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A question about speakers and crossovers (from an electronics dunce).

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Royston

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Can I start by saying I am not an electronic engineer, I'm a musician? I have managed in the last 20 years to put together various PA systems and instrument amps and cabinets without blowing myself up, due to my rudimentary knowledge of Watts and Ohms. I can also spot a black, frazzled resistor when I see one, and solder in a new one. That's about as far as my knowledge goes, hence my question.

I am building a new speaker cabinet for my double bass. So far I have two 10" 200W 8Ω cones, and I want to wire them up in parallel to give me a 400W 4Ω cabinet. My amplifier puts out 350W at 4Ω, but the performance is frankly a bit rubbish with an 8Ω cabinet. So I want to keep my cabinet as close to 4Ω as possible.

I would quite like to add some sort of tweeter to the system. The performance of the cones rolls off quite sharply at 2kHz, and I would like to make the sound just a little bit more lively than that. I looked at high-impedance peizo horn tweeters, which do not require a crossover, but I have been told they can be problematic in a high-powered rig like this, and they don't really kick in until 4-5kHz, so there would be a big hole in the middle of the sound.

So I started looking at low impedance compression drivers instead. I found a couple that had a good range, they kick in at about 1.7-2kHz, they are 8Ω (so I could use two to give me a 4Ω load if necessary). I heard good reports of their performance, but I would need a passive 2-way crossover. Now, here's where my ignorance of electronics kicks in.

Almost all of the crossovers I have looked at are quoted at 8Ω nominal impedance, but I want a 4Ω cabinet. So my questions are:

If I put a 4Ω low frequency load and a 4Ω high frequency load onto a crossover, do I get a 4Ω system, or is the crossover itself going to give me an 8Ω load?​

My schoolboy physics suggests to me that if I want to run a 400W @ 4Ω load through a crossover which has been quoted at twice that impedance, I need to get a crossover with double that power rating: for 400W @ 4Ω I need an 800W @ 8Ω crossover. Is this anywhere near correct?​

Does all this messing around with impedances change the characteristics of the crossover in any way, like the crossover frequencies, or the degree of roll-off?​

I would appreciate your comments on my electronic naïveté. Once you have finished laughing, of course. Thank you.
 
The crossover frequency depends upon the speaker impedance, thus you have to calculate the component values for the specific impedance (see Passive Crossovers, Capacitor and Coil Calculator).

And, as you surmised, the power handling capacity of the crossover (inductor wire size) would need to increased for 4 ohms.
 
Most woofers have a peak in the response before the output drops off at higher frequencies. Your crossover frequency must be below ther peak frequency or the woofer will shriek.

The piezo horn tweeter sounds like a whistle that is full of peaks and dips.

I heard compression driver horn tweeters in the Canadian figure skating championship TV show last night and they sounded awful. Peaks, dips and distortion. Like the noise at a hockey game.

If you use a 4 ohm tweeter with an 8 ohm crossover network then the network is damped too much which causes a droopy response at the wrong frequency.
 
Now I've been out of the muso scene for a while now , but this is how we used to do it. 3 way passive xover after the mixer feeding into 3 power amps for low mid & high. Gives a lot better flexibility for setting o/p levels of the three ranges.
 
I'm with Ross 100% I always thought the idea of passive crossovers after the amp to be a criminal waste. Though I'd go with active crossovers, the cutoff frequencies can be dramatically higher, which will give you less overlap.
 
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Now I've been out of the muso scene for a while now , but this is how we used to do it. 3 way passive xover after the mixer feeding into 3 power amps for low mid & high. Gives a lot better flexibility for setting o/p levels of the three ranges.
I am not building a full-range PA system, I am building a cabinet to play my double bass through, a 2x10" bass cabinet with some very nice Eminence bass cones in it, to be used with a very nice valve amplifier head. The Eminence cones give a wonderful, punchy mid without being too boomy at the bottom. They just lack a little at the top end, which for a double bass is a shame, especially when I am playing arco (with the bow).

My choice is between a one-way system in the cabinet, or a two-way system in the cabinet. I only want to carry around one big, heavy double bass and one big, heavy cabinet. I don't mind the extra weight of a passive crossover and a couple of tweeters inside the cab, but no extra boxes, no extra amps thank you.

The crossover frequency depends upon the speaker impedance, thus you have to calculate the component values for the specific impedance (see Passive Crossovers, Capacitor and Coil Calculator).

And, as you surmised, the power handling capacity of the crossover (inductor wire size) would need to increased for 4 ohms.
Thanks for the link, I did some reading on this last night, and I think I have a plan: a first-order low pass filter to keep the woofer response as flat as possible, and a second order high pass filter to keep the tweeter(s) protected from all those watts at the bottom end. The only thing is, I am now into the realms of buying components instead of a finished assembly. I found a blank board on eBay that you can build a custom crossover onto, but I am completely in the dark about what sort or size of capacitors and inductors I need to buy. Ceramic? Film? Electrolytic? Niobium?? Ah well, more reading I suppose...
 
A first order lowpass filter for the woofer is just an inductor that produces a very gradual slope. It will allow the high frequency peak to be produced which sounds bad.

The tweeter has a second-order filter that uses one capacitor and one inductor.

Cheap speakers use a non-polarized electrolytic capacitor that has a wide range of its value so the crossover frequency is not accurate. Good speakers use a 5% big and fairly expensive film capacitor.
 
**broken link removed** a 2kHz 2-way crossover that can be used with either a 4Ω or 8Ω woofer and an 8Ω tweeter. It's good for 300W.
 
Thanks - I followed the link and found a European distributor for Dayton products in Germany. I'll need to get my son, who speaks German, to help me order the right thing now. I don't mind learning a little electronics for this project, but I'm not learning another language!

Thanks again for all your help.
 
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