# Directional Microphone

## with switchable characteristic

The directional microphone published in the July-August 2002 issue of Elektor Electronics does its job, but does not have the right directional characteristic. The authors have developed a new circuit which satisfies all the requirements in practice as well as in theory.The microphone published in the July-August 2002 issue of Elektor Electronics was designed to reduce the risk of feedback when used with a jazz harmonica. However, it does not exhibit the ideal cardioid directional characteristic. It can be shown mathematically that as the characteristic is adjusted from omnidirectional at one extreme to figure-ofeight at the other, it is never strictly cardioid or supercardioid: instead a null is gradually introduced, like a belt being tightened around the omnidirectional response until it becomes a figure-of-eight.
Products
Gerber file

Elektor recommends its business partner Eurocircuits as the company of choice for its own prototypes and volume production in selected cases.

Extra info / Update
The equation in the article text on page 26 contains a wrong component reference and should be amended to read: R8 = d / (c x C7)
Component list
Resistors:
R1,R3,R39 = 4k?7
R2,R4,R37,R38 = 470?
R5,R15,R26,R32 = 47k?
R6,R7,R28 = 22k?
R8,R16,R17,R18,R31 = 1k?
R9,R24 = 100k?
R10 = 3k?3
R11 = 1k?5
R12,R13 = 1k?1
R14 = 2k?4
R19-R22 = 220k?
R23 = 150k?
R25 = 68k?
R27 = 33k?
R29 = 15k?
R30,R33,R34 = 10k?
R35,R36 = 47?
P1 = 47k? preset
P2 = 1k? potentiometer, logarithmic
Capacitors:
C1,C4 = 2nF2
C2,C5,C8,C13 = 330nF
C7 = 47nF
C9,C10,C24 = 100nF
C11 = 150nF
C12 = 220nF
C14 = 470nF
C15 = 680nF
C16,C23 = 1µF 63V MKT, lead pitch 5mm or 7.5mm
C17 = 1µF5 63V MKT, lead pitch 5mm or 7.5mm
C18 = 2µF2 63V MKT, lead pitch 5mm or 7.5mm
C19 = 3µF3 63V MKT, lead pitch 5mm or 7.5mm
Semiconductors:
D1 = zener diode 7.5V 1.3W
D2 = 1N4148
T1 = BC546B
IC1 = TS925IN (ST), e.g., Farnell
IC2 = TLC2272 (Texas Instruments)
Miscellaneous:
K1 = 3-way SIL pinheader + 6.3mm jack socket, chassis mount with separate ground contacts
K2 = 3-way SIL pinheader + XLR plug (male) chassis mount
S1 = 2-pole 6-way rotary switch, PCB mount
S2 = 1-pole 12-way rotary switch, PCB mount
BT1 = 9-V battery 6F22 with clip-on leads
MIC1,MIC2 = electret microphone element (e.g., Monacor MCE2000)
PCB, order code 030046-1, (available from The PCBShop