Hi guys.
I'm going to build simple input stage on opamp (unity gain).
But I've got a question in regards to LPF.
It should be placed as close to sockets as possible.
But how about values or R and C ?
Let assume that DAC output is between 50-100R and cable capacitance in not that important (it should be 100pF per one meter).
Which values are the best compromise between noise (high value resistors are more noisy), but higher value capacitors will increase distortion due to overloading on source output stage.
So I'm asking you about your personal values of good LPF at input stage.
Or maybe I should replace RC with LC?
But in some book I read that L will add more distortion to signal (I read that in some PDF and it was measured on opamp output which was isolated with L instead od R).
How about building more advanced few stages LPF for audio input purposes?
Does it make any sense (we are not talking about money now) in terms of quality and overall stability?
Thank you for your time and effort to share your personal expierience 😉
I'm going to build simple input stage on opamp (unity gain).
But I've got a question in regards to LPF.
It should be placed as close to sockets as possible.
But how about values or R and C ?
Let assume that DAC output is between 50-100R and cable capacitance in not that important (it should be 100pF per one meter).
Which values are the best compromise between noise (high value resistors are more noisy), but higher value capacitors will increase distortion due to overloading on source output stage.
So I'm asking you about your personal values of good LPF at input stage.
Or maybe I should replace RC with LC?
But in some book I read that L will add more distortion to signal (I read that in some PDF and it was measured on opamp output which was isolated with L instead od R).
How about building more advanced few stages LPF for audio input purposes?
Does it make any sense (we are not talking about money now) in terms of quality and overall stability?
Thank you for your time and effort to share your personal expierience 😉
Are you talking about a low pass filter that goes directly after a DAC chip? If so, which chip are you planning to use?
No, I'm talking about PRE/AMP input stage 🙂
I just want to build a good LPF for incoming RF noise 😉
I just want to build a good LPF for incoming RF noise 😉
Define "good quality".
To build a filter first you have to design it. To design a filter first you have to decide what you want it to do. What do you want it to do?
To build a filter first you have to design it. To design a filter first you have to decide what you want it to do. What do you want it to do?
OFC the point is to build first-order lowpass
filter to remove incoming RF before it has a chance to reach the opamp and demodulate
into the audio band.
So I'm asking about good compromise in terms of component values and it would be nice to know if LC would be better than RC LPF.
@Scott I'm just trying to write correctly, but english is not my native language, so I don't know many words and technical jargon.
BTW I always prefered graphical versions instead of pure text or mixed type of tutorials.
I'm not that advanced and skilled to know everything even in theory.
filter to remove incoming RF before it has a chance to reach the opamp and demodulate
into the audio band.
So I'm asking about good compromise in terms of component values and it would be nice to know if LC would be better than RC LPF.
@Scott I'm just trying to write correctly, but english is not my native language, so I don't know many words and technical jargon.
BTW I always prefered graphical versions instead of pure text or mixed type of tutorials.
I'm not that advanced and skilled to know everything even in theory.
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Well, your English is better than many native speakers so don't worry about that. Re, the technical jargon- that's not English ha ha
Ok. maybe I should use slang word instead of jargon 😉
It's always good to hear that my english is not that bad as my teacher in school said 😀
So guys how about that kinf of LPF?
Most common is 100R-1Kohm resistor with 100pF-1nF capacitor.
I never used LC LPF and I heard is more expensive but it gives better RF rejection.
It's always good to hear that my english is not that bad as my teacher in school said 😀
So guys how about that kinf of LPF?
Most common is 100R-1Kohm resistor with 100pF-1nF capacitor.
I never used LC LPF and I heard is more expensive but it gives better RF rejection.
I have a design for one of those on my blog, here - http://www.diyaudio.com/forums/blogs/abraxalito/968-line-input-rf-reject-filter.html
I use and recommend a first stage filter using only a 47pF across the input socket Hot and Cold pins.
This combined with the incoming cable's resistance and inductance creates a single pole filter at one turn-over frequency and a 2pole filter at a different turnover frequency.
If you choose the 47pF to have ultra low inductance and fit it direct to the incoming socket with effectively zero length leads, it will operate up to very high frequencies, maybe even to the 2GHz to 4Ghz interference we are getting in our homes.
Then an RC filter (at the other end of the input cable) on the amplifier PCB. Here I use an RC time constant of ~700us (1k + 680pF, or 2k + 330pF, but there is a lot of leaway and you will see variation from 200us to 1500us) this usually uses leaded components so it has an upper limit where the filtering effect is starting to lose performance, but it is effective from ~1MHz to ~100MHz, if you can keep the lead lengths and components small.
This combined with the incoming cable's resistance and inductance creates a single pole filter at one turn-over frequency and a 2pole filter at a different turnover frequency.
If you choose the 47pF to have ultra low inductance and fit it direct to the incoming socket with effectively zero length leads, it will operate up to very high frequencies, maybe even to the 2GHz to 4Ghz interference we are getting in our homes.
Then an RC filter (at the other end of the input cable) on the amplifier PCB. Here I use an RC time constant of ~700us (1k + 680pF, or 2k + 330pF, but there is a lot of leaway and you will see variation from 200us to 1500us) this usually uses leaded components so it has an upper limit where the filtering effect is starting to lose performance, but it is effective from ~1MHz to ~100MHz, if you can keep the lead lengths and components small.
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