The EQ5P from Tonelux is a discrete parametric style equalizer housed in a 500 series enclosure. There are no dedicated Q controls on the front panel, but rather the filter width is internally adjusted based on the boost and cut settings using a "Constant Energy Curve." Also known as "Proportional Q," the circuitry keeps the bandwidth wider at lower boost/cut levels, and becomes narrow at higher boost/cut levels, in the process delivering a musically pleasing result.
The curve is identical to many classic equalizers and is one of the reasons they are preferred over newer designs. When the peak button is engaged on the three lower bands, the width is kept at a constant 1/3 octave for more precise control. The high frequency has a shelving button instead of a peak button. Since the Constant Energy Curve is so broad in the lower frequencies, there is no need for a low frequency shelf button.
Each of the lower two bands have slightly different frequencies, allowing the lower band to go to 16 Hz, while the upper two bands are identical. The "In" button is a silent in/out button that does not bypass the unit, but it bypasses the filters. An output transformer adds the signature iron sound that many engineers enjoy.