Prior to starting our review, you should be aware of the fact that the best headphone amp under 100 at this price is a budget device with an initial level of sound quality. However, even such units will allow you to feel the difference in the use of headphone amp.
From the name, it is not difficult to guess that the best portable headphone amp is engaged in signal amplification and allows you to get the required volume at the headphone output. Accordingly, the question arises, why, for example, portable amplifiers are needed for phones, laptops, and players if the volume from the latter is enough?
Some devices combine a high-quality external DAC and headphone amplifier. The amplifying part of these devices is a logical continuation of the analog part of the DAC, and in this case, the placement of two functional blocks inside one housing is more a plus than a minus. The path is shortened since there is no need to use additional wires (inter-unit between the headphone amplifier and an external DAC).