Resonant speaker.....

Discussion in 'Straight Keys - CW Enthusiasts' started by KB4MNG, Jul 1, 2019.

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  1. KB4MNG

    KB4MNG Ham Member QRZ Page

    Followed this video to make a resonant speaker. I could not believe the difference it makes. It's like a filter, it will bring out signals you can barely hear and make them loud. Give it a try!!

    KL7KN likes this.
  2. N2EY

    N2EY Premium Subscriber QRZ Page

    A resonant speaker IS a filter. But you still want a filter in the receiver IF.
    WA7PRC and K7TRF like this.
  3. KB4MNG

    KB4MNG Ham Member QRZ Page

    A very good filter..
  4. N2EY

    N2EY Premium Subscriber QRZ Page

    Sort of.

    A resonant speaker is essentially a 1 pole filter, which means a sharp peak but wide skirts. It also has harmonic responses.

    In receiver design, best performance is obtained by putting the selectivity as close to the antenna as possible. That's why an IF filter is better than an audio filter. Not that audio filters are bad, just that the issues need to be understood.
    AF7XT and WA7PRC like this.
  5. KB4MNG

    KB4MNG Ham Member QRZ Page

    good info thanks
  6. N2EY

    N2EY Premium Subscriber QRZ Page

    There is a very good article in QST for January, 1957, called "What's Wrong With Our Present Receivers?" by W1DX. It's as relevant today as it was then.
  7. K7TRF

    K7TRF Premium Subscriber QRZ Page

    FWIW, I run various audio filters when I'm digging weak CW signals out of the noise and they definitely have their place. Your resonant speaker is an acoustic audio filter so it behaves similarly to a tight filter placed in the audio chain as Jim points out and can be a very handy addition to a CW station.

    Some of the issues with relying on audio filtering are AGC pumping and potential intermodulation products in the later stages of the receiver when there are strong signals within the IF passband. Basically if you have a relatively wide IF filter like an SSB filter and are relying on audio filtering you can end up in situations where you've got one or more strong signals that fall within the SSB filter passband and create artifacts that the audio filter generally cannot remove.

    AGC pumping can occur when you're trying to copy a weak signal and there's one or more strong signals within your IF passband and inside the portion of the receiver controlled by the AGC loop. The AGC will respond to the total energy within the IF passband and if that includes signals much stronger than the one you're trying to copy then the AGC will tend to follow the strong signal(s). Basically when the strong signals key up your receive sensitivity decreases and when they stop keying your receive sensitivity increases again. So even though you've got an audio filter running and perhaps don't hear those stronger signals directly your receiver's noise floor is pumping up and down in response to those stronger signals inside the receiver's AGC loop which can make copy of the weak signal very difficult. Ten Tec had a number of rigs like the early Omni series and the Corsair series that included an audio filter for CW work but placed their AGC detector after the audio filter so that filter was actually inside the AGC loop. That system worked really well in terms of eliminating AGC pumping but also created some other artifacts like a loud popping on the first key down character of strong signals which could be really annoying when tuning around the band and encountering a strong CW signal.

    The other potential problem with relying on tight audio filtering for pulling out weak signals is that two or more strong signals within the receive chain passband can mix together in later stage mixers to create intermod products that land on or very near the frequency you're trying to listen to and that can mask out weaker signals even though you've filtered at the audio stage. Good receivers tend to have a lot of dynamic range in the first mixer stage that helps reduce the chance of intermod products right at the front end but ideally there's a good filter narrow enough for the mode you're operating right after that first mixer. In recent years these have been called roofing filters, but they're just the first IF filter and ideally they're placed very early in the receive chain so that any stages down stream don't have to deal with strong off frequency signals. You really want to get rid of strong adjacent channel signals as early in the receive chain as possible which is the opposite of audio only filtering that tries to take care of those nearby signals at the tail end of the receive processing chain.

    All that said, narrow audio filtering whether something like an audio DSP unit, an old school op amp based peaking filter (e.g. the Autek QF-1 or QF-1A) or a resonant speaker like you built can be a great addition and very helpful for pulling out weak signals but in crowded band conditions some good filtering earlier in the processing chain can be very important.
    N7BDY and N2EY like this.
  8. W2VW

    W2VW Ham Member QRZ Page

    The simplicity of a paper towel roll cardboard form factor as used as an acoustic filter is akin to magic.

    Simplicity and performance.

    Enjoy the moment.

    Enjoy amateur radio.
  9. KF5LJW

    KF5LJW Ham Member QRZ Page

    KU4X, N2EY, AD5HR and 1 other person like this.
  10. KB4MNG

    KB4MNG Ham Member QRZ Page


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