Audio Processing for AM

Discussion in 'Amplitude Modulation' started by WA3VJB, Aug 30, 2016.

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  1. N2DTS

    N2DTS Ham Member QRZ Page

    Don't feel bad, I parted mine out as well, although I did use it for a while long ago.
    Lucky for me I still had the transformers in the junk box and could re create it.

    Put some extra stuff in along with it.
  2. G0GSR

    G0GSR Ham Member QRZ Page

    Yes, worked for BBC transmitters..
    Night time is the problem with all the close neighbours in Europe, there is something on the majority of adjacent channels.

    During a night shift, I once put a MF receiver onto the TX antenna... Wow, I could hear the whole world. The QRM was amazing....

  3. K4KYV

    K4KYV Premium Subscriber QRZ Page

    The transformers (input and output) are one reason I parted mine out. 10K is too low Z for either application. The line amp I built on the chassis uses a UTC LS-51 30k to 500 ohm output transformer, and the HA series (forget the number) that is about 500 ohms in to 80k at the push-pull grids. I think I still have the meter round here somewhere. It would be easy to replicate the thing with better audio transformers. Luckily, I scored a two-channel line amp from a BC station that had a pair of LS-51 output transformers in it. I use the other one in my balanced D-104 pre-amp. Didn't really have any use for two amplifiers built on one chassis. But my BL-40 modulimiter works a lot better than the AM-864, or the Collins 26-U that I sold for enough money to buy my Sherwood SE-3 sync detector.
  4. W7TFO

    W7TFO Platinum Subscriber Platinum Subscriber QRZ Page

    ....I pledge allegiance, to the tubes made in America. One technology, underused, with emission and linearity for all...

    From a guy that just hates SS stuff; I pushed it enuf for a living.

    WD4IGX and AC0OB like this.
  5. N2DTS

    N2DTS Ham Member QRZ Page

    I find it easy to build tube stuff as its simple and its what I have in the junk box.
    I do not have an issue with solid state stuff or computers/sdr, its all good and fun.

    The transformers may not be correct for the am 864 but it seems to work just fine.
    Its interesting that its a balanced circuit, kind of rare back in those days I think.
  6. W7TFO

    W7TFO Platinum Subscriber Platinum Subscriber QRZ Page

    Virtually all tube-based limiters used push-pull topology end-to-end in the audio chain.

    A single control tube is usually found bossing the gain reduction.

    I've seen the AM-864 in action, it did a pretty good for such a simple device. Not a lot of 'coloring' to the sound.

    It's all in how it is set up.

  7. WA3VJB

    WA3VJB Ham Member QRZ Page

    Frank - yes! Radio reform school! The 9105A also labels its pre-emphasis knob as a "correction."

    Fascinated by your VOA processor, the UREI 1A. Found a link with a good history of that unit. Would like to hear a recording of it in use somewhere.

    Thanks much for your posting of it.
  8. K4KYV

    K4KYV Premium Subscriber QRZ Page

    I wouldn't modify it by taking out the pre-emphasis circuitry or diode hard-limiting features. The stock configuration would make a nice unit for QRN/QRM battle conditions. Maybe switch to different unit, such as the UREI BL-40, when condx are more favourable. Or else, wire in a switching circuit to manually disable the clipper as desired. If adjusted properly, the clipping shouldn't add tremendously to the overall distortion, since it is designed only to mop up residual overshoot occurring with the peak limiter.

    I wonder if the 1A uses the optical driven compressor, like the BL-40. Basically a sample of the audio feeding an incandescent lamp placed next to a photo-transistor that controls the gain.
  9. W2VW

    W2VW XML Subscriber QRZ Page

    Solid state BA slicer.
  10. W7TFO

    W7TFO Platinum Subscriber Platinum Subscriber QRZ Page

    It seems I remember an Edison-based carbon lamp here in a metal box with a Weston Sulphide cell stashed around here somewheres...hmmm...early attempt?

    KD2ACO likes this.

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