Ton Giesberts
About the article

Loud and Clear

A portable sound system with feedback suppression

Loud and Clear
With personal PA systems there’s a fairly good chance of feedback howl. This occurs when the headset microphone picks up the sound from the loudspeaker, amplifies it, and emits it again via the loudspeaker, which produces an irritating howling tone. For this reason, we incorporated a previously published anti-feedback mechanism in our design: a frequency shifter.
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Gerber file

CAM/CAD data for the PCB referred to in this article is available as a Gerber file. Elektor GREEN and GOLD members can exclusively download these files for free as part of their membership. Gerber files allow a PCB to be produced on an appropriate device available locally, or through an online PCB manufacturing service.

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Extra info / Update

- Maximum output power 10W/8 Ohm (with 14 V supply)
- Feedback suppression by frequency shifting
- Suitable for electret and dynamic microphones
- Low current consumption
- Can also be used without the frequency shifter as a portable active speaker

Download the BOM list
The BOM (Bill of Materials) is the technically exhaustive listing of parts and other hardware items used to produce the working and tested prototype of any Elektor Labs project. The BOM file contains deeper information than the Component List published for the same project in Elektor Magazine. If required the BOM gets updated directly by our lab engineers. As a reader, you can download the list here.
Want to learn more about our BOM list? Read the BOM list article for extra information.
Component list

Frequency Shifter
Resistors (SMD 0805)
R1,R5,R6,R8,R9,R11-R22,R28-R34 = 10kOhm 1% 0.125W
R2,R3,R23-R27 = 1kOhm 1% 0.125W
R10 = 8.2kOhm 1% 0.125W
R4 = 4.3kOhm 1% 0.250W
R7 = 5.1kOhm 1% 0.250W
R35,R36 = 1MOhm 1% 0.125W
Capacitors (SMD 0805, except C17,C20)
C1 = 220nF 10% 25V, X7R
C2 = 4.7nF 10% 50V, X7R
C3,C13,C14 = 47nF 10% 50V, X7R
C4,C8,C9 = 10nF 10% 50V, X7R
C5,C23-C34 = 100nF 10% 50V, X7R
C6,C7,C16 = 1nF 10% 50V, X7R
C11 = 1.8nF 10% 50V, NP0
C12 = 270pF 5% 50V, NP0
C15 = 6.8nF 10% 50V, X7R
C10 = 100pF 5% 50V, NP0
C17,C20 = 50pF 25V, SMD trimmer, AVX type CTZ3E-50C-W1-PF
C18,C21 = 22pF 5% 50V, NP0
C19,C22 = 12pF 5% 50V, NP0
C35 = 1.5nF 10% 50V, X7R
IC1,IC2,IC4 = LM6134AIM SMD (SO-14)
IC3,IC5 = 74HC4066 SMD (SO-14)
IC6,IC8 = 74HC4060 SMD (SO-16)
IC7,IC9 = 74HC74 SMD (SO-14)
X1,X2 = 8MHz quartz crystal, SMD, AVX type CX49GFWB08000H0PESZZ
K1,K2 = 2-pin pinheader, right angled
K3 = 3-pin pinheader, right angled
PCB # 090675-2

Microphone & Output Amplifier
Resistors (SMD 0805 except P1,P2)
R1 = 220Ohm 1% 0.125W
R2,R8,R9 = 2.2kOhm 1% 0.125W
R3 = 10kOhm 1% 0.125W
R4 = 1kOhm 1% 0.125W
R5 = 3.3kOhm 1% 0.125W
R6 = 6.2kOhm 1% 0.125W
R7 = 8.2kOhm 1% 0.125W
R10,R11 = 5.6kOhm 1% 0.125W
R12 = 12kOhm 1% 0.125W
P1 = 20kOhm 20% 0.250W, SMD, Vishay Sfernice type TS53YJ203MR10
P2 = 10kOhm 20% 0.250W, SMD, Bourns type 3310Y-001-103L
Capacitors (SMD 0805, except C1,C21,C23,C27)
C1,C21,C23 = 33µF 20% 16V, SMD chip type 5x6mm
C2 = 220nF 10% 50V, X7R
C3,C10,C11,C20,C22,C24,C25 = 1µF 10% 16V, X7R
C4 = 470pF 5% 50V, NP0
C5,C6,C7 = 150nF 10% 50V
C8,C9,C16,C17,C26 = 100nF 50V 10%, X7R
C12,C14 = 10nF 10% 50V, X7R
C13 = 1.5nF 10% 50V, X7R
C15 = 2.2µF 10% 50V, X7R
C18,C19 = 100pF 5% 50V, NP0
C27 = 220µF 16V 20%, SMD chip type 10x10mm
Inductors (SMD 0805)
L1,L2 = Murata type BLM21PG221SN1D (220Ohm at 100MHz, 50mOhm, 2A)
L3 = Murata type BLM21PG600SN1D (60Ohm at 100MHz, 25mOhm, 3A)
D1 = PMEG2010AET (SOT-23) (1A 20V SMD Schottky diode)
IC2 = TS922ID (SO-8)
IC3 = LP2980AIM5-3.3 (MA05B)
K1,K2,MIC1 = 2-pin pinheader, angled
K3,P2 = 3-pin pinheader, angled
LS1,BT1,BT2 = 2-pin pinheader, straight
2 battery holders for 2 AA cells
Loudspeaker, 2W 8Ohm, e.g. Eurotec International type 59-F67.00-01FR
Enclosure, dim. 165 x 100 x 32 mm, e.g. MULTICOMP type MCRH3165 (Farnell # 1520395)
Switch, double pole double throw, 2A
3.5mm stereo jack socket for panel mounting
PCB # 090675-1

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