simplest board from a pinout standpoint.

USB
For this one, we've taken some inspiration from the MCHStreamer and DIYINHK kits. The input section is very simple, since it is limited to USB. Because we have such severe space constraints for this board, we will probably not be able to add support for other inputs like S/PDIF.

The output section is much more sophisticated:

- 7 isolated outputs for IS
- 10 isolated outputs for PDM
- 1 isolated output for S/PDIF
- 1 IC interface
- 12 GPIOs
- +5V and +3V supplies

The 18 isolated outputs are perfect for the 3 Si866x low power six-channel digital isolators that we have included on our PCB sketch. For their parts, the 12 GPIOs are motivated by the needs of the DAC chip (more on this below). And we have yet to work on adding support for an external clock.

DAC
A quick analysis of the AK4499EQ's datasheet shows that we need:

- 12 inputs for controls
- 12 inputs for audio signals (including clocks)

The former motivate the 12 GPIOs that we are planning to provide out of the USB board. Of course, we could transmit all these control signals over a protocol like IC, but having dedicated GPIOs should make everything a lot easier.
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