Article

Stereo Microphone Preamplifier

For MP3 equipment

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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Materials

060237-PCB.pdf

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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PCB

Download Gerber file: 060237-1.zip

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.

Component list

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

Discussion (0 comments)

Emil Almberg 4 years ago

Can someone please clarify the citat:

The indication with connection point ‘C1’ should be moved one level down, i.e., from GND1 down to the same level as C9.

Thanks.

MarcelC 4 years ago

Hello
I am the author of the article. Where did you read this citat? I don't understand without giving me the context, the article is old ...

Marcel CREMMEL

Emil Almberg 4 years ago

Hi, thanks for the answer, it's up there under Extra info / Update, not in the article itself.
The drawing is apparently wrong in 2 places.

By the way, do you think I should build this design today?
It has the advantage of not using expensive special components but maybe there are better solutions today?
I'm interested in building good audio equipment myself, just because it's fun.

MarcelC 4 years ago

Hello Emil
The technician who drew the diagram made a mistake in the positioning of the equipotential bonding identifier C1 (see diagram extract). "C1" is the name of the equipotential connected to the "+" input of IC1.
Already at the time of the publication of the article, there were high-performance integrated circuits to make a microphone preamplifier.
But, as an electronics enthusiast like you, I was motivated by achieving similar, if not better, results with "drawer bottom components".
You can have fun changing transistors and analyzing the results ...

capture-20210408121416.PNG (14kb)

Emil Almberg 4 years ago

Thanks again for the reply.
But I do not understand, can we draw a little so everyone is involved?
See my sketch, according to Elektor, the microphone's inputs should be connected to the transistor's base connections, is that correct?
Also, there's something wrong with the opamp's non-inverted input, where I get lost.
Can you please draw how it should be, thank you!

Again, I do this just because it's fun and of course you want solutions that you can play a lot with, and avoid special circuits!

micamp.png (158kb)

MarcelC 4 years ago

Hello.
In view of the many errors in copying the diagram, the easiest way is to provide you with my original schematic.
Note that the PCB conforms to this schematic. There is no scratch track!
Kind regards

11Préampli_micro.pdf (704kb)

Emil Almberg 4 years ago

It was very kind of you, thank you very much.
This rase the old question, is any article in the magazine E really completely flawless?
Thinking about changing your design a bit, I want to remove shutdown via microphone connector, as I do not need that feature, and add a low battery indicator.