Mic Preamp IMPEDANCE of the Yaesu FTDX1200

Discussion in 'General Technical Questions and Answers' started by IZ0FYS, May 22, 2020 at 2:34 PM.

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  1. IZ0FYS

    IZ0FYS Ham Member QRZ Page

    Hello, I cant understand what impedance is the the yaesu ftdx1200 input microphone preamplifier .
    Any good technician who can analyze this BJT stage?
    I add the schematic:

    [​IMG]

    thanks
     
  2. KM3F

    KM3F Ham Member QRZ Page

    If standard parallel Ohms law is used and not knowing the value of the mike impedance, it figured out to about 2200 ohms. This assumes the mike is a Dynamic type that has some dc resistance to complete the parallel path to ground.
    The series resistance in parallel with the 2.2k from Base to ground is ten time higher so does not have much effect on the total DC resistance. The Caps to ground have more effect via their AC reactance and affects the audio frequency response that gets amplified.

    The first stage audio amp is Cap coupled from Collector to the IC chip and appears to have further frequency shaping and bypassing.
    Actual mike impedance is not all important as long as it has enough output to drive the radio to acceptable levels on SSB, AM and FM.
    Just as a extra comment, the input frequency shaping puts some limitations on using mikes of wider frequency range without amplified outboard equalizing to try forcing a wider band audio into the radio, and up to the limit of the Tx filtering or method of audio band width control at conversion to RF.
     
    Last edited: May 22, 2020 at 4:53 PM
  3. G0GSR

    G0GSR Ham Member QRZ Page

    Mic impedance or anything external does not affect the input impedance of the circuit.

    It's [R1277(22k) || R1276(100k) || R1278(22k) || Q1044 base (>68k)] in series with R1279(2.2k)
    So about 10k.

    Capacitors C1520 and C1472 are RF decoupling and have negligible effect at audio but there is significant bass roll-off caused by C1470 and C1421

    Frank
     
    AB6RF and K7TRF like this.
  4. IZ0FYS

    IZ0FYS Ham Member QRZ Page


    Frank, you are super !
    A friend of mine in these last hours found a simple trick to measure the impedence injecting a little mV on the BF path and with a variable resistor found the point where the voltages was the same on both sides, then checked the ohm of the variable resistor in that position....and....the result was 10 K !!!!
    I wrote to Yaesu and they simply replied "it's a standard 600 ohm like all radios"....what a shame!!!!!
    Thank you Frank!
     
    Last edited: May 23, 2020 at 6:00 PM
  5. IZ0FYS

    IZ0FYS Ham Member QRZ Page

    they are making Hi-impedance preamps ......that's why these radios have muffled audio with a lots of mics.
    It's like putting a dynamic mic into a standard Line-IN.....and not into a MIC input....
    Years ago they used to make mic preamps with lower impedance and mics sounded clearer and with less basses...
    They call "warm audio".....but I think it is not a good idea, mics must be set on the right impedance, usually 2 to 10 times the mic value.
    If we have a 150 ohm mike it would be 300 to 1.5K.....
    for a 600 ohm mike it would be 1.2K to 6K.
    10K is too much......
     

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