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Markacoustic AH250 input impedance question

Discussion in 'Amps and Cabs [BG]' started by Seba, Jan 13, 2012.


  1. Seba

    Seba

    Oct 4, 2010
    Antwerp, Belgium
    Hi Guys,

    So I finally got myself the Markacoustic AC121 combo, which sounds absolutely amazing! It has the AH250 head, I'm a big fan. I do have a question about the input impedance of the two channels. Channel 1 is 500 Kohm (unbalanced jack, meant for pick up), channel 2 is 1 Mohm (unbalanced jack and balanced XLR, meant for microphone). I'm using a fishmann full circle pu on my upright.

    I tried both channels with the eq and filters flat and I find that channel 2 (1 Mohm) has a much more open and bright sound, more natural. I'm definitely using that one as my main channel. Problem is that doesn't have a mid control (which might come in handy in loud situations) and that I won't be able to combine the pick up with a microphone as I'm allready using the microphone-channel. Too bad because this was one of the main reasons that I bought this combo.

    Question: why does the channel with the higher impedance sound lighter and more natural (the volume is significantly lower, though) and is there a way to change the impedancy of the first channel to 1 Mohm?

    Thank you!

    Seba
     
  2. GrowlerBox

    GrowlerBox

    Feb 10, 2010
    Nude Zealand
    Piezo pickups shown too low an impedance will generally sound pretty "honky", and depending on which one you're using, may sound best with anything between 1 and 10 Mohm input impedance. This is best achieved with an extant preamp, and TBer fleck (Francis Deck) makes a very small, very effective little box (and has the schematic for it on his website if you'd prefer to build it yourself) with a 10 Mohm impedance, a high pass filter and phase reverse switch and a volume pot for something like $50 if he builds it, or you get the parts and build it yourself for nothing (to him).

    There is a wealth of information about this over on the double bass side of TB - search for HPF-Pre.
     
  3. Matthijs

    Matthijs

    Jul 3, 2006
    Amsterdam
    Why would they make an amp dedicated to people using piƫzo's with a 500k input? You need more, as growler stated. Seems a waste to have to include an extra pre before a amp that is meant for this purpose, or maybe they're already assuming we're all using a pre?

    You might not need the full Fdeck pre though, just a impedance buffer might be enough. On Fdeck's site there's also a design for a extremely easy to build, cheap and very small buffering pre that works really well. I installed it in my EUB with good results.
     
  4. Seba

    Seba

    Oct 4, 2010
    Antwerp, Belgium
    Thanks a lot for your replies! The funny thing is that I've tried using my fishman platinum bass di and that the sound of both channels on the ah250 still is significantly different. The pre has input impedance of 10 mOhm which should be perfect for the pu, right? On the db forum jaromir said something the channels being voiced different, but I don't really see what he means by that...
     

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