CP750 Calibration and sound issues

[Michael Richardson]

Hi there,

I work at an smaller theatre, and last week we had an issue that caused a bunch of speakers to blow, that has since sent me on a quest to find and fix our audio issues. We have 6 rooms total, this is what we are using:

5.1 Sound
Christie CP2210s
Dolby CP750s
Doremi Showvaults
QSC 900 amps
JBL 8330a surround speakers
Not sure what the front speakers are, I have to take down the screen to access them
USL XTM-300 crossovers (Only for front and center channels)
Powered subs
Audio analyzers(I can check the models but these shouldnt be the issue)

So far, I have created test DCPs that just play the same audio, one channel at a time. Doing this, I discovered several issues, such as:

Broken terminals on front channel horns

Surround speakers popping, also they dont play anything. Looking into it, its an issue at or before the CP750 as the audio visualizer on the 750 doesnt show anything ever playing there

Center channel horn intermediately stops working

Super quiet sub

One side/channel significantly louder than the opposing

These issues arent all in the same rooms, but happen in at least one or more of the rooms. Looking into it, I noticed the amps the JBL 8330as are plugged into, dont have any crossover settings, they're just all disabled. I see on the XTM-300 crossovers they have info on what settings to use on the amps, but these arent set either.

The setup is currently Showvault to Christie. Cristie has 2 ethernet cables to CP750s. The CP750 has outputs that go to an analyzer. The analyzer has a standard THX input, as well as pre-amp inputs, and outputs meant to go to the amps. Instead they are wired so that the input blocks have the wires that go from the Main audio output on the CP750, using smaller individual wires that come out occasionally to those input blocks, and then the wires that go to the amps are also sharing that input block, so for example I could disconnect the anaylzer and everything else would still be connected. So after the analyzer, they go to the amps. They are all set with the gain maxed. They seem to be wired correctly

I would like to ideally fix these issues. My current plan is disconnecting amps and measuring the impedance of the speakers at the amp, looking for differences. If anythings wonky, take and test individual speakers. Then connect to the amps through just a 1/4 cable to a PC and see how they sound, starting at a lower volume just to see if there are any differences there. Then set the gain on the amps correctly( Not sure how to do this, please advise if you have any insight) hopefully to make any popping or anything like that less noticable. Setup the built in crossovers on the amps. Get a calibrated mic, and rerun the alignment on the CP750s. I'm open to suggestions on mics to use or anything like that.

On the audio issue where we dont have audio coming from the surround speakers, I have no thoughts. The audio setup in the showvault is the same to everything else, the only thing I can think of is that the ethernet cable or connectors has been damaged, so I was going to replace those. I have a ton of spare parts from where we were a 10 plex but downsized and havent liquadated any spare parts yet. If you have ANY thoughts please let me know. We were abandoned by our initial integrator, and I'd like to see if I can fix anything before we spend 10k having someone come out to fix.

I dont want to know how long these issues have been around, most likely since we went digital 10 years ago, and that honestly scares me. No one ever really noticed or commented on any issues, besides the thing that started my crusade. Kinda scary tbh

[John Thomas]

Normally I'd immediately go to "you need a tech" for this many issues, but you sound fairly capable.


Some advice:

Simplest thing first, likeliest thing next. I'd start with sweeping amplifier gain pots. You may find dead or dirty spots that are the cause of a lot of the issues you describe. Careful to return them to where they should be for correct balance, and don't blow your speakers in the process.

Make troubleshooting moves where you're looking forward or backward in the audio chain in order to sus out the exact failure point, or rule out what hasn't failed. For example, you can temporarily swap XLR's or speaker lines at the amp to compare channels. Depending on what you swap, you can find out if the problem is in a speaker, in an amp, or before the amp. You're looking for insights like "the problem moved when I did this, so it's not that" or "the problem didn't move when I did this, so the problem is that."

Label everything if it isn't already so you don't mix yourself up when you're making troubleshooting swaps.


When it comes to actually tuning the room... you should call a tech. But be as friendly and accommodating as you can while learning as much as you can from them while they're there.


Some notes, not to pursue immediately but possibly keep in mind:

QSC USA's use non-sealed pots, so a good amount of your issues may just be dirty-pot related.

JBL 8330A's don't need a crossover, as they're "full range." They do have a passive crossover inside that can on occasion get blown, but I've seen far more blown drivers than crossovers in these. A 50hz highpass on them at the amp is fine, if the amp supports it.

CP750's can exhibit a level-dropping issue where individual channels just get quieter over time. Typically about 1-2 dB in a year in my experience. Make sure your 750's have enough space in the rack for ventilation.

Assuming JBL's behind the screen, the plastic on those spring-loaded thumb terminals can degrade and fail. You can get replacements, or as a quick-n-dirty just drive a self-tapper through the hole and now it's a screw terminal.



Good on ya for pursuing these issues in your multiplex. The multiplex world needs more people like you to fight the good fight, despite what audiences are often willing to sit through.

[Carsten Kurz]

When we went digital, we noticed during the install that quite a few low frequency drivers in our 8330s did not work, or created noise only at very high levels. We had no idea since when this had been going on. We replaced all these drivers with a suitable non-JBL replacement, and use these speakers until today. It was a bit of an issue to test the individual surround speakers. I finally went around on a ladder, disconnected all speaker wires, and played test signals from my iphone through a simple amp and long cheap wiring in the auditorium, because I needed to apply realistic levels to trigger the fault. Pink noise and sinus sweeps were the most efficient. I recommend https://studiosixdigital.com

We have a reoccurring issue with our two way JBL front systems that usually develops every couple of years when we play a very loud movie for a week or more. A capacitor in the passive crossover comes loose and disconnects HF temporarily. It took me a while to identify the component, and now I can repair it in very little time by resoldering without even taking out the crossover from the speaker. Before, I was suspecting loose cable connections, blown horn drivers, etc. The issue showed on all three front systems over the course of 10 years or so. It was hard to notice when it happened on L or R, but immediately audible when on Center, since dialog sounded very muffled.



- Carsten

[Marcel Birgelen]

On the audio issue where we dont have audio coming from the surround speakers, I have no thoughts. The audio setup in the showvault is the same to everything else, the only thing I can think of is that the ethernet cable or connectors has been damaged, so I was going to replace those.

Swapping UTP cables or DB25 connector is an easy and cheap start obviously. Still, if you suspect the AES connection between audio processor and server, it would still be a pretty big coincidence that just the left and back surround channels have been hit.

Nothing but occasional popping sounds would make me believe there is an amplifier problem. I'm not exactly sure how the visualizer on the 750 behaves. If there is e.g. a short circuit on the output of the channel, it will disable the channel to protect the preamp circuitry. I'm not sure if it still shows visual data for those channels affected.

How are your surrounds hooked up to your QSC900s?

[Marco Giustini]

While 8330 are full-range speakers, the 8330 are fairly low power and cannot handle anything lower than, say, 80Hz. It's good practice to enable a high pass filter if you have one. It will save the speakers and make the sound cleaner.

A high pass filter is recommended mostly all the time IMHO. Some soundtracks carry silly low frequencies where there really shouldn't be any: if your front speakers can't handle anything below 40Hz, then add a 40 (or maybe 30) HPF. You want the LFE to get as deep as possible but only if it's adequate for the room - which is rare. If the sub is struggling, then I found that adding a 30Hz HPF will relieve the speaker, make the sound cleaner and punchier.

If one of your AES is missing, then you should be missing two channels at the time as each AES carry two channels.

To test channels I do recommend a "channel ID" DCP. There will be a voice identifying channels one by one - that will do what your DCP is but also making sure your channels are routed correctly.

Finally, bear in mind that the 750 "level disease" is pretty common to find: on affected units some channels (or all of them) will be much lower and even non-linear anymore.

[Marcel Birgelen]

Finally, bear in mind that the 750 "level disease" is pretty common to find: on affected units some channels (or all of them) will be much lower and even non-linear anymore.

The 750 level disease is a good one to account for, although I've never experienced it with popping sounds in the affected channels. But, those popping sounds could also be a different thing.

Potential compounding issues are always a joy to analyze.

[Marco Giustini]

I think you are correct; I wasn't associating that to the popping noise, just a general consideration (maybe for the "super quiet sub" comment).