PA levels
Do you mean the ANS system. PA systems are usually preset to a given base output level that is increnmentally increased as doors sre shut, engines started etc. The highest level is for decompression. On the B747-100/200 the amplifiers had a adjustment for PA output level on the front panel of each of the 3 amplifiers. A test switch was hels to test and the output adjusted to align the meter needle on the amp to a reference mark. There was also a position on the test switch to output a test tone to the speakers to allow a speaker check in the cabin. There was no autogain system such as the B777s ANS. The B747-400 sent PA via the Access sytem and was also preset. On all these aircraft there is provision for the cabin crew to increase the PA volume for each zone, they can't decrease it below the current base level though. I hope this helps.