atcomsystems.ca/forum Forums Wiring - Telecom Carriers - Trunks & Services Security, Alarms and Paging 70v BGM system

I got called in to a job site to resolve a problem with a back ground music system.

Of course this problem started with the electrician. They used standard 8 ohm speakers and volume controls, amplifier with no impedance matching control. When this didn’t work they brought in a “home audio expert”. He quickly realized this is a commercial application so he changed the volume controls to 70v attenuators and the amplifier to 70v line. This appeared to fix the problem except occasionally there is a distortion.

Okay, so this is where I come in. I listen to the music and realize the distortion is on the peak of the bass. So I go and look at the amplifier, see that it is 70v with a bunch of home runs wire nutted together. So I figure I’ll check the taps on the speakers add them up to see if they are exceeding the 30 watt rating of the amp. Well to my surprise there are no taps on the speakers, they are plain old 8ohm speakers. I was really surprised that this amp. is still running at all with 18 speaker.

So now my plan is to install taps on all of the speakers and set them for 1 watt. This should put an 18 watt load on the 30 watt amplifier. This should be good.

My only questionable thought is: Are the 70v volume controls providing the proper load to the amplifier by themselves? Will installing the taps on the speakers cure the trouble?

I’ve always used and understood that you use 70v amp, 70v volume controls (when needed) and 70v taps set to work with the amplifiers rating. I've never had a problem using this foumula but I've never tried to use only 70v volume controls with no taps.

I think the taps will cure it.
I am too.

I do not think that having 70V volume controls without the appropriate speakers would do anything. I had an 8 Ohm amp hooked up to a regular speaker and a 35 watt volume control and even at the highest setting I did not get a whole lot of volume.

I don't beieve it's possible for 8ohm speakers to work on a 70volt line. It's possible that there's an 8 ohm-70v transformer (or more than one) someplace in the cable run. Transformers should be mounted on the speaker, but I've seen crazier things before.

Sam

Electrically, the 8 ohm speakers could work on a 70 volt line without 70V to 8 ohm transformer, but the distortion should have been worse. Only thing I can think is those attenuators are protecting everything. If you put in the transformers, and tap down to 1-2 watts, you can, and probably will have to, pull the attenuators. Do 1 drop at a time, you'll probably find other oddities. Like several 8 ohm speakers in series, or parallel, etc. I'm glad it's you and not me! John C. (Not Garand)

I am just wondering, would 70 volts AC not damage a straight 8 ohm speaker?

I don't beieve it's possible for 8ohm speakers to work on a 70volt line.

I am just wondering, would 70 volts AC not damage a straight 8 ohm speaker?

Well do the math and tell me haw many watts an 8 ohm load is going to try to pull with 70 volts across it. Hint- power is cheap these days and amplifiers in excess of 612 watts are a dime a dozen.

I remember hearing about an old trick to find those "rogue" speakers- those that somebody added to a system by connecting them directly to the line without a line transformer. Simply disconnect the feed from the amplifier output and put 120 volts on it for a few seconds. The speakers with transformers will be unharmed but those 8 ohm voice coils will never bother you again.

SST, I would check those attenuators that the idiot supplied. Atls AT series (or equal)autoformer type attenuators are what you should be using. Since this is a music system I would remember that crap speakers and crap transformers equal crap music. You want big iron in the transformers and quality speakers. Don't expect to do your shopping at Radio Shack, MG Electronics and others.

Distributed systems always get a bad rap for anything other than paging. I'm here to tell you that they can sound just as good as the stuff sold by "home audio experts" for many times the cost.

-Hal

Hal, I am not sure what you mean/how you got 612 watts?
What I meant was something like your taking 120 volts and going right into a speaker, it would kill it...

Put the transformers on all the speakers.
You may get slightly better quality by stepping the Amp output down to 25 volt which it may or may not have.
As for 25 or 70 volt working on 8 ohm speakers,
I see this all the time from DIYs. :rofl:
MrG

You may get slightly better quality by stepping the Amp output down to 25 volt which it may or may not have.

Not true. It all depends on the quality of the transformers then the speakers. The more iron in the core of the transformer the lower the frequency at which it will start to saturate. So heavy transformers will have a better low frequency response. Quality manufacturers like Quam and Atlas will spec the frequency response of their transformers.

Hal, I am not sure what you mean/how you got 612 watts?

Jeff, Jeff. Have you been sleeping in math class again? No more Tech Talk on school nights!

P= E squared over R

P= power in watts
E= voltage
R= resistance in ohms

-Hal

Add a transformer to each speaker and leave the attenuators for zone volume adjustment. Make sure the amp is off or speakers disconnected as once you are down to the last couple the amp could damage the voice coils still connected directly

No Hal, I was sleeping in electronics class in high school

Hal -

Please correct me if I'm wrong, but 18, 8 ohm speakers connected in parallel should be as close as possible to a dead short. Therefore P=5000/.00001 (or something similar) would have to be one hell of an amp.

I have seen one rogue speaker in a run, and you're right it screwed things up monumentally. But 18!

Unless I'm missing something very basic.

Maybe they're wired in some series-parallel layout?

Sam

Sam, that's why I made the comment about speakers in series. I'm guessing 4-5 speakers in series on several runs. You'd get a reasonably high impedance, but the low end would probably be 'mushy'. The cheaper the speakers, the more like 'oatmeal'! Wait!, that was part of the problem! Oh, yeah. What Merritt said, except turn the amp OFF except when you are testing, or have eveything finished. It IS a 70 volt output and it WILL hurt if some idiot pages while you are fiddling with connections. The surprise will knock you off the ladder! John C. (Not Garand)

Please correct me if I'm wrong, but 18, 8 ohm speakers connected in parallel should be as close as possible to a dead short. Therefore P=5000/.00001 (or something similar) would have to be one hell of an amp.

0.44 ohms @70.7 volts is 11,246 watts actually. If you put that load on the 4 ohm output it will do something however.

No, I'm sure that the speakers are not in series. If they really were on the 70 volt output most likely the volume controls used presented a high enough impedance to the amp at least when they were turned down. A series resistance with the speakers connected to them actually. The amp volume had to be all the way up. I think if you turned all those volume controls up the amp would go into protection.

-Hal

I didn't go through all of the wiring or actually even count how many home runs only the speakers. But I was told all speakers were home runs and it looked like it. A 2" diameter bundle of overly tie-rapped speaker wire with two big 'ol wire nuts to a single lead to the amps 70v output.

The volume is kept very low, this maybe why the amp hasn't produced a layer 1 smoke application.

Betcha it's zip cord or Radio Shack speaker wire or Monster Cable. Those are not permitted to be used as wiring within buildings. These are ceiling speakers?

Amps are pretty well protected unless it's some piece of junk. They will just go into protection rather than go up in smoke.

-Hal

Amps tapped at 70 volts usually go into protection. Amps tapped at 25 volts just cook.
Of course, you knew that, because the volume setting was higher than your marks and the top of the amp was hot enough to fry an egg.

The K-Mart employees used to try adding 8 ohm speakers. You just went to Automotive, Sporting Goods, or receiving and asked the most likely looking dude "Who added speakers?"

Sometimes though, the problem was in the grid because the careless installer, usually me, forgot to carefully trim the tails and they were touching the hat or tile bridge and grounded to the ceiling grid :-)

However, some extra advice is don't let a colorblind person wire speakers. Violet and Black must look the same or the guy is not very smart.

Carl

I agree with Sam, a 8 ohm speaker would short out system, and the volume would be rubbish must have some transformer to remove the 70v or the speaker would lock up. It sounds like the speakers just cant handle the frequency range and you could be going to a lot of trouble for nothing, Try putting on one good 75v 1-2 watt speaker and see what it is like.You might have to replace them all

Ken

When I see ceiling speakers done by the clueless they are usually from Radio Shack or equal. They are the type that you cut a 6 1/2 inch hole for and pop them in. So if you are going to add a line transformer what are you going to do, throw it up in the hole before you put the speaker back? There is no way to mount it to the speaker.

I agree, if this is anything like this I would be replacing the speakers with JBL or TOA that have the transformers built in and will fit the existing hole. Been there, done that.

-Hal

If you've only cut a 6 1/2" hole in the tile, then it's gonna be hard to fit the Quams in the space..it might be time to try those Bogen ones that you just pop into the tile.

I love your idea of the 110v AC on the grid to pop out bad speakers, Hal. I don't think I have enough nerve to do it, but it might be fun to try.

Carl

Yeah, I'd be afraid of the sparks setting fire to all the garbage and dust up there.

Regular 8 inch speakers and baffels need a larger hole, you can just make the hole bigger. Even with a quality speaker like the Quam 8C10FEPAX and TBH70 transformer they are about half the cost of something like the JBL Control 24CT (that pops in a 6 1/2 inch hole) and can sound very good. But the nice thing about the JBL is that you are getting a "known quantity" in that they are a system and will sound the same regardless of how you install them. Speakers and baffels can lose a lot on the low end if you install them with a small back can.

-Hal

I noticed that our systems here use 25V and not 70V. Is this so they get around having to run conduit?

Jeff -

If your local codes require conduit for circuits >50 Volts, then yes. That's what they were designed for.

Sam

That's how the 25 volt system came into being. Some municipalities still insist on it based on their code that was probably written 75 years ago.

This actually goes against the NEC because pulling a low voltage speaker wire into conduit is not a class 1 wiring method which is what I assume they are after.

-Hal

But with 25V you would need to use larger wire to cover a longer distance, right? What about the tap settings? Would they be the same as on a 70V system?

Larger wire generally yes, depending on distance and wattage.

There are different transformers for 25 volt, 70.7 volt and 100 volt (yes there is a 100 volt system popular in Europe). Sometimes you will find "combo" transformers with the taps marked for one wattage at 25 volts and another wattage at 70.7. I know that most speakers that have the transformer built in, like the JBL Control series give you a rotary switch with which to select the wattage taps. There are settings for all three voltage systems.

Except for those instances you mention, 25 volt systems are not really used.

-Hal

I used some excellent speakers on the last big job - Tannoy - a British company. They came with built in transformers that offered taps for 25, 70 & 100 volt taps (I've been told that in Europe they also have 200 volt speaker systems, but haven't seen them myself).

Some of the taps offered double duty - x watts if used at 70 volts, y if used at 100, etc.

One tap was 15 Watts at 70 volts and only 2 watts at 25!

Sam

Some of the Quam transformers list the 25 and 70 volt tap values, and the Bogen trumpets do also.

They are sort of inverted. In the Bogen, there are some settings that are invalid for 25 volt or invalid for 70v sources.

Carl

a transformer can be used on any voltage as long as you don't over power the amp or speaker. The power difference between 25 and 70 volt is about 8 times the difference. a transformer marked at 1 watt 70 volt would give you about 1/8 watt at 25 volt.

here is some info
and some more

Merritt, I think that is what had confused me...so you can use the same transformers but it would just give out less power.

Ok, I think you are confused because the constant voltage system was designed to make it easy to connect many speakers to an amp without having to work with impedances or calculate wattages. So you don't get to see what is actually going on, only what the markings on the transformers tell you.

The concept is really quite simple. You know that you are limited in the number of speakers that you can connect to the (nominal) 8 ohm output of an amp. Most amps will safely drive a 4 ohm load, below that you will cause problems. So we are talking two 8 ohm speakers, four if you happen to have 16 ohm ones. Keep in mind also that the total output power will be equally divided between the speakers as long their impedances are equal. With two 8 ohm speakers each will receive one half the output power of the amp.

Somebodies going to say you're talking about putting the speakers in parallel. Why not put them in series or a series-parallel arrangement? The main reason that's not done is because it would be nearly impossible to troubleshoot an installation or to add or remove speakers without rearranging everything. Another reason not to is if the speakers contain any kind of passive crossover network you will have all kinds of frequency response problems.

So we can see that the problem here is that the output impedance of the amplifier is too low and the speaker impedances are too low. So how about we make the amplifier output impedance higher and make the speaker impedance even higher? You can't do too much in the way of changing a speaker voice coil so how about connecting a transformer to the 8 ohm voice coil? Now you can make it into any impedance you want depending on the turns ratio of the transformer. We can even have taps so that a range of impedances can be selected.


Now lets say we have a 100 watt amplifier with an output impedance of 50 ohms.
>One 50 ohm speaker will receive the full 100 watts.
>Two 100 ohm speakers will each receive 50 watts.
>Up to one hundred 5,000 ohm speakers will each receive 1 watt.
>10,000 ohms= 1/2 watt- you get the idea. Mix and match for the wattage you want but don't exceed 50 ohms.

This works great and was actually the way it was done before the "constant voltage system" was thought of. Problem with this is you have to keep track of impedances by doing what can be complicated parallel calculations when you design a system with varying speaker wattages. More importantly, what happens when you replace the amp with say a 200 watter? Now every speaker gets twice the power. Not good.

Ok, how about we change the amplifier output impedance according to it's power? We'll keep the constant of 5000 ohms/1 watt so nothing on the speaker side changes and we can just label our transformer taps with wattages. So our 100 watt amp has an output impedance of 50 ohms, a 200 watt amp will be 25 ohms, 400 watts will be 12.5 ohms, 800 watts will be 6.25 ohms. (Surprise, 612 watts is 8 ohms.)

You can see that our constant of 5K/watt works out pretty well up to about 1200 watts of amplifier power where we get down to that 4 ohm limit again.

You might also note that if you calculate the voltage necessary to cause 1 watt of power to flow through 5000 ohms it will be 70.7 volts. It follows then that for any of those impedance/wattage calculations above based on 5K/watt, the voltage will always be 70.7. The amplifier output voltage will always be 70.7 volts when operating at it's maximum rated output. Hence the constant voltage system.

70.7 volts was chosen because (again) there are codes that require any wiring with a peak voltage of 100 volts to be run in conduit. 70.7 is the RMS value of 100 VAC peak.

To acheive the 25 volts of the 25 volt system the constant is 625 ohms/watt. That means that only a 100 watt amp already has an output impedance of 6.25 ohms which limits the size of 25 volt systems.

Merritt is right on, 5000/625 is 8 to 1.

-Hal

Thanks Hal, I think I understand now.
PS Should this be moved to the sound/security section?