About the article

Microphone Preamp

with professional specs

Microphone Preamp
Tape recorders have been obsolete for ages now, replaced with laptops fitted with a good-quality ‘USB sound card’. Now while a PC’s ‘line’ input generally offers acceptable characteristics, ‘mic’ inputs typically have only mediocre noise and distortion levels. Above all, they offer only a mono signal and are not suitable for connecting professional microphones with a balanced output, which often also require 48-V “phantom” powering. Here I’m suggesting a preamplifier that gets around these two shortcomings and is bound to please those who own ‘top range’ sound cards.
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Products
Relevant websites
Component list
Component List – Preamplifier
Resistors
All metal film
R1 = 220Ohm, 1%, 250 mW
R2, R3 = 6.8kOhm, 0.1%, 500mW
R4 = 470kOhm, 1%, 400mW
R5 = 1kOhm, 1%, 600mW
R6,R12 = 2.2kOhm, 1%, 400mW
R7 = 6.8 kOhm, 1%, 400mW
R8 = 22kOhm, 1%, 250mW
R9 = 68kOhm, 1%, 250mW
R10 = 220kOhm, 1%, 250mW
R11 = 47Ohm, 1%, 250mW
R13 = 12kOhm, 1%, 250mW
R14, R15 = 22Ohm, 1%, 250mW
Capacitors
C1 = 220µF, 100V, 20%, pitch 5/7.5mm, max 17.8mm
C2 = not fitted
C3,C4 = 100nF, 100V, 5%, pitch 5/7.5mm, PET
C5 = 15µF, 63 V, 10%, pitch 5/7.5/10/15/22.5mm, PET
C6 = 2.2nF, 1%, 160V, pitch 5/7.5/10/14.6mm, polystyrene, axial
C7 = 680pF, 2.5%, 160V, pitch 5/7.5/10/14.6mm, polystyrene, axial
C8 = 220 pF 2.5%, 160V, pitch 5/7.5/10/14.6mm, polystyrene, axial
C9 = 68pF 2.5%, 160V, pitch 5/7.5/10/14.6mm, polystyrene, axial
C10 = 22pF ±1pF, 160V, pitch 5/7,5/10/14,6mm, polystyrene, axial
C11, C12 = 220µF, 50V, 20%, pitch 5mm, max 10mm
Transformer
TR1 = LL1935, 1:10 (1:1:5+5), Lundahl, 38 × 23 × 16 mm
Semiconductors
LED1 = green, 3mm, T-1
LED2 = blue, 3mm, high intensity, T-1
IC1 = OPA134PAG4, DIP-8
Miscellaneous
K1 = audio XLR connector, 3 contacts, female, PCB mount (Neutrik NC3FAAH1)
K2, K4 = 2-way PCB screw terminal block, 0.2’’ pitch, 630V
K3 = 3-way PCB screw terminal block, 0.2’’ pitch, 630V
K5 = 7-pin pinheader, angled, 0.1’’ pitch
K6, K7 = not fitted (pos. opposite K5)
S1 = 2-pole, 6-way rotary switch, make-before-break, Lorlin type CK1060
PCB # 140426-1
Case type 1455K1201, Hammond Manufacturing
Other transformers that fit the board:
TR1 = Pikatron ÜP 3095 M, 1:10, 18×14,5×13,2 mm (see [3])
TR1 = Pikatron ÜP 3096 M, 1:15, 18×14,5×13,2 mm (see [3])
TR1 = Jensen transformers JT-115K-EPC, 1:10, 31.75×31.75×23.62 mm (see [6])
Component List – Power Supply
Resistors
R1 = 2.2kOhm, 5%, 0.25W, carbon
R2 = 1.0kOhm, 1%, 0.6W, metal film
R3 = 39kOhm, 1%, 0.25W, metal film
R4 = 22kOhm, 1%, 0.25W, metal film
R5 = 68Ohm, 1%, 0.4W, metal film
R6 = 12Ohm, 1%, 0.25W, metal film
R7 = 470Ohm, 1%, 0.25W, metal film
R8 = 100kOhm, 1%, 0.25W
R9 = 33kOhm, 1%, 0.25W
Capacitors
C1,C3,C5,C7,C14,C17 = 100nF, 100V, 5%, pitch 5/7.5mm, PET
C2,C6 = 10µF, 50V, 20%, pitch 2mm, max 6.5mm.
C4,C8 = 2200µF, 50V, 20%, pitch 5/7.5mm, 17.8mm max.
C9,C10,C11,C12,C19,C20,C21,C22 = 10nF, 200V, 10%, pitch 0.2’’, X7R ceramic
C13,C16 = 220µF, 100V, 20%, pitch 5/7.5mm, 17.8 mm max.
C15 = 15nF, 100V, 5%, pitch 5/7.5mm
C18 = 470µF, 100V, 20%, pitch 5/7.5mm, 17.8mm max.
C23, C24 = 100nF, X1, 440VAC, pitch 10/125/15 mm, polypropylene
Inductors & Transformers
L1 = B82724J2142N1, choke, common mode, 2×27mH, 1.4A
TR1 = Block type FL4/15, prim. 2×115V, sec. 2×15V, 2VA, PCB mount
TR2 = Block type FL6/24, prim. 2×115V, sec. 2×24V, 6VA, PCB mount
Semiconductors
D1,D2,D3,D4,D5,D6,D7,D8,D9,D10 = 1N4007, 1000V, 1A, DO-41
D11 = 1N4148, 100V, 200mA, DO-204AH
D12 = BZX55C12, zener diode 12V, 0.5W, DO-35
LED1 = green, 3mm, T-1
T1 = TIP31C, NPN, TO220 (100V / 40W / 3A)
T2, T4 = MPSA42, NPN, TO-92 (300V / 625mW / 500mA)
T3 = BC547C, NPN, TO-92 (45V / 500mW / 100mA)
IC1 = 7812, TO-220-3
IC2 = 7912, TO-220AB-3
Miscellaneous
JP1 = 0.6mm diam. wire (see text)
K1 = 3-way PCB screw terminal block, 0.2’’ pitch, 630V
K2 = 2-way PCB screw terminal block, 0.2’’ pitch, 630V
K3 = 2-way PCB screw terminal block, 0.3’’ pitch, 630V
F1, F2, F3, F5 (*1) = fuse, 100mA, slow blow, 250V, 20×5mm
F4 = fuse, 160mA, slow blow, 250V, 20×5mm
F1,F2,F3,F4,F5 = fuse holder, 20×5 mm, 500V, 10A
F1,F2,F3,F4,F5 = fuseholder cap , 20×5 mm
HS1 = heatsink, Aavid Thermalloy type SW25-4, 13°C/W, 34.5×12.5×25 mm
PCB # 140426-2
*1. For 115 VAC : 200mA, slow blow, 250V, 5×20 mm
Case type 1455N1601, Hammond Manufacturing
From the Store
PCB 140426-1
PCB 140426-2
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