Making sense of RC RL LC filters - recognizing high pass versus low pass filters - and a question

Discussion in 'Amp Tech Center' started by peteb, Sep 7, 2019.

  1. gusfinley

    gusfinley Tele-Holic

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    Please take a look at the Duncan Power Supply Designer ( Free Download )

    Configure a PSU with a Choke-Filtered Power Supply, and run a simulation. Then lower the choke value to something like 5mH and perform the same simulation. This will show you the effects of the choke in the power supply.
     
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  2. peteb

    peteb Friend of Leo's

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    Thanks Gus, very helpful.


    I looked at the description, the Duncan PSU software looks great and would show the results well.


    If I managed to run that software I would put a choke in and then take the choke out and see what it takes to get equal performance.




    Can we agree that what a capacitor is, what it does and how it reduces AC in a DC power supply is fairly straight forward and easy to understand and the same is not true about a choke?





    Now I’m curious about field coil speakers and how the speaker magnet is the choke and if that looks like a fender choke and does it support a filter cap in the same way. Not looking to extend the thread, but that is my next question. (Now that the filter choke has shown itself difficult to pin down or to make it reveal its true nature)




    Thanks all




    That should do it



    81BEE1C1-51C7-4D42-BDA0-98FE7A05C2C4.jpeg




    It DOES have its own filter capacitor!
     
    Last edited: Sep 10, 2019
  3. raito

    raito Poster Extraordinaire Silver Supporter

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    That is an incorrect statement and will cause you great pain in the future if you rely on it.

    The cutoff frequency for an RC filter is the frequency where a signal at that frequency is attenuated by 3dB. Everything below is not passed on because some lower frequencies are still attenuated. Everything above is not passed on because those frequencies are present, though attenuated.

    A single-pole RC filter attenuates 6dB per octave.

    The so-called 'brick wall' filter is concept, not reality.
     
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  4. gusfinley

    gusfinley Tele-Holic

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    Did you check out what Aiken has to say about the subject:
    https://www.aikenamps.com/index.php/chokes-explained
     
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  5. peteb

    peteb Friend of Leo's

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    I get that. That’s why I was saying earlier that the resistor moves the cut off frequency away from the target frequency, making the filter more effective.



    And I’m sure it gets even more complicated.


    A high pass filter passes highs and attenuates lows. Low cut.
    A low pass filter passes lows and attenuates highs. High cut.



    But when a low pass filter is made of a high pass filter that pass highs to ground, things change and the above statement is no longer true.

    And, when they say pass, I don’t think they mean 100%.





    I consider Aiken a top source. I have read that twice and had problems with it. I will read again.


    Thanks
     
  6. raito

    raito Poster Extraordinaire Silver Supporter

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    Study the voltage divider (especially how it loads the output's next circuit and is loaded by the input's previous circuit). That'll give you a much better idea of what the resistor does. Especially get to the point where you understand why one might use, say, a pair of 10K resistors to divide a voltage in half instead of a pair of 100 ohm resistors.

    I don't think you will be able to fully understand analog filters in isolation. The usual progression is Ohm's Law -> Kirchoff's Laws -> voltage dividers -> filters. And somewhere in there will be LC and LR and LRC series circuits.

    It's also very useful to understand that one of the inductor's properties is that the voltage drop across the component is propertional NOT to the current (like a resistor) but to the rate of change of the current.

    The filters you're looking at are just voltage dividers with time constants, really. The only thing is which side of the output the time constant is connected on.

    Once you understand how the physical makeup of an inductor and a capacitor makes them act the way they do, it gets a lot easier. WHY does a capacitor pass relatively high frequency? WHY does an inductor pass relatively low frequency? What happens with each then the source is DC and not AC?

    [/QUOTE]
    A high pass filter passes highs and attenuates lows. Low cut.
    A low pass filter passes lows and attenuates highs. High cut.



    But when a low pass filter is made of a high pass filter that pass highs to ground, things change and the above statement is no longer true.

    And, when they say pass, I don’t think they mean 100%.
    [/QUOTE]
     
  7. peteb

    peteb Friend of Leo's

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    Thank Raito, Nice reply!


    I think I have all of this:







    and this



    and not this yet






    this is useful







    I just reread Aiken, after the research I went thru written in the thread above, it IS making a lot more sense, especially the part about a choke and a resistor being interchangeable, which is the main thing that I have learned.
     
  8. raito

    raito Poster Extraordinaire Silver Supporter

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    No, not interchangeable. Look up the relation voltage has to current in a resistor and in an inductor.

    Even Aiken says they're not interchangeable. Second paragraph.
     
  9. peteb

    peteb Friend of Leo's

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    Ratio,

    I appreciate your help.


    I reread Aiken’s page on chokes. I get more out of it, like I said, but I still have a problem accepting all of it.




    In post 12 I did my best to show how a choke can filter with a resistor and what I found is that where the cutoff frequency is means it will not filter 60 Hz or 120 Hz. Maybe my calcs are off.





    My problem with understanding a choke is that I only see a choke in amps with the choke having its
    own filter cap to ground, the cap being a filter in its own right, and most amps filter without a choke.





    Count me as not impressed with with the affect of a choke, from what I can tell. But that’s OK, it doesn’t really matter.







    There was a day in 1999 or 2000 where I was on the phone at lunch with Angela Instruments, I thought I was talking to Steve himself. I was going to place an order for parts and I could not decide what would be the best amp possible. I couldn’t choose between a 5F2 and a F52-A. I was asking the guy at Angela’s about it. He said, this is DIY. I was a little put off. I was about to order hundreds of dollars of parts, and the guy didn’t help me decide. That afternoon i went downtown and bought a vintage fender champ for less than the parts. The question about the choke has since lingered.
     
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