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Audio feedback problem

Discussion in 'General Electronics Chat' started by Space Varmint, Oct 16, 2008.

  1. Space Varmint

    Space Varmint New Member

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    Wrong again Nigel.

    You are only concerned with frequency deviation up until the point that you demodulate the signal. Then and only then will you be concerned with dealing with the intelligence in a class A fashion.

    You are trying to tell me that class C will not amplify frequency deviation?

    No no. Wrong.
     
  2. Space Varmint

    Space Varmint New Member

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    That's a very good example. It shows what a strong SSB signal sounds like with out BFO, then he turns on the BFO and ultimately set the proper beat note. I think it sounds just fine.

    2KHz or 3Khz, not much difference because you add the carrier. The carrier adds allot to the modulation intelligence.

    This is the beauty of SSB. In a normal AM signal you transmit a carrier and two sidebands. The actual intelligence is in the two sidebands but only one is needed. So think about the energy needed to continuously broadcast this carrier and also to modulate the two sidebands. For 100% modulation you need twice the power of the carrier. So if you are transmitting 100 watts, your power output will peak up to 200 watts with 100% modulation.

    Now consider an SSB signal. You have no carrier so there is half the power not needed already but it even more than that because, unless there is actual intelligence being sent such as a guy saying the word "mommy", there is no output at all. When the operator does speak, he is only transmitting one sideband so it is 1/4 of the power needed to send what a normal AM signal would put out for the same effective signal strength being heard in a receiver.

    In fact is really neat to watch your power output meter as you talk when transmitting SSB. It will bounce up and down with your modulation. If you do not speak, there will be no output power at all provided you have everything tuned correctly.

    One of the things you have to pay particular attention to in a linear amplifier is to set the proper standing base current. This is because you never want the the amplifier to turn off or you will have something other than class A operation and this degrade an SSB signal horribly. The IDC is usually spec'ed in the data sheet.
     
    Last edited: Oct 26, 2008
  3. Space Varmint

    Space Varmint New Member

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    Oh brother....how trivial. If you think that a filter which is all the capacitor is, is a "high pass" filter....will completely cutoff all the low frequencies then you are dead wrong!

    All it does is attenuate the frequencies, in this case the lows. The components are still there.

    Imagine if just by adding a capacitor we could completely remove undesired frequencies. WOW! How easy and convenient would that be??????:rolleyes:
     
  4. dave

    Dave New Member

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  5. audioguru

    audioguru Well-Known Member Most Helpful Member

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    I showed you that your high-pass filter started cutting frequencies below 13.3kHz.
    You want to hear 100Hz to 3kHz but your low value coupling capacitor has reduced 100Hz to 1/100th and reduced 3kHz to 1/4. The result is a severe peak at 2kHz and your sound is like a whistle.
    Why don't you filter out the hum instead of filtering out the audio?

    The other SSB audio that was posted sounds good and not like your whistle.
     
    Last edited: Oct 26, 2008
  6. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Not at all, it will amplify anything - BUT a class C amplifer requires a VERY strong input to work at all, completely useless for a receiver.

    As you're so 'knowledgeable', perhaps you could point us to a receiver that uses class C IF stages? (or anything at all other than class A).
     
  7. Space Varmint

    Space Varmint New Member

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    Anyway, here is a straight AM recording of the receiver. I put in a switch to widen the band pass a little. It's just a recording of Brother Stair...lol.

    You can hear some hum. I'm using an old power supply I built a few years back. I'm stuch with using that one because the receiver runs off 6 volts and I have a 5.5 volt outlet on it. I do not have any enormous capacitors to put on it right now.

    The hum is definitely the power supply.
     
  8. Mikebits

    Mikebits Well-Known Member

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    All the caps in the land will bypass squat if your AC hum is on your ground. I would suggest you play with different grounding techniques to reduce hum, assuming all your other circuits are well bypassed with 120Hz filter caps.

    That's what I am thinking... But then again I have not built a radio since moby dick was a minow.
     
  9. Space Varmint

    Space Varmint New Member

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    Actually, have you ever looked at a good linear power supply? I'm talking clean. If you got to "Ham Radio Outlet" (HRO) or "Amateur Electronics Supply" (AES), they sell "Astrons (lower end) and others like a higher end "Kenwood" power supplies.

    In any one of them, look at the filter caps. I mean they are the size of a can of dog food. I'm not lying and they work too. I used to have one.

    Sure there are some techniques you can use like choking the power leads with ferrite toroids etc. But if you want zero hum, you need a whopping capacitor. Now you don't have to do that with a switcher but a swicther won't hold up to various types of transmissions like SSB and especially CW (Morse code) which is something I like to use allot.

    That sudden current demand when you key up a 75 to 150 watt transmitter, and much higher in some cases, has to be a good linear power supply. They have a very low surge impedance. Consequently you need one of those giant caps in your power supply or the other guys receiver will pick up your hum. It's kind of embarrassing if you know what I mean.

    See, one of the first things we do when establishing a QSO (contact) is to give a signal report. We do it like this:

    Your RST is: example 599 is best

    RST is Readability, Signal Strength & Tone

    So 5 is highest on readability and 9 is highest on other two

    But actually on the receiver I only get the hum when the signal is real strong. I guess on weaker signals the noise hides it. I'm really pretty safe with SSB. You heard my SSB recordings. I think the filter cap in my power supply is only like a 470 uf. You need something like 400,000 uf. No kidding.
     
  10. Mikebits

    Mikebits Well-Known Member

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    Well, not to belabor this subject, but I use to work for a company called SWAN Electronics then became Cubic Comm Inc. All we made were HF radios. We used switched supplies, and sometimes linear, but never had hum from the supply. We did get a few birdies from the switchers, but really not much.

    If you are getting hum, I would suspect grounding issues or power.
    Maybe to much gain somewhere?
     
    Last edited: Oct 28, 2008
  11. transistor495

    transistor495 Member Forum Supporter

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    Hi Space-Voomit,
    Your power supply is working fine! Problem is with your power amplifier TDA2822 which doesn't have a ripple filter. Since your radio is working at mono mode look at the excellent TA7368P from Toshiba which has inbuilt filter. I guarantee it will work great at 6V. Vcc(2-10V, average 720mW).
     
  12. Space Varmint

    Space Varmint New Member

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    Man, look. I know you are the audio guru. There is no question in my mind. You have helped me many times as well as others and I appreciate you to. Your a good guy but sometimes you are a little harsh and like I said before, you might frighten some of the newbies to electronics. Me, I can take it. But speaking on behalf of some of the newbies, you really should try to be a little nicer. :) Smile a little when you slam somebody...lol.

    *edit*

    One thing I've noticed about you is you can take allot of backlash yourself, you mean old fart. Hey, your alright though.
     
    Last edited: Oct 29, 2008
  13. Mikebits

    Mikebits Well-Known Member

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    On behalf of AG, "Sorry Miss"...:)
     
    Last edited: Oct 29, 2008
  14. transistor495

    transistor495 Member Forum Supporter

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    I've noticed that audioguru's comments are neither acidic nor basic means pH is 7(neutral):D
     
  15. Space Varmint

    Space Varmint New Member

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    Miss? You trying to get on my bad side? Your doing a good job.
     
  16. Space Varmint

    Space Varmint New Member

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    Hey! Where did you come from? I almost didn't walk back a page but thanks to Mikebits insult...lol.

    That is very interesting. Hmmmm. Let me start by looking at the data sheet but I do know that the power supply must be clean clean clean to transmit a hum free signal. Thanks man :)
     
  17. Space Varmint

    Space Varmint New Member

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    Hey Mike? I can count 2 birdies on this one from 3 t0 30 MHz. Not bad huh? I might have missed one or two but not sure yet. I think only two.

    Anyway, you say it came from switches? Hmmmm...that's very interesting. I have always wondered where birdies came from and never really got a straight answer.

    I assume it is some sort of strange mixer product in the way I interpret them. I tend to think it is a reflected impedance since no amount of shielding seems to effect a particular formula.
     
  18. Space Varmint

    Space Varmint New Member

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    Alright. I got the data sheet.

    But ya know. I kinda like old brother Stair. He's gonna retire soon. It's like the end of an era. It makes me very sad though he told me I was going to hell about 8 years ago just because I quoted Luke 22:36 to him on the air.

    He can be kind of arrogant. A friend of mine, a preacher, told me he met him personally and went to one of his sermons and he opened his bible and brother Stair told him to close it when he preaches. He said brother Stair told him he wanted to meet him out side...lol :D
     
    Last edited: Oct 29, 2008
  19. Space Varmint

    Space Varmint New Member

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    Think so? Comments to you maybe. He is a really smart guy. You have to look beneath the surface. He dishes out some slams I assure you.
     
  20. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Sure I am an old geezer. I retired 8 years ago.
    Sometimes I am nasty on forums to school kids and most come from a single 3rd-world country and send me hundreds of PMs demanding me to design circuits for the lazy ****.
    I don't use smileys here because there is no smiley button like there is on other forums and I forget is it is a (;) or a :)).

    The TDA2822 power amp has a typical supply hum rejection of 30dB which is pretty good.
    As I said before, your hum comes from the base bias resistors on your transistors and the divider for the opamp all of which do not have any decoupling.
     
  21. Roff

    Roff Well-Known Member

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    No smiley buttons? They are right next to the advanced reply window.
    :):D:p:(:eek::mad:;):confused::rolleyes::eek:
     

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