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impedance question

Discussion in 'Microphones' started by DonnyThompson, Jul 22, 2015.

  1. DonnyThompson

    DonnyThompson Distinguished Member

    ( also @Boswell @audiokid )

    Hi Dave... I have a question about the ADK AP Preamp... I know you have one, so I thought I might pick your brain about on thing in particular...

    Last night I was tracking guitar for a song, and I was using an MXL 860XL Ribbon mic ( and a Senny 421) on a Marshall amp ( the mic combo sounded great, by the way - as of course did the amp as well - but that mic combo is one of my go-to favorite arrays for guitar amp miking) and I noticed that the Ribbon mic's signal was incredibly hot going into the DAW from the ADK pre. This had never happened before, I had the gain on the ADK set at only around 30% or so ( this was just the position it happened to be in when I turned it on), and the signal was very hot, bordering on distortion. I had to to turn the gain down to only about 20% or so to avoid overloading.

    I then noticed that the impedance button on the pre was engaged, ( which I had never used before while using this ribbon) so, I turned it off, at which point the level dropped considerably, and I was able to bring the gain up to about 75%, with a nice, quiet, clear, healthy level before distortion occurred.

    (FWIW, I was using the Line In of a Focusrite i/o to get the signal into the DAW, which had no added adjustable gain, and the XFO/OpAmp combo in the ADK was a Sowter/JH990c)

    In the end, after getting the gain structure right, the result was a great sounding track, the ribbon worked wonderfully, very warm and smooth in its tone.

    So my question is... have you ever found the same thing to be evident on your ADK pre when using a ribbon mic and the impedance button? I couldn't believe the amount of gain that was added by inadvertently engaging that impedance switch... or did the transformer combo have anything to do with this? I have noticed gain differences between different XFO's in the AP, but never anything this dramatic...


  2. Boswell

    Boswell Moderator Distinguished Member

    I've not seen any schematics for the AP-1 or -2 pre-amps, but it is possible that the impedance switch has different effects on different transformer plug-ins. One explanation for your observation could be that, with the Sowter transformer, the impedance switch selects different turns ratios on the primary. This would give both an impedance change and a voltage gain change.
  3. pcrecord

    pcrecord Don't you want the best recording like I do ? Well-Known Member

    I always get a db boost when choosing higher impedance on my ISA preamps. But not so drammatic as you experienced Donny.
    But I can say that this db boost is not present on my LA-610... so I think Bos is on to something, either yours activate something else in the preamp or it made the mic go crasy..
    It's a bit rare that someone would complain about the output level of a ribbon. how was the sound ?
  4. DonnyThompson

    DonnyThompson Distinguished Member

    @Boswell @pcrecord

    Thanks guys....

    Sound wise it was great - after I was able to get the gain structure set right. I think Bos has it - I had another tracking session last night (live drums, where I used the ribbon as a room mic) - but for last night's session, I used a different XFO combination this time - a Jensen XFO and a Seventh Circle Op Amp, and I didn't have happen what had happened the night before with the Sowter/JH990C.

    Hind-sight being what it is - I wish I would have paid more attention and had actually measured the gain difference / DB boost that the Sower/JH990 combo added when the Z function was engaged; but I was working under a time limit and I had to get things running smoothy and fast. When I get a bit of free time, I might replicate this scenario, and this time, see if I can measure the actual db difference.
  5. Davedog

    Davedog Distinguished Member

    The impedance switch is actually a series impedance loading circuit designed for vintage type mics that have little gain. Its not a wonder that your ribbon which is of a more modern design was as loud as it was. I don't know if you used the other setup with the impedance switch in place but it does make a lot of difference in some configuations..

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