Need A Modulation Transformer For That 500 Watt Final?

Discussion in 'Amplitude Modulation' started by W2VW, Jun 4, 2020.

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  1. WZ5Q

    WZ5Q Ham Member QRZ Page

    The transmitters capability of BOTH handling positive peak modulation over 100% and high average modulation is the key for loud and clean transmissions. My natural voice asymmetry has 120% positive peaks and I do nothing to either enhance or detract from this natural asymmetry. I build my transmitters with Headroom, Headroom, Headroom to accommodate my voice.
  2. W2VW

    W2VW XML Subscriber QRZ Page

    Have you not stated your hearing is somewhat damaged?

    Negative modulation also is important in recovered audio volume.

    Everything in an AM system is a balance between effectiveness and unwanted distortion.
    AC0OB and W2BTK like this.
  3. N2DTS

    N2DTS Ham Member QRZ Page

    Yes, very.
    But I can hear loudness and distortion ok, I just can't hear anything over 3000 Hz.

    I have a hard time thinking anyone could hear the difference between 100% both ways and 120% positive and 100% negative.
    Maybe in an A/B comparison, but in any random signal?
    Can you listen to someone (just listen) and tell me they are doing 113% positive?
    How much difference can you hear, the difference between 110 and 120% positive?

    Just wondering how you would hear that.

    I look on the scope and see people with crazy peaks sometimes but with low average audio.
    I also see people with 110% negative and 90% positive that sound loud and clear.
    N3juh Jim often sounds and looks like that, he is mostly loud and clear, and easy (for me) to understand.
    Without the scope I would think Jim has lots of positive peaks as he sounds loud and clear, while some others with positive peaks exceeding 120%
    do not sound as loud and I would think (without the scope) that they have 60% modulation.
    I can't think of what peaks sound like...can you hear the difference between negative peaks and positive peaks?
    How do they sound different?

  4. W2NBC

    W2NBC Ham Member QRZ Page

    Bringing up the “average” is the yellow-brick-road to loudness. However, as WZ5Q mentioned, its all about the headroom with iron and a flat frequency response.

    A transmitter capable of 150% positive and close to 100% NEG (important* W2VW) with a flat and broad audio response can be controlled with an average modulation profile and audio response any way you want with outboard processing. If the transmitter can only produce 100% positive due to inadequate iron, ratios, and restricted response, then processing is much less effective.

    There is also a caveat:

    Average increase in “loudness” produced by processing can stress transformers and components. Gates transmitters have a whole lot of paper-weight and blown mod iron AFTER the rush to “loud” via processing was the new normal..

    If you want to test the difference between 150% positive vs 100% AM check the sideband peaks in USB/LSB mode. The actual signal detected will have more energy (RF signal) at 150% vs 100, therefore greater signal/noise. When you combine both side bands with carrier (AM) the detected audio will be louder especially if a higher average index is applied.

    There’s no free lunch with this stuff but there is a discernible difference!
    K5UJ and AC0OB like this.
  5. K5UJ

    K5UJ Ham Member QRZ Page

    I'm not against headroom but I think more carrier and 100% up and down is the way to go for shortwave AM if there's a choice between that and less carrier and higher positive peaks.
    Another thing that really makes a big difference is good interstage iron, p.p. driver stages and low distortion between the mic. and the modulator. It isn't about frequency response so much as it is having clean clear audio. When trying to copy a signal that's in and out of the noise, distortion free audio makes a big big improvement in readability. This is where ham rigs that have a lot of single ended stages and lower quality iron start failing to come through. That slightly distorted audio that's barely getting through the noise level makes intelligibility a real challenge compared to a weak but clean clear sound.
    W2NBC likes this.
  6. N2DTS

    N2DTS Ham Member QRZ Page

    Jim often runs a bc610, now with a replacement mod transformer which is an auto transformer, and does not allow 100%
    positive but he still sounds loud and clear and very understandable.
    Some, like KG2IR (who knows something about audio) runs 80% modulation and more carrier.

    Sure, its good if a transmitter can do 150% positive without any distortion with a response 20 to 20,000 but is it worthwhile
    for voice?

    As my hearing has gotten worse with age, I enjoy hearing audio voice equalized and find the hifi stuff less understandable.
    I used to think most Globe Kings were junk and a bc610 was no good, but now I really enjoy working them.
    I still think a KWS-1 sounds BAD though....
  7. W2NBC

    W2NBC Ham Member QRZ Page

    To quote my prior post:

    “A flat and broad audio response can be controlled with an average modulation profile and audio response any way you want with outboard processing”.

    Totally agree that “20-20K” is a goal that in practical use on the air is NOT good operating practice, period! However, in the design of the various stages it’s nice to know that with “headroom” ANY limits to frequency response (bandwidth) and averaging (density) is made with external processing and not because of transmitter design limitations.
    K5UJ and WZ5Q like this.
  8. N2DTS

    N2DTS Ham Member QRZ Page

    True from a technical aspect, and fun to try and get to that point, but really, why any better then running a mostly stock globe king or bc610/T368?
    Its not like they sound really bad mostly stock fed with good audio and run correctly...
    The only BAD sounding rigs I have heard in the last few years are rice boxes and a KWS-1.
  9. W2VW

    W2VW XML Subscriber QRZ Page

    The only thing setting many AM operators apart from those who use ssb is the pursuit of the art of AM generation.
    AC0OB, K5UJ, WZ5Q and 1 other person like this.
  10. K4KYV

    K4KYV Subscriber QRZ Page

    The greater part of being a ham radio operator is about construction projects and talking to other hams about stuff we built or modified.

    Something that is increasingly a lost art, but still alive within the AM community.
    AC0OB, K5UJ, AG5CK and 2 others like this.

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