 These used several valves (tubes) to provide RF amplification, detection, and audio amplification.Early TRF sets only operated headphones, but by the mid-1920s it was more common to use additional amplification to power a loudspeaker, despite the expense.The sound quality produced from "moving-iron" speakers used on such sets is sometimes described as torturous, although by the late 1920s the Kellogg-Rice dynamic (moving-coil) speaker had begun to find favor due to its superior sound-reproduction ability.