70V line transformers

One place set up a PA with a bunch of 8 ohm speakers on #18 zip cord. They got no volume out of it at all. The problem was (1) too much wire resistance going to the speakers - and even with all that series resistance, they had so many lines in parallel at the amplifier that (2) the impedance presented to the PA amp was way too low, and it couldn't provide enough output current to produce much power.

They had no transformers, and they wanted a quick fix, so I put the speaker lines in series, and connected them to the 70V output. I set up crude series-parallel combinations to split the power more appropriately. Not ideal, but MUCH louder! It was a warehouse environment, and they didn't need very precise power splitting.

Figure 100 watts and a "70V" line... in these systems, that means the amplifier puts out 70V RMS, and it is capable of 100 Watts. It doesn't mean it will force 100 watts down the line, it only means it can provide it. It's like an AC power distribution, 70V and 100 watts mean 1.43 amps, and 70 ohms and 1.43 amps mean about 50 ohms load for rated power in this case.

The main advantages of line transformers are: (1) The line impedance is on the order of 50 ohms, instead of like 0.5 ohms with a bunch of 8 ohm speakers in parallel. This reduces resistive losses in the cable from the amp to the speakers. (2) By using taps on the transformers you can adjust or select the level to each speaker. So you could give only a little audio to the speaker in the coffee room, while you could give a lot of audio to the speakers in the warehouse and garage.

Although its load impedance will be about 50 ohms in this example, the output impedance of the amplifier is very low. This way it puts out the same voltage whether there is a load, or not. It can provide up to 1.43 Amps RMS. You can add speakers to the system until you have so many that the load impedance to the amp is too low. You could then get a more powerful amplifier that could provide more current. Like most speaker amplifiers, the PA amplifier is designed to provide its rated power into a load impedance such that it produces the rated voltage at that power level. It has negative feedback of some sort so that its source impedance is much lower than its optimum load impedance.

Such PA amplifiers are designed to withstand a very light output load without oscillating or producing overvoltage, or overheating their output devices. Note that with tetrode and pentode tubes, screen dissipation can get high if the plate load is very light. Of course, solid state amps are mostly used today, but amplifier stability is a big consideration. Remember that a 100 watt PA would be expected to drive that 70V down the line, even if only one little speaker is connected and set up to take maybe 1/2 watt. 1/2 watt from 70V would require an impedance of 9.8K ohms! And the amp is designed to drive full power into 50 ohms. So PA amp design is a little more demanding than the usual.

And the line transformers might convert 8 ohms to 10K (1/2W), 5K (1W), 2.5K (2W), 1K (5W), 500 ohms (10W), etc. The transformers usually expect 8 or 16 ohm speakers, and the taps are specified in watts.

These systems work, and they show that impedance matching, as such, is not the be-all and end-all of power distribution. Optimum and appropriate impedances are usually the way to think.