Film-Tech Forum ARCHIVE: USL XTA-1 Crossover

This is topic USL XTA-1 Crossover in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

To visit this topic, use this URL:
https://ft-forum.com/ft/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=1;t=010373

Posted by Tom Wienholt (Member # 1510) on 03-10-2010, 12:04 PM:

I would like to upgrade a current system by bi-amping and installing JBL4675's. I have never installed a bi-amped system before. I will be using QSC 1400's with the plug in USL XTA-1 crossovers. So here is what I understand so far:

-The XTA-1's plug into the QSC 1400's.

-The signal for each channel is connected to the Channel 1 input of each amp.

Is this correct?
Here is what I do not understand:

What outputs for each amp correspond to the hf and lf?

Are the gain controls on the amp used for the hf and lf output?

Or is the hf gain control on the XTA-1 used for hf output?

Also, when setting sound pressure levels, how do you match the hf and lf of each channel for 85dbc?

Basically, what is the proper method for performing a B-chain alignment when bi-amping?

Hopefully these are legit questions. Bi-amping is new to me so I am a bit confused. I have installed many systems that were not bi-amped so I a familiar with performing A and B chain alignments, just not when bi-amping. ANy help would be appreciated. Thanks.

Posted by Tony Bandiera Jr (Member # 2365) on 03-10-2010, 01:15 PM:

Tom:

I will provide the answers best that I can, but I cannot remember for sure the right answer to the first one, but here goes:

What outputs for each amp correspond to the hf and lf?

I think it was Ch 1 LF and Ch 2 HF Or wait for Clint to chime in.
Oh yeah, VERY IMPORTANT: Don't forget to set the little DIP switches on the amp properly or the crossover WON'T work, you could damage the crossover and the speakers!!

Do you already have the extra wire to the stage for each speaker? You will need a pair for both HF and LF.

Are the gain controls on the amp used for the hf and lf output?

Yes. You will use them as described below.

Or is the hf gain control on the XTA-1 used for hf output?

Only if needed to boost or cut the HF output if the amp's gain controls don't give adequate range in the HF. I have traditionally set this to mid-point and left it alone.

Also, when setting sound pressure levels, how do you match the hf and lf of each channel for 85dbc?

You don't adjust them of 85dbc each, per say, but for the flattest response BEFORE any EQ is done, using an RTA. You CAN NOT set this or do a proper B chain without using an RTA!!!

Basically, what is the proper method for performing a B-chain alignment when bi-amping?

After properly connecting all of the speakers and making sure they are all in phase (remember JBL's LF bins are electrically out of phase with the HF in many cases), you will set up the mics and multiplexer for your RTA in the room, set ALL EQ on your processor to FLAT, then using pink noise and one channel at a time, set the HF and LF levels on the amps to get the most even (flattest) response possible before applying any EQ.

I'm not going to go over the details of a proper room EQ from here, it would take far too long to explain properly. It is something best done by a technician who knows how to do it and has a good ear, as the best job is done by balancing measured results with adjustments made by ear.

Your best bet, to preserve your sanity, equipment and possibly your customer base, is to hire a tech to do this for you. It will cost nothing compared to the potential harm of not doing it right.

Posted by Tom Wienholt (Member # 1510) on 03-10-2010, 02:15 PM:

Tony,

Thanks for the help. In response to your suggestion, I have eq-ed many theatre auditoriums before so I am familiar with the process. Bi-amping is what I am new to. I was asking about setting SPL's for this reason...I usually set the processor outputs to optimize the s/n ratio and do not touch them during B-chain alignment. I set the sound pressure levels using the gain controls on the amps. So it seems that this cannot be done the same when bi-amping since both gain controls are used for each channel. Meaning that after adjusting the gain controls on the amp for the flattest response and then eq-ing with the processor, the SPL levels should be set using the processor output controls? Hope that makes sense.

Posted by Aaron Sisemore (Member # 145) on 03-10-2010, 03:10 PM:

Tony is correct - ch. 1 is LF, ch. 2 is HF.

DIP Switches (this applies to all QSC Series One amps):

1-2-3-4 UP - 5-6-7-8 DOWN.

-Aaron

Posted by Tom Wienholt (Member # 1510) on 03-10-2010, 06:59 PM:

Ok thanks. So how about setting the SPL's after eq-ing? See my post above.

Posted by Sean McKinnon (Member # 612) on 03-11-2010, 11:27 AM:

You will set the SPL's by adjusting the channel output's of the processor. Once you set the amp's for as flat a response as possible (before any eq) you must leave them alone and only use the processor controls to set SPL.

One added thing when you are doing that first step to get a flat response between the LF and HF make sure you leave yourself enough gain on the amps to reach a proper SPL later. Dont set the amps for only a qaurter of the way up on gain with a flat response set them say 3/4 of the way up with a flat response so that later on you will have enough gain to reach a proper SPL using only the processor's channel controls.

Posted by Tom Wienholt (Member # 1510) on 03-11-2010, 02:38 PM:

That is what I suspected. Thanks for the help.

Posted by Marco Giustini (Member # 4544) on 03-11-2010, 04:03 PM:

You can still optimize S/N ratio using amps, the problem is that it is much easier to do with just one trimmer for each amp. Set the processor for the best level, then set the amp for the best EQ *AND* the right level.

After EQ you will find that level needs to be adjusted. You can now compromise a little bit using the processor level (it won't be so bad since it adjusted previously) or play with the amps' gain to increase/lower the HF/LF SPL without modifying the curve. The amps gain will move the whole LF and HF sections so you are not changing the curve's shape in fact. It wouldn't be impossible to achieve what you're looking for.
It would be harder if the amps have not "free to move" trimmer, without clicks (sorry, can't think of anything else to explain!)

Thanks

Posted by Tony Bandiera Jr (Member # 2365) on 03-11-2010, 06:12 PM:

quote:
...or play with the amps' gain to increase/lower the HF/LF SPL without modifying the curve.

Sorry Mario, that is NOT true!! The levels of the HF and LF sections are independent of each other, and they do not respond in the same linear fashion (ie, going down say 1/4 turn on HF will NOT have the same effect as going down 1/4 turn on the LF.)

In a passive system, of course the level of the amp will respond in a linear fashion over the full range.

Once the HF/LF levels are set at the amp, LEAVE THEM ALONE!! All level changes for that channel need to be done by the processor's output level adjustments.

Don't overthink this folks...it will only cause you to mess up the desired results.

[Big Grin]

Posted by Gordon McLeod (Member # 33) on 03-11-2010, 09:39 PM:

Actually the best way is set the output of the processor (assuming a cp200) at 300mv and then set all the gains on the amps
that is the "right" way to do it
The SN is best optimised that way

Posted by Tony Bandiera Jr (Member # 2365) on 03-12-2010, 11:41 AM:

True Gordon, but depending on the amp/speaker combination that doesn't leave enough gain in larger auditoria.

And in Tom's situation we're trying to get him through this with minimal complications. Hopefully he will heed my advice to hire in a tech to help him out. [Big Grin]

Posted by Sean McKinnon (Member # 612) on 03-12-2010, 12:01 PM:

quote: Gordon McLeod
Actually the best way is set the output of the processor (assuming a cp200) at 300mv and then set all the gains on the amps

One of my biggest annoyances is seeing almost every theatre I have ever been in set-up with the amplifier gains "wide open" and the processor outputs way low. I think it adds a terrible hiss to the system. I totally agree with Gordon on this one.

Posted by Marco Giustini (Member # 4544) on 03-12-2010, 03:02 PM:

That's ok Tony but increasing or decreasing the gains on the amps will move the HF or LF curves without changing their shape.

HF and LF sections will be completely independent of course, but you can still play with them after the EQ to raise the entire channel's level without changing the shape of the response. As you said to increase channel's level of, say, 2dB you could need 2 "clicks" for LF and just one for HF. But the final curve will just be translated, moved to a new position.

The only issue are when your amps have trimmers with fixed positions, like QSC. After the EQ sometimes you can't move LF and HF of the same amount, and this is where some compromise may be taken into consideration! But just a little bit, if you end up with processor's levels at 100mV there's something wrong!

Marco (not Mario!)

Posted by Gordon McLeod (Member # 33) on 03-12-2010, 04:21 PM:

Proper amp sizing to match room volume and speaker efficiency is part of the engineering work one needs to do
The best way is to set the processors gain at the best for its noise floor (THX and DOlby usually say 300MV) then with the analyzer set up raise the low frequency till it is on the 85db line of your analyzer and then raise the HF to match and there should be a very small notch at the crossover point depending on how accurate time alignment was

Posted by Tom Wienholt (Member # 1510) on 03-12-2010, 08:11 PM:

Tony,

I appreciate your help and suggestions. And I am not insulted by your suggestion to seek the help of a technician. However, I do have significant experience installing cinema sounds systems. Like I said before, I am just new to bi-amping. In addition, most people seem to have different opinions/methods for all aspects of cinema sound system design and install. This is obvious just from the differing responses on this thread alone. Basically, from past experiences on this site, I like to be thorough and very clear as to what the best procedures are for certain tasks since there are usually several different suggestions made. Once again, I appreciate all of the help/suggestions.

Posted by Richard B. Perrine (Member # 409) on 03-20-2010, 10:05 AM:

Since you've got this topic going......We have a CP-500 that is fed into the USL 3 channel bi-amp frame (forget the model #)....with 125HZ cross-over modules.
Problem is when we play an academy mono print....all sound goes to the horn.
I wanted to change to the the 500HZ XTA-1 modules to correct this.
Would the module built for the QSC 1400 work in the 3 channel frame.....or are they wired differently ?
RBPerrine@att.net

Posted by Tony Bandiera Jr (Member # 2365) on 03-20-2010, 01:44 PM:

Yes Richard, the modules are exactly the same and will work just fine in that frame.

But..changing from 125hz crossovers (and what oddball speaker requires bi-amp at 125hz?) to 500 hz crossovers means you WILL have to do a B-Chain Eq and level reset as it will radically alter the system's output.

You may find some serious issues with response as well, as it seems that the horn is supposed to take all of the "mid-lows" away from the LF driver, which may not have enough output above the 125hz crossover point...if that is even the correct crossover point for those speakers.

It would be helpful if you could post the make and model of your speakers..

It would not be a "plug and play" type of thing.

If all the sound is coming from only the horn in mono it sounds like there is some other problem happening, as IIRC even Academy mono has some content below 125 hz...

Posted by Clint Koch (Member # 1179) on 03-21-2010, 12:02 AM:

USL has two standard frequencies, 500Hz and 800Hz. We will modify them for any frequency the customer ssks for. Like Tony said, 125Hz? Odd number. Do you know who modified them? USL? When were they modified?

Posted by Gordon McLeod (Member # 33) on 03-21-2010, 12:18 PM:

What stage speakers are you sure it isn't 1.25KHz

Posted by Tony Bandiera Jr (Member # 2365) on 03-21-2010, 02:33 PM:

Good pint Gordon..and if the current xover is 1.25k and you change it to 500hz you run the risk of blowing out the HF drivers.

Hi Clint!! [Big Grin]

Those encoders I mentioned a while ago are still going strong... [Smile]

Posted by Richard B. Perrine (Member # 409) on 03-22-2010, 01:20 AM:

Ok Been away for a few days here....
I don't remember the name of the speakers....if this helps..
the system was installed in 2000....The Speaker line was sold to Big sky a few years later....we got no documentation on it at all.
I started to look into this problem when we ran a mono print of Woodstock and it sounded like a ballpark stadium.

The amp rack is duel bi-amped...
CP-500 to the ULS bi-amp EQ frame.....then LCR have 2ea QSC Mx 1500A amps in stereo mode...one complete amp for the highs....one for the lows.
Behind the screen are low, mid and high cabinets with another cross-over unit.
So...for the left channel there are 4 pairs of #12 running to the speaker from the amps.
The speaker "name" was something like SRT model 3500...it's buried in my file at the theater and actually when you look at the units behind the screen they are componets from severl different Mfg's.

Here's the simple thing.....when running a mono print...the signal present light never flashes on the bass amps. That's out of this ULS Cross-over. The modules do have a 125 hz sticker on them and are XTA-1.
If it was a 1.25Khz....there would probably be some drive on the bass amp.
Doly digital and SR sound fine.

I've also taken a couple of reels of mono prints that I have and run them in mono, SR, and bypass with no perceptable change.

I beleive it's the 100hz rolloff and the 125HZ cross-over frequency on the modules that's giving me the problem.

To answer a couple of questions you asked me.....the systems was installed new in 2000 and spec'd by then NCS cincinnati by Mitch with him and John Harvey installing it.
We did discuss this with Mitch after running several mono prints and he did think that changing the X-over would blow the speakers.
I did call Big Sky once and talk to them..They said the X-over frequency was almost a matter of personal taste.

I do have 2 XTA-1 modules 500Hz marked QSC 1400 only....so I'm wondering if they will work for a test.

i'd like to hear more on the possiblity of blowing the speakers before I try just running a test.

RBPerrine@att.net

Posted by Louis Bornwasser (Member # 3063) on 03-22-2010, 05:05 AM:

Change crossover to a more suitable frequency and replace stage speakers with "normal" speakers (like JBL), retune and get normal results. Louis

Posted by Kenneth Wuepper (Member # 1174) on 03-22-2010, 07:34 AM:

TOM,

Let's take a more scientific approach to this problem.

Did the system EVER sound right?

If it did, then what has changed?

Without proper equipment and knowledge of the basic engineering involved, you would be well advised to seek competent help with this problem.

This sounds like a classic case of out phased speaker systems. The more discrete the signals to the speakers are, the less the effect of the out-phasing will appear. Thus the DD and SR formats will show the least effect from the problem.

To begin you must have a known good and stable signal source, like the Dolby Pink Noise source. Then you will need a Real Time Analizer to see the results. Since there are many places in this system for phasing errors to occur, one should start with the alignment of the drivers in each of the speaker arrays. There is a good description of how to do this in the Altec notes on the old Altec site. Then you can proceed to the phasing between the units with the amplifiers and speaker cabling.

You mention tri-amplified and then say there are 4 pairs of wires to each stage speaker stack. Please elaborate on where these wires are connected. Tri-amplification requires 3 pair, one for each bandpass speaker. Also you state there are crossovers in the speaker arrays as well. Where in the system are these placed? Are these merely high or low pass filters to protect the drivers of that particular range?

Well, without seeing the equipment or hearing the results of these tests, I will await your response.

Sincerely
KEN [thumbsup]

Posted by Gordon McLeod (Member # 33) on 03-22-2010, 09:36 AM:

it sound that in fact you have a 3way system on your screen channels and 125hz would be probable for the bottom end speaker If memory serves bigsky used the ribbon drivers for the high end

Posted by Ron Lacheur (Member # 1126) on 03-24-2010, 03:42 PM:

Just a thought,

What is your mono trim and EQ set at on the CP500?

Posted by Robert Throop (Member # 61) on 03-27-2010, 07:27 AM:

quote: Tony Bandiera Jr
Good pint Gordon

Gordon loves a good pint! [beer]

javascript:void(0)

Posted by Mark Gulbrandsen (Member # 72) on 03-27-2010, 08:02 AM:

Dang RObert... you beat me to it!

Louis is 100% correct! You may as well have the "Horizon" brand name on those speakers!