The "volume control" on a power amplifier is an input pad. A power amplifier can only be driven by a pre-amplifier. There is no input for a mic or guitar on a power amp.) The "volume control" changes the sensitivity of the input. It is not a gain stage and it is not a limiter. Setting the "volume control" to 20% just means that it takes more voltage on the input to drive the amp to full output. Setting the "volume control" to 100% means that it will take less voltage at the input to drive it to full output. Many manufactures still use the old standby sensitivity of .77v at the input will drive the amp to full output.
There are lots of ways to hookup a pile of equipment and get some sound out of it but if you hear distortion THERE IS SOMETHING WRONG.
Seventy volt and 25 volt transformers and systems.
Generally schools intercom systems are 25 volt systems like Bogan for example. There is NO set rule saying that 25 volt only systems can be used in schools. In todays world some schools are going to a simple 70 volt system with just an all call page.
Most of the older commercial systems put in the 1950's and earlier were 25 volt.
The 100 volt system are common in Europe are are not a norm for say stadium systems. Been doing this type of work for many many years all over the country and have not run across one 100 volt system yet.
How 70 volt systems work...
The amplifier has been designed to run an internal 70 volt transformer or is hefty enough current wise to do so example Altec 9446 that will run two 600 watt external 70 volt transformers one per channel. Other commercial rated 8/4 ohm amplifiers are capable of running external transformers.
The 70 volt transformer steps the voltage up to 70 volts on the line. Each speaker is equipped with a step down transformer to lower the voltage and restrict the power to the given speaker.
Using a 8ohm speaker on a 70 volt line or using a 8 ohm speaker with a commercial transformered volume control.
Trying to use an 8 ohm speaker on a 70 volt line will result in an overloaded speaker line. Checking the load with an impedance meter will result with a load of 8 ohms and a meter reading in excess of 900 watts. It is easy to find because it will be the only speaker that is screaming.
Have had numerous times when I have pulled a 8 ohm speaker off a 70 volt transformered volume control. When wired this way the load is still in excess of 900 watts. The speaker still needs a transformer on it.
So, my suggestion is this....
I would purchase a power amplifier to use with a preamplifier to run the JBL speakers. By using a pre-amp and a power amp it allows you to upgrade later to a higher power amplifier.
Do NOT believe the speaker ratings...... I have seen more damage to speakers with amplifiers rated at less than the manufacturers ratings than I ever have with amplifiers that are larger. Why? Simple. Its because the small amplifiers clip easier and produce a distorted signal. When this happens damage is done to the drivers in the speaker. I currently run amplifiers that are rated 4 or 5 times the power rating of the speakers without a single problem.
Hope this helps.
With respect to comments on distortion in some past threads. The human ear starts hearing distortion when it reaches 1%. Your not going to hear distortion of an amplifier rated at .1 as opposed to an amplifier rated at .001
1) The output stage is sub ohm impedance. In a solid state amp, it probably is but you can't be sure without more research.
2) The feedback point is not taken from the secondary of the transformer. If it is it will have to be re-compensated for direct connection - not a trivial task.
I would highly discourage a novice from simply removing the output transformer in a PA amp without some expert analysis of the circuit.
Arny, are you a little desperate for attention?
Despite being 9 months late with your reply, I'm sure you know that a pads attenuation depends on the source and load impedances, while a voltage divider is typically used to bridge a low impedance source.
If you're going to nit pic, at least be correct.
IOW any four terminal circuit's operation depends on Z(source) and Z(load)
Pads of all common kinds are four terminal circuits.
Therefore any pad's operation depends on Z(source) and Z(load)