Selection of loudspeaker impedance for the amplifier.

In general, the basic rule is that the impedance of the loudspeaker must not be lower than the minimum impedance specified on the back of the amplifier with which the amplifier works. On the back of the amplifier, as a rule, the speaker impedance on the Western minimum is written, the Polish has a number and the oma symbol. Why ?? because anyone who knows the law of ohms knows that reducing the resistance by 2 times at the same voltage will increase the current by 2 times. As a consequence, the loss power of the power transistors of the control transistors increases, and the load on the power supply increases. The result of this is in 70% of cases thermal damage to the power transistors (collector-emitter breakdown or more often a complete breakdown between each junction of the transistor). Due to the galvanic connection of the elements in the power amplifier, it often causes an avalanche of other damages before the fuses react. And it is not, as often uninitiated people write, that connecting a 4-ohm loudspeaker to an 8-ohm amplifier always gives a 2-fold increase in power. This is complete nonsense resulting from ignorance of how the amplifier works. 90% of amplifiers have a symmetrical unstabilized power supply, which shows considerable voltage fluctuations during intense control. With a higher load, this voltage "squats" even more, so in intensive control it is even lower than for 8 ohms. Consequently, there is no 2-fold increase in power. Yes, there are cases that a 2-fold increase in power is especially in the Polish amplifiers of the older date, made with a huge reserve of power and without short-circuit protection. Therefore, I personally think that it is really not worth doing such experiments. However, you can connect a loudspeaker with an impedance higher than the minimum specified by the manufacturer. It is a very advantageous solution for the amplifier because the current and, consequently, the power loss decreases. The amplifier simply heats up less. The disadvantage of this solution is the power drop. In this case, it is rarely 2x with a 2x increase in impedance. This is also due to the operation of the power supply. Less load = less current = greater instantaneous voltage.
Crossover and its impedance distribution
Generally, when building amateur loudspeakers, the easiest way is to take the impedance of the bass speaker as the impedance of the entire loudspeaker. This is a big simplification because, as it is known, in practice, different speakers work for a given frequency, e.g. at a crossover frequency of 500hz for a crossover of 3 watts 6db / oct for a frequency of 500hz the amplifier sees both the impedance of the bass and midrange drivers because they both carry this frequency. However, in amateur conditions, it is usually not possible to measure the crossover well and plot the impedance characteristic of the loudspeaker as a function of frequency. Another argument is that in the bass band the most power is "located". As a result, the greatest currents flow there and it is the impedance of the bass speaker that has the greatest impact on the load on the amplifier. Another simplification is that in an amateur speaker all speakers should have the same impedance. It is possible to use a tweeter with an impedance greater than the woofer in order to equalize the efficiency of the loudspeaker (then the use of an attenuator is avoided), However, a speaker with a lower impedance should not be used. The fact that there is less power there does not entitle to the fact that in the range of medium or high tones one can load the amplifier with super-low impedances. Generally, you should aim at using simple crossovers at low powers. This is due to the inability to test an extensive crossover, and theory is theory, practice is practice. An extensive multi-way, poorly designed crossover network is an unfavorable burden for the amplifier and may lead to its destruction.