Folks do not usually visit an electronics hobby forum, such as Electronics Point, simply to seek information on where to obtain COTS (commercial off-the-shelf) equipment. The search engine Google does a pretty good job at that. For example, using the search string "amplifier for square wave signal generator" elicited a lot of inexpensive Asian "solutions" to your problem, and some of these might actually work for your application (whatever that might happen to be). Although the broadband audio amplifier that Harald suggested should meet your rather loose "specification," it appears to me to be a bit expensive.
So, here is a
link to a sub-woofer amplifier that may also work. It probably operates in class D mode, meaning the output is rapidly switched on and off by a digital control signal and the resulting "chopped" waveform is then filtered to remove the audible artifacts introduced by chopping the input signal. In your case the input signal is a "square wave," presumably from a function generator of some sort, in the frequency range of 20 Hz to 500 Hz with a variable duty cycle between 10% and 90%. The desired output is a 24 V waveform (positive? negative? symmetrical about zero voltage?) capable of producing 50 W (at any duty cycle?) into some sort of load (what resistance or impedance load?). Other than that, there are no specifics about the waveform needed at the load. Are the rise and fall times of the "square wave" edges an important factor in your application? Is the waveform AC-coupled with zero DC component, or does the variable duty-cycle affect the DC component of the waveform and therefore DC-coupled amplifiers are required?
Notice that the sub-woofer amplifier module cited above does not require any soldering. It does require a separate DC power supply with sufficient voltage and current capability. It may or may not be suitable for your application. Someone skilled in the art of electronics or electricity should work with you to determine what your
real requirements are, which will depend on what you are
trying to DO. You have not yet told us what you are trying to do, choosing instead to tell us how you want to do it, i.e., add an amplifier to an existing waveform generator.
For better answers, provide us with better information.
Edit: I just now read your response to Harald. This is the information we need. I will wait and let someone else jump in with a suggestion of how to purchase a suitable controller for your LED illumination application. We will probably need more information in order to suggest a specific solution.
