Portable Microphone Preamplifier

High headroom input circuitry, 9V Battery operation

This circuit is mainly intended to provide common home stereo amplifiers with a microphone input. The battery supply is a good compromise: in this manner the input circuit is free from mains low frequency hum pick-up and connection to the amplifier is more simple, due to the absence of mains cable and power supply. Using a stereo microphone the circuit must be doubled. In this case, two separate level controls are better than a dual-ganged stereo potentiometer. Low current drawing (about 2mA) ensures a long battery life.

Circuit Operation:

The circuit is based on a low noise, high gain two stage PNP and NPN transistor amplifier, using DC negative feedback through R6 to stabilize the working conditions quite precisely. Output level is attenuated by P1 but, at the same time, the stage gain is lowered due to the increased value of R5. This unusual connection of P1, helps in obtaining a high headroom input, allowing to cope with a wide range of input sources (0.2 to 200mV RMS for 1V RMS output).

Portable Microphone Preamplifier Circuit diagram:

Parts:
P1 = 2.2K
R1 = 100K
R2 = 100K
R3 = 100K
R4 = 8.2K
R5 = 68R
R6 = 6.8K
R7 = 1K
R8 = 1K
R9 = 150R
C1 = 1uF-63V
C2 = 100uF-25V
C3 = 100uF-25V
C4 = 100uF-25V
C5 = 22uF-25V
Q1 = BC560
Q2 = BC550

Notes:

• Harmonic distortion is about 0.1% @ 1V RMS output (all frequencies).
• Maximum input voltage (level control cursor set at maximum) = 25mV RMS
• Maximum input voltage (level control cursor set at center position) = 200mV RMS
• Enclosing the circuit in a metal case is highly recommended.
• Simply connect the output of this device to the Aux input of your amplifier through screened cable and suitable connectors.

Source :  http://www.ecircuitslab.com/2011/06/portable-microphone-preamplifier.html

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Quality Stereo Wireless Microphone or Audio Link

This stereo FM wireless microphone also makes a great quality audio link. We tested it to beyond 50 meters and it was rock solid. It’s certainly not the first wireless microphone we’ve ever published but this one is a little different. It’s stereo, providing surprisingly good quality sound. Second, it has a really good range. We tested it at well over 50m and it was still performing very well – noise-free, in fact – but at the time we couldn’t get our receiver any further away. So it’s likely to have even better range than that.

Complete Project:

Its easy to build, requires very little setup... and its cheap! In fact, the low price might turn some people off, thinking its low quality. Try it - and be pleasantly surprised! Third, it really is simple to build – the hard work (the transmitter module) is already done for you. It’s just a matter of assembling the microphone module, which contains the electret mics themselves, preamp and level controls, and soldering the transmitter module onto it, "piggy back" style.

FM Transmitter:

Finally, the transmitter module is crystal-locked, so you won’t have the drift probles of some earlier wireless microphones. And just in case you were wondering, that doesn’t mean the output is locked to one particular frequency – it has a nifty synthesis circuit built in to give you the choice of seven different frequencies between 106.7MHz and 107.7MHz. On-board preset pots adjust the sensitivity of each channel to take into account mic differences or if you require different levels in each channel.

By the way, the transmitter module is quite capable of operating at line level if you want just a line level transmitter (eg, to feed an audio program around your home). Sensitivity is about 100mV. Oatley Electronics, who designed the kit, have the transmitter module available by itself if that’s what you’re after. But more on that anon.

Parts Layout:

You also have the choice of two power supply levels – 3-6V or 7-15V DC. The latter results in a lower current drain. The transmitter module also has a "5V out" rail to supply power to the preamp module.

Circuit Diagram:

Two electret microphone inserts are supplied in the kit. These can be soldered direct to the PC board to make it a fully self-contained project or they can be attached to the board via suitable lengths of mini shielded coax cable. A third option is to use "proper" microphones – they can be electret or dynamic types – but no provision has been made for plugging these in.

Typical Specifications:

Audio response:.....................20Hz-15kHz.
Channel separation: ........................40dB
Total Harmonic Distortion: ...............0.1%
Output Frequency:...........106.7-107.2MHz
Pre Emphasis: ..................................50μS
DC supply voltage range: ................3-15V
Supply Current: .......................30mA @ 9V

Source: Silicon Chip 28 May 2005

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Compressor For Electret Microphone

The ‘FM Remote Control Receiver’ (available on this website in Infra-red circuits section) has a connector where an analogue output is made available. To make a simple intercom or P.A. system the associated transmitter needs a microphone pre-amplifier that outputs a signal at the correct level. And that is exactly the function of this circuit. Actually, this design is adapted from a circuit published last year (‘AM Modulator for Intercom’). A few things have been changed so that it can work with the 5 V supply from the transmitter module. The OTA (IC1) used here is the single version (CA3080), which has slightly different characteristics from the dual CA3280.

The quad opamp is the same rail-to-rail TS924IN, made by ST. The turnover frequency of the filter (3rd order 1 dB Chebyshev) has been increased slightly to improve the intelligibility of speech and is now about 5.5 kHz. The filter now amplifies the signal by a factor of 10. In practice it is possible that due to various tolerances and the fact that the opamp is not perfect, the filter characteristic shows some deviation from that required. In our prototype it was necessary to change R15 into 2k7 to straighten the response curve. The DC current variation at the output of the OTA and the resulting offset variation at the output of current/voltage converter IC2d is such that the gain of IC2d has to be substantially smaller than in the ‘old’ design.

Otherwise the output could easily rise to the supply voltage at low signal levels. The value of R6 has therefore been made smaller by a factor of 10. This has reduced the gain of the circuit by 20 dB, which is compensated for in the filter. The amplitude of the signal from IC2d is fed back as a control current to the OTA by peak rectifier D1/C3 and inverting amplifier IC2b. R7 limits the loading on IC2d. P1 can be used to adjust the amplifier between a fixed gain and maximum compression. Figure A shows clearly what effect the circuit has. 0 dBr corresponds to 100 mV. The maximum gain, with P1 set to maximum compression, is about 48 dB (250 Ω) for small signals.

The minimum gain is about 20 dB (10 Ω). The OTA is then slightly overdriven and the distortion becomes several percent! With a fixed gain selected (P1 shorted) the gain is about 42 dB (125 ×). The middle curve was measured with P1 in its central position. The curve drawn for a fixed gain (the straight line) doesn’t finish at the edge of the graph because the end of the line corresponds to the maximum possible output level, which is 25 dBr (≈1.76 V or 5 / 2√2). Figure B shows the frequency response. The low turnover frequency is mainly determined by C8 (and to a lesser extent by C1) and is about 120 Hz.

The current consumption is about 7 mA When the circuit is battery powered we recommend the use of three AA cells, because the circuit still works perfectly at 4.5 V. If you want to use a higher supply voltage (maximum 12 V for the de TS924IN and 30 V for the CA3080, but you should also think of the voltage across the electret microphone!) you have to keep in mind that the maximum current through R9 (which is IABC) is only 2 mA. When we consider a maximum chosen current of 1 mA and the maximum output voltage of IC2b (half the supply voltage, which is 2.5 V), then the value of R9 should be (2.5 – 0.7) V / 1 mA = 1.8 kΩ. The value of 0.7 V corresponds to the potential between pin 5 and earth.

For a larger safety margin R9 is calculated with the full supply voltage and a current of 2 mA: (5 – 0.7) V / 2 mA = 2k2 (rounded upwards). Of course the regulation will then be different (a little less gain). This circuit and the transmitter module can therefore be fed from the same 5 V supply. Because the transmitter requires a DC offset at its input, a resistor is connected to +5 V via a jumper, which biases the output to half the supply voltage. With the jumper open R17 functions as a load resistor when the output is not connected, because C9 still has to charge up even without a load. If you’re designing a PCB for this compressor then it makes sense to include the transmitter module as well. The current consumption then increases by about 10mA.

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