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Thread: I'm a giant idiot, pin difference in Teensy - version 4.0

  1. #1
    Member propa's Avatar
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    I'm a giant idiot, pin difference in Teensy - version 4.0

    Hello,

    I just got a couple of teensy 4.0's and didn't realise they needed a new audio board. I now have to redesign the control the board I'd made for rev c and 3.2/3.6.

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    that's the old control board, but seems to use some pins the new audio board uses now. I just swap them over and I should be good to go again?

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    This is also a random curiosity, I've had a decent level of signal out of the audio boards as far as I can tell, is it necessary to add an op amp to amplify the sound or add ferrite beads when using the audio board in conjunction with 4.0?

    I've seen some low power OPA(some number) that's a low power Opamp, would there be any discernible benefit from adding an amp circuit like that?

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    An opamp buffer would provide a much lower output impedance, useful for driving long cables for instance.

    Low power opamps aren't great for audio, they have much higher distortion at high audio frequencies due
    to the open loop gain dropping off. My favorite 5V rail-to-rail audio-capable opamp is the AD8656 as it can
    directly drive most headphones due to its high output current, and has generally good all round performance.

    Basically expect gain-bandwidth product of 20--50MHz or so (yes, really) and 4 to 5mA of current consumption
    per opamp for an audio rated device, together with low distortion, low noise (voltage and current).

    Unless you have +/-15V supplies already, true rail-to-rail opamps are needed.

  3. #3
    Member propa's Avatar
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    Quote Originally Posted by MarkT View Post
    An opamp buffer would provide a much lower output impedance, useful for driving long cables for instance.

    Low power opamps aren't great for audio, they have much higher distortion at high audio frequencies due
    to the open loop gain dropping off. My favorite 5V rail-to-rail audio-capable opamp is the AD8656 as it can
    directly drive most headphones due to its high output current, and has generally good all round performance.

    Basically expect gain-bandwidth product of 20--50MHz or so (yes, really) and 4 to 5mA of current consumption
    per opamp for an audio rated device, together with low distortion, low noise (voltage and current).

    Unless you have +/-15V supplies already, true rail-to-rail opamps are needed.
    OK thanks for clearing that up for me, I actually built a euro rack modular power supply that delivers +/- 12v, I have some TL072's knocking around, if I'm going to redesign I may try and sharpen my chops with circuit design by adding an op amp. I have a half decent sound card, I've just been amping up the signal using the pre-amps so far.

  4. #4
    Senior Member PaulStoffregen's Avatar
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    Usually Euro-rack synths need signals like +/- 10 volts (or 20 volts peak-to-peak). The audio shield defaults to only ~1.3V peak-to-peak. You can increase it by software control, but the maximum is only about 3.1Vp-p. You're probably going to need an opamp to increase the signal level up to that much higher voltage swing that other Euro rack modules expect.

    Most modules are designed with a 1K resistor between the opamp output and the 3.5mm jack. This is done for protection, in case a patch code is accidentally plugged into another 10Vp-p output. In a theoretical worst case, your opamp might be trying to drive +10V and the other one is driving -10V. If both have 1K resistors, the result is 20 volts across 2K for a safe 10mA current. If the other side has no resistor, it's still only 20mA.

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    Member propa's Avatar
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    Quote Originally Posted by PaulStoffregen View Post
    Usually Euro-rack synths need signals like +/- 10 volts (or 20 volts peak-to-peak). The audio shield defaults to only ~1.3V peak-to-peak. You can increase it by software control, but the maximum is only about 3.1Vp-p. You're probably going to need an opamp to increase the signal level up to that much higher voltage swing that other Euro rack modules expect.

    Most modules are designed with a 1K resistor between the opamp output and the 3.5mm jack. This is done for protection, in case a patch code is accidentally plugged into another 10Vp-p output. In a theoretical worst case, your opamp might be trying to drive +10V and the other one is driving -10V. If both have 1K resistors, the result is 20 volts across 2K for a safe 10mA current. If the other side has no resistor, it's still only 20mA.
    1.3V that's brilliant to know. So if I'm aiming for 10v, I need a gain of 7.5ish@1.3v? This may be a silly question but can you set it lower that 1.3v? for instance 1.2, then apply gain of ten for 12volts?

    This is where I've got to with the amp circuit, I wanted to test out hard and soft clipping, but there's a lot of switches now!

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    Opamps don't usually break if you short the outputs or set them up to duel, they are designed to current-limit internally
    and indefinitely. 100 ohms or so is usually used on the outputs to prevent instability into capacitive loads, that's all you
    really need. 1k won't hurt though unless driving _very_ long cables.

  7. #7
    Senior Member PaulStoffregen's Avatar
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    Quote Originally Posted by propa View Post
    This may be a silly question but can you set it lower that 1.3v? for instance 1.2
    Yes, you can. Or 1.22V anyway. Hopefully that's close enough?

    This is documented in the audio library design tool, under the SGTL5000 control. Here's a direct link:

    https://www.pjrc.com/teensy/gui/?inf...ontrolSGTL5000

    On the right side documentation panel, scroll down to "Signal Levels".

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