freeDSP V2.0 (ADAU1452) developement thread

Let's talk DAC

Let's talk about output stage for DAC, while I work on the PCB layout for DSP.
I've settled on AK4458VN and PCM1690 (if AKM is still not available). So at the output of the DAC there are 8 balanced voltage outputs...

Providing single 8x single ended output:
This configuration would be probably the most versatile. You could mix balanced(by cloning output in DSP and inverting polarity) and unbalanced as you wish. If you want to use balanced input(amp) for unbalanced output(dac) you need to modify the cable(to provide balanced impedance for "cold"), but that's not a problem. Nice if you have equipment with more common unbalanced input. An opamp(with a common mode voltage tolerance) with a buffer in feedback loop would do.

Going with 8xbalanced output:
Rather straightforward, I've narrowed it down to two options.
Inverting MFB filter with TPA6120 and DC servo. Or MFB filter with OPA1632 diff. in, diff. out, no frills.

At the moment, I prefer bal in/out opa1632 variant.

What are your preferences?
 
I would also prefer a MFB filter with the OPA1632 Opamp and differential in and out. Great project!

If you want to use PGA after DAC for analog gain control then you will need additional unbalancing stage somewhere (not a big problem anyway).

CS4272 evaluation board has (after DAC output) an additional instrumentation amplifier with 5x gain before filter stage, it is there to lower the noise contribution of the 2nd order filter. Wonder what kind of improvement that kind of circuit would offer?

See:
http://www.cirrus.com/en/pubs/rdDatasheet/CDB4272-2.pdf
 
Thank you for your input Piersma.

I've changed my mind a bit about connectors. I've switched to non shrouded headers for majority of the IOs. There are 3 full(all four signals) I2S for IN, 3 for Out. One I/O I2S goes to Rpi. Rest are aux MP,ADC, I2C, power supply... Just crimp/solder separate headers to your DAC cable and connect them as you wish.

1011800-2.jpg


There is quite a lot of free estate in top corner, I'm considering including TPS65131 and some post regulation. So complete setup would include only two boards, some nice enclosure and a wall wart.

Another idea was to include step-down converter so one could use more common 12-24V wall wart, instead of quite beefy 5V.

Bottom corner is reserved to EEPROM in a socket and maybe small arduino nano socket - if space permits. There will be a jumper so you can connect incoming MCLK instead of the default oscillator. I was considering getting rid of multiple oscillators and pricey ADM7150, but since I was able to fit it in designated area, and it is already designed, I've left it there as it is.

Yes, creating smaller version, just to fit RPis footprint is not a problem.
Actually I was considering moving Optical IO to bottom right corner and making the board same width and just a tad longer than RPi, but since DAC board will be quite wide, I decieded to stay with 10cm wide PCB. The dimensions are 10cmx10,5cm.

Thank you for the link mhelin, I'll look into it :)
 

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Hi,
just to let you know that there is some progress.
I've added:
input CM filter and replaced fuse with poly-switch.
TPS65131(+-6V) with postre-gulation to -+5V(TPS7A4701 & TPS7A3301)
Status LEDS - probably way too many, I'll have to decide which to keep
EEPROM - in DIP package, I'll probably use combined footprint DIP/SOIC

Since I had some free IO in I2C port expander, I've wired it to do some basic power management. So you can disable opamp PS and DSP PS, Only thing that is always on is 3V3 digital.

I've also Refactored and cleaned up the schematic and crunched some numbers regarding current consumption and power looses.

I'm starting to see the light at the end of the tunnel :)
 

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Major Redesign

Hi, quite some actually.
I've attended a 3 day high speed design course with Lee Ritchey(SpeedingEdge, highly recommended). So with newly gained knowledge I've obtained proper capacitor models and redesigned whole power delivery system... (minus switcher for +-6 for opamps - this one is yet to be done). Polymer tantalum seems to be quite nice stuff :) . The oscillator shares power supply with PLL and ADC now.

I also got rid off stuff that I don't need... BBB connection, 2 oscillators (clocks are still switchable to external clk.). I played with IBIS models and SI simulation, as a result I went with UFL connectors for MCLK.

The board seems more empty now, free space is occupied by VCC planes. Less is sometimes more. Now the whole thing will fit on 4L board even with proper power supply design :) (previous design was not going to be 4L board...).
So far only 2 channels of ADC had to go - no way to route them, otherwise it seems rout-able.

Pitrsek
 

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hmmm what a mess... did not know about the issue, TY for heads up :up:

Luckily there's a polymer aluminum cap in the same package(EEFCX0G221XR), cost almost the same.

I have to source a proper/detailed model for RPi, so I can see if there is room for a ordinary polymer aluminum in a can package.

Regarding power supply impedance modeling - I use models from Panasonic and TDK. Although mounting inductances are calculated by ALTERA PDN tool, i use spice for the simulation(PDN tool does not work properly in OO).
 
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Hi All,
I am following this thread with great interest, as I concluded the original Freedsp is a bit limited for my needs.
I am planning to Use a DSP as a 2 x 4 channel crossover. I currently use vinyl, Flac media or radio streaming. I have an RPI with IQaudIO.com Pi-DAC with a Audiophile TI Burr Brown 32-bit/384kHz DAC (TI PCM5122) running Rune Audio interface. Works really well into a TPA3116D2 Amp powering my 4 way transmission line speakers.

I am planning to go active and was looking at MiniDSP, these are my concerns;
The Noise floor, Maximum output voltage 0.9V, 48kHz sampling rate, I think a minimum 96kHz for hi res audio, Linux support. I would need the 2 x 8 mini DSP, $300.

REW integration and ability to adjust parameters on the fly are big plus points.

My system idea, 4 x TPA3116D2 boards, FreeDsp 2.0 and dac & RPI combo. With the ability to measure and adjust on the fly. Like MiniDSP + REW.
Does this sound a reasonable goal?

Many Thanks
Ian
 
Not dead :)

Hi,
sorry for a long time without any updates/replies. I was spending my summertime outside. I got back to the project at the begging of September. It took way longer to get the board to shape that I liked(enough plane capacitance, good grounds etc.) on 4 layers.... Finally, it's done.
I've just placed order for 10 prototype boards at Elecrow, let's see how log will it take - my estimate is 2-3weeks. It's just guess, I haven't ordered from them before.
The design is a bit different from last time. I wanted to have something to play with and play on(my speakers are waiting on it), so skipped larger 10cm wide variant with smps for opamps in favor of smaller RPi HAT module(changed name to reflect it).

Power supply:
LP5907 for oscillator, pll, and ADC
LM3370SD for 3v3 and 1v2

Clocks:
single crystel oscillator
selectable external clock
4 MCLK outputs via U.FL

I/O:
bidirectional to RPi
3x full I2S outputs
3X I2S input with shared bit clock and world clock
4X MP
4X AD
SPDIF IN - 3,5mm mini toslink :)

@Piersma - if you mean analog volume control, no, I did not consider it. I will most probably use volume in dac, or in DSP

@webby - sorry i do not know what is rew. Last time I checked AD site, the auto tune feature was not ready for ADAU1452. Changing parameters on the fly is possible with AD programming board. Dunno how difficult it is when uC is in control of ADAU. I haven't really researched this area yet.
 

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Question is how good the ADAU1452 is at removing the jitter from the RPI I2S port which contains quite a lot jitter. Anyway, RPi can be used as a control front end to DSP anyway, which enables the use of wireless technologies. So you could for an instance have an iOS or Android GUI controlling the volume and the input source selection for the DSP running the digital crossover application.
 
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I've just placed order for 10 prototype boards at Elecrow, let's see how log will it take - my estimate is 2-3weeks. It's just guess, I haven't ordered from them before.

I've ordered from Elecrow and they are very fast - the times they state (4-7 dats) are more or less correct, and they will take a picture of your pcb before they send it out (as 'proof' that they're not faking shipping time as production time). The longest waiting time is probably for the shipping, depending on what shipping option you chose, and where you live.
 
Ah, that's nice. Thank you for the info drojf.

I'm now pondering about the DAC. Two options:
1) 10cm wide board with balanced output, as envisioned earlier in this thread.
2) small, elegant add-on module for RPI+DSP combo, similar footprint,most probably single ended
What would you prefer?
Good news, digikey is stocking AK4458VN :)