This circuit is from a publication from the 70s. The original text was kept. The medium frequencies, in the track and audio are responsible for several effects, including the sophisticated "presence" that is difficult to obtain. The 5,000 Hz frequency is chosen randomly as the best one to be emphasized, to give the "presence" in musical performances. The circuit we present must be located between the pre and the amplifier, it is a filter in twin-T layout, with the central frequency at 5,000 Hz and a gain of 12 dB. The presence effect is regulated by R2. it is desired to change the central frequency of 5,000 Hz, capacitors C2-C3-C4 must be changed, keeping the following proportion: C2 C3 and C4 = 2 x C2.
LIST OF MATERIALS
C1 = 1 non-electrolytic uF
C2 = 4,700 pF
C3 = 4,700 pF
C4 = 10,000 pF
C5 = 10 uF x 16 v electrolytic
C6 = 1 uF, non-electrolytic
C7 = 10 uF x 16 v electrolytic
C8 = 100 uF 25 v., Electrolytic
R1 = 22K
R2 = 50,000 linear
R3 = 22K
R4 = 3,900
R5: = 6,800
R6 = 39K
R7 = 6.8K
R8 = 22K
R9 = 3.9K
R10 = 56K
R11 = 56K
R12 = 12K
Rl3 = 1.5K
TR1 = BC107 or similar
