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Posted by Antonio Marcheselli (Member # 373) on 10-05-2001, 02:23 PM:
Hello everyone.From my theater I get one 46cm JBL speaker from our broken subwoofer. It is model 2240 (if I remember correctly) from our old 4668 (same).
I found a room where I can build my HT without problems with neighbours. 
So I would like to build my own Subwoofer using the JBL speaker.
Question: I bring the speaker in an Hi.Fi shop where they tested it and the gave me some project of speaker but the lower frequence was not enough for me (60Hz, is it a subwoofer? 
)and I asked them to make others better project.
I wonder if it is more simple to copy the 4645C JBL subwoofer that has a more powerful speaker but with the same diameter.
What so you think? I saw that 4645C can handle 35Hz without EQ and 22 with EQ. So you think that I can obtain more with a bigger cabinet? Perhaps JBL optimized that subwoofer to work better with more than one cabinet...
I would like to be sure before building it!
Equalization. I could buy a parametric equalizer for the subwoofer and I've read on these forum few time ago that setting parametric equalization with ears is not so difficult. Why?
Last trouble: I get the theater's amplifier. It is a crown 2401. Just for fun I've tested the speaker in my bedroom to see if it works but when I turn off the amplifier I can hear a loud "TOC" and the cone goes forward in a bad way. I can't realize if the "toc" are frequencies coming from the amplifier or if is the coil that hit the bottom of the speaker; should the crown 2401 have some protect at shutdown? I didn't recall any "Toc"s when I powered my subwoofer in the cinema...
Thanks for all your suggestion!
Bye
Antonio
Posted by Barry Floyd (Member # 385) on 10-05-2001, 04:01 PM:
When I started working as a mobile DJ back in 1984, I tried building my own subwoofer. What I found - that by the time I spent building the cabinet and buying the electronics, I could have bought one cheaper. Another downside of home-built speakers - they have NO re-sale value, no matter what they're loaded with.I've got 3 subs now (one for each of our systems). Two of them are Cerwin Vega L36P's (two reverse fold 18" drivers) - each about the size of your average washing machine. The third is an Electro-Voice subwoofer, and it is just incredible. It only has one 18" driver - but it blows the Cerwin Vega's away!!
If I could afford it, I'd replace them all with EAW's.
As far as your amp is concerned. An old gray-haired DJ taught me one night of the best "rule-of-thumb" I've ever learned. When powering "up" your system - the amplifier is the LAST thing to be turned on - and the FIRST thing off when powering down. Those "TOC's" can fry a voice coil in a heartbeat.
------------------
Barry Floyd
Floyd Entertainment Group
Nashville, Tennessee
(Drive-In Theatre - Start-Up)
Posted by Greg Mueller (Member # 17) on 10-05-2001, 04:35 PM:
Just to add to what Barry said. JBL and other manufacturers have had the time and money to do all the research to make that speaker work it's best in the cabinet that they put it in. They know all the free air resonance type statistics and all, so they can make it work very well. Unless you copy their cabinet exactly, you're going to have to reinvent the wheel so to speak. Also, subwoofer and other bottom end speakers are kind of hard to get real solid so they don't vibrate and buzz. Unless you have lots of wood working tools and a fair amount of time, I'd go for a used cabinet. Maybe you can find one with a blown speaker or no speaker. Good luck------------------
Greg Mueller
Amateur Astronomer, Machinist, Filmnut
http://www.muellersatomics.com/
Posted by Antonio Marcheselli (Member # 373) on 10-06-2001, 02:18 PM:
Barry,I agree with you. However I had that speaker and amp for free, that is cheaper of any existing subwoofer... 
I know that Amplifiers have to be turned on after all, I do it every day in my theater. However the "TOC" happened when I turned *off* the amplifier...
Greg, I know what you mean. I will build the cabinet in my spare time and, probably, in a lot of time... But when the subwoofer will be completed, it will be a big satisfation for me!
I cannot copy the original cabinet of my speaker because the 4688 is a two-speaker cabinet and is bigger than TWO washmachine!!!
Do you think that put the 2440 in the 2441's cabinet will be wrong? It will sound as a 4645C at half spl...
Doesn anyone knows the 4645C internal measures? Or: does anyone knows the thickness of the wood?
But most important: does anyone knows the diameter of the port hole and the lenght of the reflex tube?
Bye
Antonio
Posted by Barry Hans (Member # 477) on 10-06-2001, 03:45 PM:
Go for it! Your costs are limited due to you already having the components. I like wood working so I enjoy building the cabinets and many of the designs I have found are truly first rate and do rival mass marketed subs at a far less cost ( I know- I just set myself up for flames ) . Searching the web I have found many top notch designs for home theatre. http://www.madisound.com/cgi-bin/discuss.cgi is a good general speaker building site. Here is a link to one of many I have come accross for subs http://www.decware.com/diyboxes.htm Be forewarned you can get addicted to the whole do it yourself speaker building hobby.
Posted by Antonio Marcheselli (Member # 373) on 10-07-2001, 12:04 PM:
Barry,Many thanks!!! I just take a fast look at DecWare website and I found it very very intresting! I will surely follow the HT bulding forum.
I think that copying JBL 4645C won't be a good idea not bacause JBL's subwoofers are not good but because, I think, Cinema products are intended for using in a difderent way than Home Theater. For example, I beleive that 4645C are designed to work with more than one cabinets so, perhaps, single speaker SW was designed to have maximum spl by each subwoofer but opimal frequency response with two or more modules. It is just my opinion. Since I have different requirements perhaps a different cabinet will sounds better.
And since I have the "do it by yourself" hobby since I was 10...
Your opinion?
Thanks again, your links are a good start point!!
Bye
Antonio
Posted by Antonio Marcheselli (Member # 373) on 11-02-2001, 02:37 PM:
I would like to understand better the "TOC" problem of my amplifier.
I will try it with older and cheaper speaker to see what is the problem but can the protection circuit has gone?It is a Crowm 2410.
Bye
Antonio
Posted by Zac Manning (Member # 1031) on 11-04-2001, 01:57 PM:
My two cents on using a parametric equalizerTypically on systems that I have looked at where people used their ears to eq, they end up boosting the hell out of lows and the highs (the fletcher munson curve explains this) so that it isn't flat like you want it to be. Also, you said that with eq you were able to get it to 22hz, or 20hz. What software was used to determine this? and when it reached 22, where was it compared to the rest of the frequency spectrum. I think that is sort of fighting the natural properties of the speaker when you push a frequency out of it that doesn't want to some out, are you able to hear 20hz? If you can't why worry about it. Very few people can hear 20hz or 20khz
You might be over heating the coil by forcing the frequencies that want to roll of 10 or 15dB from maybe 30hz to 20hz.And usually it doesn't sound very good when you forse those frequencies into a speaker. And in boosting those frequencies you are in turn causing a lot of phasing issues. I would suggest trying to get it to sound the way you want by adjusting the levels on the amp first before you even touch the EQ.
Don't forget the room either, what are the dimensions of the room? You might have some standing wanves or room modes that accentuate the low end, especially if the dimensions are multiples of each other.
i usually never boost with the Eq, only cut out the places where you have spikes, and a parametric eq is a great way to do that.
Posted by Zac Manning (Member # 1031) on 11-04-2001, 02:01 PM:
Greg is exactly right in terms of JBL having found the best cabinet for that speaker. Simply swapping cabs rarely allows you to realize the potential of that speaker.
one other thing.Pink Noise will be infinitely better for equalizing than music.
Posted by Antonio Marcheselli (Member # 373) on 11-04-2001, 03:01 PM:
Zac,The two different low end of 4645C (with EQ and without EQ) are wrote on JBL data sheet of 4645C!! I never made an equalization and I don't have the tools to make it!
I ask for parametric equalization just because, since I don't have an RTA, I would like to understand if parametric equalization will be the best way to equalize "by ear"!!!
More, I think that noone can "hear" 20Hz, but I think that everyone can "feel" it!
Finally, I'm searching altarnative cabinets because:
1. original cabinet is TOO big
2. original cabinet works with two drivers and one is broken.
Perhaps I will repair it to have more power but in the meantime I would like to start with just one 18"!!
Bye
Antonio
Posted by Matthew Bailey (Member # 601) on 11-04-2001, 04:26 PM:
A couple of thing I need to mention about speakers.
Is there any thing needed to build your own THX speakers for home or
cinema use? Some non-THX suff I need to mention:I am wondering
if you can use car audio amplifiers for use in the home or
in a cinema sound rack provided you have a suitable 12 volt
power supply. I would use a couple of Rockford Fosgate car
speakers,subs & amps but what would the audio quality be
if used in a home theatre system or in a cinema auditorium
especially behind a cinema screen?
Posted by Brad Miller (Member # 2) on 11-04-2001, 05:18 PM:
The quality would be shit. Car speakers and amplifiers were made for cars, not theaters. Also, the spec sheets on them are highly bloated and no you are not really getting 1000 watts of "real" power from that $99 car amplifier.
Posted by Zac Manning (Member # 1031) on 11-04-2001, 06:08 PM:
I think they would sound like shit for a lot of reasons. The "Q" or directivity factor of those speakers vs speakers designed for cinema playback is going to be very different. Also, dialogue is rarely intelligible without the use of horns when you are behind screen, at least that's how I hear it. And yes, those specs are going to be a little wonky, and those amps probably don't sound that good to begin with i.e. lots of harmonic distortion, poor dampening factor etc. But you might actually be getting 1000 watts, it just matters what load they put on the amp. You can check for yourself by taking a 1000hz sine wave and sending it into the amp, on the output take a scope and look at the wave form till it clips, read the A.C. output voltage accross the output load with a VOM, calculate the R.M.S by taking that voltage and squaring it, and dividing it by the impedance. So there is not "real power" per say. If your R.M.S. value is 400, you can get as much as 400 watts if the load is one Ohm, or as little as 25 watts if the load is 16 ohms. But i still would never you anything on a car in cinema. And I personally think THX is sort of over rated.
Posted by John Walsh (Member # 168) on 11-04-2001, 06:22 PM:
People tend to forget that THX only "inspects" equipment to see if it passes their standards. Any manufacturer could submit their equipment to THX to see if it passes. THX themselves really does not make anything (well, their crossover network, but that about it.)I do think we are better off having THX; at least the theater has undergone *some* kind of inspection to insure a decent sound system.
Car speakers are tailored for use in a car, I doubt they would sound very good in a open space like a theater.
Posted by Gordon McLeod (Member # 33) on 11-04-2001, 06:28 PM:
I would not say THX is overrated just misunderstood. One does not have to use THX's crossover and designs to have a good sound system they just guarentee the components will work togather and then certify that it does
As for car components I think they are the basis for another brand of lesser sound system
Posted by Jon Bartow (Member # 241) on 11-04-2001, 08:29 PM:
A word of caution about your amp. You say that when you connect the speaker and turn the amp on the cone goes "forward in a bad way" If you mean the cone goes forward and stays there (or backwards), then you have a serious problem. Most likely DC output. Simple test: put a dummy load on the amp (8ohm 50 watt resistor for example), put something through it (pink noise, 1000hz tone) and test the amplifier outputs with a meter or scope. You should have plenty of AC but no DC (on some meters the DC needle/display will bounce around but it will not stay at one point with AC output)
If your amp is putting out DC it is a good way to kill the speaker very quickly (or even catch it on fire!)
Jonathan
Posted by Antonio Marcheselli (Member # 373) on 11-06-2001, 02:41 PM:
Jon,No, the cone goes forward when I turn OFF the amp and the cone doesn't stay in a different position. I hear a loud "TOC" when I turn off my amp but I canno realize if the TOC is caused by the voice coil hitting the bottom of the speaker or if it is just a sound created by the amp.
It is strange because when the amp was in the theater connected to that woofer I never heard TOC's in the auditorium.
Bye
Antonio