"Crossover" question

[fallenmunk]

Bear with me if this is a simple question, but I've been unable to find the answer by using search, and I'm a noob to speaker design.

I'm building a pair of Killatone near field monitors John Sayers' Recording Studio Design Forum • View topic - The Killatone DIY speaker project

My question is this: If I'm planning on trying a 4 ohm speaker instead of the 8 ohm speaker specified, how do I adjust the values of the "crossover" components.

My best guess is that I would need to halve the inductance and resistance, and double the capacitance to get the same results.

Please somebody help before I screw this up.

Thanks!

[chrisb]

Originally Posted by fallenmunk

Bear with me if this is a simple question, but I've been unable to find the answer by using search, and I'm a noob to speaker design.

I'm building a pair of Killatone near field monitors John Sayers' Recording Studio Design Forum • View topic - The Killatone DIY speaker project

My question is this: If I'm planning on trying a 4 ohm speaker instead of the 8 ohm speaker specified, how do I adjust the values of the "crossover" components.

My best guess is that I would need to halve the inductance and resistance, and double the capacitance to get the same results.

Please somebody help before I screw this up.

Thanks!

my

The above noted project from 2003 was for the sadly departed Fostex FE127E, and more importantly the circuit was IINM a combination of Zobel and BSC contour.

To accurately recalculate the math for either section of this network for different driver(s) of any impedance, you'd likely want measurement data for it/them.

Before even undertaking that however, what driver did you have in mind? Among other considerations, the TSP for the chosen driver might also require an enclosure of different type / dimensions than above, and not all small full range drivers or applications might even need BSC, particularly in near field/small room situations. For example Markaudio CHR70 / CHP70 / Alpair 7 .

As "near field" use implies to me desk-top use in a computer based system, even if they did require EQ of some type, there's been substantial advances in computer audio applications with quite decent equalizers since the above project.

free advice is worth what you pay for it - keep the change

[fallenmunk]

chrisb,

Thank you for your input. I am specifically looking at trying the Markaudio CHR70. I understand it is more complicated than just swapping out the driver, that's why I'm here.

[planet10]

It is more complicated. You need to start from scratch. Fortunately the CHR-70 is much better matched to this kind of enclosure than the FE127.

Before even thinking about a filter, just put the CHR into a 5-7 litre sealed box, break them in (100-200 hrs minimum), and then evaluate if you don't have enuff bass.

The CHR will start out with way more bottom than the FE127.

dave

[fallenmunk]

Dave,

Thank you. I'll get a pair to break in and then figure out how much bass I need to filter out to make them lo-fi. With this application it's more about crippling the response to isolate the high bass and mids, but you also don't want the highs to become tiring to the ears. Kind of a weird thing to do with such perfectly good drivers. The design is to try to approximate the crappy sound of the Auratone speakers for mixing tracks in a recording studio situation.

If this FE127 was still available and cheap I'd just get those and avoid all of this.

Thank you for your expertise.

[planet10]

The filter shown is a HF shelving filter (Baffle step compensation) so it cuts the highs. The LF rolloff is the natural rolloff of a sealed FE127.

dave

[fallenmunk]

Originally Posted by planet10

The filter shown is a HF shelving filter (Baffle step compensation) so it cuts the highs. The LF rolloff is the natural rolloff of a sealed FE127.

dave

I think I have this just about nailed down with the help provided here and a lot of research. The baffle step components just needed values divided in half in order to use a 4 ohm speaker instead of the 8 ohm it was designed for. The Zobel wasn't that simple, but I got it figured out to where the response curves basically lay right on top of each other aside from just a bit better low end response and a bit flatter response curve in the high end. The CHR70 looks like it will work really nice for this application at least good enough to give it a listen.

I'm almost ready to order components, but I am puzzled by one other thing. The schematic shows 4 resistors in a series/parallel configuration. Why not just use one resistor of the combined impedance? Every example I've seen online just uses one resistor. Is there a good reason for this that I'm not seeing?

Thanks!

[planet10]

Originally Posted by fallenmunk

The baffle step components just needed values divided in half in order to use a 4 ohm speaker instead of the 8 ohm it was designed for.

remember 4 & 8 are nominal impedance and only have a loose relationship to the actual impedance. It also does not take into account the FR of the driver. The CHR has a natural bump, wheras the FE tends to roll-off.

The Zobel wasn't that simple, but I got it figured out to where the response curves basically lay right on top of each other aside from just a bit better low end response and a bit flatter response curve in the high end.

Zobel has to do with the imedance curve, not the response curve.

You really need to start from scratch, and forget about the filter in the original.

dave

[fallenmunk]

Originally Posted by planet10

You really need to start from scratch, and forget about the filter in the original.

dave

Ok, thank you. I have much to learn.

[fallenmunk]

planet10,

Just wanted to say thank you for the advice. Using the actual specs of the CHR70 I can see that my earlier ideas would have not worked out very well.

I've got the component values worked out using the new driver specs in case I want to add them later, but my plan is to break in the drivers first and see if I even need any filtering at all. If I can figure out a test rig I'll be able to know for sure...time for more research on that.

Thank you for advising me to start from scratch.