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Thread: Incorporating Teensy 3.6 and audio shield into mixer

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    Incorporating Teensy 3.6 and audio shield into mixer

    Hello all,

    I am working on building an 8 channel mixer based on the design of the attenuation version of the mixer schematic here. What I would like to do is use the teensy as an effects module post fader but i am not sure how to hook it up as such and i am getting conflicting info online. Is it possible at all to do this? My thought was to run each input into the teensy prior to the Op-Amp stage where I would be able to select and control an effect and then output each signal to the Op-Amps. After looking into the Teensy more and trying to understand how audio works with it I am led to believe that I may not even need Op-Amps at all?

    Any help would be greatly appreciated!

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    Senior Member+ Theremingenieur's Avatar
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    In all the mixer schematics which you linked, the first post fader OP amp is required to decouple and to build the sum of all input signals. It's after that that you would feed the sum signal into the Teensy to apply effects.

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    Quote Originally Posted by Theremingenieur View Post
    In all the mixer schematics which you linked, the first post fader OP amp is required to decouple and to build the sum of all input signals. It's after that that you would feed the sum signal into the Teensy to apply effects.
    If I take the signal from each input (input> Pot>) directly into the Teensy and allowed the mixing to take place there and use the Teensy Audio I2S Adapter as the output would I even need the Op-Amp stage? Also, If I do it as you referenced there wouldnt be any way to apply effects to each channel would there?

    I basically want to use the Teensy as an effects loop.

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    Senior Member+ Theremingenieur's Avatar
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    The Teensy Delta-Sigma ADC inputs can not build the sum of several signals while decoupling the channels in-between themselves (preventing input A flowing back into input B) as the virtual earth technology does. The Teensy ADCs need also a defined low source impedance to work correctly which is not given with faders. So, the first OP amp is definitively required.

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    Quote Originally Posted by Theremingenieur View Post
    The Teensy Delta-Sigma ADC inputs can not build the sum of several signals while decoupling the channels in-between themselves (preventing input A flowing back into input B) as the virtual earth technology does. The Teensy ADCs need also a defined low source impedance to work correctly which is not given with faders. So, the first OP amp is definitively required.
    Is there a way to do the first Op-Amp stage and then break the inputs back out? Or since they are cheap enough, would it be reasonable to do an Op-Amp on each input? Im using TL072 so I wouldnt be opposed to doing it this way since it would be that much additional space used.

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    Senior Member PaulStoffregen's Avatar
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    I'm confused... can't understand whether you want to apply effects separately to the individual channels, or if you want to mix everything together first, then apply an effect only to the final signal.

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    Quote Originally Posted by PaulStoffregen View Post
    I'm confused... can't understand whether you want to apply effects separately to the individual channels, or if you want to mix everything together first, then apply an effect only to the final signal.
    Sorry for the confusion, the goal i am trying to achieve is to have the option to apply effects from the teensy to individual tracks and THEN mix them together with the virtual earth mixer so they will eventually get mixed down to a stereo output. The teensy would strictly be handling effects but i am trying to figure out where in the signal path i could place it to do such.

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    Senior Member PaulStoffregen's Avatar
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    Building the CS42448 hardware requires pretty advanced skills with surface mount soldering. Even then, the only published board gives 6 inputs and you need 8. Adding more would require quite advanced skill - first a new PCB design, plus circuit details to add another chip (as described partially in the CS42448 datasheet) to the CS42448's extra I2S port, and then work in the software to enable it.

    Maybe I'm reading too much into your questions, but I can't escape the feeling this quite advanced electronics project may be far beyond what you should attempt to build.

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    Quote Originally Posted by PaulStoffregen View Post
    Building the CS42448 hardware requires pretty advanced skills with surface mount soldering. Even then, the only published board gives 6 inputs and you need 8. Adding more would require quite advanced skill - first a new PCB design, plus circuit details to add another chip (as described partially in the CS42448 datasheet) to the CS42448's extra I2S port, and then work in the software to enable it.

    Maybe I'm reading too much into your questions, but I can't escape the feeling this quite advanced electronics project may be far beyond what you should attempt to build.
    The soldering is no concern and I have a quite a bit of experience with the coding side. I generally work with the Arduino but the Teensy is just better suited for an audio project like this.

    I did see the CS42448 board but i guess i am confused by why it is needed if the Teensy can accept audio signals via the ADCs? I already have my 8 inputs mounted in the project housing (Its pretty slick, a big darth vader novelty helmet) and was wiring up the Op-Amps to complete the virtual earth mixer i previously mentioned but stopped before I completed it to see if there was any way to bring the teensy into it somehow. All 8 inputs would not necessarily have to be able to run effects but i thought I would at least be able to get a couple channels running on it based on what I had researched and using the audio board extension. I may just need to go back to the drawing board and find a different board that could do what I want.

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    Senior Member PaulStoffregen's Avatar
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    Quote Originally Posted by massahwahl View Post
    why it is needed if the Teensy can accept audio signals via the ADCs?
    The ADC input code only accepts 1 signal. It does not have the ability to use multiple pins.

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    Quote Originally Posted by PaulStoffregen View Post
    The ADC input code only accepts 1 signal. It does not have the ability to use multiple pins.
    Aha! I was under the impression there were multiple ADC's on the 3.6. I know there are two DACs but when I was researching it I thought I had read the audio board expanded this.

  12. #12
    Senior Member PaulStoffregen's Avatar
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    There are 2 ADC converters, and many input pins they can use.

    However, the audio library currently only supports 1 ADC. Someday in the (like distant) future it may support both ADCs. But for now, the software only supports one.

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    I would suggest starting with a stereo effects processor and an analog mixer with effects inserts as separate projects. Looking at how to scale your inputs and outputs is important. You will want to protect your teensy/codec/ADC/DAC with buffer op amps and current limiting. Buffer op amps instead of just the resistor into single inverting mixing op amp also allows you to individually control a gain boost if you need it. The Mutable Instruments open source projects are a good place to look at some analog audio circuit design with MCU. You can actually find the full schematics for Mackie mixers in their service manual section, 802 VLZ3 would be the one to look at to compare to your project. It has a bit more going on with the effects bus and equalizers. Honestly I would just get a cheep analog mixer (Behringer) with effects send/return and focus on the effects processor side of it. If you want per channel inserts the Mackie 802 VLZ3/4 has 2 channels, not gonna find that feature on a cheep mixer unfortunately.

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