by the way, i'll post a teaser pic in the next couple of days of where i'm at with the new 8 channel board. It's starting to shape up. No XMOS schematic as yet, still working on the analogue stages and elements of the onboard power supplies. I've been very busy all week at a new position, so only just opened the layout again tonight. Squeezing a high performance 8 channel balanced dac, 2 clocks (NDK) and most power supplies i(all local regulation) into a properly useful, compact form factor (100 x 100mm, with a separate prereg board underneath to supply raw voltages) has been a fun project. the way it is being designed, it could totally be leveraged into an Rockpi, should a software solution for mulichannel i2s be forthcoming and the RPI and xmos could be removed and used in another project. I will not be talking a great deal of detail about parts selection, until the real board is built and tested. I would rather not wake up to find the idea copied somewhere before i've built it and the particular design is not up for discussion. the parts selection and layout are unlike most we see in audio frequency designs (although they are well specified for audio), although i'm not inventing the wheel. I expect FR to be flat out to ~90->100khz (and only then because thats where i'm putting the LPF, the parts are capable of low distortion up into the many MHz range, so careful layout is required)
I think we need to talk a bit more about (or at least confirm details for) control and what people are expecting of the board for control of the dac parameters. being it will be interfaced with a very capable digital volume control in the form of camilla, it may not need much, but real-world tactile control for master volume should be an option, even if its simply interfacing back to moOde/camilla via isolated GPIO. the xmos can pass i2c over USB if required. these control elements will influence the cost in time and money significantly for the project from my end (and thus the end cost) if an additional complex MCU is required. there isnt a whole lot that can be done with control at the ESS chip that cant be done in camilla and xmos. the upsampling and oversampling and use of digital filters is more powerful than what is onboard the ESS IMO, you dont really want to turn the THD reduction off or anything like that and more useful distortion generators are available as plugins on linux if thats your bag
. Volume control and channel allocation is more usefully controlled in camilla too IMO. Perhaps a basic hardware volume control knob and setting the supply sequencing and input type, lock status, frame-rate and mute settings (and their reflected status) etc is all that's required? people could also add their own arduino based controller like dimdims could be adapted if desired. the XMOS can do a lot, but we dont really need a great deal from it IMO. With Henrik's permission, we could even design a custom wrapper for the GUI. i'm not too shabby as a graphic designer in both vector and pixelated/raster image, so could come up with something cool that is consistent with the design aesthetic.
myself, since a nice web control app already exists, I see no need for a big display. I will be sticking to a nice–if a little bit expensive–kinda retro-cool VFD display
https://noritake-vfd.com/cu22042-y1a.aspx. I think full colour displays look tacky on dacs and most people will use their phone or tablet. i'll leave that decision up to you guys, as its not really my end.
thoughts? I kind of need a couple details fleshed out before I can work towards finalizing the schematic, BOM and therefore have a more useful idea of cost, so I could arrive at a more informed ballpark figure, for those intent in nudging that out of me before i'm finished
. Final cost will still depend on how many I/we make in the initial batch. Of course after my tests on the prototype, I would hope to send it off to Amir for testing before making any changes if required and sending 1.0 PCBs off to print. I have a basic reflow oven and hot air setup, so small scale production is doable here. anything more, we'd take it from there. i'm using SMD plastic film caps for some analogue bypass and filter positions, so you have to be careful with the reflow profile. Worst case just those could be soldered by hand, or with a small hotair pen. I will have a very small number of parts on the underside, which is what they want, for obvious reasons reflow soldering is difficult (but not impossible) to do on 2 sides of a board
i'll probably reflow the top side and then just use a hot air pen on the underside.. i'm using nearly exclusively lead-less parts (no legs of any kind) with the DAC and XMOS being the only parts that have any so far.
Does it interest people to implement a higher current buffer on the output of 2 channels, so that it could be used as a high spec balanced headphone amp on those 2 channels? it wont have high output voltage, but with a buffer would handle most headphones admirably (better than many dedicated headphone amps). I'm designing for around 4.5 ish vrms, so its already a reasonably healthy output at line level, without any voltage gain. perhaps that could be an optional facility for extra $$ over the standard board? I will cost out a design with it on all 8 channels, but it isnt required IMO and it would increase the cost and physical size (as a percentage of a small board) quite a bit and require an additional slightly higher voltage +]- rail for L/R. they could perhaps be the top 2 channels, so the outputs would be available as well as–rather than instead of–the channels used for speakers (for anyone only needing 3 way or under crossovers vs 4 way/8 channel operation)
I do love that i'm designing this for people of like mind, who wont object to my exclusively SMD design. I will not be providing much (if any) means for people to modify their board. i'm already using best in class parts for most positions (within reason and budget) and it wont be an easy task to mod such a compact (probaby 6 layer at this point) layout. Some parts are as small as 0402, I ruled out 0201 as that is just too small for my soldering abilities
. Of course, those contributing to the project significantly should be able to have access to the hardware, perhaps after the prototype has been sent to Amir, it could be forwarded onto
@phofman and
@notabenem to play with (since you live close) and i'll work towards the final revision based on my findings and feedback. then perhaps it could be raffled off for the forum, or you could draw straws for it?
At my new job, I apparently have indirect access to a CNC, i'll be leveraging it for cutting some speaker baffles with integrated waveguides, so we'll see what the capabilities of the machine are. If it can cut T6 6061 alloy, maybe I can work on a case as well, or at least elements of it. we could use an off the shelf case and machine some features into it, or just a nice front and back panel (which could alternatively be ordered by the user at front panel express. If someone on the forum has some contacts for better pricing and quality than they can provide, or access to it directly, that would be awesome. On that note, what connectors are people going to use to connect to their amps? XLR? just those connectors would be bigger than the dac board. i'll probably use mini XLR, lemo, or a mixed signal DSUB wiring harness like used on PCI soundcards (could include a power-good signal for sequencing power).
