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Need help in 88-108MHz preamp circuit!!

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by rahulan999, Aug 25, 2008.

  1. radiomate

    radiomate New Member

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    Patrick C's superregenerative circuit is ok for begginers, give it a try if you never assembled a superregen before, but don't expect great results.. supperegen circuits are
    better suited for narrow band (communications) like commercial planes or police... they use slope detection (check the literature), so u have to tune to either side of the station, not the center. Cambre's circuit was designed for minimum parts, so it's not the best superregen in the world, do a search on the net to find better circuits.

    Check out the FM stereo receiver I made a few weeks ago:
    http://www.midcenturyradios.com/HM-FM-Stereo-2.html

    Good luck with your projects :)
     
  2. rahulan999

    rahulan999 New Member

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    radiomate
    You are a kind of person I was searching for:rolleyes:. really impressed on ur great efforts! I need ur valuable suggestions for my projects. Can u give me the circuits for the transistr radios?
    Especially that 2tr MW and 2tr-1IC fm?
     
  3. audioguru

    audioguru Well-Known Member Most Helpful Member

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    I think the FM preamp circuit will be overloaded by 12 local stations. It doesn't have sharp tuning parts at its input like a good radio has. It is already fairly wideband.
    Adding AGC will be the same as turning it off.

    I think that Patrick Cambre's Radio shack Special is a manually adjusted regenerative receiver, not a Super-regen because it has a manual sensitivity control and does not have quenching parts. It overloads more easily than a normal super-het radio.

    I think Patrick Cambre also made a TEA5711 radio using Philips' chip. It seems to be the same as my Sony Walkman radio that is 16 years old. It overloads easily because its antenna input is not tuned so it has a local-distant switch which is an attenuator when in the local position.
     
    Last edited: Aug 27, 2008
  4. dave

    Dave New Member

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  5. radiomate

    radiomate New Member

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    The radio shack special is a self-quenching superregen.

    The TEA5711 receiver has a tuned circuit at the antenna input and tuned RF stage, and it doesn't have an attenuator switch. It does not compete with quadruple or triple tuned FM receivers obviously, but its makes up a pretty decent receiver.
     
  6. audioguru

    audioguru Well-Known Member Most Helpful Member

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    You are right. Thanks for correcting me.
    I see that the 0.0047 capacitor connected to ground from the choke in series with the source of the FET charges and discharges which is the quenching oscillator.
    Then I don't know why the circuit has a sensitivity control. Maybe because the original circuit that was copied had the control so it could receive narrow-band signals.

    The TEA5711 receiver has a tuned circuit at the antenna input and tuned RF stage, and it doesn't have an attenuator switch.[/QUOTE]
    But the input "tuned circuit" does not tune one station, it tunes the entire FM broadcast band. So with a few strong stations the RF amp will have plenty of cross-modulation.

    Why did Philips change their name to NXP?
     
  7. rahulan999

    rahulan999 New Member

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    Now I'm planning to add this crystal detector to the amazing all band receiver keeping the schottky diode for detection there itself.
    http://solomonsmusic.net/FM_CrystalRadio.html
    I have doubt on his claim on the circuit that will work with a few inches lengthy antenna as a xtal receiver. But if its enough for a slope detection, I think I can power the circuit by attaching the allband receiver circuit which has got a good sensitive detector and high gain amp. How it will be?
     
  8. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Detecting an FM signal with an AM detector diode and using "slope detection' to demodulate the FM causes distortion.

    The crystal "radio' is missing a de-emphasis circuit (treble frequencies cut) that is on all FM radios. De-emphasis is used to reduce hiss and to equalize back to normal the pre-emphasis (treble frequencies boost) that is used by all FM radio stations.
     
  9. rahulan999

    rahulan999 New Member

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    Any discrete component circuit is there(without IC) that uses limiter,descriminator etc for fm detection without slope detection?(because I want to make everything using discrete components wherever possible!). What is the use of quenching circuit?It also used in slope detection right?
     
  10. audioguru

    audioguru Well-Known Member Most Helpful Member

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    A super-regen radio has an AM detector. The AM detector can detect FM (and static interference) with its AM detector if the radio is tuned to one side of a station then it slope-detects.

    A quenching circuit allows the voltage gain of the transistor to build up with positive feedback until its gain is so high that it oscillates at the radio frequency. Then the quenching capacitor charges to the peak of the RF which cuts off the transistor and causes the oscillation to quench. Then the capacitor's voltage drops a little and the transistor begins to work again.

    The quenching circuit keeps the transistor close to its max amount of voltage gain.
    The frequency of the quenching must be well above the audio frequency range and well above the 53kHz high frequency limit of the stereo subcarrier frequency or beat noises will be heard.

    About 40 years ago FM radios had a discrete ratio-detector transformer or FM discriminator transformer. Those parts are not available today.

    Since the transistor of a super-regen radio oscillates and it is connected to the antenna then the circuit radiates radio interference. Its harmonics might cause TV interference.
     
  11. rahulan999

    rahulan999 New Member

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    Done it! Pre-amp is working fine on top tower and with my TEA 5711 based stereo receiver.
    tried Patrick Cambre's 1 transistor fm project with the same 2N3819. but it turned to a big fail. Audio section is up. but the receiver section seems to be deaf. I can only hear the static from the LM386 amp set on high-gain mode.
     
  12. radiomate

    radiomate New Member

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    rahulan999, who's the manufacturer of your TEA5711 receiver and what model number is it? Or did you make it yourself?! As far as the superregen, the 3819 might be dead, or not enough regeneration to get it kicking, check the capacitor between source and drain, it might also be a bad choke, I used a ready made 4.7uH wich worked fine, also check the regen potentiometer setting. As I mentioned b4, you can try an MPF102 transistor instead of the 3819, they are better suited for superregens.
     
    Last edited: Sep 17, 2008
  13. rahulan999

    rahulan999 New Member

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    Its nice to see you again.
    For my receiver, I have bought a ready-assembled board.It's a local made one,even though the quality of assembly is very good and it performs awesome!. The audio quality far better even comparing to sony CXA based boards.I have put it inside a box with power supply and extra accessories :)
    Regarding Patrick's project, I have experimented with different pieces of 2n3819. I could hear sharp clicks when I'm inserting the transistor into the socket. But its not oscillating I think. that 4.7uh choke means source coil right? Drain coil must be anyhow ok its not critical , I made it using 8 turns of 22swg on 8mm former. Both coils have same diameter. Source coil has 25 turns of 32swg on a 8mm former.
    Also the source-drain cap I used is 4.7p. I tried with 5.6p. no result.
     
  14. radiomate

    radiomate New Member

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    Can you show a picture of the inside of the TEA5711 receiver? What did they use for the stereo amplifier?
    You should not insert the 3819 when the receiver is powered on! The 4.7uH is the source coil yes, I made the coil Patrick suggested and measured it to be around 5uH, so I used a ready made 4.7uH. You might be too low as far as the frequency, don't follow the drain coil instructions to the letter, try decreasing the turns or expanding/compressing the coil. If your frequency is too low or too high, the receiver might not oscillate at all. Try a source-drain cap from 5 to 15p. Also, don't connect an antenna to the tap, the radio should work without the antenna first, add the antenna later. As you see there are many variables you can fiddle with to bring this type of circuit to life, and after that is done, the performance is not worth the trouble :)
     
  15. rahulan999

    rahulan999 New Member

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    Also I used a polyester cap for quenching section (ie, 472p). Is it ceramic necessary?
    As I told, the TEA 5711 receiver board has a transistorized preamplified output.A single transistor(BC 547 or 9 based) voltage amplified output for each channel. This stereo output I connected to a stereo socket mounted on the backside of the tuner box. I'm connecting it into the Line-in socket of high-quality power amplifier. Creative HD50 is working very well!!
    As I'm not in my hometown now I will send you the pic some times later sorry :)
     
    Last edited: Sep 17, 2008
  16. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The TEA5711 is (was) an AM-FM stereo superhet receiver IC made by Philips (now called NXP Semi). A complete stereo radio schematic and pcb design is on its datasheet. Patrick Cambre made one years ago..

    I think the circuit is the same as Sony's AM-FM stereo IC that is in my cheap Sony Walkman radio.

    Digikey lists the TEA5711 IC but doesn't have any. NXP Semi says it is discontinued.
     
  17. radiomate

    radiomate New Member

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    Last edited: Sep 18, 2008
  18. radiomate

    radiomate New Member

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    rahulan999, doesn't matter if the 472pF is ceramic, polyester, mica or other in this case.
    Answering your q's, I made the circuit with a MPF102. Like I said, try the circuit without the antenna at the tap first to make sure it comes alive. Source-drain cap values from 5 to 15pF should be tried. A 5K for the regeneration pot is better, and the source coil should be close wound, not the drain coil. Your chances with the 102 should be better than the 3819. The drain coil is the critical one obviously as it determines the frequency with the tuning capacitor.
     
    Last edited: Sep 17, 2008
  19. rahulan999

    rahulan999 New Member

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    Its strange circuit which performs at a certain satisfactory point only. Let me find out that for my 3819. Anyway, how is the audio quality? Any distortion? Hows the audio frequency range?
     
  20. radiomate

    radiomate New Member

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    Maybe your components placement (layout) is causing the problem. The frequency range depends on your listening device. The superegen is a slope detecting circuit, so you have to tune to either side of the station, not the center, and you'll always have distortion.
     
  21. audioguru

    audioguru Well-Known Member Most Helpful Member

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    I just noticed that the TEA5711 radio IC has AFC for its FM.
    That is to keep the radio tuned to stations that have terrible frequency stability?? Hee, hee.
     

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