+ Post New Thread
Page 1 of 2 1 2 LastLast
Results 1 to 20 of 31
  1. #1
    Advanced Member level 5
    Points: 13,558, Level: 28
    Achievements:
    7 years registered
    neazoi's Avatar
    Join Date
    Jan 2008
    Location
    Greece
    Posts
    2,773
    Helped
    12 / 12
    Points
    13,558
    Level
    28

    Two audio filters. Please help me modernize them

    Here are two audio filters for receivers.
    I would like to test them but I want to make them using NPN transistors instead. Is it enough to connect replace the PNP transistors with NPN and put a positive VCC where a negative exists now?

    Also any guess of their operation principles?
    Professional engineering is the top, but amateur engineering is more fun.
    It is when you cross the barrier between these two, that things become really fascinating!

  2. #2
    Super Moderator
    Points: 43,914, Level: 51

    Join Date
    Apr 2011
    Location
    Minneapolis, Minnesota, USA
    Posts
    10,845
    Helped
    2158 / 2158
    Points
    43,914
    Level
    51

    Re: Two audio filters. Please help me modernize them

    Quote Originally Posted by neazoi View Post
    Is it enough to connect replace the PNP transistors with NPN and put a positive VCC where a negative exists now?
    Yes, that is the general rule. Also reverse diode directions, but there are none in your schematics.]

    - - - Updated - - -

    Also check capacitor orientations.


    1 members found this post helpful.

  3. #3
    Super Moderator
    Points: 43,914, Level: 51

    Join Date
    Apr 2011
    Location
    Minneapolis, Minnesota, USA
    Posts
    10,845
    Helped
    2158 / 2158
    Points
    43,914
    Level
    51

    Re: Two audio filters. Please help me modernize them

    The first schematic amplifies two bands: 70-500 and everything above 1500 Hz.

    It has the series LC which diverts a band to ground. So there is reduced gain at 500-1500 Hz.

    This may obey an equalization curve, in order to restore audio fidelity.


    1 members found this post helpful.

  4. #4
    Super Moderator
    Points: 60,649, Level: 60
    Achievements:
    7 years registered
    Awards:
    2nd Helpful Member
    betwixt's Avatar
    Join Date
    Jul 2009
    Location
    Aberdyfi, West Wales, UK
    Posts
    10,067
    Helped
    3228 / 3228
    Points
    60,649
    Level
    60

    Re: Two audio filters. Please help me modernize them

    They are not going to give the sharp peak you were looking for in your other thread.

    I thought about your requirements for a peaking circuit that avoids using feedback to increase 'Q' and an idea came to mind. I have not tried this and I can foresee some impedance matching problems but bear with me and maybe try it for yourself:

    My preferred selective network is a twin-tee. It uses easily available components, is simple to construct, has in theory an infinite 'Q' (although not in practice!) and most importantly, it conducts DC between it's input and output. The problem is they work exactly in reverse to what you want, they have high impedance between in and out at the resonant frequency, making them a notch rather than peak filter.

    The obvioius way to invert the notch is to place it in the negative feedback path of an amplifier instead of in the signal path. If the negative feedback is notched, the gain is highest at that frequency so a selective peaking effect is achieved.

    My next thought was you don't want to use ICs and you do want to keep it simple, so what is the least complex amplifier that you can add a notch filter to. I think the simplest is a single transistor common emitter stage with the 'notch' between the base and emitter pins. The bonus that the filter conducts DC can be used to simplify the bias circuit.

    What I would propose is you try this out: use a single transistor amplifier stage, input fed to the base and output from the collector, you can use NPN or PNP as long as you provide the appropriate supply polarity. It needs a reasonably high emitter resistor to work, I would suggest 4.7K as a good starting value and it must not be bypassed with a capacitor as you need signal on the emitter for this to work. Then wire a twin-tee network between the base and emitter using two 10K series resistors with 44nF to ground and two 22nF capacitors with 5K to ground. Those values will give a peak at about 750Hz which is nice for CW. Note the values need to be exact but you can make 5K from two 10Ks in parallel and 44nF from two 22nF in parallel.

    See what happens and let me know. If it shows promise, there is a simple modification that allows true bandwidth adjustment.

    Brian.
    PLEASE - no friends requests or private emails, I simply don't have time to reply to them all.
    It's better to share your questions and answers on Edaboard so we can all benefit from each others experiences.


    1 members found this post helpful.

  5. #5
    FvM
    FvM is online now
    Super Moderator
    Points: 222,033, Level: 100
    Awards:
    1st Helpful Member

    Join Date
    Jan 2008
    Location
    Bochum, Germany
    Posts
    38,109
    Helped
    11666 / 11666
    Points
    222,033
    Level
    100

    Re: Two audio filters. Please help me modernize them

    I remember that I built variable band-pass/band-stop audio filters as a student, to be used for audio effects and as a measurement tool. They always used operational amplifiers in a "state-variable" respectively "universal active filter" configuration. https://en.wikipedia.org/wiki/State_variable_filter


    1 members found this post helpful.

    •   Alt8th January 2017, 11:03

      advertising

        
       

  6. #6
    Advanced Member level 5
    Points: 12,380, Level: 26

    Join Date
    May 2015
    Location
    Melbourne, Australia
    Posts
    2,123
    Helped
    713 / 713
    Points
    12,380
    Level
    26

    Re: Two audio filters. Please help me modernize them

    Agree with Brian, the twin tee in the feedback will give you a huge gain spike at one particular frequency.
    Its extremely effective.
    Cheers, Tony.


    1 members found this post helpful.

  7. #7
    Advanced Member level 5
    Points: 13,558, Level: 28
    Achievements:
    7 years registered
    neazoi's Avatar
    Join Date
    Jan 2008
    Location
    Greece
    Posts
    2,773
    Helped
    12 / 12
    Points
    13,558
    Level
    28

    Re: Two audio filters. Please help me modernize them

    Quote Originally Posted by betwixt View Post
    What I would propose is you try this out: use a single transistor amplifier stage, input fed to the base and output from the collector, you can use NPN or PNP as long as you provide the appropriate supply polarity. It needs a reasonably high emitter resistor to work, I would suggest 4.7K as a good starting value and it must not be bypassed with a capacitor as you need signal on the emitter for this to work. Then wire a twin-tee network between the base and emitter using two 10K series resistors with 44nF to ground and two 22nF capacitors with 5K to ground. Those values will give a peak at about 750Hz which is nice for CW. Note the values need to be exact but you can make 5K from two 10Ks in parallel and 44nF from two 22nF in parallel.

    See what happens and let me know. If it shows promise, there is a simple modification that allows true bandwidth adjustment.

    Brian.
    Something like this?
    An estimation for the bias resistors R1 and R2? Maybe R2 can be omitted.
    Professional engineering is the top, but amateur engineering is more fun.
    It is when you cross the barrier between these two, that things become really fascinating!



  8. #8
    Super Moderator
    Points: 60,649, Level: 60
    Achievements:
    7 years registered
    Awards:
    2nd Helpful Member
    betwixt's Avatar
    Join Date
    Jul 2009
    Location
    Aberdyfi, West Wales, UK
    Posts
    10,067
    Helped
    3228 / 3228
    Points
    60,649
    Level
    60

    Re: Two audio filters. Please help me modernize them

    hmm.... I would add a collector load resistor or the output will be Vvveeerrrryyyy quiet!
    Yes, miss R2 out and calculate R1 according to the transistor Hfe.

    The problem with such a simple implementation is that for best results, the input impedance should be very low and the output load very high impedance, both of which this isn't able to satisfy. Try it though and see how it performs. Remember the R/2R and C/2C rule and make the values by parallel resistors/capacitors rather than using nearest values.

    Brian.
    PLEASE - no friends requests or private emails, I simply don't have time to reply to them all.
    It's better to share your questions and answers on Edaboard so we can all benefit from each others experiences.



    •   Alt8th January 2017, 18:09

      advertising

        
       

  9. #9
    Advanced Member level 5
    Points: 13,558, Level: 28
    Achievements:
    7 years registered
    neazoi's Avatar
    Join Date
    Jan 2008
    Location
    Greece
    Posts
    2,773
    Helped
    12 / 12
    Points
    13,558
    Level
    28

    Re: Two audio filters. Please help me modernize them

    Quote Originally Posted by betwixt View Post
    hmm.... I would add a collector load resistor or the output will be Vvveeerrrryyyy quiet!
    Yes, miss R2 out and calculate R1 according to the transistor Hfe.

    The problem with such a simple implementation is that for best results, the input impedance should be very low and the output load very high impedance, both of which this isn't able to satisfy. Try it though and see how it performs. Remember the R/2R and C/2C rule and make the values by parallel resistors/capacitors rather than using nearest values.

    Brian.
    I made it. I put a 4.7k on the collector to vcc and a 470k on the collector to base. I removed R2 (base to gnd) totally. I have used ceramic 22nf capacitors and two of these parallel for the 44nf.

    The circuit had no audible effect on the sound, maybe it narrowed the bandwidth a bit, because I could not hear very high pitch tones and hiss, but not the desired effect, not at all.

    Any suggestions are appreciated.
    Professional engineering is the top, but amateur engineering is more fun.
    It is when you cross the barrier between these two, that things become really fascinating!



    •   Alt8th January 2017, 23:29

      advertising

        
       

  10. #10
    Advanced Member level 5
    Points: 12,380, Level: 26

    Join Date
    May 2015
    Location
    Melbourne, Australia
    Posts
    2,123
    Helped
    713 / 713
    Points
    12,380
    Level
    26

    Re: Two audio filters. Please help me modernize them

    I feel there is just insufficient voltage gain in your single transistor amplifier to get the gain spike you are hoping for.

    It would work much better with an op amp if you have one to try.

    What might work would be a Wein bridge oscillator circuit, held just below the threshold of oscillation.
    Last edited by Warpspeed; 8th January 2017 at 23:47.
    Cheers, Tony.



  11. #11
    Super Moderator
    Points: 60,649, Level: 60
    Achievements:
    7 years registered
    Awards:
    2nd Helpful Member
    betwixt's Avatar
    Join Date
    Jul 2009
    Location
    Aberdyfi, West Wales, UK
    Posts
    10,067
    Helped
    3228 / 3228
    Points
    60,649
    Level
    60

    Re: Two audio filters. Please help me modernize them

    That is what I feared. In fact the feedback path from the emitter has no voltage gain at all. When I've used it in the past it has been in the feedback of op-amp circuits where the gain is high and the impedances better controlled. It might be possible to add gain by using a second transistor stage but as the function of the twin-tee is a notch you have to be careful to not to invert the signal while amplifying the voltage, that may not be so easy.

    Just as an experiment while you have it built, try injecting the signal at the emitter instead of the base. I have no idea if it will make any difference but it makes it a common base amplifier which might exhibit slight gain.

    Brian.
    PLEASE - no friends requests or private emails, I simply don't have time to reply to them all.
    It's better to share your questions and answers on Edaboard so we can all benefit from each others experiences.



  12. #12
    Advanced Member level 5
    Points: 13,558, Level: 28
    Achievements:
    7 years registered
    neazoi's Avatar
    Join Date
    Jan 2008
    Location
    Greece
    Posts
    2,773
    Helped
    12 / 12
    Points
    13,558
    Level
    28

    Re: Two audio filters. Please help me modernize them

    Quote Originally Posted by Warpspeed View Post
    What might work would be a Wein bridge oscillator circuit, held just below the threshold of oscillation.
    You mean like my second circuit in post #1?
    Professional engineering is the top, but amateur engineering is more fun.
    It is when you cross the barrier between these two, that things become really fascinating!



  13. #13
    Advanced Member level 5
    Points: 12,380, Level: 26

    Join Date
    May 2015
    Location
    Melbourne, Australia
    Posts
    2,123
    Helped
    713 / 713
    Points
    12,380
    Level
    26

    Re: Two audio filters. Please help me modernize them

    Quote Originally Posted by neazoi View Post
    You mean like my second circuit in post #1?
    Similar idea, but I would do it differently.
    wein-filter.jpeg
    Tweak the pot until it oscillates at 1Khz, then back it off just below the threshold of oscillation.
    It should have plenty of gain there at 1Khz.
    Cheers, Tony.



  14. #14
    Advanced Member level 5
    Points: 13,558, Level: 28
    Achievements:
    7 years registered
    neazoi's Avatar
    Join Date
    Jan 2008
    Location
    Greece
    Posts
    2,773
    Helped
    12 / 12
    Points
    13,558
    Level
    28

    Re: Two audio filters. Please help me modernize them

    Quote Originally Posted by betwixt View Post
    Just as an experiment while you have it built, try injecting the signal at the emitter instead of the base. I have no idea if it will make any difference but it makes it a common base amplifier which might exhibit slight gain.

    Brian.
    No, it did not make an audible difference.

    - - - Updated - - -

    Quote Originally Posted by Warpspeed View Post
    Similar idea, but I would do it differently.
    Click image for larger version. 

Name:	Wein filter.jpeg 
Views:	7 
Size:	23.0 KB 
ID:	135135
    Tweak the pot until it oscillates at 1Khz, then back it off just below the threshold of oscillation.
    It should have plenty of gain there at 1Khz.
    I have done that with the second circuit in post #1 in this page Regenerative CW filter help needed It worked to some extent but read that thread for the problems I encountered.

    I am curious about the Wein bridge method mentioned in post #10 here. Is that the second circuit in post #1?
    I have not tried it yet.

    - - - Updated - - -

    I have also found this circuit. but it is much more complex than an opamp. It seems to be a combination of twin-t and phasing filter. promising?
    Last edited by neazoi; 9th January 2017 at 01:04.
    Professional engineering is the top, but amateur engineering is more fun.
    It is when you cross the barrier between these two, that things become really fascinating!



  15. #15
    Advanced Member level 5
    Points: 12,380, Level: 26

    Join Date
    May 2015
    Location
    Melbourne, Australia
    Posts
    2,123
    Helped
    713 / 713
    Points
    12,380
    Level
    26

    Re: Two audio filters. Please help me modernize them

    To be really effective, I think it would need it to be within a hair of oscillating.
    That is about the best you can probably do with only a single stage.
    Cheers, Tony.



  16. #16
    Advanced Member level 5
    Points: 13,558, Level: 28
    Achievements:
    7 years registered
    neazoi's Avatar
    Join Date
    Jan 2008
    Location
    Greece
    Posts
    2,773
    Helped
    12 / 12
    Points
    13,558
    Level
    28

    Re: Two audio filters. Please help me modernize them

    Quote Originally Posted by Warpspeed View Post
    To be really effective, I think it would need it to be within a hair of oscillating.
    That is about the best you can probably do with only a single stage.
    Thanks Tony, we cross posted. See the circuit and the description in the previous post. It seems he uses Q3 to cancel out, out-of-phase signals. Not a one transistor circuit, but it uses the twin-T not in the negative feedback way (as the circuit Brian suggested), but at a notch filter. When the filtered output is applied to Q3 along with a out of phase full-signal, all the frequencies are phased out, except from the ones that existed in the notch. They do not affect the imput signal.
    I do not know how much better will it work than the single transistor regenerative filter described in this thread Regenerative CW filter help needed but at least it won't oscillate.
    I think one might be able to do it without Q1, by just feeding the input signal at the base of Q3, if the input signal is high enough?

    - - - Updated - - -

    Now that I am thinking of it, this filter should be called regenerative and phasing filters combined into one. Regenerative, because there is positive feedback and phasing because it works by phase cancellation. Please correct me if my thought is wrong.

    However, the regeneration here, is not used to narrow the bandwidth and increase the gain of a phasing oscillator, but to narrow the bandwidth of the notch in the notch filter.

    In that sense I believe that the notch amplifier will not easily fall into oscillation, as it amplifies all frequencies except from the notch, in comparison to the phasing oscillator method, that amplifies just one frequency (hence easily oscillate into that frequency).

    Please correct me if I am wrong in this aspect.
    Last edited by neazoi; 9th January 2017 at 01:48.
    Professional engineering is the top, but amateur engineering is more fun.
    It is when you cross the barrier between these two, that things become really fascinating!



  17. #17
    Advanced Member level 5
    Points: 12,380, Level: 26

    Join Date
    May 2015
    Location
    Melbourne, Australia
    Posts
    2,123
    Helped
    713 / 713
    Points
    12,380
    Level
    26

    Re: Two audio filters. Please help me modernize them

    Yes you are quite right. The twin tee notched feedback just allows the amplifier to operate at its natural maximum gain.
    For a single transistor amplifier that may not be a lot. An op amp offers far more potential.

    The Wein bridge has positive feedback, and potentially a much taller and narrower peak. But its tricky and potentially unstable.

    Only other consideration might be that the Wein is easier to tune requiring only two equal value resistors, which is dead easy. If that is a required feature.
    The bridged tee, requires three tunable resistors, which becomes a bit more difficult.

    I can see advantages and disadvantages either way.
    Cheers, Tony.



  18. #18
    Advanced Member level 5
    Points: 13,558, Level: 28
    Achievements:
    7 years registered
    neazoi's Avatar
    Join Date
    Jan 2008
    Location
    Greece
    Posts
    2,773
    Helped
    12 / 12
    Points
    13,558
    Level
    28

    Re: Two audio filters. Please help me modernize them

    Quote Originally Posted by Warpspeed View Post
    The Wein bridge has positive feedback, and potentially a much taller and narrower peak. But its tricky and potentially unstable.

    Only other consideration might be that the Wein is easier to tune requiring only two equal value resistors, which is dead easy. If that is a required feature.
    The bridged tee, requires three tunable resistors, which becomes a bit more difficult.
    Please tell me which circuits you refer to? The second circuit in my post #1 is the Wein bridge you refer to?

    - - - Updated - - -

    Quote Originally Posted by betwixt View Post
    That is what I feared. In fact the feedback path from the emitter has no voltage gain at all. When I've used it in the past it has been in the feedback of op-amp circuits where the gain is high and the impedances better controlled. It might be possible to add gain by using a second transistor stage but as the function of the twin-tee is a notch you have to be careful to not to invert the signal while amplifying the voltage, that may not be so easy.

    Just as an experiment while you have it built, try injecting the signal at the emitter instead of the base. I have no idea if it will make any difference but it makes it a common base amplifier which might exhibit slight gain.

    Brian.
    Maybe I judged your idea earlier than needed.
    Look what I have found about twin-t networks
    "The use of poorly matched capacitors in the twin-T net work will result in a low-Q filter. Capacitors should be 1% silver mica for CI, C2, C3, or they should be matched on a bridge. Do not attempt to substitute ceramic capacitors."

    I used ceramic capacitors unmatched. Maybe the Q was too low because of this. If I could use lower value of capacitors (up to 470pF-2.2nf max) I could substitute them with silver mica 1% types.
    Then maybe this single transistor circuit could work.
    But I find the idea of post #14 interesting.
    Professional engineering is the top, but amateur engineering is more fun.
    It is when you cross the barrier between these two, that things become really fascinating!



    •   Alt9th January 2017, 09:43

      advertising

        
       

  19. #19
    Super Moderator
    Points: 60,649, Level: 60
    Achievements:
    7 years registered
    Awards:
    2nd Helpful Member
    betwixt's Avatar
    Join Date
    Jul 2009
    Location
    Aberdyfi, West Wales, UK
    Posts
    10,067
    Helped
    3228 / 3228
    Points
    60,649
    Level
    60

    Re: Two audio filters. Please help me modernize them

    I suspect ANY regenerative or 'Q multiplier' filter will suffer the same problems of instability as the audio levels change and a degree of hysteresis in the regeneration control. Neazoi has already noted the effect of the filter starting to oscillate and how much louder it is than the signal being filtered.

    The schematic in post #14 is what I was describing in post #11, adding a second stage to increase the feedback in a standard twin-tee arrangement. I am unhappy about the way the tuning works in that schematic though as R1 & R2 must always be the same value and R3 must always be half that value or the Q rapidly drops to almost nothing. As noted with capacitor values, the resistors are just as critical.

    If a second stage has to be added, I see no problem with using #14 schematic but using the fixed component values already tried but omitting Q6 and all the associated metering circuits (who wants a meter to tell how loud it is anyway?) and omitting everything after the volume control. I would leave the 47K resistor in series with the volume control there though so reduces loading effects of subsequent stages. 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.
    PLEASE - no friends requests or private emails, I simply don't have time to reply to them all.
    It's better to share your questions and answers on Edaboard so we can all benefit from each others experiences.



  20. #20
    Advanced Member level 5
    Points: 13,558, Level: 28
    Achievements:
    7 years registered
    neazoi's Avatar
    Join Date
    Jan 2008
    Location
    Greece
    Posts
    2,773
    Helped
    12 / 12
    Points
    13,558
    Level
    28

    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.
    The filter will be placed at the output of the second transistor (regenerative detector) in this circuit http://qrp.gr/regenrxcer/index.htm after the 10k resistor (used for isolation).
    Do you believe Q1 stage in the filter, would be needed?

    I have also found this one http://www.qsl.net/va3iul/Homebrew_R..._CW_Filter.gif This is an Elektor project, straight forward. 3 transistors, but no tuning or weird filter effects. I thought that the 100mH chokes (the complete article, which I have on my phone SD and can't upload now, states that any 100mH choke can be used) were expensive, but on ebay, 5 of them cost about 3 euros, so it shouldn't be a great deal.
    The filter has something like 400Hz bandwidth, at -6db but the responce at lower dB is better than the regenerative filters, which seem to have a low bandwidth nose but "open" quite rapidly.
    If these components at the twin-t must be kept so precise (hard), an lc filter could be an easier to implement option. hm... difficult decision.
    Last edited by neazoi; 9th January 2017 at 11:13.
    Professional engineering is the top, but amateur engineering is more fun.
    It is when you cross the barrier between these two, that things become really fascinating!



+ Post New Thread
Please login
Page 1 of 2 1 2 LastLast
--[[ ]]--