Watts and Ohms on Ceiling Speakers

[tedanderson]

I was working on a situation today that I think I am able to figure out but I would like some input and a few other brains thinking this through to see if I am in error.

Anyhow, I went to complete an installation that a previous vendor abandoned so the task at hand was to connect the system using what was there.

I am connecting an audio system in a building that has several 100W ceiling speakers (brand unknown) wired up throughout the facility and terminated in a closet. The speakers are similar to these speakers in terms of specs and so-forth and I have a set of Audio Source AMP-100 amplifiers to work with.

These are 2-channel amplifiers that are rated at 160 watts but it promises to deliver 100 watts RMS bridged or 50 watts a channel in stereo mode. This model amp also has 2 sets of outputs (A and B) making it possible to connect 4 8-ohm speakers to a single amp.

But one thing that I also noticed is that some of the rooms that have 4 speakers are daisy chained in pairs. 2 left and 2 right. No problem. In reading the instruction manual, the amplifier can hold a load as low as 4 ohms on each channel. The trade-off is that at 4 ohms, the amp delivers 75 watts RMS. It's still not a big deal. The system is only for background music and not for sound reinforcement.

This started becoming a dilemma for me when I realized that the client has slightly more speakers than amp capacity. So I figured that one way around this problem might be to install a 6 or 8 zone resistor based speaker switcher. Another idea that I had was to wire some of the pairs in series (giving me 16 ohms) so that when I parallel them back at the amp, I will have an 8-Ohm load that serves 4 speakers or perhaps a 4-Ohm load serving 8 speakers on a channel... but I don't want to get carried away either. Then the other solution I thought of was to wire in one of those ceramic 1/4 watt resistors to add some load potential to the speaker circuit without wiring them in series.

So here are my questions-

#1. Is it a bad idea to run 8 100-watt speakers on a 100-160 watt amp if I am satisfied with the volume level and sound quality?

#2. If my amp is maxed out according to what I describe in Question #1, is it a bad idea to add a speaker switcher to expand my capacity or to add resistors to the circuits to maintain a 4-8 Ohm load?

3. Generally speaking, is it better to have an amp that has more power than the speakers can handle if it is dialed back a little or is it better to have speakers that can handle more than what the amp can deliver?

4. Going back to my description of Question #1, am I in any danger of destroying the speakers or amplifiers in this seemingly mis-matched configuration?

I didn't want to press my luck so I figured that if I went with no more than 8 speakers per amp, my load would be right at 4 ohms and I would be fine for the time being. But I am not sure what the consequences would be (if any) if I manipulate the resistance to get more speakers in operation.

Any ideas?

[sempei13]

This is exactly the problem that I deal with at work. I'm most concerned about the crossovers on the speakers. Let your ears guide you. In series, if the speakers each have crossovers, you'll be sending the signal through two crossovers. It sounds like you wouldn't be using it bridged, but stereo at 4 ohms. So that means you'll 2 speakers per channel (if in parallel only) using up to 75 watts per channel. So, instead of one speaker using up to 50 watts, you'll have two using 37.5 watts each. In the same size room, that's more power, not less. Now if you both parallel and series for four speakers per channel, you'll have 50 watts divided between 4 speakers. It will give you more even coverage, but the same amount of power as one 8 ohm speaker. Of course, it could be that there'd be too much audio coming out of one speaker in one location at 100%, but the same amount coming out of 4 speakers spread around would be fine.

Paul

[Gracetech]

Originally Posted by tedanderson

#1. Is it a bad idea to run 8 100-watt speakers on a 100-160 watt amp if I am satisfied with the volume level and sound quality?

You can put as many speakers on an amp as you want as long as you keep the ohm load in a working limit. The problem becomes trouble shooting something if a wire gets cut or comes loose somewhere. Also 100 watts spread over 8 speakers is like 12.5 watts a piece and if these are low sensitivity speakers you will be running the amp wide open all the time just to get enough level.

Originally Posted by tedanderson

#2. If my amp is maxed out according to what I describe in Question #1, is it a bad idea to add a speaker switcher to expand my capacity or to add resistors to the circuits to maintain a 4-8 Ohm load?

usage of ohm taps is good to keep a healthy load. Speaker switchers seem to be counter productive when it comes to background music.

Originally Posted by tedanderson

3. Generally speaking, is it better to have an amp that has more power than the speakers can handle if it is dialed back a little or is it better to have speakers that can handle more than what the amp can deliver?

I always try to match or over shoot the power ratings of the speakers being used. You will have more of a chance of blowing speakers by running the amp into clip then to send full power to the speakers.

Originally Posted by tedanderson

4. Going back to my description of Question #1, am I in any danger of destroying the speakers or amplifiers in this seemingly mis-matched configuration?

This is always the worry. Unfortunately we generally aren't around when some overzealous DJ gets hold of the system and wrecks it. The only thing that is going to cause problems is pushing the amp into clip and sending over volt squared waves to the speaker.


I always recommended going 70 volt for ceiling speaker setups unless the amp is sitting in the same room as the speakers. The problem is that you are going to loose watts to distance/resistance and you are going to barely have any sound coming out of your speakers when it is all said and done and then someone is going to come along and "solve" the problem by cranking everything up. Which will lead to disaster. Try to match the system up a little better to avoid this issue and if you can switch the system to 70 volt you will be much happier.

crt

[tedanderson]

Any ideas as to where I can buy some decent transformers? Every place that I've been has the lower wattage transformers which I wouldn't mind using on those cheaper flat-faced speakers but I doubt that a 10 watt transformer is going to be adequate for both the amp and the speakers when I step up the line and then step it back down.

The amps are relatively cheap from what I've investigated so I might buy a couple out of my own pocket in order to ensure a more reliable system. But when I connected everything up and powered the system, I was getting a good volume level at 40%. But if the system gets destroyed as the result of using it for a karaoke party or to accomodate a DJ, that will be on them. I will just have to make sure that they understand that their sound guy should not be allowed to patch a microphone and keyboard into the system if they decide to have a sermon in the lobby.

[Johnnycat]

For transformers you can look at Parts Express

www.parts-express.com

The AudioSource amps are good for the intended application that they were made...residential home use.

For a commercial grade amp at an affordable price look to the Behringer line, the Euro line has been a decent line, up from there you have QSC, Crown, etc...

Always have more power than you need, the reason is you want enough headroom so you don't cause distortion or clip the amp.

In a perfect world you would want to look into a 70V application for your speaker distribution.

If you would like any more help let me know... johnnycat1700 (at) yahoo (dot) com

I deal with this stuff every day and design systems too...let me know if I can help.

[Gracetech]

ditto on Parts express. They have 100 watt 70volt transformers for 45 bucks a piece. Parts express also has load taps as well.

crt

[pdc]

Ditch it all and do it right. That is the only way. Transformers are going to cost you more than the amp and new ceiling speakers.

There are no real specs for that amp. We don't know if that is 20-20k, 1k, for 30 seconds or 24 hours. We don't know what the distortion is. It could be 50% at that rating.

100W ceiling speakers are not needed in a commercial application. I just did an install today for a church. The 70V Narthex speaker system is 12W @ 430 ohms. We installed 10 speaker 70V systems in Cracker Barrels that ROCK on 50W. It is all in how you engineer it based on the desired speaker coverage, speaker Q and proper tapping.

[stevelam]

I agree with PDC. The previous guy obvisously had no idea what he was doing. If you want to do it on the cheap then go with Yorkville CA1 or CA1T and Yorkville CM5/70 is a great little speaker for the price. I'll message you.

[Brad Weber]

For one thing, the AMP-100 may have two sets of outputs but looking at the specs it looks like they are two physical outputs in parallel off a single amp channel and not separate channels of amplification, so it is still effectively a two channel amp.

The 160W bridge mode rating is at 1kHz and 1% THD, it is also at 8 Ohms. For full range, quality sound it might effectively be more like maybe 140W and should not be used with less than an 8 Ohm load. The stereo 20-20kHz (still with a relatively high 1% THD+Noise) ratings are 50W per channel into 8 Ohms and 60W per channel into 4 Ohms, the small difference in the rated output between an 8 Ohm and 4 Ohm load indicates that the amp is probably pretty severely current limited with the lower loads.

You noted having eight speakers and two of the amps. Using both the A and B outputs for both channels on the two amps provides eight total outputs, so it would seem to make sense to run a speaker to each output. You could use the Line 2 In and Out connections on the amps to parallel the inputs on the two amps. That would be 30W per speaker (minus line losses) which is probably more than sufficient. However, it sounds more like they parallel wired a pair of speakers to a single output on each channel.

You noted left and right speakers. Are they actually trying to run a stereo ceiling speaker system or were you simply referencing that they were connected to the left and right channels of the amps?

As far as whether it works, if the speakers truly are presenting an 8 Ohm load and not something lower, if the sound level provided is sufficient and if the system is not pushed, then it should work. But it might not be something you would want to warranty or support as an appropriate solution.

[tedanderson]

The system is in and it is working satisfactorily. I did more of my "trickery" in areas of the building that is less populated and made the connections closer to the manufacturers recommendations in the more populated areas where they are more likely to turn up the volume.

Again, because I have to work with what is there, scraping the system and redoing it is not an option for the client. Thanks for you input everyone!

[stevelam]

nicely done! I am impressed by your persistence and "no retreat no surrender attitude".

Good work Ted!