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  1. #21
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    Re: Two audio filters. Please help me modernize them

    Quote Originally Posted by betwixt View Post
    The purpose of the Q1 stage is to present a constant impedance at the summing point (base of Q3) but depending on what you feed it from, you may be able to leave that out too.
    Brian.
    http://www.edaboard.com/attachments/...919275-fl3.jpg
    Do you think I could leave Q1 out, if I connect it after the 10k resistor in the output of the regen detector (through a 100nF DC blocking capacitor) in my receiver circuit?
    http://qrp.gr/regenrxcer/index.htm
    I consider the constant impedance thing you mentioned.

    Would you also leave the input signal attenuator near Q1, to adjust the input signal level to Q3 for correct cancellation. Or it is not needed, because of the R4?
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    •   Alt10th January 2017, 12:47

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  2. #22
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    Re: Two audio filters. Please help me modernize them

    Ok, this filter worked nicely for me.

    However, when I increase the output volume potentiometer it oscillates badly. I have tried to reduce it's gain by reducing the 500uF electrolytic at the transistor emitter, but this affected the bandpass characteristics of the filter.
    I suspect that it oscillates because in combination with the audio amplifier that follows the filter (not shown here), the system has high overal gain. I conclude on it because, the filter shouldn't be affected by the output potentiometer setting anyway.

    Are there any things I can try to fix the problem?
    Professional engineering is the top, but amateur engineering is more fun.
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  3. #23
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    Re: Two audio filters. Please help me modernize them

    Increasing the volume should not cause instability in the filter itself. I suspect you have signal conducting along the supply line. Try adding 100 Ohms in series with the supply and 100uF from the filter side to ground.

    Brian.
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    •   Alt12th January 2017, 15:14

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  4. #24
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    Re: Two audio filters. Please help me modernize them

    Quote Originally Posted by betwixt View Post
    Increasing the volume should not cause instability in the filter itself. I suspect you have signal conducting along the supply line. Try adding 100 Ohms in series with the supply and 100uF from the filter side to ground.

    Brian.
    I did that and it cured the problem in a great extent.
    Do you think I shall do the same for the audio amplifier as welll or even the regenerative stage too?
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  5. #25
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    Re: Two audio filters. Please help me modernize them

    It's all supply decoupling so you should really ensure that none of the supply carries any signal. If in doubt, add low value resistors between each stage and a few uF to ground in each stage. For the RF stages, I would be inclined to also add a 100nF ceramic cap as well, electrolytics are not too good at coulpling high frequencies.

    Brian.
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    •   Alt13th January 2017, 21:54

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  6. #26
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    Re: Two audio filters. Please help me modernize them

    Quote Originally Posted by betwixt View Post
    It's all supply decoupling so you should really ensure that none of the supply carries any signal. If in doubt, add low value resistors between each stage and a few uF to ground in each stage. For the RF stages, I would be inclined to also add a 100nF ceramic cap as well, electrolytics are not too good at coulpling high frequencies.

    Brian.
    Thanks Brian

    I have tried to do so for the other stages but there was no difference. In the audio amp, it lowered the gain as well, because the 100R in series with the 32R phones is bad.
    So up to now, the schematic is the one shown at the end of this page http://qrp.gr/regenrxcer/

    There is a slight problem I need to correct, before I am fully satisfied with the circuit.
    When the audio volume pot is set to maximum it works ok. However, at this point, if a loud sound comes in (eg if someone switches on the lights at home), the audio section falls into oscillation.
    It seems that this volume spike causes the audio chain to oscillate, which shows me that there is too much gain probably.
    However this gain is needed when the regeneration is set too low (in order to reject the LSB), so I can't afford loosing it.

    How can I prevent the oscillation? Probably put some limiter anti-parallel diodes somewhere?
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  7. #27
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    Re: Two audio filters. Please help me modernize them

    The very old circuits in post #1 use antique geranium transistors. Modern silicon transistors will be biased wrong.



  8. #28
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    Re: Two audio filters. Please help me modernize them

    antique geranium transistors
    sounds like flower arranging for elderly people with gender issues!

    The problem still seems to be audio getting back to earlier stages. It is possible it is through ground line conduction but I'm assuming you have used reasonable construction techniques. I didn't mean to put 100 Ohms in the supply to the audio output stage, only to the stages before it. One resistor and cap in the +12 to the second transistor stage and from that filtered supply, another 100 Ohms and cap to the first stage. I suggest the regeneration control and all +12V to the left of it is fed from after the second 100 Ohm resistor so it is as clean as possible. If you haven't already fitted one, a big electrolytic across the audio output stage supply wouldn't be a bad idea.

    Consider than any audio on the supply line will reach the regeneration control and modulate the regen level. I think that's the reason it goes unstable as the output stage draws more current.

    Brian.
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  9. #29
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    Re: Two audio filters. Please help me modernize them

    I still have some AC127 and AC 128 germanium or geranium transistors and flowers. They might sell on ebay at a high price to somebody making an old geetar fuzz pedal.



  10. #30
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    Re: Two audio filters. Please help me modernize them

    Quote Originally Posted by betwixt View Post
    sounds like flower arranging for elderly people with gender issues!

    The problem still seems to be audio getting back to earlier stages. It is possible it is through ground line conduction but I'm assuming you have used reasonable construction techniques. I didn't mean to put 100 Ohms in the supply to the audio output stage, only to the stages before it. One resistor and cap in the +12 to the second transistor stage and from that filtered supply, another 100 Ohms and cap to the first stage. I suggest the regeneration control and all +12V to the left of it is fed from after the second 100 Ohm resistor so it is as clean as possible. If you haven't already fitted one, a big electrolytic across the audio output stage supply wouldn't be a bad idea.

    Consider than any audio on the supply line will reach the regeneration control and modulate the regen level. I think that's the reason it goes unstable as the output stage draws more current.

    Brian.
    I did all these but the problem was not cured, neither reduced. I forgot to mention, I am currently building this into breadboard, which is probably bad.
    Maybe an audio limiting mechanism would be a better idea?
    The audio volume on the phones is high, when this oscillation happens after all.
    Last edited by neazoi; 15th January 2017 at 01:01.
    Professional engineering is the top, but amateur engineering is more fun.
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    •   Alt15th January 2017, 00:49

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  11. #31
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    Re: Two audio filters. Please help me modernize them

    Regeneration is something that's easy to like for its benefits, but it's also got its quirks. My experience with regeneration was when I made a simple shortwave radio (on a breadboard). I adjusted the potentiometer looking for that 'magic spot' that the instructions talked about. Suddenly a faint broadcast came in strong. It was fun to see how well it worked.

    However it was a bit of trouble to get the gain right. I had to re-adjust every time I tuned to another station.

    I saw an article saying regeneration once was built into old-fashioned living room radios. A knob on the front was attached to a plain inductive loop located in the center of the tuning coil. Turning the knob changed the relative position of the coils, enhancing or reducing the regenerative effect. A listener would turn the knob to different positions, in an effort to bring in faint broadcasts. Evidently not many customers went to the trouble to learn how to use the regeneration knob, so it was omitted eventually.


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