LP filters, resistors and damping factor

[Tenson]

Hi I hope you can help me with something that has been puzzling me a little.

First of all the main question - does adding a series resistor in the LP filter of a woofer destroy the damping factor?

The reason I ask is because I have been designing an xover for some speakers and I find the woofer needs to have the bass and midrange lowered or it sounds too 'fat'. So my solution was to put a series 2R7 resistor before the 2nd order LP filter. But my friend commented that he thought it would destroy the damping factor as it would be like raising the source impedance of the amp by 2R7. Is that the case?

If so, does it help to move the resistor to be after the 2nd order filter components? Like that it would be directly in line with the woofer and I don't see how that would be any different from having a voice coil with a slightly higher DCR, and as far as I know having a speaker with a higher impedance doesn't destroy the electrical damping.

I guess I'm not sure if it should be seen as raising the source impedance or raising the load impedance! Or both... in which case it would kind of cancel out wouldn't it?

Sorry if I am taking rubbish!

 

[theAnonymous1]

Rubbish!

If you want to pad the woofer you need one resistor in series and one resistor in parallel with the driver; this is before the low pass filter. It will have no effect on the low pass filter.

If your driver is 8 ohm and you want 3dB attenuation you need a 2R3 in series followed by a 19R4 in parallel.

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

Here is a calculator for the l-pad circuit.....

http://www.carstereo.com/help/Articles.cfm?id=55#lpad

 

[Tenson]

Hi,

Thanks for the response, but I don't want an L-pad. I want to change the response so it slopes down slightly toward the low frequencies.

Otherwise when the LP filter is applied it takes away rising response the woofer has and makes it sound fat and a bit overblown. I want to tilt the response a bit.

 

[theAnonymous1]

Sorry, your description didn't sound too specific to me. I thought you just had a level matching problem.

A series resistance will raise the Qts of the driver and make it sound even boomier.

 

[Tenson]

Any ideas how else to tilt the response then?

 

[theAnonymous1]

Sorry I haven't built a passive XO in a long time and it would take some thinking on my part. You can try one of the many XO tools at this link........

http://www.lautsprechershop.de/english/index_tools.htm

Or wait for a passive XO expert to chime in.

 

[darkfenriz]

I must agree with your friend partly or specifically, you won't hurt DF as it is a property of an amp, but you will destroy Q.
On the other hand 2.7R won't do much to attenuating bass noticably, I'd find the reason elsewhere. Is it bass reflex enclosure? what are the drivers and crossover?

 

[Tenson]

Hi,

Having been thinking about using an L-pad I have had another go in Speaker Workshop and actually it can help...

The problem is that the LP filter required to get the right xover point flattens the slight rising response of the woofer. But if I do add an L-pad of a few dB then I don't need such a strong LP filter to achieve the slope I wanted in the first place, so it seems like it can work out like that. Yay!

P.S. Anybody know a calculator for ferrite core based inductors? I have a 2.1mH ferrite core and need to unwind it to 1.9mH for the revised Xover. I only know of air core calcs.

 

[Svante]

First of all the main question - does adding a series resistor in the LP filter of a woofer destroy the damping factor?

Well, "destroy" is not the word. "Alter" would be better. When it comes to damping in the loudspeaker, the amount should be appropriate, not as high as possible.

The damping is directly reflected in Qts, which in turn depends on Qes, which in turn depends on Re and any series resistance. And Qts should be appropriate for a particlar design, not as low as possible.

There is a common misconception that a low series resistance is very important for the sound, that the loudspeaker would "loose control" otherwise. It is true that a series resistance can affect the response, but a skillful designer takes the resistance into account when designing the system.

Here is the effect of adding 1 ohm series resistance to a closed box system with a slightly too low Qtc. It could be argued that the response of the system with the series resistance is flatter.

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