# Open Full Range with boxed, passive, helper woofer

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#### TiMBoZ

a Low Pass filter question:

At moment I have semi open/U-frame, full range direct to amp, no filtering. With a low pass on a boxed 'helper' woofer - both parallel to a burly S.S amp.

I wanna do a 2nd or 3rd order filter at 150 Hz on the helper woofer

At 150Hz the amp is seeing a combined parallel load of 4 Ohms: the full range at @10.5 Ohms & Woof at 6.5 Ohms.

My Question?:

Do the coil/cap values for the low pass 'helper' woofer need to be at the combined value of 4 Ohms or the woofer input value of 6.5 Ohms?

I'm confused for some reason.

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#### picowallspeaker

As they are not pure resistance and we are talking about audio, what do
those numbers come from ?
I guess that you have just measured the DC resistance of the coils, which is
a factor of the lenght of the wire and its diameter.
The amplifier works by trying to mantain stable voltage ( potential) at its output . The load represented by a speaker presents something called complex load, so there is an argument (phase) and a module ( impedance).
The amplifier should be specified at (max power) and which load to be sufficiently stable ( phase deviation margin).

#### TiMBoZ

what do those numbers come from ?

The numbers come from the graph that comes with the drivers. It's a fairly straight forward question isn't it. I'm hoping for a fairly straight forward answer. Thanks Pico.

#### picowallspeaker

Uh, sorry: so you just pointed your finger at 150 Hz at each graph and you derived which impedance.
My skills made me writing what I wrote, my mathematic mind told me that it was just right so I submitted the post to the Forum.
To understand what I wrote ...well that's a mistery !

At 150 Hz probably you are very near to FR's Fs, on the descending side of the bell shaped graph of the impedance, which peak is located at Fs.
For the woofer side, it should be already a (nearly) straight line.
A boxed woofer presents an inductive type of load (the argument of the impedance, i.e. the phase angle) before Fs and a capacitative load above Fs.

#### TiMBoZ

Thanks Pico. Anyone else?

Do I set the filter values at the combined parallel load of both drivers or the just the load of the woofer at 150Hz?

#### TiMBoZ

Just to clarify, the impedance of woofer is pretty flat at zmin at 6.4 Ohm @ 150Hz. the woofer Fs = 27Hz. The impedance of Full range is just starting to rise toward Fs (42Hz) at 150Hz from zmin approx 9 Ohms up to 10.5 Ohms.

The low pass filter is being applied to the woofer only.

Yes, these numbers are approximate, not precisley engineered, but will do for me, for a 'DIY helper' woofer to fill in under the open full range.

Is the impedance created by the box interering with impedance curve of the woofer to such an extent we can't guestimate a basic value?

#### picowallspeaker

Hi, yes, I hate to read A&Q made by only two guys
And I don't measure anything, just slap a big coil and some cap to model
the knee ...very basic
BTW haven't you read anything about F.A.S.T. ( fullrange and subwoofer tecnique) ?
Also a series crossover might work, just a coil and a cap ...
but it depends on which drivers you use.
BTW(2) Which drivers are those ? For a minimum guessestimate, like diameter, sensitivity....

#### Johno

As the low pass filter is in parallel with the full range you can ignore the the FR and the filter calculations need only consider the woofer impedance.

#### TiMBoZ

Phew! Thanks Johno. Was hard to get my head around for some reason? So the values need to be relative to the input of woofer.

Ride safe.

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