This provides 2.5 V of signal swing, which is enough for driving headphones to a considerable volume. The op amps are supplied by 5 V and sink to 0 V (GND). Therefore, Spectrum raises the ground point to a virtual ground of (VDD / 2), or 2.5 V. This is not possible with a single supply. Traditionally, operational amplifiers use a bipolar supply of +VDD and -VDD, with the ground point of the signal being 0 V. Spectrum uses a 5 V power supply that is regulated by an L7805 LDO. The circuit must be powered by an external wall adapter or any other 5 V source that can supply approximately 250 mA of current. The resulting output can be played through stereo headphones. Spectrum effectively uses two identical mono circuits to modify both channels of stereo audio. A graphic equalizer only operates on a single mono signal. Typically, the signals are used to drive separate 'left' and 'right' speakers. 'Stereo' uses two separate audio signals that differ slightly but can be used to give the perception of spatial audio. This is a single audio signal that plays through all the speakers. The most basic form of electrical audio signals is 'mono'. The resulting signal is the same audio but with each frequency range adjusted with a desired gain. The modified signals are mixed back together into a single audio signal which appears at the output. These ranges can then be individually amplified (boosted) or attenuated (cut). Background Graphic EqualizersĪ graphic equalizer is a piece of audio equipment that isolates different ranges of frequencies from an audio signal. The circuit requires an external power source of at least 5 V at 250 mA. Each band can be individually amplified or attenuated using linear potentiometers.

It uses active band pass filters in a multiple feedback configuration.

Spectrum is a 10-band stereo graphic equalizer circuit.

Audio equalizer circuit