Can somebody help with a simple 12dB filter?

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#3
Well, passive filters at 200 Hz is often problematic due to the impedance curve of the driver. Below is an example of a cookbook filter (green curve) and what happens when it is applied to the complex load of the driver. When the values are "fiddled" a bit, it looks a bit better (blue curve).

An externally hosted image should be here but it was not working when we last tested it.


I think this highlights the need for simulations when designing passive filters 😀 . The impedance of the blue system is probably too low to be useful, though.
 
#4
I'll try the PLLXO route then. It seems easy enough (even for me). I have no idea how to choose R1 though. But if I were to choose 10kohms would this be correct for a 2nd order lowpass @ 200hz?

R1 = 10kohms
C1 = 80nF
R2 = 100kohms
C2 = 8nF

Is this right?
 
#5
GuyPanico said:
I'll try the PLLXO route then. It seems easy enough (even for me). I have no idea how to choose R1 though. But if I were to choose 10kohms would this be correct for a 2nd order lowpass @ 200hz?

R1 = 10kohms
C1 = 80nF
R2 = 100kohms
C2 = 8nF

Is this right?
Click to expand...

You pick R1 and R2 as a compromise between insertion loss and the impedance your preamp sees. You need to know your preamp's output impedance, and your amplifiers input impedance, too get an idea of how much wiggle room you have.

For example, if your amplifier's input impedance is 100kohms, then your above filter would result in about 3db insertion loss. If the input impedance is 10kohms, then insertion loss is closer to 11db. As you reduce R2, insertion loss gets smaller, but so does R1.

Since R1 is the lowest impedance the preamp will see, you can't reduce it forever. I think 10x output impedance is a minimum, but hopefully someone with more knowledge than I will chime in with a better answer.

Dan
 
#6
Hi, is there no way you can use an active filter before the amps. You will get a much more predictable crossover and better control of the drive units as there will be less resistance between the amp and the driver coil.

Regards,
Andrew
 
#7
gfiandy said:
Hi, is there no way you can use an active filter before the amps. You will get a much more predictable crossover and better control of the drive units as there will be less resistance between the amp and the driver coil.

Regards,
Andrew
Click to expand...


+1 active is always better than passive... cheaper and better sound quality. 🙂
 
#9
davidlzimmer said:
I'm in planning stage of similar project. Planning to use a DVC sub woofer. Was going to use active filter but concerned about Power supply. Going to run car radio with ac power supply. An active filter would require another amp and would be considerable challenge to power supply.

See if this helps. http://www.apicsllc.com/apics/Misc/filter2.html
Click to expand...


AC PSU? Look into getting +-5v from your cars electrical system, it shouldn't be too hard. 🙂 Try not to use AC in your car unless you are making your own SPMS. :smash:
 
#10
ry not to use AC in your car unless you are making your own SPMS.
Click to expand...

Maybe I wasn't clear. The project is a portable boom box. Based on car stereo which will give me FM, CD, and external inputs such as MP3 player.

Plan to use AC power instead of battery. Tho, a batery could be used if no power available.

Am now considering a matched component system and be done with it.

Like this
 
#13
The thing is, for not a great deal more than the list price, I think I can build something with much more power and better sound that will have an aux input for my and other's MP3 players.

BTW, I'm 62. Maybe I AM too young for this project. 😀

The project is for Beer and BBQ parties. Not to be carried around!:whazzat:
 
#14
Or less, I've built several portable party systems with a used car audio unit/co-ax drivers for next to nothing. I imagine using more recent deluxe systems with their considerable DSP will sound better overall.

GM
 
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