Connecting to a "house system"

[Dick Rees]

I just had a not so interesting problem crop up while providing sound for a conference. I was to provide a small PA for a set of entertainment and use the house system (speakers in ceiling on several zones) for the keynote address and Q&A with a four person panel afterward. We also had a second "cafe" entertainment area and two break-out rooms with SOS systems.

We loaded in and set up on Friday, everything testing out 100%. The connection to the house system was via an XLR mic input in the wall behind the stage. This ran into the house system which was configured:

______ < SCM810 auto mixer zone 1 < Crown amp
Splitter <
______ < SCM810 auto mixer zone 2 < Crown amp

Zone 1 also had a consumer grade "volume control", Zone 2 did not. Each zone had four sub-divisions on on/off toggle switches.

My feed was:

Four condenser mics, four dynamic mics and four wireless mics into my MixWiz 14:4:2 < GraphiQ < transformer iso box < Shure line pad set to -15dB < house mic input. The transformer iso was there to separate the house system and my PA as well as to make sure that the phantom 48v coming from the SCM810 wouldn't mess with my Wiz.

Everything was just fine on Friday, client signed off on all aspects of the setup. Saturday show time......house system is fuzzed out in Zone 1 (the one with the volume control in line) and Zone 2 was weak. We ran the keynote address and follow up session through the entertainment PA and it was OK. After the keynote address I went in and did the "twist the pot back and forth" a bunch on the Zone 1 volume knob, brought the feed to the house back up at my board and the fuzz was gone, both Zones 1&2 putting out the same quality of sound but at limited volume.

The LED meters on the 810's always showed optimal levels, one yellow LED at the most. Amps showed "signal present" and absolutely no indication of clip or overload. After the Q&A I shut the house system down and checked all the cables between the output from my board and the wall jack. I took a small sub-mixer, ran the signal into it and listened on the headphones. Everything was clean going into the wall.

I ASSume that the problem was somewhere in the house system. The new owners of the building had had no other large events in there prior to this, so were unfamiliar with the system.

Now here's the thing I can't figure out. After everything was done I returned everything to the original configuration with a single condenser gooseneck mic on the lectern, powered up the system and everything was back to 100% in every way.

Can anyone identify the possible failure points in all this? Keep in mind that everything worked fine for several hours on setup day, glitched out on show day, then went back to apparent normal at the end of day 2.

Where were the gremlins?

 

[Charlie Zureki]

Re: Connecting to a "house system"

Hello Dick,

I think that the 70v system had a speaker or speakers miss-wired.....using the 4 or 8 ohm terminals on the speaker(s) instead of the appropriate terminals. I've had the exact same thing "fuzzy" thing happen in a theatre's 70v system for the dressing room and office speakers.

And... it seems that the volume control was incorrectly wired (building up a rather large capactiance (impedance change) or just plain intermittant (with the level drop)

Good Luck,
Hammer

ps...if you're feeling up to repairing their system... change out the volume control regardless, and add another local control for the other zone, and then check all speakers in the system ...starting with the one in the control room, then any lower mounted....hallways and offices.... It's very common for the surface mount speakers to be REALLY jacked-up, as people love to play...

 

[Dick Rees]

Re: Connecting to a "house system"

Charlie.....

Those were some of the things that came to mind. I'm going in today to do exactly as you suggested in re the volume pot(s). The thing is it worked fine, then didn't, then did sort of, then didn't, then did again. The cheapo volume pot is my bet for Zone 1. As far as checking the wiring and more extensive revisions I'm going to refer it to a friend who has done a lot more of that work than I have.

The only other thing I can think of is that the room where the rack was located is not heated.....at least not to the level of the main rooms. The temperature dropped below zero for a couple of days here, the room was closed and locked after setup on Friday and opened early Saturday a couple of hours before show time. There is a possibility that some condensation may have occurred. That might also explain the capacitance/minor short thing. I couldn't get into the rack to pull connections to check that. The connectors on the 810's are barrier strip, but they're being fed from a splitter which just as likely has XLR ins and outs. I've found that a tiny speck of condensation in an XLR can cause similar problems.

Anyhow, it seems to be their problem ........ and mine only for that day.

Anybody else got ideas?