Broadcast Audio limiter/compressor in audio chain for newbie

Discussion in 'Amplitude Modulation' started by KD6CXW, Jul 28, 2019.

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  1. KD6CXW

    KD6CXW Ham Member QRZ Page

    I'm new to AM and heavy metal. Been reading a lot and understand that a BC transmitter needs the audio limited and compressed .
    I'm in process of acquiring a Bauer 707 and will need a limiter/compressor.
    was wondering if a Behringer DSP 8024 and a vx 2000 would do the trick ?
    already have the dsp 8024 and the person whom has the Bauer has the vx 2000 .
    will post when the Bauer is home and status of unit.
    would like input whats available at reasonable price.
    thanks
    John
     
  2. KA0HCP

    KA0HCP XML Subscriber QRZ Page

  3. W2VW

    W2VW Ham Member QRZ Page

    The 8024 works fine. Eventually several electrolytics will fail and make the unit appear like it's bricked at boot. They are easy to repair.

    The VX-2000 is a nice all in one concept but it's compression pumps and causes more problems than it solves.
     
  4. AC0OB

    AC0OB Subscriber QRZ Page

    I assume the Bauer has been converted to 160m and or 80m?

    My philosophy is to place as little processing as possible between the transmitter and the source so as to not "color" the audio to any extent.

    The better AGC/limiter box in my view is the Inovonics 222 with the NRSC and the Low-Pass functions set to ON.

    You will need a microphone amplifier to increase the audio level for subsequent processing. There are currently some Rane MS1S mic amps available on ebay.

    Rane has a decent write up here wrt mic preamps:

    https://www.rane.com/note148.html


    If you're into homebuilding circuits I have some mic preamp circuits (both tube and SS) I'd be willing to send you or you can google "microphone preamplifier circuits, " or see the writeup for balanced mic preamps at:

    http://www.thatcorp.com/datashts/AES129_Designing_Mic_Preamps.pdf


    Pheel
     
    Last edited: Jul 28, 2019
  5. WA7PRC

    WA7PRC Ham Member QRZ Page

    When I worked as the ET at Symetrix, I became aware of how good their 528E 'Voice Processor' (datasheet) is. It's a single channel unit in a 1U rack-mount box. It contains:
    • a very good preamplifier (w/ phantom voltage)
    • a de-esser
    • a compressor/expander
    • a 3-band parametric equalizer
    Each section can be bypassed from the front panel, or patched into on the rear panel.
    [​IMG]
    [​IMG]
    click for bigly image(s)

    I use the preamp & compressor-expander. Even with significant hard limiting, the compressor is quite clean. The expander reduces gain a bunch when I pause, minimizing the effect of background noise. I have the EQ set to dump LF and slightly emphasize HF for contesting & DXing. I leave it bypassed for ragchewing.

    When I built the shelf unit for my desk, I cut a T-slot underneath, and mounted my 528E there:
    [​IMG]
     
  6. KL7AJ

    KL7AJ Ham Member QRZ Page

    Hi John:
    I would rephrase it to "A BC transmitter can benefit from judiciously applied compression and limiting." If you don't know what you're doing, it's easy to overdo it.
    You definitely want to closely monitor your modulation, and if you don't have a modulation monitor, you need to know how to do this accurately with an oscilloscope....both for positive and negative peaks. I really suggest reading some ARRL Handbook articles and QST articles from the early to mid 1950s.
     
    WA7PRC and (deleted member) like this.
  7. K5UJ

    K5UJ Ham Member QRZ Page

    That's usually true. Broadcast rigs were designed with a lot of audio power with lower modulation transformer step down. The result is that with dynamic audio (human voice) it's easy to clip the carrier off with negative modulation. You can avoid that by speaking in an unnatural even tone keeping the same distance from the mic. at all times, but that's an unenjoyable way to operate and your average audio power will be low. The design assumption originally was that every station would always have someone at the transmitter manually riding gain. Then later on when audio processing began to become widespread, the design assumption was that everyone had peak limiters. Tube ham rigs were never designed for any of that--the design assumption was that a typical ham would plug in a D104 and talk. To keep them out of trouble, modulators were weaker, and the mod. transformer step down was high. This is why some stock ham rigs can barely make it over 90% modulation. If you run a D104 right into a broadcast rig (with impedance matching) yikes, you'll find it impossible to not clip the carrier.
    I'm not familiar with those items but as far as I know, Behringer makes performance and studio gear; not transmit processing gear. Your equipment may sort of work and be better than nothing, but what you really need is something designed for AM peak limiting. These are limiters that are very fast acting and can grab and hold short duration, almost transient peaks, using look ahead or feed forward designs. Keep a lookout for affordable peak limiters made by CRL (Circuit Research Labs) and Inovonics. The CRL line consisted of the PMC 300, 400 and 450. Inovonics made the model 222. The 223 is in production now. There are a lot of these out there and information about them including manuals is on-line. This is all you need if you have already tailored your audio to your voice.
     
    KD2ACO likes this.
  8. W2VW

    W2VW Ham Member QRZ Page

    Hi John,

    You may find audio processing discussions to be similar to discussions of religion.

    I suggest taking advice from folks you can actually listen to on the air with heavier weight.

    Good luck and keep the furniture blankets handy.

    Dave 2VW.
     
  9. AC0OB

    AC0OB Subscriber QRZ Page

    Hi John,

    I don't have the Behringer you speak of but I do have a Behringer SPL3220 two-band AGC/Limiter processor touted as being usable for broadcasting, but no longer use it as I found it to be inferior to the Inovonics 222.


    Pheel
     
  10. N1BCG

    N1BCG Ham Member QRZ Page

    The pitfall with this unit and others that are designed for studio use is that using the EQ to boost any frequencies will cost you modulation. With the compressor placed before the EQ, boosted frequencies such as in the presence range will be louder than the others, causing overmodulation (distortion and splatter) or will require the overall modulation to be reduced to compensate.

    The insert jacks on the 528E can be used to reorder the functions so that the compressor is the last stage before the transmitter. But, given that this isn't intended to keep peaks at bay, a true limiter such as the Inovonics 222 as mentioned would make an ideal compliment to the 528E as-is.
     
    AC0OB likes this.

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