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Author Topic: USL XTA-1 Crossover Tom Wienholt Master Film Handler Posts: 371 From: Towson, MD, USA Registered: Dec 2002 posted 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.  |  IP: Logged Tony Bandiera Jr Film God Posts: 3067 From: Moreland Idaho Registered: Apr 2004 posted 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.  |  IP: Logged Tom Wienholt Master Film Handler Posts: 371 From: Towson, MD, USA Registered: Dec 2002 posted 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.  |  IP: Logged Aaron Sisemore Flaming Ribs beat Reeses Peanut Butter Cups any day! Posts: 3061 From: Rockwall TX USA Registered: Sep 1999 posted 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  |  IP: Logged Tom Wienholt Master Film Handler Posts: 371 From: Towson, MD, USA Registered: Dec 2002 posted 03-10-2010 06:59 PM                     Ok thanks. So how about setting the SPL's after eq-ing? See my post above.  |  IP: Logged Sean McKinnon Phenomenal Film Handler Posts: 1712 From: Peabody Massachusetts Registered: Sep 2000 posted 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.  |  IP: Logged Tom Wienholt Master Film Handler Posts: 371 From: Towson, MD, USA Registered: Dec 2002 posted 03-11-2010 02:38 PM                     That is what I suspected. Thanks for the help.  |  IP: Logged Marco Giustini Film God Posts: 2713 From: Reading, UK Registered: Nov 2007 posted 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  |  IP: Logged Tony Bandiera Jr Film God Posts: 3067 From: Moreland Idaho Registered: Apr 2004 posted 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.  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 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  |  IP: Logged Tony Bandiera Jr Film God Posts: 3067 From: Moreland Idaho Registered: Apr 2004 posted 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.  |  IP: Logged Sean McKinnon Phenomenal Film Handler Posts: 1712 From: Peabody Massachusetts Registered: Sep 2000 posted 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.  |  IP: Logged Marco Giustini Film God Posts: 2713 From: Reading, UK Registered: Nov 2007 posted 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!)  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 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  |  IP: Logged Tom Wienholt Master Film Handler Posts: 371 From: Towson, MD, USA Registered: Dec 2002 posted 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.  |  IP: Logged

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