Cobbled PA system – Shure Bros speakers


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For parades and occasional shows, we have a PA system that has been cobbled together over the years. The people that put it together have moved on, and I'm trying to collect information on maintaining and using the system.

The amp is a Realistic MPA-30 20 watt amp. I was able to download a manual for that.

The speakers are Shure Brothers Vocal Master speakers, and in my quest for information on the internet, I suspect that these may be 20+ years old from a portable PA system made by Shure Brothers (before they became Shure Inc). (The speakers have handles for easy carrying.) On the back of each speaker are two unlabeled inputs for "large jacks" and a single volume dial. We run one speaker wire from the PA amp to one of the jack inputs on a speaker.

We can power the system from a vehicle using a Jensen 300 watt inverter, although on at least one occasion, that didn't work, but the PA amp has a factory-wired "cigarette lighter" jack as an alternative power source, and that worked fine.

We use an output from an ordinary "boom box" into the PA amp to provide the source of the music.

My questions are about the Shure Brothers Speakers. What are the two jack inputs for? What is the speaker wattage? Are the PA amp wattage and speaker wattage "mismatched"? How should the speaker volume dial be used in relation to the volume dial on the amp (for example, should the speaker dial be left at, say 5, and then use the amp dial exclusively?) (To say the least, I have no experience with sound systems.)

I'm also curious why the Jensen inverter didn't work, while the PA amp cigarette lighter jack did work. (Although I never tried testing the Jensen inverter using another piece of equipment – maybe the Jensen is just broken.)

The two jacks may be wired in parallel to allow "daisy chaining" of the speakers. If possible, I would remove the back panel and check this.

Whilst you are there, you may be able to see the speaker rating on the magnet or on a sticker inside the box somewhere. Impedance can be roughly worked out with a multimeter. An 8ohm speaker will show about 6ohms of resistance on a multimeter. However, this is not going to tell you the wattage.

The pot on the speakers is not something that I have had much experience with and it may either be volume or tone. If it were volume, I would be inclined to set it at 80-100% and drive the volume from the desk/amp. I think that they would be used more if you wanted to balance out the sound in an odd shaped room or one where you hade speakers in separate areas. Again – have not really seen these outside the home entertainment arena.

With the inverter not powering up the amp. As you have said, it may be the inverter but I also wonder if you have powered ti up via the mains supply? You may find that there is a ‘switch’ somewhere that selects either AC or DC inputs. Again, a bit of a wild guess here.

Given the age of these boxes, I would contact Shure and tell them that they are still going strong after all these years but you would just like to find as much information on them as you can. You never know, they may send you a new set and take these to display in their head office. Don’t laugh; I have seen this happen before.

Hope this helps.
Sounds like your running a house sterio system for a Personal address system. 20 could yell as loud, what are we looking at. 45Db at the most? lol j/k i think you should starting building up for a better system.

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