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Thread: XLR Input on Audio Shield

  1. #51
    The shared gain is a great idea. Us the other half of 1k dual pot with the op amp by connecting it from the negative input to ground via another 47 ohms resistor and a largish capacitor (470 uF at least). The feedback resistor sets the maximum gain. Note that you should really use a "C" taper pot here, though a linear "B" works well enough. You could also reverse wire a log taper "A" pot and mark it "Attenuation" but it might be confusing and odd for the users.

  2. #52
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    The unity DC gain idea is a good one on the op amp.

    I've been batting around which taper would be better. THAT suggests reverse audio (log) tapers for the pots, as you note, so I'll start with C (US) taper and see where that takes me.

    Because we're multiplying the gains of the two stages, B starts to look promising, as the "squared" curve somewhat approaches inverse log.

    I'm going to mock things up and see which taper, and gain setting configuration on the final op amp, provides the most "natural feeling" result. While I can model the outcome to my heart's content, the real test will be how "natural" the gain changes feel when turning the knob!

  3. #53
    Just ordered my Teensy 4.1 and audio board 😬 Super excited for this! Shout out to Paul for making these awesome products. 8 MB of flash memory 😮

  4. #54
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    Good to hear that your T4.1 is on the way!

    I've been trying to sort out the gain issue that mhelin identified in my spare moments this week.

    So far I haven't found an elegant solution, as the THAT chip really needs a 5k reverse audio (C) taper pot. These are very rare in dual-ganged variants - none of Mouser, digikey and Element14 stock suitable pots (even in larger physical packages).

    Anything else (50k is quite common, or A / B tapers) gives a very sharp taper response (see red curve = 50k; orange is the kind of taper we want).

    As the final stage gain and this one are controlled by the same pot, the effect is squared - giving an unusably sharp taper (like the grey curve, which has 95% of the gain coming in the last 10% of rotation.)

    Click image for larger version. 

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    I'll keep playing with it to see if there's different solution, and also hunting for suitable pots.

  5. #55
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    Sleeping on it provided the answer:

    - Go back to a simple switched gain arrangement for the THAT chip 20/40dB.

    - The pot controls the gain (0-20dB) of the final stage.

    Let's try and finalise things this week - so that you can get your preamp boards ordered!

    BTW: I've put the files on my github: https://github.com/palmerr23/audio_preamp

  6. #56
    I知 glad the answer came to you over a good night痴 rest. By the way I知 definitely in no rush so definitely take your time.

    The files and board look awesome on github 👍

  7. #57
    By the way, does this circuit have to run off the 5V pin? Just asking out of curiosity. I知 wondering because when I run the Teensy off a battery, I知 concerned it won稚 be able to supply the 5V without a boost converter if I am running off a 3.3V battery for example.

  8. #58
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    There's no real reason why it won't work, but the current draw from 3.3v would be nearly twice that from 5V.

    I'll try the PSU modules out on the lower supply voltage and see!

  9. #59
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    The +/- 12V boost module works OK down to 3.2V with a 100Ma load on one leg.

    At 3.0 V input the output is down to 7.5V, which is still adequate for our needs.

    I don't have one of the phantom boost modules in hand yet, but it's based on the same chip, so hopefully the characteristic will be similar.

    I hope you have a beefy LiPo battery, particularly if you are using phantom - as the current draw may be up towards 300mA @ 5V with a couple of phantom powered mics in use.

  10. #60
    Great to hear! I知 so excited 😬😬 Yes, you池e absolutely right about needing a big battery. I was planning on using 1-2 18650 cells and a TP4056 charging module. So for each cell that should hopefully be 1800 mah at a minimum.

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