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EDY19

Thu Jul 27 2006, 05:19PM

Registered Member #105 Joined: Thu Feb 09 2006, 08:54PM
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Posts: 408

I just got a new head unit in my car (kenwood MP332) and I like it, but I would like to add a small subwoofer at some point. The amp has front and rear preouts, but it does not have a subwoofer preout. I think I can get an amp (or make one

)that has a low filter so only the lows get put through to the sub (from one of the front or rear preouts) , but how would I go about filtering out some of the lows from the other 4 speakers? If i filter out some of the lows, they can get more of the high freq. stuff, and less stress will be on them. I looked at using a single capacitor as a high pass filter, but the cap would have to be 265uf for a 4 ohm speaker and 176uf for a 6 ohm for a crossover freq of 150Hz, which seems really high- and physically large.

HV Enthusiast

Thu Jul 27 2006, 05:27PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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You could always go with an active high pass butterworth filter using a single op-amp a a few resistors and capacitors. This will give you good roll off as well, but does require a power source, which might not be any big deal anyways since you have 12V readily available.

Do a search on active and passive audio filters on GOOGLE. There is LOTs of info out there. . .

EDY19

Thu Jul 27 2006, 06:26PM

Registered Member #105 Joined: Thu Feb 09 2006, 08:54PM
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Aren't these active crossover designs for preouts, not speaker level outputs (22W)?

HV Enthusiast

Thu Jul 27 2006, 11:58PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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Yeah, your right. A series capacitor should do the trick, although its just a simple first order filter, and yes, the capacitor will be pretty big uF wise.

Even a 2nd order filter using an L and C will still require a pretty big C for around 120Hz corner frequency.

Steve Ward

Fri Jul 28 2006, 01:51AM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Look at motor run caps, some of them are pretty large uF wise and not too expensive if you look in the right place.

An active crossover is the better solution of course.

EDY19

Fri Jul 28 2006, 02:58AM

Registered Member #105 Joined: Thu Feb 09 2006, 08:54PM
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I would do that, but I don't have a 4 or 2 channel amplifier for the speakers, so using the onboard amp in the reciever is my only option right now- My stock speakers handle suprisingly well, so maybe I'll turn up the mids and the highs to them(+8db), then leave the bass at 0db, then send one of the preouts to an amplifier for the subwoofer, and use a low pass there so the amp still gets plenty of signal, and the the lows are somewhat "cut" from the 6x9 and 6.75 inch speakers.

HV Enthusiast

Fri Jul 28 2006, 10:48AM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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For your head unit which i'm guessing puts out less than 30W per channel RMS, you only need a low voltage capacitor, so its still going to be quite physically small. You probably only need a capacitor rated to about 20V and this is pretty tiny, even at a few hundred uFs. If you have an audio shop near you, they usually stock these types of capacitors (usually bipolar) and are very inexpensive - couple of bucks each.

Steve Conner

Fri Jul 28 2006, 10:58AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Meh, filtering the head unit's speaker level outputs doesn't solve the problem, that the bass will make the head unit's power amps clip early. It needs to be filtered out before the power amps. All the more so, because they're relatively low power, so their power should be saved for mids and treble to help them keep up with the subwoofer.

Doesn't the head unit have an option to do that? My cheap crappy car CD player didn't, and I never got round to busting it open and installing an active crossover inside. (just as well really, because it broke and my work would have been wasted.)

EDY19

Fri Aug 11 2006, 05:41PM

Registered Member #105 Joined: Thu Feb 09 2006, 08:54PM
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I just got around to playing with my bass guitar amplifier, the speaker is a 4 ohm impedance. I found that a 150uf capacitor does about what I want, I used a cap from a computer power supply rated at 400V so the voltage reversal wouldn't hurt them, although It probably wouldn't anyway because if was fairly low volume. I figured that a 150uf cap and a 4 ohm load cuts frequencies centered around 260Hz. I then put a 6 ohm speaker (the 6x9 inch ones in my car) into the equation with this 260Hz, and found that a 100uf capacitor would suffice for my needs. Could I use some of these high voltage polarized electrolytics in my high pas filter? It does work really well

And I'm not too concerned about serious clipping as the volume control goes up to 35, and I only have had it up to 20 for any period of time.

Steve Ward

Fri Aug 11 2006, 07:02PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

The output of an amplifier is AC, you are using a DC cap. Expect it to sound like garbage, and or, the cap to fail in short time. DC caps will only stand maybe 2-3V reverse before they basically go short circuit and no longer act as a capacitor, so they wouldnt do much good for filtering.

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