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AM audio processing for the homebrewer

Discussion in 'Amplitude Modulation' started by KI4YAN, Jul 22, 2020.

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  1. KI4YAN

    KI4YAN Ham Member QRZ Page

    Working on an audio processor for driving an 807-based modulator. The modulator's frequency response is pretty Hi-Fi, so I'll need to filter pretty heavy to keep from eventually sending out a very wide signal. (-3dB points are 38hz to 29Khz at 45W out of 65W design output!)

    Currently, I have a four-pole version of the all-pass phase rotation filter, of the type suggested by W3AM Here:

    From there, I need to do some high-pass filtering and ditch everything under 80hz, and some low-pass filtering to cut everything off over 4.5Khz. This should be my "brick wall" filter, to make sure I'm never much wider than 10kHz on the air. To do this, I've got a 4th order sallen-key filter for both the low and high pass filters-that's 24dB/octave slope.

    But, the limiter/compressor is what is eluding me. Is a simple diode clipper followed by the high-pass filter going to be OK? Or do I need to dive deeper into variable gain amps and the like?
  2. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    I assume you are talking about this modulator?

    And if I understand you, you are looking for a solid state solution.

    Here is a limiter which allows you tight control over both positive and negative peaks.


    Attached Files:

  3. KI4YAN

    KI4YAN Ham Member QRZ Page

    That is correct. I'll draw the limiter up tonight and try to get it arranged, ideally the output voltage from the audio processor box will be around 3-4 volts peak-to-peak.

    Am I correct in assuming the signal chain should be mic-mic amp(with internal leveller)-phase rotation filter-high pass filter-limiter-low pass filter?
  4. N1BCG

    N1BCG Ham Member QRZ Page

  5. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    Clark's solution above is very interesting and pretty much defines the audio chain.

    My design needs 0 to 13/5V peak-to-peak voltages nominal for input so you may have to have a X4 gain amplifier ahead of it if you decide to use it and in your system and a /4 voltage attenuator after it. Any discrete limiter such as this produces a certain amount of harmonic distortion when in limiting so a steep slope LPF is needed after it.

  6. KI4YAN

    KI4YAN Ham Member QRZ Page

    That link to the MAX audio processor is very cool. Can't see how to order the board or kit, but all the schematics are available. It even has one thing I was hoping to figure out, but hadn't yet-the gain gate function.

    I'll send over an Email and see if boards/kits are available. If not, I may crib from that design in some places, especially the gated limiter.
  7. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    Well Clark, You and Rick have come up with an excellent design.:cool:

  8. N2DTS

    N2DTS Ham Member QRZ Page

    I started building mine last night, but ran into a snag, the parts list seems to be wrong.
    I mounted the surface mount chip, and sockets for the rest of the IC chips, then started on the caps, but some are clearly supposed to be electrolytic caps in the power supply but are listed as .01 bypass caps in the parts list.

    The MAX is not a kit, its more of an unbuilt product, its not step by step like an Elecraft.
    I wonder what the cost would be if the board was populated in China....
    That seems to be the way everything is going...

  9. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    Does it have SMDs already soldered in?

  10. N2DTS

    N2DTS Ham Member QRZ Page

    No, but its not a small surface mount part, its a larger one with big pitch.
    I have done some that were like hair, the peabury V2 had a very fine pitch surface mount D/A converter chip that was a bit tricky.

    The way my max kit is now, you find a part hole on the board, look on the schematic for the part, find its value, then find the part and install it.
    The instructions say, step 3 install the caps....step 4, install the resistors...

    Its quite a large board, it does a lot of things, love the multi input and outputs each with their own level controls.
    You also have the option to install controls on the front panel, or as set and forget trimmers on the board.
    Its designed by AM guys so it has what we need/want...

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