Hi:) and a passive cross-over question

Hi all,

I was given this site by Xristo on Car Audio Australia, who said you might be able to help me out :)

I'm looking at upgrading my car audio system with a new front stage. 8inch morel Hybrid mid/woofers, coupled with a pair of Arvus (TBC) tweeters. These will be going into my BMW E32 7-series, and driven from an Eclipse EA3422 Amp bridged into each channel.

I have a set of Canton Pullman RS3.22 3-way passive crossovers, that I'm trying to dissassemble and make a 2-way crossover for a 8inch Morel and a tweeter (Yet to be decided).

I spent some time trying to document the crossover circuit layout and have come up with a layout I'll put up very soon. I'm a little unsure of some areas of the crossover layout, values, and thier purpose though.

The main section that has me confused, aside from the component value's of the inductors is the tiny ceramic cap, and 12ohm 10W wirewound resistor section that feeds the tweeter output. The tweeter section appears to be 12dB high pass (w/attentuation) which is fine, but the small ceramic is in series with the pos feed?

The mid seems okay, with a 6dB high pass and 12dB low pass.

The woofer again being a 12dB low pass.

I'm thinking I should just be able to drop the 6dB high pass from the mid section and use that as my mid/woofer output?

Can anyone see any potential issues I may be overlooking here?

I'm also tempted to put a polyswitch in line with the tweeter for protection(Thanks for the advice Jason) but can't seem to find one that will trigger around 120WRMS?

http://img180.imageshack.us/my.php?image=cantonpullmanrs3220os.jpg


Sorry for the long post....

Thanks in advance...

Ben
 
I have an active 3-way crossover already in the sytem that I could use to run the front stage active.

I wish to run passives as I'm bridging the 4ch amp into front and rear and feeding front and rear left and right signals through 'Y' pieces respectively. If I were to drop back to active I'd only be able to feed 50WRMS into the 4ohm mids and around 13WRMS into the 16ohm tweeters, as opposed to 120WRMS+ into the mids and 30WRMS into the tweeters :)

I'm greedy :) I realise the losses associated with passives, but the extra 3-4dB from the bridged poweris too tempting;)
 
I think you're going to be disappointed... A load of work for a IMO net loss.

The passive crossover you are trying to cannibalise will be designed specifically for the original drive units. There is no such thing as a standard 4 ohm crossover, despite what you may see in catalogues, etc. You need test equipment, experience and time to design a passive crossover properly.

As you have a 4 channel amp and active crossover I strongly recommend you stick with that. It will provide superior sound quality and because of the way that active systems distribute power, it will play louder than the equivalent rated passive system (even ignoring passive losses).

The amp might not like running bridged into 4 ohms, it may say it can do it on the box, but sound quality will suffer.
 
Thanks for the responses.

The ceramic might be a polyswitch, it just appeared to be physically much smaller then polyswitches I'd seen previously.

I would love to run active and am looking for another amp atm, to be able to run one 4ch bridged into the 8's and then use 2channels for the tweeters and the spare two perhaps for some midbass 'rear fill'.

Oh my poor little alternator :p

Thanks again gents..
 
You'll need to know the Fs of the tweeter to design a crossover properly, it's a complicated process and I really would go active as suggested-it will make tuning much easier and you can use the amp gains to level match between the drivers. 13wrms will be fine on some tweeters and 50wrms is plenty on some decent mids, if you find you need more power trade up your amp-but TBH it will be pretty loud unless you have incredibly inefficient drivers.

Copied from another forum:
The "Real Deal" with 8 ohm drivers I've seen many people say, "Well I want to use this speaker, but I'm put off by the fact that it's 8 ohms."

Let me explain why it's ok to use an 8 ohm speaker, and why it could actually be better than a 4 or 2 ohm speaker.

First of all, using a higher impedance than what your amp is nominally rated for is always ok. In fact, if your amp is rated for 4 and 2 ohm impedances, typically you will get less power into 8 ohms meaning that your amp will run cooler and more efficiently at higher impedances. It won't hurt your amp, and in fact it's actually much better for your amp's longevity.

Now, you're probably saying how is getting less power out of my amp a good thing? Think of your amp's power reserves as your bank account. Just because you have 100 dollars in your bank account doesn't mean you have to spend it all. Quite the opposite. Wouldn't you rather spend LESS and get MORE? That's exactly what you're doing when you use less power from your amp, and get the same amount of output from a high efficiency speaker.

Remember, most speakers are rated at 2.83V. A 4 ohm speaker rated for 90db spl at 2.83v is really being rated at 2 watts! Whereas an 8 ohm speaker rated for 90db spl at 2.83v is only being rated at 1 watt. You can do the math for yourself, Power = Voltage^2 / resistance. At 2 watts, we can assume that same 8 ohm speaker is actually rated at 93db spl (remember, every doubling of power gives you a theoretical 3db gain in spl). So using that 8 ohm speaker will give you the same amount of output, at half the power required as a 4 ohm speaker. Your amps run cooler and draw less power from your vehicle's charging system, your speakers run cooler, and everyone is happy!

Ok, now let's look at another example of an 8 ohm versus 4 ohm voice coil. Typically, the efficiency of a speaker is given by:

Efficiency = ( B^2 * L^2 ) / ( R * Sd^2 * Mms^2 )

B = magnetic field strength
L = length of wire
R = resistance
Sd = surface area
Mms = mass

So for your 8 ohm voice coil, using the same wire as a 4 ohm voice coil, you would need twice the L or length to get an 8 ohm impedance. That makes sense doesn't it? A longer wire will have more resistance. Now, looking at the formula above, doubling L actually causes your efficiency to rise, even though the impedance also rises. So in this very oversimplified example, raising the impedance actually causes efficiency to go up and lowering the impedance actually causes a loss of efficiency.

What's important to remember is that it's the overall output and efficiency of the speaker that's important, not the impedance. A high impedance, high efficiency driver can get just as loud off a small amount of power as a low impedance, low efficiency driver that sucks a ton of power! Just because you have a 100 watt amp doesn't mean you have to use all 100 watts.... it's all about being efficient.

I also found this excellent post by Dan Wiggins over at carstereos.org:

"I think one thing to consider is that going to a higher impedance voice coil will result in better packing of the voice coil, meaning a higher cross-sectional-area of copper in the flux, for a given mass.

If you take a given driver, and simply swap out voice coils, you end up with more efficiency as you increase the impedance. Take a voice coil, say 2" diameter, 1" winding length, 24AWG 4 layer, and swap it with a 2" diameter, 1" winding length, 27AWG 4 layer, and you double the impedance, but the efficiency also goes up - less mass and better packing density."

It's because the moving mass has dropped, and if desired - because of the thinner wire diameter which packs in tighter - you can put more layers in the voice coil and potentially raise the BL."
Last edited by npdang; 12-05-2005 at 12:19 AM..