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pegodk 27th November 2012 10:30 PM

PA speakers using 2x Zomax P1250
I seek advice on the speaker unit Zomax P1250. First of all if anyone has any data on the driver, I can't seem to find it anywhere on the web.

I just acquired 4 of these drivers for a reasonable price and the idea was to use two of them with a HF compression driver in each cab. The HF driver will probably be Stage-Line MHD-152 since I have these from a previous project. MHD-152 - Stage-Line mid-high range horn loudspeaker 2KHz-20KHz 8 Ohm - Europe Audio

Last year I build a pretty decent PA speaker with the MHD-152 and the SM212 (Beyma) for Roskilde Festival. It had a simple 2nd order filter (2400 hz) with no attenuation on the tweeter (we just used the EQ on ipods). The drivers are of high quality, but the configuration with no attenuation of the tweeter is not very handy, on the other hand lowering of the efficiency by installing a L-pad is not perfect either.

For this years festival we would like to have 2 high efficiency 12" per channel to raise the sensitivity of the bass (and midrange) and not having to use any attenuation on the tweeter. The idea is to connect the bass-drivers in parallel to make it effective 4 ohms and raising the sensitivity by 6 dB (The drivers are said to have 97 dB/w on Jansen's page, making it 103 dB).

Further more, I've been struck by the idea of building a transmission line to support the bass (which is struggling due to the outdoors purpose + no baffle step compensation). Making a modest TL length of maybe 1 meter would support the frequencies down to 60 Hz, correct? But what kind of cabinet size is needed for this? I don't really have any values to make simulations from..

To sum it all up: The project is to make a PA speaker with 2 Zomax P1250 and 1 MHD-152 per channel, possibly a transmission line. The overall goal is efficiency above all, but also weight and size matters. A perfectly linear frequency response is of less importance.

Any ideas to the project, thoughts about the possibility of making a transmission line, experience with similar projects (possibly links?) and general knowledge about the Zomax P1250 driver would be highly appreciated!

Best regards

chrispenycate 28th November 2012 03:50 PM

Here goes. No, I don't know your 12"; are the essentially light cone designed to be efficient in the low mids or low resonance, designed for bass use?

While I agree (possibly for different reasons, but we end up with the same results, so who's counting) about L pad attenuators (although if you're winding your own inductors, I've a neat trick tapping the HF one, and a switch), have you considered electronic crossovers and multi-amping?
Arguments for:- Power amps nowadays are cheap, and electronic crossovers too.
You can do more correction for driver sensitivity and frequency response cheaply and easily than is practical with passive crossovers.
If you cross the bass off your 12" drivers with a passive crossover this is going to be a considerably more expensive and heavier affair than the HF split.
Having the HF drivers on a separate amp means that peak clipping on the power amp for the low frequencies only comes out of the cone driver, considerably less objectionable than the horns. (That's not an argument for the HiFi enthusiasts, just practical experience in doing festivals with not quite enough gear).
You can put a protection limiter on the HF amp without seriously compromising your overall loudness.
Your amps are seeing a relatively constant impedance. Oh, they'll run into strange loads, but they generally prefer something that doesn't resemble a mountain range.

Arguments against.
Cabling gets considerably more complex. If you don't plan it out well in advance it's very easy to blow drivers, unless you have a particularly meticulous rigging crew (So plan it, with multiway Speakons and colour coded cables)
Power amp racks are heavy.

As regards transmission line – as far as I've been able to tell, they're always big, and low efficiency (possibly because the radiated energy from the back of the cone is always being absorbed and converted into heat. And big because they've got to have a tube about half the length of the lowest wave being generated- over 5 metres if you're going into 25-30 Hz territory - and even folded up, that's a lot of space. Horn speakers are bigger, and don't go down quite so far, but they make up for it in loudness.)

Personally I'd put a short front flare on the twelves. Spreading them a little to reduce the beaming factor of a big cone at high frequencies they ought to increase the comprehension value on vocals considerably.

Is this an outdoor festival? If so I'd be seriously considering horn-loaded bass. Yeah, a lot of woodwork, but the extra throw means you can annoy your neighbours much further away. And if you're restricted to twelves for the low end, concentrate on getting the vocals through. Trying to bring up drum kits (particularly kick drums) on a system like that is an invitation to confused, intermodulated sound. Not only from the amplifiers, but the drivers themselves, trying to reach levels they weren't really designed for over a wide range of frequencies.

pegodk 28th November 2012 08:20 PM

For the festival it will be run on one 12 volt battery (sometimes two in series) and a cheap, but very efficient d-class amplifier. For this reason I haven't really considered the whole active crossover/multiple amplifiers, but im sure it would be easiest with unlimited power.
The speakers will not only be used for festivals tho, they will also be used as normal speakers the rest of the year. The main reason is for festival use however, so maximum efficiency is of top priority.

About the TL build: Im not expecting bass response down to 30 Hz. And I'm not gonna build separate folded horns either. I just want to maximize the bass response from the mid-caps maybe down to 60 Hz. This would require a TL length of around 1 meter, but I don't know the required volumes etc. Do you think I can just simulate it for an average high efficiency 12" speaker to get a decent result?

chrispenycate 29th November 2012 07:46 PM

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Batteries. Oh, yes, been there, done that (didn't get a T-shirt at that gig:D) At least that means that you won't be fighting electrified bands.

A transmission line doesn't tend to increase the efficiency, just slightly extends the low frequency response. A Horn actually amplifies the signal (well, I suppose technically it matches the speaker better to the air, since it can't add energy, but it's louder for the same power, and that's what we're looking for, right?

I've tried to add a drawing; I can't get my CAD program to export anything I can upload so I did a pencil sketch and scanned it, and I'm no graphist.

With a transmission line the energy from the rear of the cone is absorbed, thrown away. With a ported enclosure a small percentage is let out, to add energy in the frequencies below the driver's resonant frequency. But with a horn like this the space behind the speaker works as a compression chamber (I should have drawn that smaller, but this isn't a scale drawing or anything precise), and the expansion of the path works to match it to the air; the speaker stays e relatively high impedance, because it's pressure loaded, and most of the lows come from the horn mouth.

Yes, without very careful calculation there's going to be antiphase signal, and wobbles in the frequency response, but we're not aiming for super HiFi. And for 60Hz they're not enormous; only the expansion rather than taper makes them bigger than a transmission line.

I drew a single 12 cabinet, because that means you can aim them; one of the problems about using largish cones up to 2.5 kHz is that they get very directive. Perhaps I should have drawn it trapezoid, though that makes visualising the horn flare a lot more difficult.

As regards using a transmission line for a driver it wasn't designed for it should work, yes. It's absorbing a particular frequency half a wavelength behind the speaker, and this is not cone dependent, or resonance optimised like a ported enclosure Same thing with the horn; although it would really prefer a light cone, low excursion, high magnetic field concentrated in a small space rather than a long homogenous gap driver, it will work with almost anything.

All that biamping would require would be another cheap, high efficiency power amp, and for the same level it would hardly flatten the batteries any faster (yes, I am aware of the fact that, given the system can now get louder without excessive distortion it will get turned up, which will take more power). You could build the crossover round three transistors, or get a quad opamp and have a balanced input. :)

pegodk 1st December 2012 11:06 AM

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Hello chrispenycate, thanks for your answer! It's a great help.

I think i've come to the solution that i will just build regular bass-reflex cabinets. Horn loading would be cool, and definitely give us a higher efficiency, but the extra volume and weight will make them too unhandy i fear. Afterall its not a PA system meant for professional use, but just for festivals/parties. I've included a sketch of the system as im planning it to look like, not with completely right dimensions. By the way, would you recommend to put the tweeter between the woofers or on the top? Im thinking on the top atm, because that will take it closer to listening heights if the speakers aren't elevated that is, ofc.

Bi-amping sounds like a cool thought, and an extra d-class amp isn't a big expense. What would it take to make a simple 2nd order active filter (crossover freq. around 2-3 khz i imagine)? The filter would have to be run off the batteries as well.

pegodk 1st December 2012 11:11 AM

One more question:
Can bi-amping be done using passive crossovers, or would that ruing the whole purpose?

chrispenycate 6th December 2012 01:52 PM

Cheaper and easier to do it with resistors and caps than muck around with inductors. My web access has died, so I can't easily do diagrams, but with a transistor you can put a second (or even third) order filter round an emitter follower (zero voltage gain, impedance converter) or an op amp with the negative input shorted to the output, which does the same thing without the voltage shift. Either can work from your battery, or take little enough current that you could put them on a tiny battery alongside. One high pass, one low pass, and the values can be got from any active filters book (I'll try and draw a diagram and post it next time I've got net access).
Nice drawing. You're not going to get a transmission line into that, so go for reasonable ported enclosure. The tweeter in the middle is theoretically better for phase matching, at the end, as you've drawn it much more practically useful; you want to get the high frequencies well above the heads of those nasty moving absorbers we call the 'audience'. Make a (sealed) hole in the bottom so you can stick them up on poles – um, 'speaker stands'. And put handles on them so you can fly them, or tie them to trees.

chrispenycate 13th December 2012 12:55 PM

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Yes, I'm double posting, and yes, it's probably too late for this project, (oh, and yes, it's not the most HiFi I could manage, but easily high enough quality for this use) but I might as well see if I can upload files like this.

And who knows? Somebody else might need something like this (although they'd better have a bit of an idea about electronics; I left out values of an awful lot of non-critical components.);)

JMFahey 13th December 2012 02:01 PM

Sorry friends but there's something I don't get *at all*.
Running from a (car?) battery was mentioned, so I thought Roskilde Festival was some small event, with smiling hippyish Flower children or their modern equivalent, all acoustic guitars, maybe violins, tambourines and flutes, or similar low acoustic volume stuff.
Fine, less than 100W may do if everybody is quiet, sitting in the grass and listening.
I have done similar PA work many times.
But out of curiosity I googled Roskilde Festival: Frontpage
They speak of 7 stages with a combined capacity of 31000 people :eek:
One of the attractions is Slipknot, plus they mention "Norwegian Metal Bands" :eek:
I'm :confused:
Yes, one of the stages is "Gloria: The intimate, offbeat and low-key room" , meant for "quiet acoustic music" , but even so, it has a 1000 people capacityt.
Plus all of them have regular 230V power.
Do you reallyneed to rely on battery power.
Mind you, I understand sometimes it's needed and as mentioned have actually worked under such constrictions, in picnic type mini festivals, but in this Roskilde Festival it does not seem necessary.
Or am I missing something? :)

pegodk 13th December 2012 02:42 PM

Thanks again Crispenycate!
Haven't got around to start this project yet (got exams to finish first). Those diagrams seem quite complexs, maybe ill just do a passive 2nd or 3rd order filter.

For a 2nd order filter i would probably use 3500 hz as crossover point, 3rd i would go with 3000 hz.. Also i will have to make this project into a 1 woofer 1 tweeter project, since one of the woofers i received was broken :(... I have this horn tweeter laying around:
Foster 025N03 Cast Aluminum Horn Tweeter 3" x 7-1/4" 279-102

It has a lower sensitivity than most other horns, requiring less modification.. Its also 8 ohm (like the woofer), so i will probably just build a standard crossover around 8 ohms, and maybe a L-pad reducing the sensitivity of the tweeter by 1-3 dB.

For JMFahey: Yes it is Roskilde festival, but im just a normal participant, so i dont have access to 230V. The stages are big and plays quite loud, but before the music on stages start there will be 4-5 days warmup (no music on the stages), thats why we need our own system :)

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