I've done a couple of 70 volt audio systems in dental offices.

I have another job that will be a gym-type atmosphere. The former tenants left 5 Bogen OPS1's which are a tear drop type speaker hanging about 20 feet up.

Each speaker has a switch that allows various voltages - near the end of the dial is 8volts and then 16 volts.

I don't remember the exact model, but it is an 80 watt Crown amplifier.

I would like to use their existing 80 watt amp. If I set the Bogen's to 16 volts, they will add up to exactly 80. I figure I will get that much more loudness out of them, but that won't leave me the typicaly 15-20% of head room that I believe is usually suggested in commercial 70 volt system.

Am I playing with fire doing this? Will there be that much of a difference in loudness between 8 and 16 volts? Assuming they aren't cranking the system, will I really be in that much danger of over-heating the amp?


Customer is on a very tight budget...

The difference in volume will be 3 dB between a 8 watt tap and a 16 watt tap.

I strongly recommend leaving head room.

The taps represent watts, not volts. You would never want to max out the wattage budget.

OK, thanks

Yeah, meant to say watts not volts.

If all five speakers are running off a single volume control, can I dial some of the speakers to 16 watts and some at 8 watts in order to keep the total wattage down? I know it's messy but is it a huge deal?

When purchasing the volume control, the wattage rating just needs to be the same or higher as the highest wattage speaker in the chain of speakers following the volume control correct?

Yes, you can have them tapped at different wattages. That is one of the very useful features of 70V. If I recall correctly, I believe the VC should be rated for the total wattage going through it, which would be the sum of all speaker tap wattages. 

Setting different levels is not a problem providing it all adds up to less than the amp rating with about a 20% headroom. They'll have different volumes of course which may be a problem for the client but not a problem for the amp.


It needs to be rated at more than the TOTAL of the speaker tap wattages.

No. Don't do that.

Just because you can doesn't mean you should...

If you do that you'll end up with this lopsided acoustical environment. Remember the lowest perceptible level difference is 1dB. Your customers will notice and will complain that they can never get the sound even in the space.

Wouldn't you have to determine the logistics of the environment they will reside in? For instance, I have a job in which someone else installed ceiling speakers in a club/bar. The club side is in an adjacent room to this bar, and music is pumped on that side about as loud as you'd expect a club to be. The right side of the bar is towards the club side and is far noisier than the left side. The ceiling speakers are to pump the music at a more acceptable level to the bar side. Keeping them unbalanced would make more sense in this setup, no? I have not touched this system as we did just the TV's and CCTV.

You, as the designer, have to sense the situation, use your judgement, and create a solution using the tools you have available...

I interpreted the room described by the OP as a single acoustical environment with an array of speakers to cover the one space. In the scenario described, you would not want to tap the speakers differently 'just to maximize the power being used' as it would create undue differences in the level throughout the space.

The scenario you described is still not a good use of different tap values on the same power amp channel in my opinion. Sure they do achieve the goal of reducing the signal to noise ratio of the program audio in the bar (louder speakers for louder background noise), but they don't exactly offer the level of control that I'd design into a system. This is an instance where I'd provide a completely separate zone - different level control, different amp channel, very likely higher tap values, automatic level control via an ambient level microphone, et cetera... Will it work? Yes. Is it MY preferred solution? No. I prefer to do the calibration type things in software / DSP from the comfort of my office rather than turning tap value switches on a speaker 15' up in the air.

Yes, do that if it makes sense.


Unless it makes sense. A case in point was a run of three speakers over an eating environment about 10 feet x 25 feet. The ceiling was three feet higher at one end, and we tapped that speaker at a higher wattage than the other two.

Different tapping can absolutely be used to balance levels where the environment is not uniform.


Or you'll fix a lopsided acoustical environment. Know what you're doing!


True, but it probably takes a silent environment to detect this small a difference. Speakers are tapped in 3 dB increments because it's close enough to be practical.

In small dental office environment (4 dental chairs), what do you 'normally' tap the speakers at in:
-the operatories?
-the waiting area?
-the bathroom?
Doing a project and need to find a safe amp to run a single zone loop without overspending.

So after a quick look, it seems like Atlas Sound is one of the few that carry a 70 volt volume control rated over 35 watts - they have one at 100 watts. Can you suggest additional brands I can find - hopefully something that is at A%&D. I'm squeezed for time on this one!

16 watts can be pretty damn loud, but in a gym they will push for REALLY loud.


Which will be pushed to the limit in a typical gym situation.


The logic fails here. Back up a step and remember that a 70 volt sytem puts out its rated wattage when its output is at its maximum, which is 70 volts. If you set the speakers to 16 watts, thus drawing a possible 80, it will be possible for you to overload the amp, causing amp distortion, perhaps shutdown or destruction. If you set them to 8 watts, you won't be able to destroy the amp unless you have an overly high input signal. At this point, the concept of gain structure comes into play: make sure the line level going into each stage of the system cannot be over the input required for maximum output. That is, if that crown will give you full output with one volt in and your preamp or source stages output two volts, the amp volume control, at max, will pus the amp to twice its output voltage ( = 4x its output wattage), thus guaranteeing problems.


Not really. Crowns usually go up in a puff of smoke, not an actual fire.


Well, watts, as others have pointed out. The difference is three dB.


Which may very well screw him. Set it up quickly, without volume controls, with as much output as you can get, being sure not to feed too much line level to the power amp, and see if it's loud enough for him. If it is not loud enough, he has to change his budget or his expectations. The most wise customer I ever heard of said "Don't screw me by trying to save me money."


Do a Google search for


and you'll have lots of results. Telephone the vendor you mentioned and ask them. Don't even think of using their totally worthless website to try to find this product, or much of any.

That google search yielded a paper on 70 Volt theory that might be good, too. It's a Word doc so it just opened without giving an actual URL, but here's the google search pointer: [Link: google.com]

I actually just quoted a gym with a 70v system, my first. The gym is for rehabbing, mostly elderly patients. All the cardio machines have TV's with headphones. They want a basic system for background music. There is one open area which would get 6 pendants and two locker rooms, each with a bathroom area and dressing area. These areas will each get an in-ceiling speaker. I did mention this setup in another thread, but figured I'd ask here as there is some good info and a similar setup has been mentioned.

I did not propose any volume controls and just figured we would adjust the taps to allow for more volume in the fitness area with softer output in the locker rooms. Is there any problems with going this route? I did propose an amp with plenty of power, way more than the 20% suggested.

Special thanks to 3PedalMini for helping me figure out 70v. I owe you that Brendon Fuel still.

goldenzrule,
if the different areas are always going to be used pretty much the same, there may indeed be no need for volume controls.

Somewhere in here it was suggested that we do not set speakers to different levels, but I must repeat, that's wrong-headed. Choosing the taps allows you to balance out a system; I first learned this with home impedance-matching VCs when I realized that a VC set for x8 yielded an output 3 dB below a VC set at X4 and thus, for instance, outdoor speakers might be at x2, indoors at x4, bathroom speakers at x8, giving reasonably consisten results without necessarily needing volume controls.