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Thread: T4 WM8731 Codec - Consumer v's Pro Line Level Output

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    Senior Member houtson's Avatar
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    T4 WM8731 Codec - Consumer v's Pro Line Level Output

    Hi

    Looking for a bit of help on what I hope is some basic audio electronics.

    I'm using a Teensy 4 with a WM8731 Codec (datasheet) in a custom effect unit.

    I'm using the line level out to patch back into a mixing desk and finding I alway have to add additional gain at the mixer.

    Reading up I see that there are two different line levels, as below from here and wikipedia - Pro line level and Consumer line level and that the WM8731 outputs at Consumer line level.

    I'd like to use the unit directly with my mixer without adding gain on the mixer.

    The question is what would i need to do to amplify the line out lifting it from consumer level to pro level (single ended is fine)?
    - I'm assuming some sort of simple opamp circuit would do it?
    - Struggling to get my head around the different units of measure - what gain would I need to achieve a 12dB boost to get it to pro line level?
    - Would the supply voltage be a limitation ? I have nice clean 3.3v on the board but could also provide 5v if needed
    - How would I match impedance at input end (~10k?) and achieve a low impedance output (~100 Ohm?)

    Any input appreciated, Cheer, Paul


    'There are two types of line level: +4dBu "Pro" Line Level or -10 dBV "Consumer" line level.
    The normal level of the audio signal in a professional studio is +4 dBu or about 1.23 volts (0 dBu=.775 volts).
    The level of the audio signal in home stereo equipment, inexpensive "semi-pro" gear is -10 dBV or .316 volts (0dBV=1 volt). This lower "line level" is about 12 dB lower than professional line level.'

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    I wrote a response, and then found a typo. Went to edit it and deleted it by mistake. This was on mobile, the form was a bit strange, I think it's broken.

    Anyhow... Pro level signals are usually also balanced. Elliot Sound Products has a great article about it including some schematics here. The Rolls Promatch MB15b is a popular product that does line level matching/conversion. I'm attaching the user manual here which contains the schematic.
    Attached Files Attached Files

  3. #3
    Senior Member houtson's Avatar
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    Hi @jayshoe - thanks for those references. For this use I don't need a balanced output just looking to lift the levels.

    If I was starting again and it wasn't for the chip shortage I'd probably use the CS42448 (the additional channels would also be helpful) but I'm just trying to finish this current design by adding some amplification to the single ended output of the WM8731. Cheers, Paul

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    The supply voltage is the limitation, that chip has an output full-scale of 1Vrms (2.83V peak-to-peak) at 3.3V supply - theres no
    way to get much more than that from a single 3.3V rail.

    An opamp gain stage running from +/-18V is what you'd normally expect for professional audio, that can generate upto about
    30V peak-to-peak (11Vrms). You'd need at least +/-5V supplies and rail-to-rail opamp(s) to get enough headroom for what you
    want I suspect - probably easier to just go to +/-15 or 18V and pick a good audio opamp.

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