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How To Make Passive High Pass Filter?
How To Make Passive High Pass Filter?
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Old 29th October 2017, 06:37 PM   #1
cacao ambiance is offline cacao ambiance  United Kingdom
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Question How To Make Passive High Pass Filter?

How to i create a passive line-level 90hz high pass filter? I want to be able to filter off the low end limit of my speakers (90hz) before it goes into the (class d, tpa*) amps.
How would using a headphone output from a device affect the passive filter?
Or a audio isolator before the passive filter?

there's designs using resistors and caps but where there other passive circuits that used other components like err was it inductors??? :/

I’ve tried days of researching but a lot of what i find is for electric guitars or doesn't answer my question of what resistor and caps(?) to use for a line level or headphone input. Im a newbie that is trying to make my first understand of these related subjects and could do with a meaningful to me example , like this question to help me get started in making sense of it all
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Old 29th October 2017, 07:13 PM   #2
scottjoplin is offline scottjoplin  Wales
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What speakers are they? What order filter do you need/want? A first order filter, that is one that attenuates at the rate of 6dB per octave, would comprise a capacitor in series with a resistor to form a potential divider, the resistor could simply be the input impedance of your amplifier TLS.org | Passive Line-Level Crossover
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Old 29th October 2017, 07:59 PM   #3
ceulrich is offline ceulrich
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Default One Option for High Pass Filter

Hello cacao ambiance,

If I understand your question correctly, I had a similar issue a while ago. Due to room placement, my right speaker had a bit more low bass output than the left speaker, so I wanted to tame down the one with more output. My solution was to insert a capacitor between the output of my preamp and the input of the power amplifier for the right channel. I worked up a little spreadsheet to calculate the capacitance needed for the known input impedance of the power amplifier.

The first figure shows the simple schematic and the calculations for determining the -3dB frequency. The second figure calculates and plots the response curve.

In my case I used a pair of 0.1 MFD MusicCaps in series to get 0.05 MFD or 50 nF, which I thought was close enough to the calculated 47 nF for this application. This solution worked fine to tame the low bass output so the two speakers were better matched.

I hope this will be of some help.

Cheers,

ceulrich
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Old 29th October 2017, 08:35 PM   #4
Fast Eddie D is offline Fast Eddie D  United States
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If you know the input impedance of your power amp, you can calculate it according to the formula

C = 1/2pi(Zin)(f3) where

Zin = amplifier input impedance in Ohms
f3 = desired -3dB cutoff frequency in Hertz
C = series capacitor in farads (NOT microfarads)

(This equation is a simplified derivation of above equation.)

Example Zin = 47K, f3 = 90 Hz, C = 1/2pi(47,000)(90) = 38 nF.

If the amplifier has an input capacitor, then remove it and replace it with your series capacitor.

Output impedance of driving device needs to be much lower than the input impedance of the amplifier. Typical mobile devices have an output impedance ranging from a few ohms to around 40 ohms, which would not affect the performance of the circuit. Typical preamps have an industry standard of 47 ohms output impedance, but again this varies.

So you need to know the input impedance of your amplifier to design this circuit.

Last edited by Fast Eddie D; 29th October 2017 at 08:37 PM.
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Old 29th October 2017, 09:35 PM   #5
cacao ambiance is offline cacao ambiance  United Kingdom
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oow goodie help already!

was hoping for something generic that i can swap between amps.
I use tpa3116/8, irs2092,tpa3250, tas5611, tpa3255, tk2050. I mainly use the tpa3116 black board for these speakers. I have attached the 2 Input Impedance related pages from the datasheet for the tpa3116. next looks like i need to find out what gain resistors are used to know the Input Impedance, right?

next ill get photos of the speakers
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File Type: pdf tpa3116.Input.Impedance.pdf (125.6 KB, 8 views)
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Old 29th October 2017, 10:03 PM   #6
cacao ambiance is offline cacao ambiance  United Kingdom
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heres the photos of my speakers and the amp i use with these ones, which is in fact YJ SMD version of tpa3116 boards:
Yuan Jing Audio - ON SALE! - TPA3116 Class-D 2.0 Stereo Amplifier Board + Anti-Pop - USD $14.99

I think these speakers are European or German - made or designed. there was very little info about them, think i remember.
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File Type: jpg IMG_6793.JPG.q80.jpg (999.0 KB, 155 views)
File Type: jpg IMG_6791.JPG.q80.jpg (638.1 KB, 146 views)
File Type: jpg IMG_6790.JPG.q80.jpg (978.0 KB, 145 views)
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Old 29th October 2017, 10:11 PM   #7
cacao ambiance is offline cacao ambiance  United Kingdom
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I”m not sure what i need. How do i work out how many *dB per octave i need?
I even wonder would a passive post amp speaker "crossover" (not really a crossover, just a freq limit filter.) is what i want? would be generic and work with any amp but i lose efficacy. The speakers don’t have cross over. the tweeter seams to have its own thing cus it was just wired in parallel with the mid/bass speaker.

I should mention come to think of it, these speakers where originally 100v ones but i removed the transformer to turn them into normal speakers. The transformer had a fat cap across it. i can get a photo of it? still got them laying around.

Last edited by cacao ambiance; 29th October 2017 at 10:23 PM.
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Old 29th October 2017, 10:18 PM   #8
Fast Eddie D is offline Fast Eddie D  United States
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Quote:
Originally Posted by cacao ambiance View Post
Im not sure what i need. How do i work out how many *dB per octave i need?
With passive you're pretty much stuck with 3 dB/octave. Otherwise go active.

Quote:
I even wonder would a passive post amp speaker "crossover" (not really a crossover, just a freq limit filter.) is what i want?
Will probably require a large, expensive capacitor.

Quote:
was hoping for something generic that i can swap between amps.
Will only be "generic" if the input impedances of your amps are the same.

Do you know the input impedance of your amplifier?
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Old 29th October 2017, 11:27 PM   #9
cacao ambiance is offline cacao ambiance  United Kingdom
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Quote:
Originally Posted by Fast Eddie D View Post
With passive you're pretty much stuck with 3 dB/octave. Otherwise go active.
ok, thx. knowing that is quite helpful.
Quote:
Originally Posted by Fast Eddie D View Post
Will probably require a large, expensive capacitor.
i could potently live with that.

Quote:
Originally Posted by Fast Eddie D View Post
Will only be "generic" if the input impedances of your amps are the same.

Do you know the input impedance of your amplifier?
nope. a prev post was about my efforts to try and see about working that out.

Last edited by cacao ambiance; 29th October 2017 at 11:52 PM.
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Old 29th October 2017, 11:48 PM   #10
cacao ambiance is offline cacao ambiance  United Kingdom
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Here is are photos of the original transformer from the speakers from when they where 100v ones. it had this 15uF kerpen cap and parallel 220R resistor in series with what i guess(?) was the ground of the user/installer connections to the speaker transformer.
Can this cap be used for a speaker side/post amp passive low freq filter? or did it do something else to do with it being a 100v line speaker?
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File Type: jpg IMG_6795.JPG.q80.jpg (662.7 KB, 40 views)
File Type: jpg OrginalSpeaker100vTransferConnections_IMG_6796.jpg (918.6 KB, 38 views)

Last edited by cacao ambiance; 29th October 2017 at 11:50 PM.
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