I'm having a difficulty in choosing a right Keyboard Amp. I have experiences with some of the leading brands of Keyboard Amp like Roland and Laney, but they are really expensive and the sound is not really good for my opinion. That is because I have a long experience of connecting my keyboard directly to the Mixer with the use of DI Box, and the sound that I was looking for was perfect. I also tried connecting it to a vocal monitor and it was also perfect. So I'm thinking a shift from keyboard amp to a vocal monitor. I started from learning the technical specs and one of it is to get the best range of frequency response of an electric keyboard and match it to the vocal monitor. The Frequency of an Acoustic Piano is 27Hz to 4.2KHz, is this the same for an electric Piano?
A keyboard amp is a small PA system. It aims to give an even frequency response, without distortion, across the full range of human hearing, or as close as it can get within cost and size constraints.
In ideal conditions, humans can hear from 12Hz to 20Hz -- although adults lose their sensitivity to the highest frequencies.
The lowest piano key, A0, is 27 Hz, so to reproduce it accurately you'll need an amp and speakers which handle that frequency. However, note that even bass guitar amps typically only target 31Hz (the lowest note on a five-string). The main audible part of a piano note that low, is in the harmonics.
Although the root frequency of the highest piano key, C8, is 4.2KHz, it has harmonics that go beyond the range of human hearing. Typical PA amps handle 10KHz or so. Without that range, you lose the character of high notes (if you still have hearing in that range). Higher is better, up to a point - 20KHz is higher than you'd ever need (unless you're performing for dogs).
Practically speaking, the speaker you can fit in a small cabinet won't be able to manage loud bass at the bottom of the piano's range - if you want loud bass, you need a big cabinet. For the highs you need a cabinet with a tweeter. Think about your playing style, and whether you actually use the top and bottom ends of the keyboard much.
Modern synthesizers put out a signal which is full band audio, intended for a high fidelity amplifier. They can produce content that is located anywhere and everywhere in the audible spectrum.
Many instruments have harmonics which extend up toward 20 kHz, the limit of human hearing. Though there aren't actually notes in the range 5 kHz to 20 kHz, that range of frequencies must be reproduced if the sound is to have a realistic timbre, and "air". If these frequencies are attenuated, the sound will lack definition, sounding muffled and "telephoney".
By "there aren't any notes", what this means is that pure tones in these high frequencies above 5 kHz or so are heard, but they are not heard as pitches which have the usual pitch relationships. If a melody is transposed into that range, it is no longer recognized as a melody.
Similarly, the sound system has to have ample bass response, even for reproducing notes whose fundamental frequencies are well about that frequency. Although a sustained/decaying 110 Hz A note does not have a 30 Hz component, the attack when the A is initially struck may require a response at 30 Hz for full realism. You want to hear the "thud" in the piano's key and hammer mechanism, and the resonance of its large wooden surfaces, not only the sound of the string. A good quality piano patch should produce all these sounds.
In other words, in order for the instrument sound to have a realistic impact, you need frequency response down to 27 Hz or less even if you don't play the lowest note on the keyboard.
Interesting technical fact: The upper harmonics can be stripped from music and speech and fake ones regenerated in their place which have the same spectral envelope (shape of the frequency spectrum). To listeners, this sounds all right: the sound is "defined" and has "air", and so forth. This is done in audio codecs such as AAC and where it is called Spectral Band Replication (SBR). It saves a lot of space compared to reproducing the actual high frequency parts of the waveform in detail. We really do not hear the detail very well in the high frequencies, but we need them to be present in approximately the right amount.
It will depend on your keyboard. 61 up to 88 notes. If you're going to use any other sounds, e.g. strings, horns,organ etc.,(and why wouldn't you?) then the frequency range will differ again. I often use a vocal p.a. amp, through a 15" speaker, with a bullet horn. The 15" gives as much bottom end as I need, with the horn often being too much on the highs. It's rated at 150 watts, so is plenty for most gigs; I actually use Roland and Laney keyboard amps, but prefer the sounds from the vocal amp ! There are usually several inputs on this sort of amp., so when you use more than one 'board, it's not a problem. Line out means you can go into the house p.a. as well.A vocal monitor is really a small version of a p.a. set up anyway, so this will work well.