Adding a 10uf cap to Ex3500 Tweeters?

haghighi

Hi All,

I am replacing Bose tweeter's in my G35 that are 4 ohm, and a bunch of people are recommending putting a 10uf cap inline to get better response. I am not sure what exactly this will do so I was wondering if anyone who knows something about crossovers could explain what this is going to do and if it will cause a problem from the Polks!

gatemplin

I dont know what you are doing and what speakers you have but a cap in series with a tweeter is a high pass filter. You are protecting the tweeter from the low frequency signals which would destroy them. I cant help with the values but 10 uF is in the ballpark.

RuSsMaN

A 9.95 uF cap inline on a 8 ohm driver will result in a 6db\oct high pass at 2000Hz. A 9.85 uF cap inline on a 4ohm driver will result in a 6db\oct high pass at 4000Hz.

Basically, part of simple crossover.

Cheers,
Russ

haghighi

Okay I know the Bose speaker I am replacing is 4ohm and has a 4.7uf cap on it, and I want to replace it with these Polks that are 4ohm, so from what it sounds like I should be fine.

What's the formula for the crossover?

jcaut

In actual practice it's more complicated than this, due to the fact that the speaker's impedance is not constant, but varies with frequency. But here's the basic 1st order, -6db/octave high pass formula to get you in the ballpark:

C (cap in uF) = 159155/Fc (crossover freq. in Hz) * Impedance of the driver in Ohms.

So at 4 Ohms if you want to cross at 2000 Hz, you'd have

159155 / (4 x 2000) = 19.9 uF

Like Russ said, if the impedance is 8 Ohms, the cap would be 9.95 uF.

Jason

haghighi

Great info guys. I am thinking with the EX3500 I will go with a 10uf cap since 4000hz sounds about right. (Ken S if you read this thread please chime in if you think this is good).

So now question is does it matter what kind of cap it is:

Radio Shack has:

10µF 35V Axial-Lead Electrolytic Capacitor

10µF 35V 20% Radial-lead Electrolytic Capacitor

DAYTON:

10uF-250V POLYPROPYLENE CAPACITOR


Does the voltage rating matter?!?

jcaut

... is that they need to be NON-POLARIZED. I'm not sure about the specific RS units you mentioned.... They do stock a few nonpolar "crossover" capacitors. A cap rated 100V is good for about 200 watts: The voltage rating DOES matter.

The Dayton caps from PartsExpress are good. They also have plain-ol' nonpolar electrolytics which are cheaper should work fine in your application.

With a first order filter (a single cap on a tweeter) you want to position the crossover frequency as far as possible from the tweeter's resonant frequency. I'd say octave-and-a-half, minimum, otherwise it will probably sound harsh and strained.

haghighi

The Polk Ex3500 kit comes with some caps on a circuit board, I'm not sure what the crossover is set at, but do I even use this? Wondering if I can have that inline with the 10uf cap behind it?

jcaut

Don't use both. I don't claim to know much about the car audio products, but if a crossover comes with the kit, then you should probably use that. It's hard to say, when you're trying to add-on to an existing system, but you either need to use one or the other, not both.

haghighi

I am trying to achieve a 4000hz crossover point. The Polk manual for the EX3500 states a second order crossover high pass filter is set for 12db/highpass octave.

Looking at the crossover circuit board I see two capacitors rated at 1.5uF @50V in-line connected to the postive line, and what seems to be a coil rated at .25mH +/- 10%

So the option is to use this Polk crossover, or replace it with a single 10uF non-polarized cap rated at 50v.

What's the difference here?

jcaut

You've about reached the limit of my ability to help you! Perhaps if you posted this in the car audio forum you might find someone who knows more about it. I can help you with some crossover basics, but I know little about car audio.

That said, the Polk crossover is a second order filter (although the circuit you describe sounds more like a third order). It also sounds like the crossover frequency of the Polk network may be higher than 4000Hz, although that's difficult to say without some more info. Heres the way I see it: The Polk crossover will almost certainly be "better" for the tweeter. However it may not integrate as well with the other speakers in the system. How the 10uF cap in series would work in this respect is anybody's guess, really. It would be more of a trial-and-error thing. One thing for certain though: The single cap in series with the tweeter is not going to offer a whole lot of protection for the tweeter from low frequencies. Polk won't warranty it if you burn it by not using their crossover. You could try and use a second order filter, but then you're getting into more complex crossover design, and more parts to trial-and-error with.

Is the tweeter you want to use intended to be an add-on, or is it part of a component system? If you don't use the Polk x-over, how about just hooking up the 4.7uF cap that was on the other tweeter and see how it works. If the impedance curves of the two tweeters aren't vastly different, it ought to work pretty well. Using the 10uF cap would result in a lower crossover freq., though it may well be a different frequency for each tweeter.

Jason