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setting the bias current (scratchy Cortado)

Home Forums Cortado Contact Microphone Forum Cortado FAQ & Support setting the bias current (scratchy Cortado)

This topic contains 23 replies, has 3 voices, and was last updated by  brach 1 month, 2 weeks ago.

Viewing 9 posts - 16 through 24 (of 24 total)
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  • #2172 Reply

    zdlAdmin
    Keymaster

    !!BRAVO!! I am SO GLAD to hear you solved the mystery! That’s really good news for a Monday morning. Have fun with your gear, and if you capture any crazy-wild sounds, be sure to share them with us.

    Glen

    #2174 Reply

    brach
    Moderator

    …I just got back in town, sorry that I wasn’t able to respond earlier.
    That’s great to hear that’s it’s working now! I was thinking there had to be more variables to this issue than I knew about because these Cortado circuits aren’t that complicated. Good job getting it figured out. I’m glad to hear that you are pleased with your Cortado and Espresso!
    -Brach

    #2223 Reply

    Nick Burge
    Participant

    So I have discovered a new issue with the cortado when used with my RME fireface soundcard. The cortado works fine when plugged into my Mackie 12/02 mixer, but when plugged into my RME fireface soundcard it immediately goes into very loud oscillation. I’ve checked the interface with other audio sources and I also compared the phantom power voltages and I note that my mixer gives 45 volts whilst the audio interface gives 49 volts. I assume this higher voltage is causing the problem. If so, is there a way to reduce that voltage within the cortado?…..(adjusting the cortado potentiometer makes no difference to the oscillation).
    Thanks Nick.

    #2225 Reply

    brach
    Moderator

    I’ve never heard of that problem before, but I don’t think the higher voltage in itself would cause an oscillation. But to answer your question, there is not an easy way to reduce the voltage in the Cortado. Your can try to tinker with the zobel network (R7 and C3) to see if you can nullify the oscillation, but even then, I doubt that’s going to have any affect on it because I don’t think it’s coming from instability in the XLR cable. My thoughts are that you are somehow hearing a lower harmonic from the switch mode power supply of your converter. SMPS’s can be pretty finicky if they don’t get the proper load they want to see…in which case they can go into “burst mode” and become extra noisy….and Cortado’s do present a very low load. Plug in another condenser mic to a different channel on the converter at the same time to give the 48V rail a bit more of a load. The Mackie has a linear power supply, so it’s much more stable.
    Good luck.
    -Brach

    #2227 Reply

    Nick Burge
    Participant

    I was wrong about changing the bias current not making a difference….I was winding the pot the wrong way!! I’m able to get rid of the oscillation when used in conjunction with my Fireface by setting the bias current high: 0.3v
    Happy to have found a solution and thanks again. Nick

    #2228 Reply

    Nick Burge
    Participant

    …..ahh but now I wonder whether setting the bias this high will damage the FETs?? I note from the original instructions that 0.15 should be top limit.

    #2230 Reply

    brach
    Moderator

    That supports my theory. By setting the bias that high, you are increasing the load on the SMPS, allowing it to stabilize. You now have 1mA going through each FET, which is fine, as long as you are ok with the sensitivity of the Cortado at that bias setting. If you need to adjust the bias back down (for sensitivity reasons) then just do as i mentioned in the previous post and plug in a different condenser mic into another channel on the converter to add a higher load to the phantom power SMPS at the same time the Cortado is plugged in to the first channel.
    Let me know if you have any more questions.
    -Brach

    #2231 Reply

    Nick Burge
    Participant

    That’s great Brach, yes I’m deliberately setting the gain sensitivity high as I am thinking of using cortados within a sculptural installation, it’s a screen of fine elastics that produce very little volume in themselves. Our existing pickup system could do with improvement, hence my interest in your product.

    Thanks, Nick

    #2234 Reply

    brach
    Moderator

    That’s really cool! Please keep us informed with how it works in your installation.
    Good luck.
    -Brach

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