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Mic 'works' in one preamp, not in another....helphelphelphelp....

Discussion in 'Preamps / Channel Strips' started by Jeemy, Jan 20, 2010.

  1. Jeemy

    Jeemy Well-Known Member


    Hoping somebody can shed some light on this, its got me confuddled.

    Basically removing all other variables, I have a pair of Charter Oak M900 SD mics and one is acting strange.

    They work fine (or seem to) when plugged directly into an RME Fireface, or a Ramsa digital mixing desk.

    However when plugged into a transformer-balanced preamp - either a JLM TMP8 or a Sebatron VMP-4000e - one mic exhibits about 20dB less gain than the other, and when the gain is brought up to match in levels, it exhibits a distinct buzz.

    I've traced and tested every cable back through and tried all of these connections directly to the pre's and via headphones so I'm sure its not any part of my signal chain.

    I can replace the M900s with HM-1s and both channels work fine - its really confusing me; does anybody have any ideas what might be causing this? I'd say the mic was faulty but it sounds OK through the non-transformer balanced pres; it seems to have a bad reaction to a transformer-balanced system.


  2. Spase

    Spase Active Member

    Is the issue channel related? If you swap the mics/cables does it stay in the same channel or follow the mic?
  3. Jeemy

    Jeemy Well-Known Member

    I've tried everything like that - it follows the mic, but only on the transformer pres, not the transistor ones.

    Following my recent flood everythings been re-cabled with 4-core Canare, I've tested every line with a circuit tester from the cable leading to the wallbox, right through to each preamp stage, and then out of each preamp right to the A/Ds.

    The behaviour is verified, and if I swap the M900s for HM-1s, it stops.

    So Mike at CO is replacing the mic, assuming its the head amplifier. But what I don't understand is why it appears to perform fine on the 'transistor' pres - the ones in the Ramsa and the RME FF800.....
  4. Link555

    Link555 Well-Known Member

    Hey Jeemy and spase, the XFMR input impedance is typically 600ohm, does your mic have a faulty FET output getting loaded? Does the mic have a transformer in it? Just thinking out loud so to say...
  5. Jeemy

    Jeemy Well-Known Member

    I think that must be it, the transformer loads the mic and forces it to reveal the head amplifier fault, but a non-loaded pre doesn't do it. Would like to learn more if anybody needs some info.
  6. Link555

    Link555 Well-Known Member

    But does that mic have a output XFMR in it?

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