Hoffman Amplifiers Tube Amplifier Forum
Other Stuff => Cabinets-Speakers => Topic started by: Dive on July 17, 2009, 11:23:27 pm
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I just have an outboarded raw Eminence Legend 1518 and not sure I am getting all the volume I can or should get out of this, but maybe it is not reproducing some of the sound and I need something that will. What configuration and speakers are suggested best for this amp? I'm supposed to be good for 2,4,8,16 ohm Z out for speakers.
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Back in the day the best speakers for that signature marshall tone were either greenbacks or G12M70's.The latter being my absolute favourite.
The Legend doesn't hold a candle to Celestions.
p.s. what exactly is an ' outboarded raw Eminence"??
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Just my way of saying a speaker with no cabinet, just as you bought it and ran some wires over to it.
:smiley:
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No cab, baffle, nothing? :dontknow:
You can't evaluate a speaker like that. Of course you're not getting all the volume... or frequency response or efficiency or anything else. That's like revving up a Boss 302 while it's laying out by itself on the ground and wondering why you can't get any speed out of your Mustang.
Put it in a proper cab and listen to it. If you don't have one/can't afford one/whatever, then put your motor back in your Mustang, run down to Lowe's, and buy yourself a 4'x8' sheet of 3/4" MDF. Cut the proper size hole right in the middle of it, screw the speaker in it, and try that. You still won't know how it might sound in a sealed cab, but this test will give you an idea how it would behave with an open or semi-open back.
Letting a speaker hang out in the breeze is only good for making some sort of avant garde sound effect for recording or building an emergency radio from what's left of your airplane after crash landing in the middle of nowhere. Definitely not for speaker evaluation listening tests.
EDIT: After re-reading this, I thought I should explain a little more so you understand why this isn't good. Speakers produce sound waves according to the electrical input signal they receive. You know this part already. The electrical signal we send them is a complicated one comprised of multiple frequencies. Some speakers are better than others at translating these electrical signals into acoustical sound comprised of the same frequencies. All guitar amp speakers, including the Legend 1518, will be capable of producing a very wide (by guitar standards) range of frequencies at least to the extent that we'll classify what we hear as "music" and not just multiple overlapped tones. The way it does this is by pushing the cone outward (air compression) and retracting it back (air rarefaction). This causes the air pressure on your eardrum to vary from the nominal 14.7psi in a corresponding manner to the cone excursion. As a result, you "hear" the speaker's movement. The problem is that a speaker is a two-way device: It produces it's sound from the rear as well as the front.
Because your Legend produces sound from both the front and rear, you can experience cancellation of like frequencies. A "+" value of anything, when in the presence of a "-" value that's equal in scope and magnitude will eliminate each other. Take an empty 8oz glass. Fill it with a positive 7oz of water. Then fill it with a negative 7oz of water (pour it out). You're left with the same empty glass you started with. As your Legend's cone moves outward, it causes a compression of air (positive). At the exact same time and looking at the speaker now from the rear, the cone is also moving inward causing a rarefaction of air (negative). The compressed air, which you should normally hear, sneaks around to the rarefaction if it can. As the cone moves the other way, the opposite happens. It's as if the cone never moved. Or it would be if the world were perfect, but even in the inefficiency of this situation, it's inefficient - not all of the air can move fast enough, so you still hear something. You'll still have some water droplets in your glass after you pour it out. To really kick inefficiency in the can, we need a baffle.
Now, if we could place the speaker into a hole in a wall inside your house, seal it up, then play it - and close the doors to that room so there's no possibility of air sneaking around from front to back - then you could hear the full capability of the speaker. Or we could just build a sealed box, cut a hole in that and do the same thing (ala Marshall 4x12). Either way, it forms an infinite baffle. We don't always desire the absolute isolation an infinite baffle provides. Sometimes a little cancellation with the right circuitry, cabinet materials, speaker(s), etc, can be magic. In that case, we'll simply cut a hole in a flat piece of wood, fold the edges around so it forms a box with not much back to it, and call it an open cabinet. That allows some isolation, but not completely.
Anyway, a speaker with only itself as a baffle just doesn't work.
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> buy yourself a 4'x8' sheet of 3/4" MDF
For gitar speaker quick-evaluation, slide to the side, HD sells 2'x4' cuts. Fits in the Civic a lot easier than a 4x8.
And I hate sawdustboard. I'd go through the 1/2" even 3/8" plywood looking and knocking for a hunk with few voids and no apparent loose-bits between the layers. If it is all junk, I'd look at the hardboard, or even pulp-board. An open baffle does not have to "hold pressure".
As you say, a speaker speaks on both sides, opposite phase. If the speaker could be infinitely small, ALL sound would cancel-out. If the speaker could be infinitely large, no cancellation, but impractical. So what about practical sizes?
Bass limit, VERY approximate:
8" frame = 840Hz
15" frame = 450Hz
30" frame = 225Hz
An early "cone" speaker -was- 30 inches across, naked in the room. This gives reasonable bass, a lot better than telephone earpieces on trumpet-horns. But the highs were ratty.
Nevertheless, if you don't have midbass (200-400Hz zone), the sound is very thin, lacks body, sounds weak.
The first good home loudspeaker was a 6" cone in a 24" open-back box. The small cone spread highs well. The larger box supported the bass; actually prevented large bass waves from coming together to cancel. On this plan....
24"sq flat baffle = 280Hz
48"sq flat baffle = 140Hz
4'x8', long edge on ground = 70Hz
A 2'x4', on ground, speaker hole low, will droop around 150Hz. However guitar speakers usually bump around 100Hz. This will work out well, or maybe too good: most guitar cabs are not this big.
A loudspeaker NEEDS a baffle of some type. Otherwise it is like a piston without a cylinder: it just sloshes, doesn't pump.
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Wow, great information! Thanks PRR...
...but no Civic - think 'Mustang'...
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Well thanks for all the input... I was really just trying to hook my recent amp build to something to see if it would make sound. Yea the speaker performance was nothing to brag about outside the cabinet etc.. OK, so now things are different. I have been playing through a 4 X 10 speaker cabinet with it and been pretty pleased. I began assembling and making a 4 X 12 cabinet, at the moment I have 2 celestion greenbacks installed at opposite corners and nothing installed in the other 2 opposite corners. The cabinet is practically usable now, but I'm trying to decide on speaker configurations and dial in a sound I will like. I figure a large volume cabinet tends to be a little bass'y for me and I dont mind bass if I can get the treble too if I want it. The unused uninstalled locatons are not cutout yet. The cabinet has a volume of appx 7600 cu in. frontal angle upward of appx 2.5 degrees. 766 sq in frontal area Height appx 26.5 in, width appx 28 3/4 inches, depth running appx 9.5 inch top to 10.5 in at bottom or so. 3/4" oak plywood for the top sides and botom, 1/2 inch birch plywood for the front, speaker mounting/cutouts. some birch ply in the back longer piece at top and a shorter piece at bottom. I have not fully ported out the ply in the back, but it has some about 5 1/2 inch window opening in the back running all the way across. Pieces not cut out within each piece yet in the back. But I only have the 2 speakers now. Giving it a try, I can tell I like the bass'y side of it, but it's not getting enough treble for me yet. I dont know if 2 more of these same speakers and port out the ply some more will deliver what I want, or if I should use 2 different speaker types for the other 2 locations maybe even going with 10" for the other two???? that lean toward the treble end. I am no speaker or speaker box expert by no means. But I'm not completely lost in the subject either.. So, any suggestions are appreciated. Thanks