HomeMade LowPass Filter?

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fourthhorseman

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looking at making a filter to add inline,between the main board and speaker of a scanner,,

Size is one issue,and the cut off cant be too low,

reason,
OEM speaker is a little too high/harsh would like to,for lack of a better term,take the edge off..

any thoughts,,im thinking its basically like the highpass filter that you make to cool the signal off
coming off a disc tap..but im not sure of the specs..i know the shack has the parts,,a capacitor,
a resistor,,presto!...sort of...lol..

thanks in advance,and ill happily post pics and findings!!
 

k9rzz

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The higher register of the speaker is probably due to it's small size and small box it's mounted in. Dunno if a filter is going to do much. What radio are we talking about? Can you just patch the audio into a stereo or PC ? I do that often and it improves audio 100 fold.
 

fourthhorseman

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yeah,,in house,or car i can go 1/8th to a pair of RCAs to input into my home stereo amp,or the direct line to my durangos
alpine,,
more or less looking to have better audio-in hand..on the porch or in traffic AM on the car stereo,and scanner in cupholder..

i totally get what your saying about the size & mount/enclosure for the speaker,,
its in a 106,,the 97s speaker isnt so high,the OEM is almost a tweeter,,its truely a
pain to my ears at times,,
 
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jackj

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K9RZZ is right on his advice. An effective low-pass filter is going to remove most of the audio the speaker is capable of reproducing and leave you with a mumbled, hard to understand sound. Another problem you are going to run into is the fact that the speaker is a low impedance, high power circuit. You will have a circuit that will have very low Q (quality factor) and won't work well. You might have better luck replacing the OEM speaker with a higher quality one or using a larger, external speaker.
 

John_M

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What the human ear can hear is 20Hz through 20kHz. So you would want to cut off the high end at maybe 18khz or lower.
 

prcguy

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20Hz to 20KHz is a very generalized audio spec, most people and especially men over the age of 40 can't hear much above 15k.

For communication use I would recommend a low pass cutoff between 3 and 4KHz with at least a 12dB/octave slope using a single capacitor and inductor. For a 12dB/octave slope I would use 3Khz as the knee and with an 18dB or higher slope I would move that up a bit since the cutoff will be sharper.

You can calculate the approximate values on several speaker builder sites and here is a link to one: Crossover Design Chart and Inductance vs. Frequency Calculator(Low-pass)

Round the values to the nearest commercially available parts and they can probably fit inside your speaker and be switched in and out if you want to get fancy.

Parts Express is a good company to get the parts from, they cater to speaker builders. Parts-Express.com – Speaker Building – Speaker Kits, Plate Amplifiers, Cabinets & Coverings, Cabinet Hardware, Crossover Components, Assembled Crossovers, Speaker Repair, Design Software
prcguy






What the human ear can hear is 20Hz through 20kHz. So you would want to cut off the high end at maybe 18khz or lower.
 

fourthhorseman

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yeah,,too muffled was my worry,knowing what subwoofer lopass filters do
in the car audio realm,,

ill admit,the reading is good,seeing the finer details,but im smart enough to know when im in over
my head,,

hows the audio that comes out of the external,what look like 4inX4in base/mobile speakers,

keeping in mind,as i now type this i sit with a broken rear vent window,from a sumbag trying
to get into my cookie jar thismorning..
so lo-pro and hidden installs are needed..go west Philly crime rate!..i listin to it,,now Im a part of the action...lol

thinking fitting the speaker-flush mount,
but hows the audio?
 

prcguy

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Sub woof filters usually cut below 200Hz, a 3 to 4KHz filter with sharp cutoff will not molest the voice band. Telephone circuits pass roughly the same range.

A 12dB/octave filter is only two parts with the cap going across the speaker and the inductor in series. A cap around 4uf and inductor about .7mH will do just fine for an 8ohm speaker and 8uf and .35mH for a 4ohm speaker.
prcguy



yeah,,too muffled was my worry,knowing what subwoofer lopass filters do
in the car audio realm,,

ill admit,the reading is good,seeing the finer details,but im smart enough to know when im in over
my head,,

hows the audio that comes out of the external,what look like 4inX4in base/mobile speakers,

keeping in mind,as i now type this i sit with a broken rear vent window,from a sumbag trying
to get into my cookie jar thismorning..
so lo-pro and hidden installs are needed..go west Philly crime rate!..i listin to it,,now Im a part of the action...lol

thinking fitting the speaker-flush mount,
but hows the audio?
 

kb2vxa

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You guys always complicate things when the simplest method is to shunt the highs around the speaker. All you need is a low voltage electrolytic capacitor across the speaker terminals, the value may be determined by temporarily connecting several one at a time until you find one that gives the desired result. Note that the higher the value the lower the tone so don't start out with something like 100uF when 10uF is more like it and be aware of the fact the capacitor will also lower the apparent volume somewhat.
 

slicerwizard

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Bingo. A small electrolytic directly across the speaker terminals will eat the harsh high frequencies. Mind you, a better approach would be a small cap (0.1 uF perhaps) on the audio amp's input, assuming that's not beyond the OP's capabilities.
 

fourthhorseman

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Just so happens ive got a couple 10uF's from a dtap project slicerwizard helped me out with a year or so ago..

thinks ill give it a go today,

and now you mention it i do remember seeing speakers,like 6x9s 5 and 6in round with a cap
bridging the terminals..now it makes sense,,
 

prcguy

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The cap needs to be non polarized and if your lucky to get the value right to start rolling off around 3KHz its only going to be about 6dB down at 6KHz. If you find a value that makes a big difference in the annoying 5 to 10KHz range it will probably make the voices sound muddy.

A cap and inductor in a 12dB or 18dB/octave arrangement is the right way to do it and you'll be very happy with the results.
prcguy


Just so happens ive got a couple 10uF's from a dtap project slicerwizard helped me out with a year or so ago..

thinks ill give it a go today,

and now you mention it i do remember seeing speakers,like 6x9s 5 and 6in round with a cap
bridging the terminals..now it makes sense,,
 

kb2vxa

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Not to be argumentative but an NP cap really isn't needed in this very non critical application, if it were a hi-fi speaker crossover network it would be another story. On the other hand what we're looking for here is a compromise at best and his ears will be the judge so now that he's on the right track please don't derail the train.

So Hoss, only a couple of 10uF caps floating around? I'll bet you have more than you bargained for if you know where to look, pretty much any old junk electronic gadget will do. Bust open an old VCR, radio, TV, computer card or anything laying around gathering dust and you have a parts gold mine.
 

slicerwizard

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Just so happens ive got a couple 10uF's from a dtap project slicerwizard helped me out with a year or so ago..
You can try a single cap, or two in parallel (connect + to -), or two in series (connect + to +)


Bust open an old VCR, radio, TV, computer card or anything laying around gathering dust and you have a parts gold mine.
Yeah, parts with short leads that don't work for most homebrew stuff, e.g. not going on to a printed circuit board. Given what caps, resistors, diodes et al cost these days, it's hardly worth it.
 

jackj

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slicerwizard....Caps in parallel; hook the caps up + to + and - to - then the values add together. Caps in series; hook the caps up + to - and the value is less than either alone. There is a formula to figure the value for caps in series but I forget it, google it.
 

slicerwizard

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slicerwizard....Caps in parallel; hook the caps up + to + and - to - then the values add together. Caps in series; hook the caps up + to - and the value is less than either alone.
Well, sure - if you don't want to take advantage of the opportunity to provide some non-polarization. Why would you do that?
 

jackj

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Non-polarization?

Well, sure - if you don't want to take advantage of the opportunity to provide some non-polarization. Why would you do that?
I don't understand how would hooking two electrolytic caps in parallel will give you non-polarization. Can you explain please?
 

nanZor

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Lowpass to a Pro-405 desktop

Here is a shot of a lowpass filter applied to a Pro-405 desktop. The speaker is 8 ohms, so I used a 10-ohm resistor, and a 10uf non-polarized cap across the terminals. My local RS stocked the 4.7 and 10uf non-polarized caps so that made it easy.

10uf with this rc circuit starts rolling off at around 1.2khz with 6db per octave slope so it was a little too mellow, and later backed it out and put in a 4.7uf which seems about right.

OH, if you trim up the excess lead lengths after soldering, be SURE to have a firm grip on them beforehand - otherwise the magnet will suck up your little trimmed lead, usually right near the voice coil. Trust me, this is a PITA to remove, and if you don't, you'll hear it vibrating. :)
 

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