A power amp is what’s needed at an absolute bare minimum for passive loudspeakers to technically work – its singular and necessary function is to amplify the power of an incoming signal so that it’s strong enough to actually move the components of a loudspeaker. That said, a plain-jane power amp lacks any kind of controls, notably a volume knob, so as you can imagine it isn’t practically viable on its own.
Integrated amps solve this by adding in a preamp, which can adjust the volume at a minimum and optimize the signal before it reaches the power amp. Integrated amps are an excellent option for running passive speakers with minimal cost and fuss.
However, most modern sources do have volume and basic EQ controls, as you probably realize, which begets the obvious follow-up question: If you have these controls on your source, then do you actually need a separate preamp?
No, you actually don’t. Pretty much any modern computer or phone has software and hardware that can do everything that a separate preamp would: basic EQ/volume control, digital to analog conversion, etc.
Non professional grade plain power amps are not all that common, but they do exist – worth mention here is the popular Nobsound Mini:
In utter contrast to a bulky conventional receiver, this product is about the size of a deck of cards and we’ve seen it go for less than $50, and it’s something that’s quite appealing for simple near/mid-field setups. (Yes, it has a volume knob and Bluetooth compatibility so it’s *technically* integrated, but it’s essentially what a penny pincher is looking for if they’re shopping for a basic power amp)
There are some arguments floating out there on the internet that claim device-onboard controls and DAC will yield an inferior signal due to things like electronic noise from other nearby components, but a lot of these claims are frankly dated and speculative. Modern digital devices have become quite decent at mitigating signal to noise ratio and information loss, and we’d say the odds that you’d actually be able to notice a difference with a pair of ~$150 speakers if you add an external preamp to the chain is very slim.