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Thread: Choosing Resistors for Band Pass Filter

  1. #1 Choosing Resistors for Band Pass Filter 
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    Hello Sir/Mam.

    When selecting resistors for a band pass filter (, low pass filter or a high pass filter,) what's the difference between choosing two high or two low value resistors which give the same output?
    What is the minimal value for a resistor and a capacitor I should choose?

    Sincerely, Michael.


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  3. #2  
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    Quote Originally Posted by Mechatron View Post
    Hello Sir/Mam.

    When selecting resistors for a band pass filter (, low pass filter or a high pass filter,) what's the difference between choosing two high or two low value resistors which give the same output?
    What is the minimal value for a resistor and a capacitor I should choose?

    Sincerely, Michael.
    You've provided such little specific information that it is impossible to provide anything useful. So I will ask you a set of questions to narrow things down a bit.

    What is the frequency range of interest?
    Is this a passive or active filter?
    Do you care about noise? If so, what noise level is acceptable?
    Do you care about linearity? If so, what distortion is acceptable?
    Do you care about input and output impedances? If so, what are the specifications or limitations on same?

    Those are just to get started. As you provide answers, there will be more questions.


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  4. #3  
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    Quote Originally Posted by tk421 View Post
    Quote Originally Posted by Mechatron View Post
    Hello Sir/Mam.

    When selecting resistors for a band pass filter (, low pass filter or a high pass filter,) what's the difference between choosing two high or two low value resistors which give the same output?
    What is the minimal value for a resistor and a capacitor I should choose?

    Sincerely, Michael.
    You've provided such little specific information that it is impossible to provide anything useful. So I will ask you a set of questions to narrow things down a bit.

    What is the frequency range of interest?
    Is this a passive or active filter?
    Do you care about noise? If so, what noise level is acceptable?
    Do you care about linearity? If so, what distortion is acceptable?
    Do you care about input and output impedances? If so, what are the specifications or limitations on same?

    Those are just to get started. As you provide answers, there will be more questions.
    It's an active bandpass filter, it's an inverting bandpass filter like in the link below:
    http://www.electronics-tutorials.ws/filter/fil51.gif

    Does it matter what frequency range I'm interested in? If I increase or decrease R1 I can increase or decrease R2 to get the same frequency range, so why does it matter? Shouldn't there me some general rule for what the smallest resistor should be? Let's take 10 000 Hz for example.

    Of course I care about noise. 0 noise is desired. But how do you go about to compensate for noise? Do you pick larger capacitors to reduce noise?
    Please tell me how you do this.

    The output is an AC voltage or rather a square wave, so I don't know about linearity, what do you mean?

    I'm very, very scared about input and output impedance. This worries me a lot, because I don't understand the concept of it.
    I know very well what a resistor is. However, I have been told that this bandpass filter, needs a voltage buffer, which generate a low impedance output.
    I don't understand why the impedance output should be low. I don't understand how you can input or output an impedance at all. I thought it was the same thing as resistance.

    I just figured out that the impedance input must be low, otherwise it adds to the total input resistance, causing unpredictable centre frequency and response. I'm not sure what is meant by "it adds to the total input resistance".
    Last edited by Mechatron; February 13th, 2014 at 02:02 PM.
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  5. #4  
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    Quote Originally Posted by Mechatron View Post
    Quote Originally Posted by tk421 View Post
    Quote Originally Posted by Mechatron View Post
    Hello Sir/Mam.

    When selecting resistors for a band pass filter (, low pass filter or a high pass filter,) what's the difference between choosing two high or two low value resistors which give the same output?
    What is the minimal value for a resistor and a capacitor I should choose?

    Sincerely, Michael.
    You've provided such little specific information that it is impossible to provide anything useful. So I will ask you a set of questions to narrow things down a bit.

    What is the frequency range of interest?
    Is this a passive or active filter?
    Do you care about noise? If so, what noise level is acceptable?
    Do you care about linearity? If so, what distortion is acceptable?
    Do you care about input and output impedances? If so, what are the specifications or limitations on same?

    Those are just to get started. As you provide answers, there will be more questions.
    It's an active bandpass filter, it's an inverting bandpass filter like in the link below:
    http://www.electronics-tutorials.ws/filter/fil51.gif

    Does it matter what frequency range I'm interested in? If I increase or decrease R1 I can increase or decrease R2 to get the same frequency range, so why does it matter? Shouldn't there me some general rule for what the smallest resistor should be? Let's take 10 000 Hz for example.

    Of course I care about noise. 0 noise is desired. But how do you go about to compensate for noise? Do you pick larger capacitors to reduce noise?
    Please tell me how you do this.

    The output is an AC voltage or rather a square wave, so I don't know about linearity, what do you mean?

    I'm very, very scared about input and output impedance. This worries me a lot, because I don't understand the concept of it.
    I know very well what a resistor is. However, I have been told that this bandpass filter, needs a voltage buffer, which generate a low impedance output.
    I don't understand why the impedance output should be low. I don't understand how you can input or output an impedance at all. I thought it was the same thing as resistance.

    I just figured out that the impedance input must be low, otherwise it adds to the total input resistance, causing unpredictable centre frequency and response. I'm not sure what is meant by "it adds to the total input resistance".
    I am sorry to be so insistent, but if I am to help you, you will have to answer all of my questions. As an example, it is obvious that zero noise would be ideal. But just as obviously, it is unattainable, so you need to tell me what noise level is acceptable. If you just say "as small as possible," that is the equivalent of no answer at all. So please reread my list of questions, and provide answers. Otherwise this will last a very long time.

    Given that you are not a circuit designer, perhaps you could start by at least describing the application for which this filter is to be used.
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  6. #5  
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    Quote Originally Posted by tk421 View Post
    Quote Originally Posted by Mechatron View Post
    Quote Originally Posted by tk421 View Post
    Quote Originally Posted by Mechatron View Post
    Hello Sir/Mam.

    When selecting resistors for a band pass filter (, low pass filter or a high pass filter,) what's the difference between choosing two high or two low value resistors which give the same output?
    What is the minimal value for a resistor and a capacitor I should choose?

    Sincerely, Michael.
    You've provided such little specific information that it is impossible to provide anything useful. So I will ask you a set of questions to narrow things down a bit.

    What is the frequency range of interest?
    Is this a passive or active filter?
    Do you care about noise? If so, what noise level is acceptable?
    Do you care about linearity? If so, what distortion is acceptable?
    Do you care about input and output impedances? If so, what are the specifications or limitations on same?

    Those are just to get started. As you provide answers, there will be more questions.
    It's an active bandpass filter, it's an inverting bandpass filter like in the link below:
    http://www.electronics-tutorials.ws/filter/fil51.gif

    Does it matter what frequency range I'm interested in? If I increase or decrease R1 I can increase or decrease R2 to get the same frequency range, so why does it matter? Shouldn't there me some general rule for what the smallest resistor should be? Let's take 10 000 Hz for example.

    Of course I care about noise. 0 noise is desired. But how do you go about to compensate for noise? Do you pick larger capacitors to reduce noise?
    Please tell me how you do this.

    The output is an AC voltage or rather a square wave, so I don't know about linearity, what do you mean?

    I'm very, very scared about input and output impedance. This worries me a lot, because I don't understand the concept of it.
    I know very well what a resistor is. However, I have been told that this bandpass filter, needs a voltage buffer, which generate a low impedance output.
    I don't understand why the impedance output should be low. I don't understand how you can input or output an impedance at all. I thought it was the same thing as resistance.

    I just figured out that the impedance input must be low, otherwise it adds to the total input resistance, causing unpredictable centre frequency and response. I'm not sure what is meant by "it adds to the total input resistance".
    I am sorry to be so insistent, but if I am to help you, you will have to answer all of my questions. As an example, it is obvious that zero noise would be ideal. But just as obviously, it is unattainable, so you need to tell me what noise level is acceptable. If you just say "as small as possible," that is the equivalent of no answer at all. So please reread my list of questions, and provide answers. Otherwise this will last a very long time.

    Given that you are not a circuit designer, perhaps you could start by at least describing the application for which this filter is to be used.
    The noise level is equal to 5.75*10^-7 dBV/Hz. And I've given you everything else; 10 000 Hz frequency range, an inverted active band pass filter, I don't care about linearity. I care about the impedance. The input into the bandpassfilter must be low, and the source must come from a voltage buffer.
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  7. #6  
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    Quote Originally Posted by Mechatron View Post
    The noise level is equal to 5.75*10^-7 dBV/Hz. And I've given you everything else; 10 000 Hz frequency range, an inverted active band pass filter, I don't care about linearity. I care about the impedance. The input into the bandpassfilter must be low, and the source must come from a voltage buffer.
    Ok, almost all of that makes no sense. So, let's start with your application, then. What is this to be used for? And of course you care about linearity -- a filter that is permitted to distort an arbitrary amount makes no sense.

    When you say that the input into the filter must have a low impedance, that makes no sense. Perhaps you mean that the filter is to be driven by a source whose impedance is low. That's a completely different thing.

    The noise level you state is as useful as a random number. You left out several important parameters, without which the number is meaningless. So I add these questions:

    Is this number the amount of noise present in the source? Or is it the maximum acceptable value? If the latter, then is that value measured at the output, or is it referred to the input? Also, dBV/Hz is not a proper unit of noise density. It's either dBV/root-Hz, or dBm/Hz. Which do you mean?

    I'm having a tough time helping you because you leave out key information. I am assuming it's because you just don't know what's important, so again, let's just start with the application you're trying to build this filter for. Then I'll just make some educated guesses based on that instead.
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  8. #7  
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    Quote Originally Posted by tk421 View Post
    Quote Originally Posted by Mechatron View Post
    The noise level is equal to 5.75*10^-7 dBV/Hz. And I've given you everything else; 10 000 Hz frequency range, an inverted active band pass filter, I don't care about linearity. I care about the impedance. The input into the bandpassfilter must be low, and the source must come from a voltage buffer.
    Ok, almost all of that makes no sense. So, let's start with your application, then. What is this to be used for? And of course you care about linearity -- a filter that is permitted to distort an arbitrary amount makes no sense.

    When you say that the input into the filter must have a low impedance, that makes no sense. Perhaps you mean that the filter is to be driven by a source whose impedance is low. That's a completely different thing.

    The noise level you state is as useful as a random number. You left out several important parameters, without which the number is meaningless. So I add these questions:

    Is this number the amount of noise present in the source? Or is it the maximum acceptable value? If the latter, then is that value measured at the output, or is it referred to the input? Also, dBV/Hz is not a proper unit of noise density. It's either dBV/root-Hz, or dBm/Hz. Which do you mean?

    I'm having a tough time helping you because you leave out key information. I am assuming it's because you just don't know what's important, so again, let's just start with the application you're trying to build this filter for. Then I'll just make some educated guesses based on that instead.
    tk421: Don't bother trying to solve the problem.
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  9. #8  
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    Quote Originally Posted by Mechatron View Post
    tk421: Don't bother trying to solve the problem.
    If specs don't matter, or you don't know what specs matter, then just follow rules of thumb. Choose resistors in the kilohm to 10k range. Specify an op-amp whose gain-bandwidth product is much, much larger than the filter's center frequency, and whose slew rate well exceeds 2*pi*f*A, where A is the amplitude of the sinusoidal output, and f is the frequency.
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  10. #9  
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    Quote Originally Posted by tk421 View Post
    Quote Originally Posted by Mechatron View Post
    tk421: Don't bother trying to solve the problem.
    If specs don't matter, or you don't know what specs matter, then just follow rules of thumb. Choose resistors in the kilohm to 10k range. Specify an op-amp whose gain-bandwidth product is much, much larger than the filter's center frequency, and whose slew rate well exceeds 2*pi*f*A, where A is the amplitude of the sinusoidal output, and f is the frequency.
    I couldn't possibly have been more clear when I said that there is a voltage buffer with a high impedance input with a low impedance output. This output is the input to the bandpass filter. The bandpass filter MUST have a low impedance input. The bandwidth should be 10 000 Hz, so the center frequency is then 5000 Hz. But if you still feel stuck about what would be the lowest resistor and capacitor value, then let's move on to something else;

    What is the Q factor of the active bandpass filter (inverting band pass filter)?
    Link to the filter in the link below:

    http://www.electronics-tutorials.ws/filter/fil51.gif

    The transfer function of the band pass filter in the link provided, is it only useful for plotting the frequency response?
    And I wonder, at what point is an impedance high or low? At which values are they high or low?

    Don't ask me why they didn't teach my about impedance, to the level I require at my University.
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  11. #10  
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    Quote Originally Posted by Mechatron View Post
    I couldn't possibly have been more clear when I said that there is a voltage buffer with a high impedance input with a low impedance output. This output is the input to the bandpass filter.
    I fully understood that. No problem there. However, you then say this:

    The bandpass filter MUST have a low impedance input.
    That makes no sense. At all. That's why I asked the specific questions I did, which you did not appreciate. So I can only offer general guidance that may or may not serve your (unstated) purpose.


    The bandwidth should be 10 000 Hz, so the center frequency is then 5000 Hz.
    Again, the second part of your sentence is not in any way a consequence of the first. The bandwidth is a function of Q, which may be specified independently of the center frequency. Stated another way, this is a second-order dynamical system, so there are two degrees of freedom. You are assuming away one of the degrees of freedom. That, and other statements/assertions like it, have conveyed your level of understanding. But you find my didactic style unpalatable, so we are stuck.

    But if you still feel stuck about what would be the lowest resistor and capacitor value, then let's move on to something else;
    We are stuck only because you lack the understanding of circuit theory to understand why answering my questions precisely is important.

    What is the Q factor of the active bandpass filter (inverting band pass filter)?
    Link to the filter in the link below:

    http://www.electronics-tutorials.ws/filter/fil51.gif

    The transfer function of the band pass filter in the link provided, is it only useful for plotting the frequency response?
    And I wonder, at what point is an impedance high or low? At which values are they high or low?

    Don't ask me why they didn't teach my about impedance, to the level I require at my University.
    I recommend looking at a wiki article (e.g.) on impedance.

    You may also benefit from studying the relevant sections in Horowitz and Hill, The Art of Electronics. There are also many handbooks on active filters that would give you useful background.

    As to the transfer function, that's just a fancy name for "complete amplification factor" in this case. It will give you the gain and phase as a function of frequency. As such, it is a complex number at each frequency. The magnitude of that complex number is the magnitude of the amplification factor. The phase of that complex number is the phase of the output relative to the input.

    There is no absolute "high" or "low" impedance (except, maybe, zero and infinity). The relevant issue is whether it's (much) higher or lower than some other impedance. That's what matters.
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  12. #11  
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    Quote Originally Posted by tk421 View Post
    Quote Originally Posted by Mechatron View Post
    I couldn't possibly have been more clear when I said that there is a voltage buffer with a high impedance input with a low impedance output. This output is the input to the bandpass filter.
    I fully understood that. No problem there. However, you then say this:

    The bandpass filter MUST have a low impedance input.
    That makes no sense. At all. That's why I asked the specific questions I did, which you did not appreciate. So I can only offer general guidance that may or may not serve your (unstated) purpose.


    The bandwidth should be 10 000 Hz, so the center frequency is then 5000 Hz.
    Again, the second part of your sentence is not in any way a consequence of the first. The bandwidth is a function of Q, which may be specified independently of the center frequency. Stated another way, this is a second-order dynamical system, so there are two degrees of freedom. You are assuming away one of the degrees of freedom. That, and other statements/assertions like it, have conveyed your level of understanding. But you find my didactic style unpalatable, so we are stuck.

    But if you still feel stuck about what would be the lowest resistor and capacitor value, then let's move on to something else;
    We are stuck only because you lack the understanding of circuit theory to understand why answering my questions precisely is important.

    What is the Q factor of the active bandpass filter (inverting band pass filter)?
    Link to the filter in the link below:

    http://www.electronics-tutorials.ws/filter/fil51.gif

    The transfer function of the band pass filter in the link provided, is it only useful for plotting the frequency response?
    And I wonder, at what point is an impedance high or low? At which values are they high or low?

    Don't ask me why they didn't teach my about impedance, to the level I require at my University.
    I recommend looking at a wiki article (e.g.) on impedance.

    You may also benefit from studying the relevant sections in Horowitz and Hill, The Art of Electronics. There are also many handbooks on active filters that would give you useful background.

    As to the transfer function, that's just a fancy name for "complete amplification factor" in this case. It will give you the gain and phase as a function of frequency. As such, it is a complex number at each frequency. The magnitude of that complex number is the magnitude of the amplification factor. The phase of that complex number is the phase of the output relative to the input.

    There is no absolute "high" or "low" impedance (except, maybe, zero and infinity). The relevant issue is whether it's (much) higher or lower than some other impedance. That's what matters.
    If you actually knew what you were talking about, you would be able to at least solve a part of the problem. Please do not attempt to help again.
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  13. #12  
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    Quote Originally Posted by Mechatron View Post
    If you actually knew what you were talking about, you would be able to at least solve a part of the problem. Please do not attempt to help again.
    I can only lead you to knowledge. I cannot make you think.

    I'll now place you on ignore. Good luck.
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  14. #13  
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    Quote Originally Posted by Mechatron View Post
    When selecting resistors for a band pass filter (, low pass filter or a high pass filter,) what's the difference between choosing two high or two low value resistors which give the same output?
    In general the resistors you choose will be based on several things:

    Output impedance of the op amp (assuming an active filter.) If the capacitors are very high value and the resistors are very low value (say < 10 ohms) the output impedance of the op amp will start being a factor in the response of the system. And since that impedance is often nonlinear it's very hard to compensate for.

    Desired input impedance. Again if the capacitors are large and resistors are small, input impedance (especially at high frequencies) will be very low; often a bad thing. In the circuit you show you will start seeing the output impedance at the input.

    Noise/PCB layout. If you use high value resistors and low value caps (say 10 megohm resistors) the resistances on the PWB itself (i.e. the fiberglass) will start being an issue.

    What is the minimal value for a resistor and a capacitor I should choose?
    I generally don't go below 1K for filter networks and I generally don't go higher than 1 meg. The exact number is going to depend on which op amp you use, your desired input impedance, what frequencies you care about, the desired quality of the filter etc.
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  15. #14  
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    Quote Originally Posted by billvon View Post
    Quote Originally Posted by Mechatron View Post
    When selecting resistors for a band pass filter (, low pass filter or a high pass filter,) what's the difference between choosing two high or two low value resistors which give the same output?
    In general the resistors you choose will be based on several things:

    Output impedance of the op amp (assuming an active filter.) If the capacitors are very high value and the resistors are very low value (say < 10 ohms) the output impedance of the op amp will start being a factor in the response of the system. And since that impedance is often nonlinear it's very hard to compensate for.

    Desired input impedance. Again if the capacitors are large and resistors are small, input impedance (especially at high frequencies) will be very low; often a bad thing. In the circuit you show you will start seeing the output impedance at the input.

    Noise/PCB layout. If you use high value resistors and low value caps (say 10 megohm resistors) the resistances on the PWB itself (i.e. the fiberglass) will start being an issue.

    What is the minimal value for a resistor and a capacitor I should choose?
    I generally don't go below 1K for filter networks and I generally don't go higher than 1 meg. The exact number is going to depend on which op amp you use, your desired input impedance, what frequencies you care about, the desired quality of the filter etc.
    You said that if the capacitor is large and the resistor is small, the input impedance will be small. But, I thought the input impedance comes from the output impedance of the voltage buffer? The voltage buffer I have in my circuit, is just an operational amplifier with identical values, where one is connected between ground and negative input pin of op-amp while the other resistor is connected between the negative input pin of the op-amp and the output of the op-amp.

    The source impedance must be low with respect to the input resistance, and normally these filters are driven from an opamp buffer. If a high impedance is used, it adds to the total input resistance, causing unpredictable centre frequency and response. A low impedance input means that it will be less susceptible to noise pickup than a high impedance one.
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    Quote Originally Posted by Mechatron View Post
    Quote Originally Posted by tk421 View Post
    Quote Originally Posted by Mechatron View Post
    I couldn't possibly have been more clear when I said that there is a voltage buffer with a high impedance input with a low impedance output. This output is the input to the bandpass filter.
    I fully understood that. No problem there. However, you then say this:

    The bandpass filter MUST have a low impedance input.
    That makes no sense. At all. That's why I asked the specific questions I did, which you did not appreciate. So I can only offer general guidance that may or may not serve your (unstated) purpose.


    The bandwidth should be 10 000 Hz, so the center frequency is then 5000 Hz.
    Again, the second part of your sentence is not in any way a consequence of the first. The bandwidth is a function of Q, which may be specified independently of the center frequency. Stated another way, this is a second-order dynamical system, so there are two degrees of freedom. You are assuming away one of the degrees of freedom. That, and other statements/assertions like it, have conveyed your level of understanding. But you find my didactic style unpalatable, so we are stuck.

    But if you still feel stuck about what would be the lowest resistor and capacitor value, then let's move on to something else;
    We are stuck only because you lack the understanding of circuit theory to understand why answering my questions precisely is important.

    What is the Q factor of the active bandpass filter (inverting band pass filter)?
    Link to the filter in the link below:

    http://www.electronics-tutorials.ws/filter/fil51.gif

    The transfer function of the band pass filter in the link provided, is it only useful for plotting the frequency response?
    And I wonder, at what point is an impedance high or low? At which values are they high or low?

    Don't ask me why they didn't teach my about impedance, to the level I require at my University.
    I recommend looking at a wiki article (e.g.) on impedance.

    You may also benefit from studying the relevant sections in Horowitz and Hill, The Art of Electronics. There are also many handbooks on active filters that would give you useful background.

    As to the transfer function, that's just a fancy name for "complete amplification factor" in this case. It will give you the gain and phase as a function of frequency. As such, it is a complex number at each frequency. The magnitude of that complex number is the magnitude of the amplification factor. The phase of that complex number is the phase of the output relative to the input.

    There is no absolute "high" or "low" impedance (except, maybe, zero and infinity). The relevant issue is whether it's (much) higher or lower than some other impedance. That's what matters.
    If you actually knew what you were talking about, you would be able to at least solve a part of the problem. Please do not attempt to help again.
    Hey, Mechatron, as an electronics engineer myself, I can tell you that tk421 very much DOES know his stuff. All his questions and the help he is trying to give you are completely relevant. You sir need to realise that you have very little electronics knowledge and your understanding is very patchy. You need to learn what impedance is - there is no need to be scared about it. Many frequency dependant filters change their characteristics if the impedances change, (and vice-versa), hence why most have buffers on their inputs and outputs. Do some research as tk421 suggests, take a course perhaps, and you will realise that all of tk421's comments are totally on point. There is no need for your petulant and badly informed replies to him.

    OB
    Last edited by One beer; May 6th, 2014 at 04:40 AM.
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