QSC REAR RACK EAR KIT - 3" Rear Support Rack Ear KitQSC power amplifiers are 19 inch rack mountable. In many cases, especially portable systems, it is desirable to mount amplifiers utilizing both front and rear rack ears to minimize vibration. This is true for both heavy and lightweight amplifiers. The longer rack rails are ideal for shorter amplifiers like the GX3, or the PLD4. 2, where the shorter rack rails can be used with longer amplifiers like the PL380, or the PLD4. 5.