. Various API 550 Equalizer Versions free download

File name API_550_Equalizer_Versions.pdf

Versions of API 550 Equalizer this info was originally by Paul Wolff and later editing and additions were done by John Klett - May 2000 The api 550 series equalizer, with the exception of the 550A-1, has a family of boost cut curves that are referred to as "Proportional Q". Proportional Q describes a relationship between the energy that is added or removed by boost and cut and the range of frequencies (bandwidth) effected by differing degrees of equalization. If you look graphically at the the lines representing flat response and the lines in the family of curves describing the equalized boost or cut responses you'll find that the area enclosed by those lines remains reasonably constant despite the boost or cut amount. The enclosed area represents the amount of energy being added in boost or subtracted in cut. With small amounts of boost or cut the equalization curve is low and very wide and, as the amount of boost or cut is increased, the Q increases and the equalized response curve becomes narrow or sharper. The bandwidth and the boost/cut amounts are proportionally related - WIDER at 2dB and NARROWER at 12dB. The API 550: Proportional Q Equalizer with three frequency selections on the top band, three frequency selections on the bottom band and five frequency selections in the middle band, four discrete transistor buffers each using two transistors, a two pole filter for the high/low pass switch (different than 550A), the EQ is switched out by tying all the filters together and taking them to ground through the in/out switch, two 2520 opamps, and long frequency knobs. The API 550A: Proportional Q equalizer with five frequency selections on the top band, five frequency selections on the bottom band and five frequency selections in the middle band, four discrete transistor buffers each using three transistors, 1x-10x high and low pass filters with same frequencies as the 550 (50 HZ to 15KHz band pass), each band went through it's own in/out switch, two 2520s and short frequency knobs. The API 550A-1: Constant Q equalizer... the bandwidth remains constant regardless of boost or cut amount. The 550A-1 was essentially the same circuit as used in the API 560 graphic equalizer but the frequency selections are distributed over three boost cut "bands" having the same frequency center points as the 550A. That's why it sounds so different. This model had an IC opamp based input and filter stages and one 2520 on the output. The design change has been explained two ways... one explanation was that API simply wanted to lower the cost of making EQ's and the other was that a customer was ordering several large consoles and expressed that he really liked sound of the 560 EQ but wanted the stepped controls and center points of the 550A... since this represented a large sale and involved about a hundred equalizers API made that change and then kept making them that way because they were less costly to replicate. API never checked the Q other t