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Audio feedback problem

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

  1. audioguru

    audioguru Well-Known Member Most Helpful Member

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    I have built hundreds of audio circuits with BC549 and 2N3904 transistors. Usually the frequency response is absolutely flat up to 400kHz. Their hfe (AC current gain, not voltage gain) is 330 at 1kHz and drops to 1 at typically 200MHz.
    The circuits have never had trouble with feedback or spurious responses even when two high power AM radio stations are next door or 30 high power FM radio stations are next door.
     
    Last edited: Oct 23, 2008
  2. Roff

    Roff Well-Known Member

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    I think he's confusing hfe and fτ.
     
  3. Space Varmint

    Space Varmint New Member

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    Yeah, your right. It's been too long since I considered those specs. You and Guru. I forgot "ft" is different. That's what I was talking about. And yes a high ft will give you more gain like in an LNA....Nigel.

    What do you think an LNA is? It is simply a high gain transistor. The more gain you get, the higher the signal to noise ratio. So they try to accomplish it at the front end. They have a high frequency transition.

    Yeah, your one of those "appliance operators". Talk a good game but never did any actual home brews. I've been a ham 36 years....WA4BJO. I didn't say you amplify with any attenuating amplifiers. That is a contradiction. I said after the post mixer amp you use an attenuator to keep noise out of the Xtal filter and lower the impedance.

    Again, in SSB the dynamic range is so dramatic that you cannot just amplify to your hearts content or you will have stages clipping all over the place. When I get a chance I will upload you an audio file so you can hear it. It's perfect audio quality. Complete class A all the way through. That's what you have to have in SSB to receive the intelligence of that kind of signal.
    .
     
  4. dave

    Dave New Member

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  5. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    LNA isn't simply a high gain transistor, it stands for LOW NOISE AMPLIFIER, and as such doesn't mention the gain at all - transistors at the microwave frequencies used only have fairly low gains that high. It's all about signal to noise (as you said), and high gain doesn't mean low noise.

    I've built plenty of equipment in the past, including valve gear for VHF.

    You claim to have been a ham for 36 years, yet you have no idea how to wire a simple volume control - you've had two tries, and neither were close to been correct :D Don't you have to take the RAE over there?.

    Remember we've all been explaining what AGC is to you - perhaps you should have listened.

    The sections of circuit you've posted has far too many stages, most doing absolutely nothing, and your audio AGC idea is what you would might on a direct conversion receiver, where you have no RF or IF amplifiers to control.

    Why would you want class A for low quality SSB signals?, in any case, all low signal audio stages are class A - it's not like you have an option :D

    Your output stage won't be class A though, and no reason for it to be so.
     
  6. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Some people heat their homes in winter with class-A amplifiers.
    Other people use a vacuum tube amplifier to stay warm.
    My class-AB amplifier gets nice and warm after a good loud workout.

    I use a telephone to talk to people on the other side of my country, not short-wave radio.
     
  7. Space Varmint

    Space Varmint New Member

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    Oh brother.... Yep, an appliance operator. I got ya.
     
    Last edited: Oct 23, 2008
  8. Space Varmint

    Space Varmint New Member

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    Well, let me put it to you this way. If you depend on the system to operate your cell phone, don't be surprised when the one guy in charge flips the switch.
     
  9. Space Varmint

    Space Varmint New Member

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    Well, let me put it to you this way. If you depend on the system to operate your cell phone, don't be surprised when the one guy in charge flips the switch.
     
  10. Space Varmint

    Space Varmint New Member

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    OK, here is a recording of the receiver with no front end filtering yet.

    I have it on 40 meters (7.00-7.5 MHz).

    So I first tune near top of band and listen to some SSB. Then I tune down to bottom onto some Morse code.

    Notice as I tune past a few AM broadcasts, with solid carriers, you only hear the beat note down to center frequency. You do not hear it go back up. That is the sharpness of the home made Crystal filter.

    The worst part about this recording is the computer's DSP. It degrades the actual receiver's audio.

    footnote:

    My electricity went out. :( It is rainy and cold here today. Must be a power line down. I am working off my laptop battery and it is so dark outside I am running a florescent lamp I made. I already posted it some time back. The one with a single stage Armstrong oscillator using a 2N3055 transistor. I see my keyboard just fine. Hope my laptop battery hangs in there a while.
     

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  11. audioguru

    audioguru Well-Known Member Most Helpful Member

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    I think it sounds awful.
     
  12. Space Varmint

    Space Varmint New Member

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    That's the computer DSP. It is clear as a bell. I guess I have to speed up the bit rate and make a larger audio file.
     
    Last edited: Oct 24, 2008
  13. Mikebits

    Mikebits Well-Known Member

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    What really matters is how it sounds to you. Your the end user, so if your happy with it, then thats all that matters :)
     
  14. wasssup1990

    wasssup1990 New Member

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    Hi Frank (space vomit) and Audioguru!

    Frank I am very interested in what you are doing with this project of yours. I don't know about everyone else but I like to see pictures and videos of projects, just like I do back over at Aaron's forum. If it's not much of a bother could you please explain exactly what your contraption does or at least boot me in the right direction to learn. I want to make something like this. You know I'm a digital guy Frank, and I see you have a DSP in your device.

    So thanks! I'll keep my ears perked up. LOL!
     
  15. Space Varmint

    Space Varmint New Member

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    .
    Mike Mike, that is very nice of you but the issue here is class A operation. That is what I have. All of you know that digital recordings can be crap, so it is not fair to me. Now I can do a higher bit rate recording for better sound quality. In fact why don't I just do it. What the hell.

    Hey Johnny! :D
     
  16. Space Varmint

    Space Varmint New Member

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    OK turned up the bit rate for digital recordings to 96 KHz (studio quality) it says. Turns out today is a DX contest day! I received this guy in Jamaica :6Y1V Contest Superstation - Jamaica

    And here is a recording from Demark Germany. I am located in Atanta, Ga USA.

    He was only on briefly so didn't get the "beat note" set just right on BFO and freq.
     

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  17. Space Varmint

    Space Varmint New Member

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    Here is a California station. BFO beat note still a little high.
     

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  18. Space Varmint

    Space Varmint New Member

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    Here is a Texas station where the beat note is set just about right. He gives frequency he transmitting on. K5TR
     

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    Last edited: Oct 25, 2008
  19. wasssup1990

    wasssup1990 New Member

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    Oh yeah, gotta make me one of those. Good work buddy.
     
  20. Space Varmint

    Space Varmint New Member

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    Here is a video of the receiver tuned to 7.229 MHZ with a guy from Honduras contesting DX today 10/25/08. His call sign is HQ3Z. You can look him up on QRZ Ham Radio


    Those are cat food cans...lol. :D
     

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  21. Space Varmint

    Space Varmint New Member

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    Hey thanks Johnny. :) Shoot me an Email and I'll give you some specs. I still have to update the schematic. The BFO was a B____ until I found the right core. It's like a T-80-06. A pretty big core but it is very stable. Many of your high quality receivers use crystals for their BFO both upper & lower sideband. This is tunable so I can get both upper and lower and use it to get the beat note right unstead of having to mess with the fine tune.

    I'm building a quick trickle charger right now. I got a 5.6 volt power supply I made for the receiver but I also got two small 6 volt sealed lead acid batteries it will run off of. I'm gonna work up a quick SSB modulator. I already have transmitters and rf linear power amps I made a while back.

    I may have to put a processor on the trickle charger because one the battery reaches the charge voltage the comparator drops the charge voltage then the battery dips slightly so it goes back into charge mode. Consequently the LED won't go out.

    edit:
    Oh, I think I know what I got to do. I have to trick the comparator into thinking the voltage is slightly higher than the actual current battery voltage charge held. I have made a processor trickle charger before but just don't think it is necessary. I have seen some Maxim circuits on the web. This one is just a few transistors and a '741 OP Amp comparator.

    Yeah...I see what's happening. The charge rate decreases because the output of the comparator is not digital but linear. So as the level is going high it dramatically reduces the charging current to the battery. The higher the comparator output the less charging current, so the battery is charging very very slowly or it has hit a neutral point. Maybe I can re-bias the LED.
     
    Last edited: Oct 25, 2008

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