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Audio feedback problem

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

  1. Mikebits

    Mikebits Well-Known Member

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    Look into Mu-metal, makes a good shield for switch supplies. Then you just have to filter the DC.

    Magnetic Field Shielding Materials

    That R3030 was a real nice radio. At the time it sold for about $10k, military will pay anything I guess.
     
    Last edited: Nov 7, 2008
  2. Mikebits

    Mikebits Well-Known Member

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    If you want to show people what you have done, produce some specs of your radio like NF or MDS. Also include your test method :)
     
  3. Leftyretro

    Leftyretro New Member

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    Several years ago I bought the first IF module from this radio from Fair Radio Sales. It was (is?) going to be the heart of my dream radio. Still working on a LO that I can build and afford, needs 40-70mhz range. Anyway it's a hell of a front end mixer, with a 15khz wide 'roofing' crystal filter. Would be so cool if I could find the 455khz filter module that this receiver uses, but I'm sure I wouldn't want to pay what it's worth even if avalible.

    Lefty
     
  4. dave

    Dave New Member

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  5. Mikebits

    Mikebits Well-Known Member

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    The 455KHz filters we used were very expensive as they were mechanical filters as they called them, but I bet you could find some on ebay for cheap. I will try and find the company name for you. I still have a few peeps I can call, wait! I just remembered. The filters were made by Rockwell International,or maybe Rockwell Collins, if that helps any :)
     
    Last edited: Nov 7, 2008
  6. Mikebits

    Mikebits Well-Known Member

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    Last edited: Nov 7, 2008
  7. Mikebits

    Mikebits Well-Known Member

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    This is what the 455KHz filters looked like. These are from Collins. Found on ebay $35.00 wrong freq range though.


    [​IMG]
     
  8. Leftyretro

    Leftyretro New Member

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    Thanks;

    Yes I tracked 455khz collins mechanical filters on E-bay for quite awhile, they always fetch pretty high prices in the end.

    Really the local oscillator design and construction should be my first priority, as affordable ceramic second IF filters at 455khz are pretty common and I have obtained a few 4 and 5 section models. IF amp should be pretty simple with the great chip amps avalible today. The AGC will be a challenge, demodulator (AM/SSB/CW) should be pretty simple and I all ready have a nice audio amp module.

    So got any ideas on a clean, stable, affordable, constructable local oscillator? I have a mini-circuit 40-70mhz VCO module but have not been able to find a PLL circuit to use it with that I feel I could successfully reproduce. A DDS module would be cool and the chip prices have come down some but I'm too old to try and solder those too many pins SMD devices.

    Lefty
     
    Last edited: Nov 7, 2008
  9. flat5

    flat5 Member

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    About 1978 I built a 40 & 80 meter ssb receiver I was very proud of :)
    I "borrowed" circuit ideas from Ham Radio Magazine and the ARRL Handbook.
    It used the Collins mechanical filter (2.8k) and the MC1496 in the if and mixer/detector.

    For 40 meters I just used a 40 meter converter to 80 meters.
    Because of a preselector in front this worked well enough.

    I miss that receiver!
     
  10. Space Varmint

    Space Varmint New Member

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    Alright Alright! Radio chatter! See? We got some radio lovers, you'all just wont speak up.

    I got a Collins 455 mechanical filter but I'm not using it in this design. I have my reasons for that.
    But Lefty, if you need a cheap 455 KHz ceramic filter, you can pull one out of about any cheap old radio. Just look for a rectangular shaped plastic thing near the audio section with 3 pins on it. That will be it. They come in everything. Little FM Receivers, ...you name it.

    Mike, I'm not trying to call you out. I know you know your stuff, but let me tell you what happened......

    Yesterday I was enjoying WWCR on 3.210MHz and at about 5 or 6 AM they switch frequencies to 9.495 MHz. I tried to follow them and could not pick them up. But I could get WWV at 10MHz. So I started reworking my preselector which has a 5 position rotary switch.

    I got so frustrated I tore the whole thing up. Not the radio, the preselector. I began reconstructiong it and got so tired I fell out. After recuperating, I'm back checking everything and still got my 40 & 80 meter coverage. But there are no hetrodynes or intermod this morning. Do you think I could have forced some intermod through the pre-selector?
    Tell ya what I hate, the dern PS from the laptop just kills the signal. It is so noisy! Don't figure I can do anything about that on the radio end?

    Oh, Lefty, man do need a VFO still? I got a some real simple Hartly oscillators in my schematic. What is the frequency of your 1st IF?
     
  11. Space Varmint

    Space Varmint New Member

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    Shew....10Hz step resolution!

    That's some sh__!
     
  12. Space Varmint

    Space Varmint New Member

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    I'll say one thing for my little radio. When you hear both ends of every conversation on a 10 foot piece of wire in a basement, something working right.

    Don't like my design Nigel? You don't have to buddy. I works! It works damn good!!! When you say it has too many stages and they are a waste, that's where your designs are crap on mine are superb. I'm a firm believer in buffering. It comes from experience Nigel. Build a couple of receivers and find out. ;)
     
    Last edited: Nov 8, 2008
  13. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Your audio amplifier has many stages with a lot of gain. Then many attenuators. You end up with reduced bandwidth and distortion.:(

    Your circuit has many transistors at max gain without any negative feedback. You should have used negative feedback to reduce the distortion. Attenuators do not reduce the distortion.

    My simulation shows a high voltage gain and 40% distortion.
    The second half of my sim has much less distortion due to negative feedback but the gain is reduced.
     

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    Last edited: Nov 8, 2008
  14. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Your stages aren't buffering, they are simply a waste and performing really badly - I don't think any single stage is designed anywhere near correctly?.

    I'm not trying to build a radio, I did all that decades ago - but never built one as poor as this.
     
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  15. Space Varmint

    Space Varmint New Member

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    Yes Sir!

    Now I respect what you say. I am sure my audio can be improved but radio wise, this guy does the job! I don't give a crap about what Nigel says. He's just jealous. He's been on my arse since I joined this site.

    Nigel, buffers are not a waist...take a lesson in radio.
     
  16. Space Varmint

    Space Varmint New Member

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    See, Nigel goes out and buys a couple of ICs that say they are the best and he thinks he's done something. Nigel, you need to leave radio to the big boys. It's out of your league.
     
  17. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Your four buffers look like differential amplifiers but they are actually just useless darlingtons that are not needed.

    A darlington has a very high input impedance that is not needed.
    A darlington has a very low output impedance that is not needed.
    A darlington has no voltage gain.
     

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  18. Space Varmint

    Space Varmint New Member

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    Well...let's see some of your old crappy designs from decades ago.

    What do got? A couple of 6L6's in some sort of direct conversion garbage?
     
  19. Space Varmint

    Space Varmint New Member

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    Ah....that's where you are wrong. They are deliberately Darlington amps. What do you think a Darlinton is? I'll tell you. It's a BUFFER.

    Yes they have high input impedance and low output impedance.

    Precisely what I was after.
     
    Last edited: Nov 8, 2008
  20. Space Varmint

    Space Varmint New Member

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    Bottom line is performance. If you can add low power with performance than I would say that is the trend.

    I'm not afraid of digital at all. I got a PLL loop for 40 Meters ready to insert. But first I wanted the S/N top notch which it is.

    Trust me, this is communications quality. But I can still enjoy some AM. It's kinda of low quality AM because the crystal filter is kinda sharp. Hey, you can only process what you are handed.

    But as far as SSB and CW..........no problem. No problem at all. In fact, quite exquisit if I do say so myself. Like I said, "I hear both ends of every conversation even on the NETs. That's with a 10 foot piece of wire as an antenna in my basement".

    Now, talk to me.
     
    Last edited: Nov 8, 2008
  21. Space Varmint

    Space Varmint New Member

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    Look at Nigel's signature. PIC processors...lol. Easy sh_t. Dude, I was programming and designing with the latest Intell embedded processors before I heard of Microchip. In fact I kept telling Intel to put some SRAM in them sand they wouldn't listen so Microchip got the bizz.

    You don't even want to see some of the device drivers I've written Nigel. Windows claims you can't access other processes memory. Bunk. I've modified NTOSKERNL.exe to add new services. Digital is just one side of electronics. There is plenty of room on the analogue side of things. Do it right! Don't cut corners. In radio....buffer and shield everything you can manage.

    Shoot, the best receiver I ever made had about 30 aluminum boxes inside the tin boxes. Shielded. Even the mixers. Everything was shielded. But this one is right up there with it. Using the self shielding properties of toroids and knowing when and when not I can get away with it.

    Shielding and buffering are key in good radio design.
     

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