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Thread: AudioToy - modular 8x8 channel Teensy 4.0 pro-grade audio mixer hardware

  1. #1
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    AudioToy - modular 8x8 channel Teensy 4.0 pro-grade audio mixer hardware

    AudioToy is a set of modules to complement an 8x8 CS42448-based Teensy 4.0 audio board, providing fully balanced, software gain-controlled input and output modules.

    Fully balanced or unbalanced microphone, line and instrument inputs are supported and the output stages can drive 600 ohm loads at +8dBm. Only a 5V supply is required.

    All hardware information is available for open access at https://github.com/palmerr23/AudioToy with full schematics, BoM, Gerbers and design notes provided.

    An ESP32 provides WiFi connectivity for remote control.

    Top view
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    From the front
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    Front panel
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    Rear panel
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  2. #2
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    great work!
    does it have differential in & out circuits? had a look to the repo but the schematics are hard to read, I'm used to 'op-amp stage' style instead of ICs,

  3. #3
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    M4ngu,

    Hard to read in what sense? Too small? Not enough information?

    They're in currently standard Eagle schematic format, and that's a bit hard to change!

    I'm not sure what 'op-amp stage style' is, can you post an image of a small section of a schematic in that style?

    R

  4. #4
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    Hi palmeer,
    with 'op-amp stage' style I mean this:
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    instead of this:
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    for me it's way easier to see what's going on looking at the first one.
    Cheers!

  5. #5
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    Yes, I agree wholeheartedly with your preference with 'op amp' style schematics. They are much easier to decipher.

    Sadly, I'm limited to what the manufacturers (or more commonly with these 'box' symbols, SamacSys or some other 3rd party who create Eagle libraries under contract).

    With the MCP604, I could easily have used an alternate 4 op amp chip as the pinouts are pretty standard. Sorry for the inconvenience!

    My pet peeve on this issue is 3 terminal regulators which seem to come with all sorts of weird symbols.

  6. #6
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    On the 'differential' issue - yes fully differential in and out, other than inputs 7-8, which are only differential at the preamp stage, reverting to single ended at the ADC.

    I couldn't find a readily available 2-channel ADC that would work in auto-detect slave mode that had differential inputs. Paul's design has the same issue.

  7. #7
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    I want to give a try to the differential IO circuit on a project I'm working on, indeed I would like to totally remove the caps for the DC offset (that can be solved with software calibration), that way the module could output slow LFOs, and DC input voltages would work too (to use audio ins as CV in too)

  8. #8
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    Yes, it's relatively straightforward to add DC bias to a single-ended input, but a little more complex for balanced. Of course, for LFOs, balanced isn't relevant.

    Small input offsets will remain, which as you say can be digitally removed - the CS42448 provides some help in doing this with a programmable ADC High-Pass Filter for DC Offset Calibration, with Freeze capability (Sect 4.2.3, 6.6.2)

    Outputs are more straightforward as you have direct control of the values, and VQ does most of the work.

    You'll need to provide better protection for the ADCs than is currently in place (I can't find any reference in the CS42448 literature about input protection diodes), if you're going to put DC voltages into the front end. Watch out for non-linearities that can be caused by a diode's reverse and forward current leakage - most are by no means "perfect" devices, especially zeners!

    If you need help with the circuitry, I'd be happy to provide some pointers. Just put a reference to your thread for the project into this thread and I'll join you in the other one.

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