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Multi-Way Conventional loudspeakers with crossovers

Impedance and Crossovers? 8ohm or 1000ohm - I'm confused!
Impedance and Crossovers? 8ohm or 1000ohm - I'm confused!
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Old 2nd August 2004, 01:13 PM   #1
Ovation is offline Ovation  United Kingdom
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Question Impedance and Crossovers? 8ohm or 1000ohm - I'm confused!

Hi All,
I'm trying to improve my Fiancee's naff old Disco speakers so that we get some better mid-tone, but at minimal cost as we're probably going to build or buy some *real* speakers later this year.

At present there is a Zomax 120W 12" woofer in each cabinet along with an 'anonymous' bullet tweeter horn wired in parallel with no crossover. We're very limited for space in the cabinet, so I really want to replace the tweeter with a horn that is going to provide a better range.

In my CPC catalogue there are Horns which are 8ohms that need a crossover, but there are also a whole load of Horns that are rated as '1000ohms @ 1kHz' - The advert for many of these says that they do not need a crossover and should be wired in parallel with the woofer.
Just to add to my confusion, they have a power rating of things like: "300W max. (4ohm)"
How can it be 4ohm if it's 1000ohms?

What exactly does the 1000ohm impedance imply? Can I still use one of these with a crossover? (The 1000ohm Tweeters with a range of 1.8k -- 27kHz would be ideal (I think the anonymous tweeter only goes down to about 4kHz so an extra 2k lower will probably help the sound) , but I'd feel more comfortable if I had a crossover connected as well)


Please bear in mind that these boxes are years old and were not necessarily 'calculated' when they were built. They've got drain pipe for bass reflex!! I'm not trying towork a miracle with these units - they're crap and always will be, but a few tweaks may help then survive a bit longer.

Can anyone explain this difference in impedance to me a I'm confused and I can't find any DIY sites that mention the idea of *not* using crossovers.

Many Thanks
Jonathan
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Old 2nd August 2004, 01:55 PM   #2
bobolix is offline bobolix  Czech Republic
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Certainly the "1000 Ohm tweeters" are the piezo ones. All of their specifications are confusing usually, as they behave quite different from dynamic, i.e. "low impedance" speakers - their impedance is quite purely capacitive. The don't need the crossover theoretically, as they have a sharp rolloff below 1,5 - 3,5 kHz (depending of type). But they can be destroyed by excessive voltage stress coming from the bass region, if they are connected parallel to the bass (or mid-bass) driver. So it is better to use them with crossover at around 1-2 kHz, but it is necessary to connect a parallel resistor 8 ohm to the tweeter. The resistor should to be dimensioned to 20 watts approx. Without this resistor the crossover is ineffective.

And, don't expect any miracles - the sensitivity of these drivers is about 93-95 dB referred to 8 ohm impedance, if measured conventionally. In turn, they can be connected more piezo-tweeters to get higher sensitivity - 4 pieces no problem.

This is very simplified - you can find more in literature searching about "piezoelectric tweeter".
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Old 2nd August 2004, 02:14 PM   #3
Swedish Chef is offline Swedish Chef  Sweden
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These "1000 ohm" tweeters are most certainly piezotweeters. In fact, their real part impedance is very high - they have a complex impedance and behave very much like a capacitor. This is one of the reasons they do not need any crossover - coz' they already got on "built in". In fact, they usually perform better without any crossover at all.

They are rated as "300 W RMS @ 8 ohm" or something like that. This does NOT mean that they are capable of handling 300 W RMS (which is a hell of a lot of tweeter power BTW). It simply means that in a fullrange system with 8 ohms system impedance driven by 300 W of normal program material they are up to handling the power in the high frequency region. Normally they can take a real beating.

All quality piezotweeters are made by Motorola, formerly CTS and rumors go that CTS have ceased production for now.

Some of these piezotweeters (like the popular but horrible KSN-1005) might need a resistor of say 47 ohms in series to prevent certain amplifiers from oscillating.

Could you post a picture of the tweeter? Zomax made some JBL2402 style bullet tweeters that were electrodynamic and hence require a crossover. Most of their LF drivers were JBL copies (they even used JBL part numbers!) and without touching the originals by any stretch of the imagination some of them were not all that bad. And there is nothing wrong with plastic drain pipe or cardboard pipe as a bassreflex pipe, in fact it is the standard.

You could have a look at the Pi Speakers Forum http://www.audioroundtable.com/PiSpeakers/ for tons of information about piezotweeters since the KSN-1038 is used in the 2 Pi's.

For the money, piezotweeters offer the highest bang for the buck ratio in the entire audio field IMO.

/Magnus
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Old 2nd August 2004, 02:17 PM   #4
Swedish Chef is offline Swedish Chef  Sweden
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As usual, somebody else got here first...

/m
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Old 2nd August 2004, 02:24 PM   #5
Ovation is offline Ovation  United Kingdom
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Thanks for that update, that's put me in the picture a bit better.

Can you define what you mean by sensitivity? 93-95dB sound like an indication that they're faily loud, but not hugely powerful.

I'm really stuck for space in this cabinet - At the moment we suffer pretty muddy bass, absolutely *no* midtone and tinny treble... I think that one of these horns will be a marginal improvement - Ideally I'd like space for a second 10"-12" cone plus a decent tweeter, but that ain't going to happen. I could try and fit two of these horns in... Can they simply be wired in Parallel?

(Incidentally, the horns I'm looking at have got some kind of burnout protection built in).
Oh, and regarding the muddy bass, the boxes are hollow - I'm just about to buy some cheap acoustic foam for this.


If you've got any better suggestions as to how I can make these boxes sound a bit better I'd be glad to hear them.


Cheers
Jonathan
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Old 2nd August 2004, 03:01 PM   #6
Ovation is offline Ovation  United Kingdom
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It looks like I have a choice of several horns given what you've all told me...

(I can stretch to the cost of a few resistors, but I'm trying to avoid buying crossovers as well - given that these *should* work without)...

I've potentially got:

CTS/Motorola - KSN-1141
Power (EIA RS426) 200w nom. 400W protected
Voltage 35v Max
Freq - 1.8k to 30kHz
Impedance 1000ohm @ 1kHz
Sensitivity 92db
Dimsneions 187mmx79mmx108mm

OR

CTS/Motorola "300W Flat Horn Piezo Tweeter":
Freq - 2k to 30kHz
Output SPL 95dB
Dimensions 178mmx95mmx115mm
Power Rating RMS 150W
Max Voltage 35v
Impedance >1000ohm

OR

CTS Piezo Tweeter - Twin Drive
Power 300W max (4ohm)
Freq 3.5k - 20k
Sensitivity 99dB
Max Volts 35v
Dimensions 162x96mm

Any ideas on which may be better??
(I was going to go for the KSN-1141)

Cheers
Jonathan
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Old 2nd August 2004, 04:34 PM   #7
Ovation is offline Ovation  United Kingdom
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I'm afraid I can't exactly provide a picture of the actual tweeter - it's totally unbranded and my catalogue (CPC, Preston, UK) have several tweeters that all look the same.
An identical one in looks and size is here:
http://custom1.farnell.com/cpc/produ...ct%5Fid=260530

There seem to be both an 8ohm and a 1000ohm version of the above tweeter available. My concern is that the 8ohm version may have been installed in these speakers without a crossover. These speakers have been owned by at least 6 different DJs and have most likely been hammered - I'd guess that the tweeters are pretty tired by now.

The sound could be described right now like thumping a tub full of water while rattling a coke can! Maybe, just maybe, the addition of a bigger and better horn, plus some internal foam may help the sound a little bit.

Incidentally, these boxes are floor standing. I was thinking about the idea of mounting them on tripods - How bad an effect might this have on the sound quality (given that there may be some movement in the speakers).

Cheers
Jonathan
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Old 2nd August 2004, 05:42 PM   #8
Swedish Chef is offline Swedish Chef  Sweden
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OH NO! THE HORRIBLE KSN-1005! Get rid of this part! Really.

This driver is extremely popular in sub-quality "pro" systems and I have never figured why. Maybe because of the "long-throw" narrow dispersion pattern. Clusters of 10 or even more of these units per channel is a very common sight in many DJ setups.

However, it severely lacks in the fidelity department. I did a quick measurement on one unit and it peaks like mad around 5 kHz. You think your ears are being ripped apart as Van Halen hits the cymbals with this one...

KSN-1141 certainly would be an improvement, it is one of the best piezos IMO. The extended frequency response (down to approx 2k) also takes some stress from your 12'' drivers and should clean up a bit in the upper vocal range. I wouldn't run the 12''s above 2k as they will start exhibit breakup distortion if you run them too high. You could connect 2 in parallel to increase the sensitivity a bit (around + 3dB).

Other than that, you could use the glassfibre-wool stuff that you usually use for house insulation to stuff the box with. Not only is it dirt cheap but it is actually one of the very best stuffing materials out there. The only trouble is that some are allergic to it and it is quite nasty to work with. In a DJ setting small amounts might also find its way through the ports when the speakers are pushed hard.

/M
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Old 2nd August 2004, 05:46 PM   #9
Swedish Chef is offline Swedish Chef  Sweden
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Oh, about the tripods. Do you run these speakers as a fullrange system or is it complemented by subs? In the latter case I would go with tripods as they raise the MF and HF drivers to ear-level. Vertical dispersion of these units is generally not the widest.

/m
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Old 3rd August 2004, 08:39 AM   #10
Ovation is offline Ovation  United Kingdom
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Regarding the tripods: At the moment we've just got the two speakers providing the full range (although at present there's not much inbetween the bass and the treble).

Incidentally, I've ordered the KSN-1141 Piezos as recommended.

If we decide to build a new set of speakers later this year I suspect we'll make them taller than these (these are only about 20" high by 15" wide).
I'm expecting that I'll go for a 12" - 15" sub, a 10" - 12" Mid and then perhaps a few tweeters to cover multiple directions, AND proper crossovers - I'm certainly open to ideas on this one.

Out of interest, does anyone know if the Zomax 12" 120W cones are any good as we might reuse them if they're really worth the >$100 that people seem to charge for them. (I think the model is P1265).
I'm not totally sure on which cones we'll want yet, but some of the Celestion range look quite reasonable.

When we upgrade the speakers, we'll also do the Amp. At the moment, Jane's got a 'Lyon Forge A120' It seems to do the job, but again I'd be interested to hear any opinions as to whether this is any good. Again, any recommendations of an Amp which is approx 200-300W with a good price/power ratio will be apreciated - We don't need Hi-Fi - It's just a Disco (Sadly, drunk people have no appreciation of Hi-Fi quality lest to suggest I might reduce the volume to a *safe* level :-) )

I'm still at a loss as to the definition of 'sensitivity' - Can anyone explain this to me?

Thanks
Jonathan
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