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

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?