Help me decide on a DAC for a poly synth

[toutant]

Hi Teensyers,

I'm building a Teensy poly synth and would like to figure out how to build out my own DAC hardware. Here's a pic of the prototype board. Currently using the Audio Board, which honestly works great, but it takes up a lot of space and have more features than I need. Can anyone recommend a good external DAC for my purposes?



Here are more details:

Project: 8 voice polyphonic synthesizer with LFO, filter and amp envelopes
Hardware: Teensy 4.1
DAC/Audio Requirements:
-Stereo line level output
-Good sound quality (note: this doesn’t necessarily mean hi-fi or 24 bit, just nice sounding for an analog modeling synth)
-Ideally under $15
-Easy to hookup — ideally something that someone has already experimented with and documented for Teensy

I noticed that someone made a useful document here with compatible hardware, but I’m not sure which one to go for.

[PaulS]

Price versus quality, I think the PCM5102A is the best choice. You can find boards for less than 5 US$. See here.

Regards,
Paul

[Frank B]

Agreed, the PCM5102A is good.

[toutant]

Sounds great! Any ideas where I could look to find a proper schematic to use a PCM5102A with a Teensy? I'm hoping to integrate it into the PCB instead of buying a breakout board.

[PaulStoffregen]

The good news is chips like PCM1502A are really simple. To interface with Teensy, you just connect the I2S data and 3 clock signals. Some chips use different names so you need to figure out which is which. But on PCM1502A 3 of the 4 have basically the same name, so by process of elimination you can figure out SCK is MCLK.

The bad news is there aren't a lot of schematics, because it's so very simple.

On the digital side, you also need to configure the chip. On these simple parts you just connect certain pins high or low. The important setting is to use I2S (FMT pin low) slave mode (PCM1502's only mode). The other settings don't matter much for the interface to Teensy.

Then the rest of the pins are the analog signals or power supply. Follow the datasheet's advice for all those.

[toutant]

OK, great! Thanks for weighing in, Paul.

I guess I'm just confused on what's going on with a breakout board like the one that PaulS linked. (The other PaulS.). If I can connect the PCM1502A with just I2S and three clock signals, what's going on with all of the SMT stuff on a board like that? I feel like I'm missing something.

[Frank B]

Here is a schematic:

taken from here:
https://electronics.stackexchange.co...hat-i-expected.

I'd just use the breakout and solder it to yours...

Edit: Here is the Datasheet: https://www.ti.com/lit/ds/symlink/pcm5102.pdf

[toutant]

Thanks, Frank.

[Frank B]

Its indeed the same schematic as in the datashhet.
Note the datasheet says, for lower frequencies (as we have) you can omit the output filters.. (but hey, it's just a resistor+cap) - adding them allows you to play with 384kHz sampling rate.
Looks like the board from the link has two voltage regulators.

[toutant]

OK, great! Thank you so much.

I'll probably end up laying it out on the board just because I could use the practice with PCB design.

[PaulS]

The board I linked to also has 2 voltage regulators on board. I assume to generate a 3V3 analog and a 3V3 digital power supply.
If you want an easier-to-use board, go for this one. Check out the wiring here.
To be 100% exact, we're talking about the PCM5102A version. Datasheet here.

Regards,
Paul

[MarkT]

The datasheet at least draws the schematic the right way round(!) And it has a PCB layout which you would
do well to follow verbatim.

Maybe add schottky protection diodes on the audio lines. Stops someone blowing up the thing by plugging the
wrong leads (happens all the time in the audio world, noone checks, and +/-15V supplies in some equipment)

I presume the diodes would go to Vneg and AVdd which are the analog rails, though that raises the issue of
whether to add crowbars to those rails too...

[JotaEfe13]

There is an interesting video about a guy analyzing the board PaulS has talked about, in case it is of your interest: https://youtu.be/VualiZkPlw0

[Frank B]

Edit:
Err.. ok its too late-
going to bed .. should not post wrong things.

[toutant]

Ahh... good point about the layout and schematic in the datasheet. That's handy.

[toutant]

Here's my attempt at building a PCM5102A into the board with a Teensy 4.1.

I think I have the Teensy pins correct, but I need to buy a pre-made board and double check it before printing this one. Any thoughts or feedback are greatly appreciated.

I also built in two voltage regulators which seems to be what both of those breakout boards do.




pdf version: pcm5102A schematic.pdf

[PaulS]

The board I referred to has 3 voltage regulators: the charge pump [pin 1] is powered separately, not interfering with the analog power.
Also the 1000uF caps at the output of the 3V3 regulators is a bit overkill.
And I would connect XSMT directly to AVDD. Datasheet states: In systems where XSMT is not required, it can be directly connected to AVDD.

Regards,
Paul

[PaulS]

There is an interesting video about a guy analyzing the board PaulS has talked about, in case it is of your interest: https://youtu.be/VualiZkPlw0

Here is another interesting video: https://www.youtube.com/watch?v=_PfUEnkqeOs

Both reviewers had not noticed though that this yellow board has the cinch jacks reversed. The jack with the red plastic insert [red = right channel] is connected to the left channel output of the DAC and vice versa...

Reagrds,
Paul

[toutant]

The board I referred to has 3 voltage regulators: the charge pump [pin 1] is powered separately, not interfering with the analog power.

Ah! Thank you. I just added that third regulator.

Also the 1000uF caps at the output of the 3V3 regulators is a bit overkill.

Knocked them down to ~100uF.

And I would connect XSMT directly to AVDD.

I decided to put a voltage divider on the mute circuit based on this guy's recommendation. He says it provides muting as the unit powers down.

Here's an updated schematic. Thanks again, Paul.