Heed Audio Q-FLT Mains Noise Filter & DC Blocker The Q-FLT is a DC blocking mains voltage filter, designed to greatly reduce the effects of a noisy, polluted and DC-shifted mains voltage which can cause inconvenient problems in audio systems. The audible mechanical hum of the toroidal transformers used in most audio devices is a common problem nowadays. The fluctuating noise that comes and goes is mostly caused by the DC component of the mains network, which is the result of many high-demand electric consumers we use in our households.