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Improvised phantom power for prelim testing

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  • #3805 Reply
    Lloyd-ss
    Guest

    I have a Cortado contact mic kit coming that I plan to use in a stand-alone set-up with its own amp and speaker for amplifying a reasonably resonant wooden tap dance practice floor. Being able to record on a smart phone would be nice too. Looks like I will need to order the Espresso Phantom power kit also and smart phone adapter cable to give a nice clean arrangement. That seems like the safest thing to do since you guys have designed and built these items to work together. Nice work, BTW!
    My question is about prelim testing of the Cortado before I have the Espresso. I don’t have any sound equipment and know nothing about it, so any advice is much appreciated. Please tell me if this will work. I have built a little power amp using a Ti LM386 chip per the data sheet, with the bass boost circuit. It seems to work fine driving a little 8 ohm speaker, so I am comfortable with that for the moment. I also have a bench top power supply that might be usable for the phantom power, but I am not sure. The ground from the power supply would go to XLR pin 1. 48vdc+ (probably more like 36v would be adequate) would go to XLR pin 2 and pin 3. But which pin would I connect to the input of the LM386 amp? And how would I connect the grounds? Sorry for leaning on you so hard for advice, but I do pay attention and learn, LOL.
    Thanks in advance for whatever help can be offered.
    Lloyd

    #3809 Reply
    brach
    Moderator

    Hi Lloyd,
    Attached is a circuit that I just drew up. It shows you how you could use your external power supply as phantom power for the Cortado. As you can see, pins 2 and 3 of the XLR jack are used for both signal and phantom power. The signal (an ac voltage) is superimposed on the DC phantom power. The signal is the separated from the DC via the two 47uF/63V capacitors. Make sure they are rated for a voltage higher than your phantom power. The XLR pins (2 and 3) are separated from each other via the two 6.8K resistors. The two 100K resistors are to give the caps a place to drain when nothing is plugged into the XLR cable. You may need to increase the value of these (to has high as 1M) if the Cortado has a hard time driving the signal past them (ie the output signal is lower than you want). Let me know if you need more explanation or if you have other questions.
    Good luck.
    -Brach
    Cortado phantom power circuit

    #3810 Reply
    Lloyd-ss
    Guest

    Brach, WOW, perfect. Thanks so much for the instant answer. Exactly what I was looking for, and its almost like you went rummaging thru my parts bins to choose the resistor and cap values. Only question I have at the moment is regarding the 1 percent tolerance on the resistors. I am thinking you mean that I should hand select the resistor pairs as closely as possible for actual ohm measurements.
    I will let you know how it works in a couple of days.
    Thanks, Lloyd

    #3813 Reply
    brach
    Moderator

    Most metal film resistors (usually blue in color) are specified to be within 1% tolerance of the given value. This is indicated by the brown stripe at the end of the colored bands. If you are using carbon film resistors (usually tan in color), they are usually 5% tolerance, indicated by the gold stripe at the end of the colored bands. If all you have is 5% resistors then you could hand match them. In practicality it’s really not that big of deal with the Cortado in your application. The weakest link in the chain by far will be how well you couple the disc to the floor boards. The audio fidelity of the system mostly hinges on that.
    Good luck.
    -Brach

    #4095 Reply
    Lloyd-ss
    Guest

    Brach, This worked great. It took me a few minutes when I was trying to set the bias to realize that the Cortado pin 1 needed to be connected to the grounds. Doh! Many thanks! I am starting another thread about trying the Cortado as a tap dance floor mic.
    [IMG]http://i226.photobucket.com/albums/dd79/loyd500/Cortado/Cortado-assy-1_zpsghknxpcu.jpg[/IMG]

    #4096 Reply
    Lloyd-ss
    Guest

    Prelim Cortado set-up for tap dance floor

    #4105 Reply
    brach
    Moderator

    Excellent!

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