Film-Tech Forum ARCHIVE: Subwoofer's amps clipping

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Posted by Antonio Marcheselli (Member # 373) on 05-10-2002, 02:49 PM:

Hi everyone

We're become crazy.
We finally had our tech to re-eq our theater and all now sounds very good. We didn't install the B6 filter on teh SW because our tech want to get more information on that because he know what is it but he usually sell EV and he don't know very much JBL.

This is the situation: 4 JBL 4645B subwoofer driven by 2 JBL MPX600 amps set to bridge.

If my calculations are correct, at 113dB we are using each subwoofer with 440W where they could handle 800W AES.

Actually in the auditorium subwoofers are not particulary loud, just as has been when EQ was ok, and as they should be, I think.

The projectionist of the theater noted tonight, during a loud scene, the CLIP led lighted on the amps. He is not sure if where both amps, or both CLIP and PROTECT on just one amp. He will check better.

If is CLIP and PROTECT on just one amp, can be the amp faulty (I remember that one is hissing with no signal)

If it is CLIP on both amps... Is it possibile???

MPX600 at 4 Ohm bridged can handle 1700W.

I will speak directly with our tech of course but I also would like your opinions. We have Star Wars on 16 and we would like to have that theater at "top"....

Bye
Antonio

Posted by Neil McGovern (Member # 1247) on 05-10-2002, 06:05 PM:

Just a small point, but I'd check you don't have a 6dB pad on your sub

I know some EQs have this.

HTH
Neil

Posted by Steve Guttag (Member # 268) on 05-10-2002, 06:18 PM:

I suppose you "might" be able to drive your amps into clipping with that arrangement. Your amps only have about 1200 watts bridged into 4 Ohms, not 1700 (be careful of the specifications, the 1700 watt figure is at 1KHz, not wide band pink noise nor real audio). Thus you only have about 2400 watts at your access. You need something like 1823 watts given 4 JBL 4645Bs with 27 meter room. 2400 watts is only about 1.5dB higher than 1823. Give or take an EQ setting also remember the 97dB figure for the JBL 4645B is for a frequency range that is typically higher than the subwoofer area and you might be on the edge.

But all of this presumes you have your fader set to 7.0 and everything is calibrated perfectly and the film was recorded to the theoretical maximums.

I'm more inclined to look at the amp and subwoofers to investigate a failure there.

Steve

------------------
"Old projectionists never die, they just changeover!"

Posted by Antonio Marcheselli (Member # 373) on 05-10-2002, 07:16 PM:

Steve,

Where did I wrote 27 meters? 27 meters is the other theater, where we blown up the 4688.

This is our THX screen. 17 meters from screen to proj. That means about 450W for each subwoofer, right?

Uhm... I believe 1700W for the amps, 1200 is another story...
(THX recommended to wire that subs in stereo mode...)

Ok, so what do you suggest? Add amps? Perhaps it is not a problem.

Steve, let's say that you have to install a new theater 17 meters long. And let's say that you choose 4645B for subs and MPX600 for amps. How many subs and how many amps you'd install in a B6 setup? I'm curious...

Neil,

Thanks, that is called B6, we spoke about that few days ago. No, we don't have it, unfortunately...

Antonio

Posted by Steve Guttag (Member # 268) on 05-10-2002, 09:43 PM:

Ok...I got the 27 meter room numbers wrong anyway...

For a 17 meter room, one only needs about 1297 watts or 324 watts per subwoofer. The subs can handle 800 watts each, the amp is capable of putting out 400W/Ch stereo into 8-Ohms. Therefore, I'd say your are covered, power-wise. You only have about 1dB of headroom though.

A safer method is to bridge the amplifiers into 4-Ohms...you will have about 1200 watts per amplifier at your access or 2400 watts total. This will yield about 2.7dB of headroom.

You will generally have lower distortion if you run an amplifier in stereo mode and that is probably why THX recommended it.

Steve

------------------
"Old projectionists never die, they just changeover!"

Posted by John Westlund (Member # 913) on 05-10-2002, 10:20 PM:

We had the same problem about a week ago. It was the way the 2 subwoofers were wired. Just a thought.
John

Posted by Antonio Marcheselli (Member # 373) on 05-11-2002, 04:47 AM:

Steve,

Our amps are already connected in bridge mode.
Let's say the room is 20 meters long, I'm not sure that 17 meters is exact.

So, for 20 meters and 113db I need 442W for each speaker. 884W for each amps that is actually wired to provide 1200W.

However, this is not enough, as we saw!

Do you believe that 4 4645B are enough for that theater? In that case we will add 2 amps.

Bye
Antonio

Posted by Steve Guttag (Member # 268) on 05-11-2002, 07:44 AM:

Not to quibble, I came up wiht 449 watts per sub but in either case, you have enough power and subwoofer for that room. I would look elsewhere to see what is going on. 4 JBL 4645B subs are plenty for a room of that size and given the specs for the amplifers, they are also big enough. There is something else amiss.

Steve

------------------
"Old projectionists never die, they just changeover!"

Posted by Antonio Marcheselli (Member # 373) on 05-11-2002, 02:35 PM:

Steve,

I'm confused, there is something that I'm unable to understand. You wrote above that
"Thus you only have about 2400 watts at your access. You need something like 1823 watts given 4 JBL 4645Bs with 27 meter room. 2400 watts is only about 1.5dB higher than 1823"

Now you say that we have enough power for the room with 450W for each sub (that is 1800W for all subs).

What I didn't understood?

THANKS for your patience.

Antonio

Posted by Antonio Marcheselli (Member # 373) on 05-11-2002, 02:47 PM:

John,

I suggest you a better face protection when you change the lamps, it should cover ALL the face!!

Bye
Antonio

Posted by John Westlund (Member # 913) on 05-11-2002, 07:44 PM:

We have that now. Sometimes you have to take a risk in an emergency

Posted by Antonio Marcheselli (Member # 373) on 05-12-2002, 02:21 PM:

We lowered the THX trailer at 5.5 to avoid clipping yesterday.

A colleague of mine told me that, at 5.5. the amps are clipping anyway.

I've already checked the phases of the wires, tomorrow I will check the speaker phases. If I record a 30Hz tone and I reproduce it softly on SW channel, can I check if subwoofers are internally wired correctly putting an hand on two cones simultaneously?

There are three different thing that challenge for out of phase:

1. The system's frequency responce is limited to 45/50 Hz... I believe that, also without B6, that speakers should go further...
2. Amps are clipping while we have enough power
3. There is no audible excess in Subwoofer volume in the auditorium.

Any other suggestions? We MUST solve this problem for May 16th...

Bye
Antonio

Posted by Gordon McLeod (Member # 33) on 05-12-2002, 02:59 PM:

Disconnect the speaker and use a battery
connect the + of the battery to the red terminal and the cone should move out
Do that on each of the drivers
Also Smart sells a good phase checker as does Goldline

Posted by Steve Guttag (Member # 268) on 05-12-2002, 03:28 PM:

Antonio, I already said I flubbed the 27 meter room numbers, pay attention.

Again given the room specs and the equipment specs, you have enough. If you don't achieve level, your problem(s) lie elsewhere.

Check the phasing, as others have suggested (you say the amps are bridged so hopefully someone wired the subs in parallel with both positive terminals going to the positive terminal of channel 1 on the amplifier and both negative terminals going to the positive terminal of channel 2 on the amplifier.

Verify the amplifer is good, unless both are doing it. But once again, given a 20 meter room, you have enough, equipment-wise.

Steve

------------------
"Old projectionists never die, they just changeover!"

Posted by Antonio Marcheselli (Member # 373) on 05-12-2002, 06:12 PM:

Steve,

Both amps are clipping so I'm excluding an amp failure, right?
I've already checked the amps-speakers wires and are in-phase. The last chance is that one subwoofer is out of phase internally, by factory. I realize that it is a non common thing but let's check this one also...
Tomorrow I will try another "brand" of equalization (that differs only for SW setting), I'm not convinced by this one...

Gordon, can I make damage to the speakers with a standard 1,5V battery?

Steve (and all others)

Thanks for your patience, I realize that sometimes (often?) I ask many times the same question because I don't fully understand your answers! Your help is always very precious to me.

Bye
Antonio

Posted by Gordon McLeod (Member # 33) on 05-12-2002, 06:32 PM:

I usually use 2 1.5 volt in serries to give me 3 volts and if not left on indeffinetly will not damge the speacker.
Make sure it is disconeected from the amp before doing

Posted by Antonio Marcheselli (Member # 373) on 05-13-2002, 03:28 PM:

Hi,

Today we made all tests.

1. I've played "Lord of the rings" in DTS at 7.0. Result: just one moment of clip. (Karen, why does new DTS disks doesn't allow to play THX trailers by keyboard???)

2. I've project tha Star Wars trailer (#3) and THX trailer. Star wars at 6.5: heavy clipping. THX trailer (broadway 2000) heavy clipping).

We have tested the speakers with a battery: are all in phase.

Sound in the theater seems correct. What can be wrong?

Actually we have lowered the amps by two step to avoid heavy clipping. We will swap the 2 MPX600 with two MPX1200 soon.

I know that it shouldn't be a problem but... it is.

Opinions?

Bye
Antonio

Posted by Karen Hultgren (Member # 395) on 05-14-2002, 11:56 AM:

Antonio,

First be sure the THX trailer is really on the disc. It will be printed on the label if it is there.

Trailers are always compiled together as reel 14 and I am not sure if you can play a compiled file with the keyboard.

Karen at DTS

Posted by Antonio Marcheselli (Member # 373) on 05-14-2002, 06:39 PM:

Karen,

With TITANIC disks I'm able to play THX trailers by keyboard. I've tried with many actual titles with no success. Is there a way?

Bye
Antonio

Posted by Karen Hultgren (Member # 395) on 05-14-2002, 07:21 PM:

Antonio,

We have not changed anything on discs for a few years. If trailers on TITANIC discs play, then trailers on SW should as well. As mentioned, all trailers on the disc are written on the label. If you do not see it listed on the label, it is not there. Not every disc has THX trailers.

Karen at DTS

Posted by Antonio Marcheselli (Member # 373) on 05-14-2002, 07:44 PM:

Karen,

Thanks. I will try with more disks. Actually I tried with Shallow Hal and Lord of the rings without success.
You don't have a "power handling" track on empirical that tests subwoofer channel, right?

Bye
Antonio

Posted by Karen Hultgren (Member # 395) on 05-15-2002, 06:23 PM:

Antonio,

There is not a "power handling" test specifically for subwoofer on the empirical test disc.

Karen at DTS
khultgren@dtsonline.com