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Adjusting AMEK 9098 mic preamp's CMRR trimpots and capacitor?

Discussion in 'Preamps / Channel Strips' started by mickeyratt, Aug 8, 2013.

  1. mickeyratt

    mickeyratt Active Member

    Dear Friends

    Please forgive me, i'm new to this forum, sol maybe this thread discussed before.
    I'm experimenting with a diy mic preamp, based on AMEK 9098 mic pre schematic.
    I'd like to know, is there a good/reasonable method to set the input variable capacitor and the two trimpots(LOW/HIGH CMRR TRIM)?
    On the attached schematic the parts: VC1,VR14,VR15.


    I heard, that the adjusment must be done at mid gain setting (30-36dB) and the trimpots are for mid frequency, and the VC1 is for high freq.
    I imagine to short the two inputs, fed with some sinusoidal sign, and with an oscilloscope, monitor the outputs to have a minimal or zero sign between the high and low differential outputs (XLR pins: 2 and 3)
    Is that correct?

    Thank You in advance!
  2. Boswell

    Boswell Moderator Distinguished Member

    Welcome to the forum!

    I'm sorry to say the schematic you posted is scarcely readable. I can't identify VC1, VR14 and VR15, so I can only give the general method to set up this type of adjustment on a differential amplifier.

    You need a signal generator and an oscilloscope. You won't be able to connect the oscilloscope usefully to both the the +ve and -ve outputs of the amplifier, so just use the +ve output (XLR pin 2) and ground (XLR pin 1). Set the generator to give a 100Hz squarewave and apply the output to both the +ve and -ve inputs of the amplifier shorted together. Adjust the waveform amplitude so the spikes in the output at the transitions of the squarewave do not clip as seen on the oscilloscope.

    It's now a matter of adjusting the three components (VC1, VR14 and VR15) to give the best compromise between minimum step at each squarewave transition, coupled with smallest spike on the rising edge and flattest wavetop. If you could find a way of posting a legible schematic I could give more specific suggestions for this particular device.
  3. mickeyratt

    mickeyratt Active Member

    Oh dear Boswell

    Thank You for your reply and help. Sorry for small sized picture, the RO has reduced the size automatically, so i upload the picture to an external link and edited the first post.
    It would be nice, if you can see the schematic.


    Thank You so much in advance!
  4. mickeyratt

    mickeyratt Active Member

    Just some photos.
    It is now "working progress..." state, but it works (without any adjusments), and i love the low noise, what i get at maximal gain thumb

    It was the longest time to arrange components in my head before soldering because i have no source to longer PCB.

    gain_sw.jpg pcb20.jpg pcb21.jpg pcb23.jpg

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