I'm not sure. All I can think of is that they did not feel the need to expose the DAC's chip dual interface through the evaluation board's external interface. I asked a question on an earlier post about that: why do they have a dual DSD interface on the chip (43..47 and 50..54)? Do you know why they did that? And did you have any particular idea in mind when asking the question? Any pedagogic intent on your part that I should be aware of?
In order to better understand where all these pins come from, you can look at the list of relevant pins on the AK4499EQ chip on this sheet. On column D, I have identified and named the pins that we should expose on our pair of ERM8 connectors. These plus the power circuits give us the list of circuits that need to be provided on these connectors, some of them being replicated. From there, we get the pinouts for the two ERM8 connectors.
In developing this pinout, my primary goal was to ensure that every single relevant interface offered by the DAC chip would be properly exposed through the ERM8 connectors to the Interface Board. That way, we can make changes to the Interface Board without having to make any changes to the DAC Board. This is important, because the Interface Board will be dramatically simpler than the DAC Board, and won't have very expensive components like the AK4499EQ. The µModules on the Interface Board are just $10 a piece. Of course, we need five of them, but still... And I am worried about availability of the AK4499EQ (none are available at DigiKey currently).
Also, the evaluation board manual has a typo: PORT100 is 12-pin, not 10-pin.
By the way, are you comfortable posting copies of the evaluation board's manual? So far, I've refrained myself from doing so, but if there is a legitimate reason for posting such extracts, this might be convenient sometimes.
Through the PSU board. The "PSU board" is a bit of a misnomer. Maybe it should be called "Interface board", because it will provide the interface to the OTOBUS™, for both signals and power. Three pins one the OTOBUS™ will be dedicated to propagating the three clock selection circuits on this particular interface board (not on the standard OTOBUS™ of course).
The stack is as follows:
- Plate Board (the one hosting the MCU) with OTOBUS™ sockets
- Interface Board (the one with the boosters) with OTOBUS™ headers
- DAC Board (the one we've been working on so far)
Clearly, the Interface Board is designed specifically for the DAC Board. In other words, the Interface Board won't be used for any other brick, at the exception maybe of the ADC Brick, which will have an ADC Board not too dissimilar from the DAC Board, but I have no idea whether the ADC Board and the DAC Board could share the same interface board. This would be great, but I have not studied the AK5578EN long enough to know whether this is possible or not. I will probably dive into its datasheet before making any prototype for the DAC Board though, just in case...
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It may be a legacy compatibility thing. For instance, IIRC, AK4137 only has separate PCM and DSD input pins. Whatever the reason for that in the past, no need for it now...unless perhaps if DSD and PCM are to come from two different sources and separate input pins could provide an easy way to switch between the two. In this case with one USB board for both digital audio formats, the extra pins aren't of much use.
Although there is no NDA for the eval board manual, AKM has apparently seen fit not to release it for download. Maybe they worry some people might criticize the Jung regulators and so forth and not bother to listen first to find out what it sounds like. It can be tempting to start trying to improve what is seen on paper rather that what is heard in air.
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Well, the 60-pin ERM8 connectors give us enough pins to expose both interfaces, so I'd better be safe than sorry. We could go with a 40-pin connector, but it would not save us much room on the board, and it would give us less ground return circuits on the North side. Of course, the North side is much less critical than the South side as far as digital circuits are concerned, but still, I'd rather err on the side of caution.
Well, I'm making the Jung regulators pluggable (with the Super Regulator board interface), so anyone will be able to play with different options. It is very likely that I will design both options and make them both available in the free kits: Jung regulators using the AD817, and Super Regulators using the AD825. That way, we'll be able to listen to both.
I wish we could do the same for the OPA1612, but you strongly advised against it, and I don't think we'll be able to find much better operational amplifiers for this application anyway. That being said, I might provide a way to by-pass them entirely. We have enough room on the board to provide a set of four connectors on which an OpAmp board would be mounted, and this board would replace the XLR board (it would include both the alternative OpAmps and the XLR connectors). That way, we could provide the modularity that we want, but without taking any chances with the original design.
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