Screen/Plate Modulated 50W Input Mod Reactor Question

Discussion in 'Amplitude Modulation - AM Fans' started by W2WDX, Jan 4, 2018.

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  1. N2DTS

    N2DTS Ham Member QRZ Page

    I just think my screen modulated stuff sounds cleaner then the plate modulated stuff.
    Next up is the AB1 modulated stuff.
     
  2. W2WDX

    W2WDX Ham Member QRZ Page

    That's precisely why I plan the way I do. Many will argue saying you only need say 200- 4k. But using that range as a brick wall end-to-end and assume nothing happens outside is poor design practice. People always think amplitude response only matters in the specified range. Not so. Things outside a given band pass will effect things within. For a variety of reasons. K4KYV touched on the effects of core saturation, but this is just one aspect. (This was the issue that prompted this thread). The old thinking of "I get fine signal reports" and "You don't need broadcast bandwidth" is so arbitrary, since receiver specifications vary greatly; as well as the issues of propagation variations. If your signal is tilted greatly or your density is not high enough, some receivers will hear you well, while others will not. What people hear should not replace measured performance. Audiophiles (and the stupid voodoo science they follow) are a great example of what happens when people rely on subjective opinions over tested and verifiable data.

    This is the reason why transmitters like Apache have audio problems. Small transformers, less of them (no reactors) to reduce cost and weight, led to the Apache having very scratchy audio. It was a combination of forced narrow bandwidth due to limits in the iron along with filters to roll-off band margins and try to compensate for the effects of marginal iron. So I always try to push the audio response within transmitter modulators and preamplifiers. If they can handle very wide response all the way through, then I can limit response width within audio processing external of the transmitter. I know the transmitter will have marginal effect. It's not that I want to transmit broadcast bandwidth; it's more a matter of what bandwidth I choose for communication quality will pass with minimal negative effects from out of band artifacts within the transmitter.

    Another problem is on the other end of the audio band. Audio HF overshoot/ringing or roll-off are also similar negative effects from limited bandpass capability of a transmitter. Making sure you have flat response beyond one octave of transmitted audio bandwidth is a good benchmark. I set my criteria goals for two octaves beyond and settle for one.
     
  3. N2DTS

    N2DTS Ham Member QRZ Page

    Clean audio is great.
    So is 20 to 15,000 Hz response.
    Many do not have the ability or the interest in limiting the audio bandwidth into the transmitter and have no idea their 20 Khz wide signal does not really
    fit between a qso on 3873 and 3885.

    I myself have no good way to limit my response.
    If I use a good microphone into the screen modulated transmitter (or even the plate modulated ones) I can have audio out to
    at least 15,000 Hz with no GOOD way to brick wall it below that.
    A nice strong 400 watt signal 30 KHz wide or more. Sounds great.
    I can turn the EQ down 12 Db at most, not very helpful...
    The best rigs are the sdr radios, as they CAN brick wall at whatever you set them for, plus you can see how wide you can be and fit in the space.
    They have profiles you can pick with all the settings (EQ, width) at the click of a mouse.

    A broadcast or class E rig can pass a lot of high frequency stuff and most have no brick wall filter.
    Some THINK they have a good filter, its 30 Db down! Well, if you are 50 over S9, 30 bd down is NOT very good.

    That is why I like to run a D104 most times, it does not really make much audio above about 5 KHz.

    I have come to like modern solid state rigs and old vintage somewhat stock rigs for their narrow bandwidth as most can sound quite good with the right EQ.

    In the very remote past, I used to try and get the best fidelity out of a rig, now I want a clean very good sounding signal that is not very wide.
    That seems to be harder to do if you have a transmitter that passes a wide response.
    What I would like is a very good filter that you could adjust with a knob, say 3000 Hz to 6000 Hz, 50 db cut or more.
    That would be very nice...
     
  4. W2NBC

    W2NBC Ham Member QRZ Page

  5. N2DTS

    N2DTS Ham Member QRZ Page

    That is fantastic, thanks!!

     
  6. W2WDX

    W2WDX Ham Member QRZ Page

    I was using CRL Systems and/or analog Optimod AM for audio processing, however either just didn't have all the functions I wanted. I ended up with a software solution. A program called Breakaway Broadcast.

    I'm lucky in that I have very good audio interfaces for my PC's, pro-audio types not computer interfaces. Breakaway allows all the capabilities of SDR's and much much more. It has all of the features you would find in a Digital Optimod like all the audio processing, like multi-band compressors/limiters, various EQ types, brickwall hi/low-pass filters, etc.

    You can use it to look at an RF sampler output and adjust things like transmitter LF tilt, HF overshoot/ringing, set asymmetry, and even perform negative peak limiting with up to 150% positive. The software acts as the scope, and I have verified it against a real one. You can also play an audio track recorded from your station mics that will play back as a loop to make adjustments, and monitor either the sampler sound or the processor output without worrying about feedback or headphone issues.

    What I really like about it is you can save profiles for different microphones used on different transmitters, so you can just have everything wired up to everything, and just switch the I/O on the audio interface via each stored profile. This way I can have two, three or four mics, connected to up to four different transmitters, with every combination stored in memory. If I want use the TLM-103 with the Gates, I select that profile. The RE-20 with the HT-37 ... select that profile. I can even set profiles up for different modes on specific transmitters and mics. No patching microphones, or moving cables around or any of that. Really nice and very effective.

    Of course PTT is a different issue, handled separately.
     
    Last edited: Jan 30, 2018
  7. AC0OB

    AC0OB Subscriber QRZ Page

    I just finished an Apache upgrade and the stock inter-stage driver and modulation transformers are sufficient for the needed bandwidth. Upper frequency roll-off starts at about 5.8 kHz.

    The crummy audio of the Apache is due to the speech amplifier's coupling caps and the Splatter Filter.


    Phil

     
  8. K4KYV

    K4KYV Subscriber QRZ Page

    Not to mention that stupid 470k resistor strapped across the audio input line that goes to the grid of the mic preamp stage. :rolleyes:
     
  9. N2DTS

    N2DTS Ham Member QRZ Page

    The break away broadcast stuff sounds very impressive.
    What is the latency through the system?
    How does it interface with the computer, just the plug in sound card?
    Anything usb seems to add 500 ms of delay on any computer I ever used....
     
  10. KM1H

    KM1H Ham Member QRZ Page

    The WW2 Collins ART 13 mod transformers are still common even NOS. They look small but run fine at 100W out from an 813 with 811 mods. I run my ART-13 with a modified audio driver and a D-104 and reports are always complimentary even tho the low end is about 300 Hz; I could care less since my voice is not low pitched. It also has a screen winding.
    A few on here have heard it on 17M.

    Knowing the specs you can back into what tubes to use for 50W.

    Carl
     
    Last edited: Feb 10, 2018

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