bass/treble output power for 8020A - Forum

Thank you for your question, it is not naïve at all, but very relevant. To put the answer very simply, the tweeter amp needs to be of that size. The reasons are more complicated.

If we look at the music energy spectrum vs. frequency, in most cases it declines with increasing frequency. Inherently this would support smaller tweeter amplifier. However, looking at the instantaneous peak levels we find that the trend is not so clear. There are high peaks at treble frequencies as well, and if we intend to reproduce those without clipping, there must be sufficient output capability.

The second issue is the sheer sensitivity of the drivers, expressed in SPL at certain distance, usually at 1 m, for specified input voltage. The common terms are SPL in dB for 1 W at 1 m. However, this is actually confusing, as Watt is a unit of power and it requires additional definitions. So what is being meant is 1 W at 8 ohms load, which is equivalent to 2,83 Volts. Now, the speaker impedance is not constant but varies with frequency, and there are nominally different impedance drivers anyway. So, it is better to define SPL at 1 m for 2,83V input for nominally 8 ohm drivers, and 2,0 Volts for nominally 4 ohm drivers. The end result is same, if we measure also nominally 4 ohm drivers with the same 2,83 Volts and reduce 3 dB from the result.

If we now have woofer and tweeter of similar sensitivity, let us say 90 dB SPL for 2,83 V, they will produce same SPL with same input voltage. If their impedance is same, the amplifier output power is also same. Sounds simple.
But is the sensitivity the same in reality? Usually it is not, but the actual sensitivity also depends on frequency. There are many reasons for that and their effects are tackled in the crossover circuitry. If, for example, the woofer sensitivity gets higher, as it does due to the front baffle, the input voltage to the driver must be reduced. If the sensitivity goes down, as it does, for example, at low frequencies when a woofer is put into a small enclosure, the voltage has to be increased to get flat response. All modern amplifiers have very low output impedance, i.e. they are voltage sources, and the actual power depends on the load impedance. So we find that we should not actually talk about amplifier output power but output voltage.

Third issue is reliability. Funny enough, too small tweeter amplifier is a risk. When overloading an amplifier the frequency spectrum of the clipped signal will change. This was learnt in 80s in sound reinforcement applications, where tweeters were burning frequently. Clipping caused actually more energy to go to the tweeter, and hence it is better to use sufficient amplifiers for the application. Extra headroom is a nice feature and it should be used for reliability.

Fourth issue is convenience in design. If the designer wants to use a single power supply, which would be reasonable in terms of reliability, it means that conveniently both (or all) amplifiers have the same supply voltage. If drivers have same nominal impedance, this leads to same power rating. Still the designer has freedom to play with driver impedances. If, for some reason, the woofer sensitivity is lower than the tweeter sensitivity, the designer can use lower impedance driver and get extra output from the same available amplifier voltage; of course the amp needs to be able to supply more current. This would then show in the specification as higher power woofer amplifier, although the actual output voltage is same.

I hope this clarifies the situation.
Best regards,

GENELEC
Ilpo Martikainen
D.Sc (Tech.) h.c.
Chairman of the Board and founder of Genelec Oy