• on Audio/Video/Photography
  • Published in issue 2/2007 on page 0
About the article

Stereo Microphone Preamplifier

For MP3 equipment

Stereo Microphone Preamplifier
There are still a lot of hobbyists who enjoy making high-quality live audio recordings of concerts, interviews, radio broadcasts, natural sounds, and so on. Professional equipment such as Nagra and the like is simply too expensive. We can thus be thankful that many MP3 players are equipped with an analogue input and a high-performance MP3 or WMA encoder.
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Gerber file

CAM/CAD data for the PCB referred to in this article is available as a Gerber file. Elektor 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. Elektor recommends its business partner Eurocircuits as the company of choice for its own prototypes and volume production in selected cases.

Extra info, Update
1. The circuit diagram shown in Figure 2 should be corrected to reflect that the negative terminal of capacitor C3 is connected to junction R10/R18/T2 base, i.e., not to the emitter of T2 as indicated. The same applies to C12 and T6. The printed circuit board design (Figure 4) is correct.
The indication with connection point ‘C1’ should be moved one level down, i.e., from GND1 down to the same level as C9.

2. On the PCB component overlay, the labels of SMD resistors R29 and R32 have been transposed. Resistor 29 (2 k?) should be the top device in the row of resistors fitted to the right of S1 and S2. R2 (6k?8) should be directly below it.
Resistors
R1,R12,R23,R34 = 470?
R2,R24 = 100?
R3,R33 = 4M?7
R4,R5,R25,R26 = 3k?3
R6,R11,R27,R32 = 6k?8
R7,R10,R28,R31,R45,R46 = 47k?
R8,R9,R29,R30 = 2k?
R13,R35 = 33?
R14,R22,R36,R44 = 220k?
R15,R37 = 2k?2
R16 à R19,R38-R41 = 22?
R20,R21,R42,R43 = 10k?
P1,P2 = potentiometer 10k? B Alps type RK09K11310KB (Farnell # 119-1725)
Capacitors
(all SMD):
C1,C4,C6,C7,C8,C10,C13,C15,C16,C17,C19-C22 = 10µF 16 V tantalum, Vishay Sprague 595D106X9016B2T (Farnell # 392-8895)
C2,C3,C5,C9,C11,C12,C14,C18 = 470nF/25 V tantalum, Vishay Sprague 595D474X9025T2T (Farnell # 116-6814)
Semiconductors
D1 = LED, 3mm, red
T1-T8 = BC856ALT1G, SMD, SOT-23, ON Semiconductor (Farnell # 114-7782)
T9,T10 = 2N7002, MOSFET, SMD, SOIC, Texas Instruments (Farnell # 102-1754)
IC1,IC2 = TL071CD, SMD, SOIC, Texas Instruments (Farnell # 958-9449)
Miscellaneous
K1,K2,K3 = 3.5mm jack socket with 3 contacts, Schurter R36310000 (Farnell # 149-933)
S1,S2 = SPST switch, angled pins, PCB mount, height = 12.7mm, C&K 7103MD9 AV2BE (Farnell # 957-5510)
BT1 = 9V battery with clip-on lead
K4 = 2-way PCB pinheader
Case with battery compartment, e.g. BC2, Farnell # 223-554
PCB, ref. 060237-I from ThePCBShop
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