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Old 4th January 2004, 01:26 AM   #1
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Location: St. Cloud, MN
Question Crossover question (refurbing Yamaha PA cab)

Folks:

I'm new to DIY Audio and a duffer when it comes to the precise science of speaker / crossover design so forgive the question if it's totally dofus-ville.

I'm refurbing an old Yamaha PA cab that has a 15" woofer and a horn driver, both toast. I have determined that both were custom made by Eminence. I plan to replace the horn driver's diaphragm and have the woofer rebuilt. As the simplest way to replace the 2-way crossover (totally gone from the cab), I plan to buy an Eminence crossover. They have a variety of crossover options and I'm guessing the original one must've been at 1600Hz (PLEASE correct me if you think that's a bad guess!). But my question concerns the two drivers' nominal ohm ratings:

woofer = 8ohms
horn driver = 16 ohms

Why would they be different?
Eminence's stock xovers state they're for 8 ohm drivers and I CAN get an 8 ohm diaphragm from Eminence (instead of the 16).
What would you advise?

Thanks!

Luther
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Old 4th January 2004, 01:59 PM   #2
sreten is offline sreten  United Kingdom
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Probably because at 16 ohms the tweeter is still
efficient enough to match the main driver.

Using a 16 ohm tweeters simplifies slightly the
crossover design, reducing the cost of components.

Using the proper eminence crossovers that include
L-pad matching and tweeter protection you should
use the 8 ohm compression driver diaphragm.

sreten.
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Old 4th January 2004, 02:18 PM   #3
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hi luther,

Advice: stay with the 16 ohms, so recalculate the filter for that.....
Don't buy a filter, build one yourself.
If you stay with 16 ohms, there are two goals:

1 Compared to the 15", the compressiondriver can stand less power so its better for the diaphragm's lifetime....

2 The compressiondriver/horn is lot more efficient, so to get the response right you'll need an L-pad or resistor network anyway.

For the cross: 1600 Hz 4th order active is ok, but if you cross passive it can be somewhat low, depends on the driver. Is it 4" diaphragm/2" exit, then its ok, but keep it 3d/4th order. If its 2"/1" exit I would say go for 2 kHz.

If you use a symmetrical 3d order filter and phase reverse the horn, you can possibly get a better vertical downward pattern because the horn is acoustically further away, and you reverse compensate a bit with that. It is absolutely not electrical correct, but a better vertical downward pattern is more useful then upwards, at least if you place the cabs above the audience's heads.... You can also try using 3d order for the horn (leave it "in phase") and 4th order for the 15" or someting like that, with the same goal.

What I normally do is use an active crossover with smaartlive measuring system, look what crossing gives the best result (with different angles etc) probably need an extra 1-3 dB lpad for the horn, (but test this with the actice crossovers hi-out level).
If I have the right cross/level/slope/phase I measure the impedancecurve of the drivers (lspcad/justmls) and design the filter passive.
If you have a cheesy comp driver, you can put some car lightbulbs (2x18W-24V paralelled orso) in series with the compdriver, (before the filter) as sort of a high limiter, to protect for too much power, quite rude but it works.

good luck!
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Old 4th January 2004, 03:51 PM   #4
JC-21 is offline JC-21  Canada
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Location: Burlington
Hey Luther,

Don't worry so much about 16ohm vs. 8ohm.
Both of those drivers are gone now.

Get replacement drivers for both Horn and WF. - 8ohms each and
Make your own X-over:

~2,000Hz...(safe side)

In line with the (+) wire on the Horn insert a 10uF cap.

In line with the replacement 8ohm 15" Woofer you buy, insert a 0.64mH "air" or "I" core inductor on the (+) line as well.

(In the least, insert the 10uF cap on the horn. This will limit low frequencies from damaging the horn.)

The 0.64mH "air" or "I" core inductor on the 15" will help the system sound less harsh, and keep some of the higher frequencies from damaging it's larger voice coil.

Either and both are a "1st order" (6dB per octave slope X-over) pretty complicated huh? - Madden Football is far more complex.

I would buy a complete new 15", since it is near impossible to guess the correct jig height for the existing Voice Coil to Gap to Spider Height to Cone relationship on your existing magnet structure. My experience is that it will be equal $ to just replace it,... if the voice coil is burnt, recycle it.

Not really my business though, if you know someone who can do it for you - and you have already set it up... go for it.

I can give you numbers for a 12dB/oct "second order" filter if you like...also not complicated. - Just cuts off unwanted frequencies at a steeper rate - and would still be less expensive than having somebody else make it. Also, if you want to reduce the horn output relative to the Woofer, you can use the L-pad idea, or a fixed pull down (a couple of resistors).

Afterthought...

That 16ohm driver was probably to be used in a system with 2 or more of those horns, or other 16 ohm units...wired in parallel to end up with ~4ohm load to the amplifier.

A 16ohm Horn, and 8ohm 15" in parallel means roughly 5.3ohms as seen by the amp... and an 8ohm horn with 8ohm 15" in parallel means roughly 4ohms total, - most transistor amps can drive either load all day.

Remember, to avoid blowing up your work once you have finished, give the speakers a decent power amp to run on. (200W or more) Lower power (~50W amps an so on) means you are going to run the amp into clipping earlier trying to get the colume you want - and start to burn the coils... damaging your latest work. -

Hope this helps!

-Jayson-
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Old 4th January 2004, 05:05 PM   #5
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Default Crossover question (refurbing Yamaha PA cab) - follow-up

Folks:

I just KNEW I'd found the right place for this question!

You've given me lots to think about. Building my own x-over is intriguing-- and would drive my kids (the guitarist and drummer) crazy. So I'm forced to shelve that idea (just for now, doktor! --I'd need to dig waaay back into my memory store and hit the books to stay with you on your suggestions). The stock Eminence x-overs DO include protection lamps, however, so I now understand their reason for being part of E's designs. And the phase-reversing suggestion is also intriguing-- and something I would be able to try, letting my ears (trained somewhat by years of working with hifi gear-- but that's another story) make the determination.

It's interesting that sreten and the good doktor offer somewhat differing advice re the impedence of the replacement diaphragm-- tho I perceive some common ground in that the 16ohm serves to roughly equalize the relative sensitivity of the two different drivers. I can see in E's literature (available as pdfs online) that the output curves of the two drivers ARE different, with the horn driver's higher than the one for the Delta 15-- the woofer that, upon inspection, seems to most closely match the "Yamaha" driver pulled from the cab.

Eminence also offers "cabinet ready" crossovers with pads for adjusting outputs-- another suggestion both replys have in common-- but those are considerably more expensive that their "board-only" cousins and so I'd need to do some more thinking on that. As it is, the cost of replacement / repair (not counting my time --which is, as you all know, "free" to one's kids) will come close to simply buying a new cab at discount (about $240 from Musician's Friend). But, where's the fun in that?!

Anyways, warmest regards and heartfelt thanks to those that replied-- or will reply, as this remains an on-going issue for me here.

Luther
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Old 4th January 2004, 06:23 PM   #6
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Default Re: Crossover question (refurbing Yamaha PA cab) - follow-up

Quote:
Originally posted by los_gatos



It's interesting that sreten and the good doktor offer somewhat differing advice re the impedence of the replacement diaphragm-- tho I perceive some common ground in that the 16ohm serves to roughly equalize the relative sensitivity of the two different drivers.


well sreten is just very accurate/correct in how he states things (accoring to the posts I read from him, he is very skilled/knowledged too)....
lets just say I'm not typing in my native language . But we sort of mean the same: If you plan to make the filter yourself take 16 ohms, if you plan to buy the prefab eminence take 8 ohms, otherwise the filter won't work proper.


Quote:
Originally posted by los_gatos


Eminence also offers "cabinet ready" crossovers with pads for adjusting outputs-- another suggestion both replys have in common-- but those are considerably more expensive that their "board-only" cousins and so I'd need to do some more thinking on that. As it is, the cost of replacement / repair (not counting my time --which is, as you all know, "free" to one's kids) will come close to simply buying a new cab at discount (about $240 from Musician's Friend). But, where's the fun in that?!

Anyways, warmest regards and heartfelt thanks to those that replied-- or will reply, as this remains an on-going issue for me here.
I know it is more expensive, but look at it like this: if you construct it yourself you know what you have. If you buy the original one its probably less better according to your own criterion (? what is the right word?) , because you didn't took the chance to tweak it yourself.
Do I understand that is has to be kid-proof? then the bulbs and lpad are welcome!

anyway good luck with it luther
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