Bad sound over public address system

[Krestent]

Hi,

I've recently been trying to fix up the speaker system for our community center. Nobody seems to know much about how the system was wired, who wired it, etc; it's been messed with a lot over the years. From investigating the wiring, I found that it's wired using a 70V constant voltage system, with about 35 speakers all connected using 5W taps. The main issue is that the sound quality just isn't great; it's hard to describe, but sounds like the volume is set too high, although they aren't very loud. The amplifier was a Bogen GS250, until it died (don't know why) fifteen minutes before an event, forcing me to swap in an ancient Bogen CT100C we had in the closet from our previous, demolished, building. The bad sound was present on both amps. I've currently disconnected 15 of the speakers as well.

Any idea what can be causing the bad sound?

Also, what would happen if I connected the rest of the speakers to the CT100C?

 

[jkowtko]

If the input signal to the amp is too hot I would think everything would sound like it's clipping. What is supplying the audio signal to the system?

 

[JD]

Regarding the loading on a 70v CV line:
Generally, the best loading is 1:1 meaning, if the amp is 250 watts, the speaker taps should add up to 250 watts. 35 speakers set at 5 watts comes to 175 watts. Although not a match, it should have worked fine. (gives a little room for expansion.)
As for the 100 watt amp, best to lower some of the speaker taps down to a lower tap so the total is around 100 watts. Even with the overload, it should not have produced as bad a sound as you were hearing.
So, here are some steps to help diagnose:
1) Try a single local speaker connected directly to the amp. If the sound is still distorted, then you may be clipping the front end of the amp with too hot a signal. If not-
2) Make sure the wiring to the 70 volt output is correct (com and 70v)
3) Check for additional phantom loading on the line and/or a shorted line.
4) Sequentially disconnect groups of speakers. 70v transformers do go bad and a shorted one will load the line.
5) Check for any step-type volume controls. Usually contain a small transformer and I've see those go bad.
6) Never assume they system is wired correctly! I did a repair at a school in Wildwood NJ where someone over the years had removed many of the 70v transformers from the speakers to make them louder! Result was an amp load of 0.1 ohms!

 

[FMEng]

All it takes to really screw up a 70V line system is one of a few things:

1. A 4 or 8 ohm speaker is hung across the line without a transformer.
2. A transformer was wired in backwards.
3. A transformer has shorted.

Any of those three will present the amplifier with a load that is nearly a dead short. The amplifier in distress will distort, or die like the original one. Test the amp by disconnecting the 70V line and listening with a known good 8 ohm speaker, driven from the 8 ohm tap. If that sounds good, then test a single speaker and transformer from the ceiling. That'll help determine if the problem is caused by horrid quality speakers, or not. Some old, distributed systems were built so badly and cheaply that it was impossible for them to sound good. If the single speaker sounds fine, then you'll need to inspect the entire system.

If you don't find a wiring error, then disconnect parts of the system until it works OK. If part of the system is found to be bad, then disconnect one speaker at a time in that section to identify a bad transformer.

Also, you want the total Wattage of all the taps to add up to less than the amplifier's rated Wattage. The reason is that the transformers have losses that are not accounted for. A transformer delivering 5 Watts to the speaker could be consuming 7 Watts from the amp. How much loss depends on the quality of the transformer. A rule of thumb is the total shouldn't be more than 70-80% of the amp's rating. On a system that big, the efficiency error could be substantial.

There are more elegant ways to locate the trouble with test equipment to measure impedances, but this is the poor man's method. Note that DC resistance and AC impedance are not the same thing, so you can't accurately measure the load impedance with a DMM. A DMM might help find a severe short, though.

 

[Krestent]

Thanks for the replies, I'll try your suggestions when I go back there this weekend.

The amplifier is driven by a bunch of microphones connected to the XLR microphone inputs on the back.

Also: How would I go about measuring the load on the speaker line? Would I just measure resistance or current with a multimeter?

 

[Morte615]

The amplifier is driven by a bunch of microphones connected to the XLR microphone inputs on the back.

Please elaborate on this. Are you sending music data (from a CD player or other device) through the microphone inputs on the amp? Or are you connecting more than one microphone to a single input on the back? If either of these are true that would cause problems right away.

If more than one microphone then you would need to run through a mixer. And if you are using a music device then you would need to change the impediance of such device, look for a Line Level to Mic Level adapter (my personal go to usually, http://www.sweetwater.com/store/detail/pcDI)

Other wise all the advice from above is a good place to start.

 

[Krestent]

Please elaborate on this. Are you sending music data (from a CD player or other device) through the microphone inputs on the amp? Or are you connecting more than one microphone to a single input on the back? If either of these are true that would cause problems right away.

If more than one microphone then you would need to run through a mixer. And if you are using a music device then you would need to change the impediance of such device, look for a Line Level to Mic Level adapter (my personal go to usually, http://www.sweetwater.com/store/detail/pcDI)

Other wise all the advice from above is a good place to start.

The amp is a mixer/amplifier, with 6 XLR inputs on the back; we're typically using 3 at once (all microphones)

 

[rwhealey]

FMEng has great suggestions, but this would be a simple service call for a competent contractor - I doubt it would cost very much to have someone come out and troubleshoot.