8 × AK5578EN + 8 × AK4499EQ ADC/DAC Boards

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The high input voltage (+15V) is mandated by the Jung/Didden circuit, but you're right, it does not mean that we have to use it. Instead, since we are planning to have an external PSU giving us +15V, -15V, and +5V, we should start from the +5V supply line. My mistake: I got totally carried away by the schematic, and I forgot that we had this +5V supply line available to us.

In that case the NCP161 would work!

Sorry for my mistake.

Chris,

Since we have +5V coming from the PSU board, what would you replace the four VDD regulator circuits of page 57 by? A single 47μF capacitor per circuit? Is there any point in adding a +5V to +5V regulator? Sorry if that sounds like a stupid question, but I must ask it to be sure...
 
Yeah, I was talking about the digital supplies though.

I guess I'd have to step back and see why you have to go from 15V all the way down if you have some lower voltages already from the PSU board. You could go from the 5V AVDD down probably without an issue (could add a ferrite between the two if you are worried).

I assume LT3471 is a typo for getting the 1.8V though, right? That's a dual output switcher.

Yep, I see you edited and you do have a 5V to use :).

Yes, it's a typo, I meant LT1965. I've fixed the original post, sorry for that...
 
Chris,

Since we have +5V coming from the PSU board, what would you replace the four VDD regulator circuits of page 57 by? A single 47μF capacitor per circuit? Is there any point in adding a +5V to +5V regulator? Sorry if that sounds like a stupid question, but I must ask it to be sure...

Hmm, if you had something like a 6V rail that would be the ideal thing to use. 5V to 5V won't really work for a linear regulator because you need at least 0.2V drop for most of them to work well. If the next highest rail you have is +15 then yeah, I guess you have to decide if you want to regulate down from that in one or two steps.

Could you do without the regulators at all? You might be able to get away with just a ferrite and capacitor off the 5V from the PSU board for each of these supplies but I couldn't say if it would impact performance.

Unfortunately the selection of IC linear regulators that accept higher voltages is not as good as the low voltage parts. There is a ton of demand for the low voltage parts.
 
Hmm, if you had something like a 6V rail that would be the ideal thing to use. 5V to 5V won't really work for a linear regulator because you need at least 0.2V drop for most of them to work well. If the next highest rail you have is +15 then yeah, I guess you have to decide if you want to regulate down from that in one or two steps.

Could you do without the regulators at all? You might be able to get away with just a ferrite and capacitor off the 5V from the PSU board for each of these supplies but I couldn't say if it would impact performance.

Unfortunately the selection of IC linear regulators that accept higher voltages is not as good as the low voltage parts. There is a ton of demand for the low voltage parts.

Now that makes sense, and I'm starting to understand the logic of it all!

Well, if getting a regulator as close to the target of the power source is important, then everything should be regulated on the DAC board, and the PSU board should provide a +6V rail instead of a +5V one indeed.

Let me work on something like that...

Thanks a lot Chris, very much appreciated.
 
Yeah looking at datasheets, the LT1965 is a decent enough part. If you're ok with the power dissipation you could probably just use those in place of the 7805s.

I am thinking of using it on the PSU board, following what is done by the OSVA PSU. If we were to output +5.5V from there, we could use something like the NCP161 that you suggested on the DAC board. Dropping just 0.5V would dissipate very little heat, and a 47μ capacitor should be plenty enough.
 
2mm Hirose Connectors

In order to save some space on the boards, we will use 2mm pitch connectors instead of the more common 2.54mm version. For the DAC board, we will use the A3C-16P-2DSA(30) male connector mounted on the underside of the board, and for the USB board we will use its A3C-16DA-2DSC(71) female counterpart. This will ensure that we have enough room to fit a 4.3mm wide 2917 capacitor in between the pins of the female connector and the AK4499EQ chip on the DAC board. And it will save an extra 5mm × 4mm area on the longitudinal side, without any major degradation of the assembly's structural integrity.
 
For now, my external power supplies will be +-15, and +8v. I will drop the +8 down to +5, +3.3 with a small regulator board attached to the eval board. That will be used to take the main digital loads off the +-15v eval board rails. Preliminary experiments showed that should clean up the audio output a bit more. Also, after testing 4 modular USB to I2S solutions, I found two that seem to work well enough. Both use NDK SDA series clocks.

First job, as I see it, is to get this thing sounding as good as I can. That way I have a point of reference for any subsequent work.

A muting circuit will have to be developed for the analog outputs of the dac to eliminate occasional pop noises produced in DSD mode when certain switching events take place. This has to work without adversely affecting sound quality when playing music.

Also, I am learning some things about programming the dac's registers, shall we say, what AKM calls 'good etiquette.' Still lots of work to do before thinking about how to reduce the dac functions to a smaller footprint.

In addition, I agree with Chris to get something working then improve upon it. Its not research exactly, but it is incremental design. I think it is necessary for high end audio. Not so much needed for simpler types of things.
 
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For now, my external power supplies will be +-15, and +8v. I will drop the +8 down to +5, +3.3 with a small regulator board attached to the eval board. That will be used to take the main digital loads off the +-15v eval board rails. Preliminary experiments showed that should clean up the audio output a bit more. Also, after testing 4 modular USB to I2S solutions, I found two that seem to work well enough. Both use NDK SDA series clocks.

First job, as I see it, is to get this thing sounding as good as I can. That way I have a point of reference for any subsequent work.

A muting circuit will have to be developed for the analog outputs of the dac to eliminate occasional pop noises produced in DSD mode when certain switching events take place. This has to work without adversely affecting sound quality when playing music.

Also, I am learning some things about programming the dac's registers, shall we say, what AKM calls 'good etiquette.' Still lots of work to do before thinking about how to reduce the dac functions to a smaller footprint.

In addition, I agree with Chris to get something working then improve upon it. Its not research exactly, but it is incremental design. I think it is necessary for high end audio. Not so much needed for simpler types of things.

I like your approach, and I can't wait to see your board. I hope you'll keep sharing as much as possible on this forum/
 
Also, after testing 4 modular USB to I2S solutions, I found two that seem to work well enough. Both use NDK SDA series clocks.

Can you recommend one or two? A lot can be learned by simply looking at boards, and I will need a good one to establish my own benchmarking configuration. For info, all my tests will be made on a pair of Focal Twin6 Be monitors. Later this year, I will add an Audeze LCD-4 headphone, as soon as I find a suitable amplifier.

Thanks!
 
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DAC PCB Layout

After a couple of days of work, here is a first PCB layout for the 70mm × 35mm DAC board. It was designed as a proof-of-concept using Adobe Illustrator, simply because it allows us to focus exclusively on component placement. Obviously, things might change once routing is designed in KiCad, but the overall design isn't expected to evolve much from now on. The only question that still needs to be answered is wether we can stick to a 4-layer routing, or whether we need to upgrade to a 10-layer design (there isn't much point doing anything in between). We will try with a simple 4-layer design, and switch to 10 layers only if we can't find any reasonable way to make it work.

Obviously, this is a very high density board, and we might have to move some components to the board's underside. Nevertheless, we've managed to cram pretty much everything on the top side, at the exception of all diodes and transistors, plus eight 180pF capacitors, and a single resistor (similar to what is done for the AKM evaluation board, at the exception of the diodes). This strategy will ensure that all the underside components have a low profile, and that we have plenty of underside room to expand the design.

The board's layout is highly symmetrical across two dimensions, reflecting the fact that the AK4499EQ is a four-channel DAC chip. Therefore, we've updated our BoM to take this underlying structure into account, grouping most components into quadruplets. This method allowed us to catch and fix many bugs in our design, and we intend to continue using it all along.

The top quarter of the board is dedicated to seven power supply regulators (L1, R1, L2, R2, +5V, +3.3V, +1.8V), while the remaining part is centered around the AK4499EQ DAC chip. The five connectors are placed as follows:

- North-East: Power inputs (connected to underlying PSU board)
- North: L2 + R2 balanced outputs (connected to overlying XLR board)
- South: L1 + R1 balanced outputs (connected to overlying XLR board)
- West: Control inputs (connected to underlying USB board)
- East: Audio inputs (connected to underlying USB board)

For the time being, we're assuming that the PSU board will provide +15V, -15V, and +5.5V. That being said, we've made sure to use the SOT23-5 version of the NCP161 LDO regulator so that we have enough room to switch to another regulator that could support a higher input voltage. For example, Markw4 is playing with +8V, and we'll test different options once we receive our evaluation board.

Next: moving all this to KiCad...

Note: A PDF version of the design has been attached to this post.
 

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The top quarter of the board is dedicated to seven power supply regulators (L1, R1, L2, R2, +5V, +3.3V, +1.8V), while the remaining part is centered around the AK4499EQ DAC chip.

The top quarter of the board is much less dense than the remaining three quarters, and I am wondering how we could put the empty space to good use. One idea is some debugging LEDs. But instead of just a couple of them, how about a 16-segment VU meter for each of the four channels? Since we have an I²C input, we might be able to implement each VU meter with a single chip and 16 LEDs. This would be totally gratuitous of course, since these VU meters will be masked by the brick's cover, but OTO is 20% engineering and 80% art...

This sounds like a cool side project...
 
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How about leaving some space to separate components a bit where capacitive coupling between adjacent devices could use a little reducing?

Happy to do that, but I'm not sure where this will make a real difference.

I still need to read the engineering book you recommended. I'm just back from 5 weeks on the road, so I should be able to dive into it as early as next week.

Also, we have a ton of space available on the board's underside. Therefore, for the signals that are not badly affected by vias, this is definitely an option that we should consider if we need to give our components some breathing room.
 
Happy to do that, but I'm not sure where this will make a real difference.

I still need to read the engineering book you recommended. I'm just back from 5 weeks on the road, so I should be able to dive into it as early as next week.

Best to keep the smallest decoupling caps as close to device pins as you can possibly get them. Some good advice in the book you mentioned. Rearranging parts a little to accomplish that might also reduce packing efficiency. Study first, design later may be better than the other way around.
 
Best to keep the smallest decoupling caps as close to device pins as you can possibly get them. Some good advice in the book you mentioned. Rearranging parts a little to accomplish that might also reduce packing efficiency. Study first, design later may be better than the other way around.

That's what I tried to do on this design, but I certainly could use some additional study time. :worship:

Also, it should be relatively easy to add additional decoupling caps on the underside of the board, especially in relation to the seven regulators that are located at the top. And for the L1, R1, L2, and R2 regulator circuits, we also have the option of moving them to the board's underside. But if we can achieve similar results by just adding underside decoupling caps, I'd rather keep the board's underside as sparse and low-profile as possible, because it will face the underlying PSU and USB boards, which will need some vertical space in order to accommodate the use of WE-SHC shields.
 
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