A circuit, such as a VCA or mixer, that will pass through DC levels and subsonic frequencies through from its signal inputs. Typically, audio circuits are AC coupled because DC offsets in audio cause various difficulties in amplification (such as amplifier clipping and loudspeaker burnout). Some synth circuits produce signals with DC offsets as an unavoidable consequence of the way the circuits work, and these are usually AC coupled to block the DC offsets from the output. However, control voltages are essentially raw DC levels, so circuits that may be required to process control voltages must be DC coupled. A typical problem is when a mixer is to be used to add control voltages; if the mixer is not DC coupled, it won't work.