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Question about DIY sub crossover

I think that the NHT 3.3 woofer crossover is a good example.
It is redrawn at the bottom of this page:
https://syclotron.com/biamping-the-nht-m3-3/

L1 12mH and C1 are on the woofer, I'd probably put 1 ohm 10W in series with
cap and leave out the resistor in series with the inductor.
You cannot use design tables for a real woofer with an inductive input impedance.
The 1 ohm resistor helps to avoid peaking in the low pass response when the
woofer has an inductive input impedance. You could use a Zoble but the 1 ohm
is a nice simple compromise.
I don't remember what the input impedance of the Super Zero looks like so it
is hard to say what to use there, probably 100 to 200 uF. Your maxSPL will be
limited if you do not filter out some bass to the sats. Super Ones would be better.

Don't worry about using NPE caps most large 3 ways use them without any
problems.
 
Question about the above post. Inductive impedance is just beyond the limit of my knowledge and full understanding. When you test a woofer with something like the Dayton DATS system, and get an impedance curve, does it take into account this inductive impedance, or is that something that occurs in a dynamic music program and/or is related to the amp, and isn't inherent to the speaker itself? In other words, is the impedance curve under test conditions enough to know whether you will have 'peaking in the low pass response when the woofer has in inductive input impedance', or not? How do you know when you need that 1 ohm resistor, or a Zobel?
 
When you measure Zin of a woofer in free air at DC (if your system goes that low)
you are measuring the DC resistance of the VC wire - you can use a DMM for this also.
As you go up in frequency the first peak is due to the mechanical resonance, moving
mass resonating with the total suspension compliance, being reflected (because a
moving voice coil speaker is a reciprocal device, voltage/current in the primary
produces motion in the secondary (mechanical side) and motion on the mechanical
side causes voltage/current in the primary) into the primary. After the first peak the
impedance of a woofer rises and this is due to the voice coil inductance. It is a coil
and it has the iron pole piece which raises the inductance. More advanced woofers
may have shorting rings or copper caps that act to reduce the amount of VC inductance.

If you are able to measure the electrical response of the inductor - cap driving the
woofer you'll see peaking in the low pass response when a resistor is required.
Sometimes more than 1 ohm is required, 3 or more ohms. A larger resistor will
also produce a more gradual rolloff in the lowpass response.

Do you have test equipment for measuring frequency response?
 
Back on this project a bit here. It just hit me about the significance of your question on the previous page.

Pete B said:
I think that I saw two different answers but will each woofer present 4 ohms to
each crossover (8||8 ohm coils)?
Click to expand...

We've been looking at a crossover for 8 ohms on both sub and mains, but the sub is actually 4 ohms. For 150Hz, the sub inductor value is 4.25 mH and the cap for the mains is 132 uF. I have no inductors near that value, darn it. I would happily move it down to 100 Hz but that's still 6.4 mH/199 uF.

Setting that aside for a moment, I'd sure like to biamp this system now, but doesn't that throw the whole xover design out the window? The input impedance of an amp is, what, 50 kohms? You can't do that with a crossover designed for 8 ohms....
 
toxcrusadr said:
Setting that aside for a moment, I'd sure like to biamp this system now, but doesn't that throw the whole xover design out the window? The input impedance of an amp is, what, 50 kohms? You can't do that with a crossover designed for 8 ohms....
Click to expand...

If you plan on bi-amping the speakers, you should use an active crossover between the pre-amp and the amps. Unlike a passive crossover network, an active crossover manages the crossover frequencies and slopes without concern for the input and output impedances.
 
Yeah, I'm just resisting buying one, because I swear I bought a Behringer CX for future use about 5 years ago or so and put the box away, and now I can't find the damn thing. I must have either loaned it out or resold it to some AKer in need but I don't remember doing it. And if I buy one, it'll turn up, you know how it is. :biggrin:

Just for fun I ran a crossover calculator with 50 kohms impedance for woofer and tweeter, and it came up with some ridiculous tiny inductor and "Infinity microfarads" for the cap. :rflmao:
 
toxcrusadr said:
Just for fun I ran a crossover calculator with 50 kohms impedance for woofer and tweeter, and it came up with some ridiculous tiny inductor and "Infinity microfarads" for the cap. :rflmao:
Click to expand...
It is possible to design a passive filter to go between a preamp and a power amp, but it has to be done very carefully. The output impedance of the preamp and the input impedance of the power amp become factors in the design and the design won't work with different source and load impedances, but it is possible.
 
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