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Old 17th September 2012, 06:08 AM   #1
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Question Passive Low Frequency Filtering

I have a two-way speaker with a full-range crossover. The tweeter gets its proper highs but the midrange (at 6.5") gets ALL of the lowest of lows. And, I don't have an active crossover available for this unit, unfortunately.

Having the midrange try and reproduce anything under 200Hz or 160Hz is just a total waste of inaudible cone movement and likely messes with the frequencies that it CAN reproduce very, very well.

Sooooooo....

Is there a way to electrically insert, right before its stock crossover, a simple single capacitor (filter) that would eliminate 200Hz to 0Hz or 160Hz to 0Hz depending on what I selected for the capacitor's value and would yield a specific expected slope? I did read that metalized polypropylene capacitors are the best and I can obtain caps, resistors, inductors, etc from a quality local dealer.

The midrange driver is rated at 75Wrms/225Wpeak into 4 ohms so I assume that the cap would be a bit large (grin)? And, while I am better with woodworking projects, I would still like to overengineer its electrical capacity to handle twice those wattage amounts (Scotty would do the same for Kirk).

Yup, I'm a newbie and just read that capacitors "limit low frequencies." Is what I'm trying to do "just that simple." (probably not)

Attached is the stock crossover's wiring diagram if that would help. Thank you!
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Old 17th September 2012, 07:06 AM   #2
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Do you have access to your amplifier ?
It may seem a little complicated , but it's the fastest way
to limit BW that goes to the speakers .
It'a all about changing the value of the input capacitor as the
formula Fc=1/2πRC sets it .
Just be very careful with the PCB thin layers , better to solder two
wires in place of the original capacitor and try different values .
Starting with 0.1 or 0.22 uF would lead to a 6 dB/oct high pass at
about 100 Hz...
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Old 17th September 2012, 08:00 AM   #3
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Quote:
Originally Posted by PrecisionAudio View Post
I
Is there a way to electrically insert, right before its stock crossover, a simple single capacitor (filter) that would eliminate 200Hz to 0Hz or 160Hz to 0Hz depending on what I selected for the capacitor's value and would yield a specific expected slope? Is what I'm trying to do "just that simple." (probably not)
Nope, a single cap will not "eliminate 200Hz to 0Hz or 160Hz to 0Hz", it will put some curve that rarely equates to 6 dB per octave in to the mix.
Go with post #2 or be prepared to go down the rabbit hole with big caps and chokes (coils) and a lot of measurement and testing and substitution of values.
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Old 17th September 2012, 09:43 AM   #4
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Originally Posted by weltersys View Post
Nope, a single cap will not "eliminate 200Hz to 0Hz or 160Hz to 0Hz", it will put some curve that rarely equates to 6 dB per octave in to the mix.
Go with post #2 or be prepared to go down the rabbit hole with big caps and chokes (coils) and a lot of measurement and testing and substitution of values.
I do wish I could modify the amplifier as "picowallspeaker" gave instructions. No can do. BUT, thanks a ton for the info. Very interesting!

First, how big of caps and chokes (coils = inductors?) would this require? Going that route may still be the cheapest here.

Second, and likely much more expensive BUT may really be worth it and make them like monitor speakers, how about I use "plate type amplifiers" that I would build into the enclosures which would have "low end adjustable filters" (ie no filter meaning full range 0Hz to 20kHz, 40Hz-20kHz, 80Hz-20kHz, 120Hz-20kHz, etc).

The plate amps would obviously accept line level inputs but would also be required to accept speaker level inputs. I have a very old Infinity subwoofer with a plate amp that can accept speaker level inputs.

Would that do the trick?

All of this to try and eliminate frequencies lower than 200, 160, 40, etc. Sorry guys because I now feel like a bother.

BUT, thanks for all your help and letting me know your advice on this.
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Old 17th September 2012, 01:23 PM   #5
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You want to read this: why it's not so easy to turn a mini into a 3-way
You have to do it electronically.
Ralf
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Old 17th September 2012, 01:29 PM   #6
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If your 6.5 can't satisfactorily reproduce frequencies down to the low 40's, your driver is pretty marginal.

Nevertheless, your simplest solution may be to get a Behringer CX2310 or DBX 223 active crossover. Either will not cost much more than the PLLXO that you will need to do what you want.

Alternatively, search for Fmod. http://www.parts-express.com/term/fmod?srch=fmod
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Old 17th September 2012, 06:12 PM   #7
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Quote:
Originally Posted by PrecisionAudio View Post
First, how big of caps and chokes (coils = inductors?) would this require? Going that route may still be the cheapest here.
For around a 200 Hz crossover with a nominal 3.3 ohm load you are looking at somewhere around a 3 mh coil in series and a 200 uf capacitor in parallel.

For experimentation to get it right it is easier to get one big cap (say 100 uf) and a number of smaller caps to add in (parallel caps adds capacity) as needed to get the response to look like what you want.

In the real world of speakers which behave nothing like a resistor of the same nominal rating, the values given can easily be off by 100%, hence the experimentation.

Have fun, good luck!
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Old 26th September 2012, 10:33 AM   #8
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Originally Posted by fastbike1 View Post
If your 6.5 can't satisfactorily reproduce frequencies down to the low 40's, your driver is pretty marginal.

Nevertheless, your simplest solution may be to get a Behringer CX2310 or DBX 223 active crossover. Either will not cost much more than the PLLXO that you will need to do what you want.

Alternatively, search for Fmod. fmod - Parts Express Ships Fast and Ships Free
Yup, that would be simplest and most effective but likely quite expensive.
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Old 26th September 2012, 10:34 AM   #9
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Originally Posted by weltersys View Post
For around a 200 Hz crossover with a nominal 3.3 ohm load you are looking at somewhere around a 3 mh coil in series and a 200 uf capacitor in parallel.

For experimentation to get it right it is easier to get one big cap (say 100 uf) and a number of smaller caps to add in (parallel caps adds capacity) as needed to get the response to look like what you want.

In the real world of speakers which behave nothing like a resistor of the same nominal rating, the values given can easily be off by 100%, hence the experimentation.

Have fun, good luck!
Thanks for the values. I am learning.
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Old 27th September 2012, 03:17 AM   #10
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Is this a car speaker??
One of those All-in-one' thingies??
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