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modified 10 years ago

RC PI Filter

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03:49:50

This is a 7 pole passive RC PI filter. It has a low-pass cut-off of jut over 1.5 KHZ. However, as can be seen, the actual 'knee' point starts somewhat before the calculated cut-off. It shows a >100 dB reduction in signal level at 100 KHZ, although in audio we are normally concerned with the 10 KHZ point.

The end load is set at a nominal 1K and in audio circuits its not normal to talk about a standard impedance as is done with RF circuitry. You can play with the 1K load and you'll see that it doesn't  have a huge effect on the performance of the filter.

Also of interest in this simulation is the ability to choose from four different points along the filter network. 

published 10 years ago

10 years ago

I wont speak of "poles" for an RC filter. Btw the first cap does not have any impact to the filter output. Its overruled by the low ac source impedance and just cause some more current to draw. Call this a "lpf of third order". Not poles. Poles are coming into the game with LC filters. Hope this clarifies a little.

10 years ago

The cutoff frequency is 475Hz. The cutoff is always taken at -3dB from the center frequency. So in thia case the center frequency attenuation is -12dB so the cutoff is at 475Hz/-15dB. Hurz how do you mean poles come with LC filters? They don't have polarity. Or is it some other kind of pole?

10 years ago

Poles exixt in every transfer function, RC, LC, RLC, RL and so on

10 years ago

You probably mean a resonance peak with LC

10 years ago

Poles in LC circuits are resonance points/frequencies. Thats my definition

10 years ago

Btw, amplifier mostly measured cut off are given at -1dB. Thats also a thing of definition. Or filter ripple (caused by underdampening) is sometimes gives in +-0.5 or +-0.1dB.

10 years ago

@thebugger, filter poles have NOTHING todo with polarity!

10 years ago

Count the number of L and C components and subtract 1 is the number of poles or maybe more precise poles and zeros!

10 years ago

Yes i got it. The peak resonance points. But especially in filters all cut off frequencies are given at -3dB.

10 years ago

Amplifiers are other things on hand. Especially in low gain preamps. You can't have -3dB when the gain itself is lower than 3dB. This would cause attenuation at the corner frequenciea rather than any gain

10 years ago

Buggzy, for cheap "HIFI" amplifiers Yes -3dB is marketing Schatzi.

10 years ago

Have you even read what i just wrote :D I said this applies in filters and not in amps.

10 years ago

I'm talking about the cut off freqyency! What are you talking about?

10 years ago

The cut off frequency too. But for filters, seeing as how he designed a filter circuit and not an amplifier circuit. Amplifiers are a whole othwr deal :D

10 years ago

You said " The cutoff is always taken at -3dB from the center frequency. " and " in filters all cut off frequencies are given at -3dB " these are your statements and I just say this is to general spoken!

10 years ago

In filters it's a rule. The corner frequency is always at -3dB. Would you like a reference?

10 years ago

No because you have and usual don't good references - remember your amplifier dream project. Forget it

10 years ago

Just write cutoff frequency in google. I needn't say more.

10 years ago

I take the point about the 3dB point. However this circuit is just an illustration of PI filters mainly to show progressive increase in filtering as the number of poles increases.

10 years ago

Also I don't make any claim about the 3dB point. Hurz, so are you saying that this would be a 6 pole filter??????????

10 years ago

For audio a 10 dB point is perhaps more useful check out jack orman! He is a well respected audio electronics dude!

10 years ago

How on earth could a 'rule' pertaining to filters not relate to amplifiers? That makes absolutely no sense at all. Any device under test can be measured for voltage dB gain or loss and this can be done with a swept frequency. This will reveal that any amplifier is also a filter since there are no perfect amplifiers.

10 years ago

@hurz, yeah I take your point about the first cap. Its totally redundant and probably counter productive.

10 years ago

For me this RC low pass does not have any RELEVANT poles. LC will have. And dB's are a matter of marketing definition, as i tried to illustrate. Anyway dont trust a user who tries to use caps as galvanic separation. Or till a few days never saw a reed relais contect but was explains the community how fast the fastest relais can be! Lol. Or does learned 2years ago how dB is related calculated for amplifier. Or thinks poles are in filter related to polarity. Or AM is limited to broadcast from 300kHz to 1.5MHz. Or AM is old technology and writing this nonsense with its QAM mobile phone. Or used voltage doubler to generate 650V to burn it then down to 250V! That guy has to learn a lot before talking bullshit to us!

10 years ago

You should update your info. Most of these things are soooo old that they don't even ring a bell. Never said caps would galvanically separate the mains. They provide extra protection in a very dangerous circuit. The dB thing for amps was an early EC bode plot error, i directly took the result from it. The thing about the AM was over 4 years ago. I was just barely scraping electronics back then. And i never burnt a voltage doubler. I don't even know from where you took that last one. On the other hand a certain know-it-all who always corrects people on technicalities, just for the kicks is someone i wouldn't trust. Oooh excuse me if i write Khz instead of kHz. How wrong of me 😱 How efficiently you're avoiding the main topic. So what result did you get from writing cut-off frequency in google. Please share. And yes i find myself more agreeing with @zorgrian, about the -3db thing applying to amplifiers as well as filters. Power amplifiers also use the same rule. Check every Hi-Fi example on the web, and mostly Rob Elliot's and then talk to me, you arrogant fart.

10 years ago

I admit i don't like to be corrected, but by God's mercy, if someone were to correct you, all hell breaks loose

10 years ago

bookmarking this for comments

10 years ago

@thebugger, sorry but my comment about the -3dB point filters/ amplifiers related directly to your lack of logic

10 years ago

Buggzy the list is much longer. You have to fart a lot more. Dont you see, this is the hard way to learn. Hard for both sides teacher and student. Every time you learned something new you take it as law and nobody is allowed to say anything against it. Because it the last corner knowledge you can imagine and nothing else matter. This is in your head like hammered in stone! This is how you "see" the world with closed eyes. Your fixed model of electronic. And any point on this list took me quite some effort to teach you its not in stone and there is more in our world. But this list is getting longer and longer. Try to understand that you should more often say "imho.." Or "as far as i know..." or "maybe its this..." Or "hope this is true...". Stop the stone age work. Makes it for us as observer and also for you much easy. Think about

10 years ago

Well these rules are not created by me. These things are really written in stone. Or more like in paper. Everywhere on the world, people follow these rules,and as a german i thought you'd be more fond of these rules. The -3dB is widely used as a corner frequency of almoat every circuit. It finds less application in low gain preamps, but as a whole it's universal. Nonetheless let's just stop here. I'm not looking for a battle of minds. I agree you know more about these things, but I'm no slacker too. For me there are a few people in this app with far superior knowledge than others. You, Secuture, there used to be a guy named Pip, and a few others too, which have had pretty successful circuits, and as far as I'm concerned I'm in there too ;)

10 years ago

Ok, close this last list fail and check this link carefully what they say about their -3dB definition. Thats marketing you as young engineer (or what ever you think you are :-) trapped https://www.google.de/url?sa=t&source=web&rct=j&url=http://www.icepower.dk/files/solutions/icepower1000aspdata.pdf&ved=0ahUKEwjE3siLtP_KAhVTSZoKHWijAIgQFggdMAA&usg=AFQjCNHRXQfjrx1GziqnhpITDYYoPg_uJA&sig2=c3XugMPqjNbFe6wJETHpWA and search for bandwidth and read the note about!

10 years ago

Here the technical view of a -3dB bandwidth "High-frequency Protection (amplifier) The amplifier has full power bandwidth from 20Hz to 20kHz. However, the output filter of the amplifier is not designed for long-term high frequency signals (>20kHz) with full amplitude. This is due to the power capabilities of the output Zobel-network. In such cases, the high-frequency protection circuit disables the amplifier for 5 seconds and then restarts."

10 years ago

It disables the amp at the presence of high frequency signals above 20Khz? That seems somewhat weird. Typically high frequency signals have much less ,,punch'' on the circuitry and speakers than bass frequencies. That's why they invented the RIAA equalization. Or you're talking about something else?

10 years ago

Aah a zobel network, yes, they usually use low value resistors, that may overheat if the output is at a continuous high frequency. But is the effect so dramatic that it needs special precautions

10 years ago

Higher frequency is actually related to higher energy! A loudspeaker is an inductive load and so this is a reactive load; necessarily this means that it will present variation of impedance at different given frequencies. So, you might expect the loudspeaker to present higher impedance as the frequencies increase. This is only one factor here, as the voice coil is designed to operate within a given range of frequencies, outside of which and importantly above which the windings will cause both vibrational damage to the throat and cone AND overheat. Low frequencies will punch the voice-coil to move slower but further (if the speaker is actually designed to handle such frequencies). All of this is why amplifiers often have impedance matching networks. These attempt to compensate for the variation in impedance presented by the speaker. These are things I use for my work, so please do not try telling me that I am wrong or that I have not applied rule no. 2361 in the classroom schoolbook! Thank you!

10 years ago

"There is no spoon" THE MATRIX!

10 years ago

RIAA equalization is a specification for the recording and playback of phonograph records, established by the Recording Industry Association of America (RIAA). The purposes of the equalization are to permit greater recording times (by decreasing the mean width of each groove), to improve sound quality, and to reduce the groove damage that would otherwise arise during playback.

10 years ago

The spoon exists only in the Matrix, which really means it doesn't exist. It's a lesson for Neo, to help him realize that manipulating the Matrix isn't about focusing on an object and trying to change it. The object doesn't exist, so he can't change it, he has to change himself. Metaphorically, it's all in his head.

10 years ago

I know why RIAA was invented, i also know the effects of high frequencies on the coil of the speaker. That's why i was surprised. High frequencies have less ,,punch'' on the coil, because its impedance increases as the frequencies do. I also noticed why hurz wrote it. The Zobel Network won't be able to handle a continuous high frequency ,,attack in bulgarian'', so thr circuitry will disable the amp for a few seconds. One thing i don't get though. How can high frequencies damage the coil if its impedance restricts the flow of electrons through it? I don't see how the winding will vibrate as such high frequencies, and moreover why would they overheat when the current through them is reduced.

10 years ago

1) it doesn't look like did know why RIAA eq. Was invented.

10 years ago

2) there's little evidence to support that you did know the effects of high frequency on loudspeaker coils as your last statement demonstrates

10 years ago

3) you ask how high frequency signals damage the coil. Remember that the voice coil is part of a complex electromotive dynamic energy transfer device or transducer. Reactive devices are not like simple resistance. Put very simply, if the energy cannot be transferred to movement then it becomes heat!

10 years ago

This is a problem of Amps itself - however, do not expect that loudspeaker have a flat impedance over 20-20kHz. Additional - Internal frequency splitter/crossover do have extreme different Z even inside this frequency bandwidth. Z is not constant and can also not be taken only inductive!! You should know if you publish 100Watt amplifier and recommend how to build them, right? i think you never build one. That's again a prove of your imaginary world of fantasy.

10 years ago

In almost every PA system that I've worked with (up to 500K rigs) it's always the tweeter system that blows first. In professional rigs we don't use crossovers at the speaker end of the amplifier, we split the signal at the output of the desk. Sometimes we split this into as many as five bands. These are then fed into amps and speakers with known response. (Or known as much as possible haha)

10 years ago

Doing this really helps in many ways not least of which is getting the maximum power out of the speakers and obtaining the minimum distortion!

10 years ago

I know what an inductor is thanks, wtf. And yes i know why RIAA was invented. Mostly because bass frequencies take more space on the record, and so the increase the efficacy of a sort. But one thing i still don't get. High frequency current doesn't pass theough the coil, as much as low frequency current. So how can it be dissipated, when it's not consumed. The reactive impedance increases, so the current flow decreases. I don't think that's ever going to be a problem. The tweeters are the first to go.

10 years ago

Now you say you know all about RIAA. But why this should fixed build into an modern amplifier as you suggested. Boy stop being like a slippery snake. And as i already said - the output of an power amp does not see a constant R or L and also not a const Z it depends on its frequency if Z eg gets a -Z. Read something about smith charts. Hope this will helps. Read google first the ask. Sorry im lossing my Erziehung

10 years ago

I know the impedance thing. It's very close to mind to know that the speaker will not exhibit the same impedance for the whole audio spectrum, as with all inductors, because a speaker is a large inductor, with an active resistance R and reactive resistance Xl and a combined resistance (impedance) Z. The R is constant the Z is variable. The thing i don't get is why would high frequencies damage the speaker speaker if its impedance is higher at high frequencies, so less current will flow through it.

10 years ago

OK I am not sure how much merit there may be in continuing like this. However, since I don't like repeating things, I must remind you that there are other considerations. One of which is that the electrical characteristics are not the only barrier to generating sufficient high frequency audio in the air in a given space. Human hearing is also not flat in terms of perception. We generally need much higher sound pressure at the top end and the worst case of this is open air auditoriums. Here is where the amplifiers and tweeters really need to be well designed.

10 years ago

Yes the human hearing is not linear, it's more logarithmic, and it's somewhat wobbly between 3kHz and 16kHz, and i get why tweeters are used. I just didn't get the coil damaging thing. I've made some experiments with inductors and the higher the frequency the lower the current through them. The lower the current the lower the total power dissipation through the active resistance, and as far as i know reactive resistance Doesn't actively dissipate power, therefore the higher the frequency should lead to less strain on the speaker, and on the amp itself.

10 years ago

Yes, and its also already said in the bang and olufson spec. Problem in this case is the loudspeaker together with internal compensating zobel network. B&O portects the amp and not the loudspeaker! I never said something different, right;

10 years ago

Check this http://everycircuit.com/circuit/5767875407642624

10 years ago

Again what B&O honestly say about the technical view of a -3dB bandwidth "High-frequency Protection (amplifier) The amplifier has full power bandwidth from 20Hz to 20kHz. However, the output filter of the amplifier is not designed for long-term high frequency signals (>20kHz) with full amplitude. This is due to the power capabilities of the output Zobel-network. In such cases, the high-frequency protection circuit disables the amplifier for 5 seconds and then restarts."

10 years ago

I imagine what such a circuit would look like. Let me give it a try.

10 years ago

I go to sleep, good night

10 years ago

I'm ready. Going to sleep too :)

10 years ago

The more I learn, the less I know. - A.E.

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