Low Noise Balanced Microphone Preamp Using TL071 IC

This is a simple design circuit which has very low noise, close to the theoretical minimum, high hum rejection and variable gain with a single rotary pot. The circuit design consists of differential compound pairs of transistors with a common mode (floating) gain control connecting the emitters of the pair. The compound pairs of 2N4403 and BC549s are far more linear than any single transistor. The circuit is differential in and out and therefore requires a balanced to unbalanced buffer to give suitable output for the next signal stages of a channel in a mixing desk. This is provided by a high performance op-amp differential gain stage, which can be a TL071 or similar IC of your choice. The stage has a gain of six or 15 dB and that sets the maximum input level at about 1.5 volts rms before clipping. This equals an SPL of over 150dB with a typical microphone. This is a figure of complete design for the circuit.


The operation of the circuit is input stage is configured for least noise and this has meant a non IC approach. There are some special ICs that can be used for microphone pre-amps, they contain a circuit like this one except fabricated on one chip. Components should all be readily available except for the 10 k ohm pot for the gain control. This needs to be a reverse log taper - or else use a multi-position switch with 6 dB gain steps covering the 60 dB range of the circuit. Make sure it is make before break. The +/-15 Volt power supply is important too, it must be regulated and low noise. If the usual voltage regulator ICs are used I recommend fitting a post filter consisting of a 10 ohm resistor and a 470 uF capacitor to remove any noise generated in the ICs.

Good quality components should be used with metal film resistors in the collectors and emitters of the input pairs for least noise. Where a resistor has significant DC voltage imposed on it in high gain circuits always use low noise types. Metal film resistors are about the best only bettered by wire wound which is a bit impractical. Avoid metal glaze, and very old carbon composition types. Also avoid bead tantalum capacitors, as they go leaky and crackle. They are just about the most fragile electronic components made. The 100nF capacitor (C6) should be mounted as close as possible to the op-amp supply pins - a ceramic cap is recommended for best bypass performance at high frequencies.

The design source is by Phil Allison

Low Power FM Transmitter Using SDM

This is a low-power FM transmitter using surface-mount devices (SMD) that will be received with a standard FM radio. There are many designs for small FM transmitters but they have some problems. First, you need an audio amplifier to get enough modulation. Second, the antenna is attached directly to the collector. Third, the coil L must be wound by hand and adjusted by stretching. This is the figure of the schematic.


How is the circuit work? The transmitter is consists of two stages: an oscillator and an output amplifier. Modulation is from an electric microphone but you can use a low power audio source. Transistor Q1 is a Colpitts oscillator where the frequency is determined by the parallel resonant circuit formed by inductor L, varactor V1 and capacitors C7 and C8. Q1 is a common-collector amplifier where the power gain counts. V1 is actually dual varactor that eliminates the possibility of forward conduction at the sine wave peaks.

The frequency of oscillation is set by adjusting the DC voltage on V1 with potentiometer R2. R4 and C3 form a low-pass filter to prevent RF from feeding back onto the DC. Capacitors C7 and C8 form an AC voltage divider to provide feedback at the emitter of Q1 to sustain oscillation. Modulation is done by superimposing an audio signal from the electret mic onto the DC bias applied to V1. R3 and C1 form a low-pass filter to prevent RF from feeding back to the microphone. R3, R4 and R2 form a voltage divider for the audio.


The output of the oscillator is fed through C9 to the Q2 emitter-follower. The output of Q2 drives the antenna through C11. The Q2 emitter-follower it ensures that the oscillator is not loaded down by the impedance of the antenna and it provides power gain to drive the antenna. This is the component of the circuit.

VU and PPM Meter Circuit for Audio

A VU (Volume Unit) meters used to be the mainstay of audio metering system. The Peak Program Meter (PPM) is notoriously bad at showing the peak signal level. This is a circuit that have function same of the explanation in above. In this circuit, the amplifier/rectifier is a simple LM1458 or similar dual op-amp, and buffers the rectifier circuit. This is the figure of VU meter;


The principle of the VU meter is a single diode is used in some, but the better ones will generally use a tiny selenium bridge rectifier or a germanium diode bridge. A capacitor is show, few budget VU meters will include it. As a result, the meter movement itself is uncontrolled in most of these meters, so overshoot is often huge, and the reading is almost useless. Because of the diode forward voltage, many of these meters also fail completely to register low level signals (< -20 dB). In figure 2, is shown of the meter's ballistic control for the VU meter. The principle of this circuit is the values of R6, C1, C2 and C3 may need to be adjusted, depending upon the ballistics of the meter movement you use. Because meters vary so widely in this respect, it is only possible to provide representative values, although they should work quite well in practice. In order to get exact VU meter ballistics, it will be necessary to test the meter with a 300ms burst waveform at full scale (+3VU). It should reach 99% of full scale with up to 1% of overshoot before dropping back to zero. If we want to know about the result of the VU meter testing, we must make the circuit.

15dB UHF Antenna Preamp Circuit

This is an UHF band TV antenna preamp circuit with 15dB gain and build by a transistor. It is formed based on BF180 UHF Transistor. This circuit is a simple circuit. This is a figure of the circuit.


The principle operation of the circuit is two stages. The first stage is an band pass filter constructed by the C1, CV1, L1, L4, C7 and C3, the second stage is a base-common voltage amplifier with low input impedance to match. Build the L1 ~ L4 as air core coil to obtain high Q-Factor. You can find out about air core coil construction and calculation here. After assembling, pack it into a proper metallic box and connect the ground of the circuit to the box to reduce noise effect.

Variable Voltage and Current Power Supply Circuit Using LM1458

This is another design for power supply that can build or based on LM1458. This is called variable voltage and current power supply. It is a once of regulated power supply. This figure below is shown the circuit;


The operation of this circuit is the power transformer requires an additional winding to supply the op-amps with a bipolar voltage (+/- 8 volts), and the negative voltage is also used to generate a reference voltage below ground so that the output voltage can be adjusted all the way down to 0. Current limiting is accomplished by sensing the voltage drop across a small resistor placed in series with the negative supply line. As the current increases, the voltage at the wiper of the 500 ohm pot rises until it becomes equal or slightly more positive than the voltage at the (+) input of the op amp.

Current limiting range is about 0 - 3 amps with components shown. The TIP32 and 2N3055 pass transistors should be mounted on suitable heat sinks and the 0.2 ohm current sensing resistor should be rated at 2 watts or more. The op amp output then moves negative and reduces the voltage at the base of the 2N3053 transistor which in turn reduces the current to the 2N3055 pass transistor so that the current stays at a constant level even if the supply is shorted. The heat produced by the pass transistor will be the product of the difference in voltage between the input and output, and the load current.

VariablePower Supply Circuit

This is a regulated power supply circuit that can adjust between 3 – 24 Volts with current limit is 2 amps. In reality, this circuit can increasing the amps up to 3 amps. But, for doing those amps will must selecting a smaller current sense resistor. Voltage regulation is controlled by 1/2 of a 1558 or 1458 op-amp. This is a figure circuit of variable power supply;


The 2N3055 and 2N3053 transistors should be mounted on suitable heat sinks and the current sense resistor should be rated at 3 watts or more. The 1458 may be substituted in the circuit below, but it is recommended the supply voltage to pin 8 be limited to 30 VDC, which can be accomplished by adding a 6.2 volt zener or 5.1 K resistor in series with pin 8. The maximum DC supply voltage for the 1458 and 1558 is 36 and 44 respectively.

The power transformer should be capable of the desired current while maintaining an input voltage at least 4 volts higher than the desired output, but not exceeding the maximum supply voltage of the op-amp under minimal load conditions. The power transformer shown is a center tapped 25.2 volt AC / 2 amp unit that will provide regulated outputs of 24 volts at 0.7 amps, 15 volts at 2 amps, or 6 volts at 3 amps. The 3 amp output is obtained using the center tap of the transformer with the switch in the 18 volt position.

Variable Brightness AC Lamp Circuit

This circuit is based on SCR operation to control the lamp using slowly intensity. The intensity of a 120 volt light bulbs by controlling the time that the AC line voltage is applied to the lamp during each half cycle. The circuit is directly connected to the AC power line and should be placed inside an enclosure that will prevent direct contact with any of the components. To avoid electrical shock, do not touch any part of the circuit while it is connected to the AC power line. This is figure of the circuit;


How is the circuit will work? This circuit using transistor NPN. A couple NPN transistors are used to detect the beginning of each half cycle and trigger a delay timer which in turn triggers the SCR at the end of the delay time. The delay time is established by a current source which is controlled by a 4017 decade counter. The first count (pin 3) sets the current to a minimum which corresponds to about 7 milliseconds of delay, or most of the half cycle time so that the lamp is almost off. Full brightness is obtained on the sixth count (pin 1) which is not connected so that the current will be maximum and provide a minimum delay and trigger the SCR near the beginning of the cycle. The remaining 8 counts increment the brightness 4 steps up and 4 steps down between maximum and minimum. Each step up or down provides about twice or half the power, so that the intensity appears to change linearly. The brightness of each step can be adjusted with the 4 resistors (4.3K, 4.7K, 5.6K, 7.5K) connected to the counter outputs.

Symmetric Power Supply Circuit

This is a symmetric power supply that is based on LM7912 IC. There are two IC’s that can used in this circuit. Both of them, is using as voltage regulator and connected with zener diodes to deliver a stable DC voltage. The rectifier and filter is built by rectifier diodes and filter capacitors C1 and C4. Capacitors C2 and C5 is using to stabilize the regulator ICs. This is the figure of the circuit;


The circuit, it becomes unconventional. Both op-amps together with the driver transistors are wired as DC voltage amplifiers. The non-inverting inputs are connected to +2.45 volts through voltage dividers R2/R5 and -2.45 volts through R7/R10. A regulated symmetrical voltage extracted from the voltage divider P1/R1/R9 and subtracted from the fixed +/- 2.45 volts. The negative from IC3 and the positive is from IC4. This results to a symmetrical voltage outputs from +/- 4.9 volts to 18 volts. Capacitors C7 and C8 are stabilizing capacitors. The resistors R6 and R11 are pull-down resistors to preload the outputs in case of an empty load.

The accuracy of the voltage symmetry is dependent on the resistor values of the three voltage divider circuits. If there is non-symmetry in the output, it is probably due to one of the regulator ICs with its 10% tolerance rating. In such case, adding a 5K trimmer in series with R1 or R9 is recommended. Adjust the trimmer until symmetry is achieved.

Sunrise Lamp Circuit Using MOSFET IRF640

This circuit is a design circuit that can used to make decoration in the house. This circuit is based on LM324 for generate triangle waveform with frequency 700Hz. It is a 120VAC lamp is slowly illuminated over an approximate 20 minute period. This is the figure of the circuit;


The circuit is supplies DC voltage by bridge rectifier to the MOSFET and the 60 Watt. A 6.2K, 5 watt resistor and zener diode is used to drop the voltage to 12 volts DC for the circuit power. The bridge rectifier should be rated at 200 volts and 5 amps or more. In pin 1 LM324 is generate signal and a slow rising voltage is obtained at pin 8. These two signals are compared at pins 12 and 13 to produce a varying duty cycle rectangular waveform at pin 14, which controls the MOSFET and brightness of the 60 watt lamp. When power is applied, the lamp will start to illuminate within a minute or so, and will slowly brighten to full intensity in about 20 minutes.

If you want to adjust this lamp, you can with adjustments to the 270K resistor at pin 9. The 2.2 ohm resistor and .015uF cap connected to the lamp serve to sub press RFI. The diodes at the pin 9 and 10K resistors on pin 8 are used to discharge the 3300uF cap when power is removed. Power should be off for a few minutes before re-starting.

LED Audio VU Meter Circuit

The LED meter circuit is simpler and smaller than its analogue counterpart, and is very common in audio equipment. This circuit is based on LM3915 IC and uses the logarithmic version. This circuit is using a single IC and a few discrete components. The extra diode (D3) is included to ensure that the DC to the LEDs is almost unfiltered. C1 is included to make sure the IC does not oscillate, and is not a filter capacitor. This allows a higher LED current with lower dissipation than would be the case if the DC were fully smoothed, and full smoothing would also require a much larger capacitor. This is the figure of the LED audio VU meter circuit;


How is this circuit work? We will explain with simple ways. L1 to L8 will normally be green (normal operating range) and L9 and L10 should be red (indicating overload). This gives a 6dB overload margin when the unit is calibrated as described below. As shown, full scale sensitivity (with VR1 at maximum) is 12 Volts peak (approximately 8.5 volts RMS). This is designed for direct connection to the speaker output of an amplifier, but is still suitable for use with preamps if the sensitivity is changed. Power comes from a 15-0-15 transformer (connected to AC1-Com-AC2). You can generally use the smallest one available, as average power is quite low. The peak current is about 120mA DC, so a 5VA transformer will be sufficient to power two meter circuits. One 15V winding goes to the terminal AC1, the other goes to AC2 and the centre tap is connected to Com (Common).

Inductance Meter Adapter Circuit

The inductance meter adapter circuit output is connected with a frequency meter and the inductance is calculated from the frequency. So, you will need a frequency meter and some calculation to get your inductor value. The circuit enables to measure inductance of the inductor which is the inductance to be measured. The operation of the circuit is built by a TTL square wave whose frequency relates to the inductance being measured. This the figure of the circuit;


How is the circuit work? The core of the circuit is the buffer colpitts oscillator(the first stage) which resonates with the unknown inductance to give a sinus wave of a particular frequency . The frequency of the sinus wave is a function of the unknown inductance and the four 1000pF C. The output sinus wave is amplified by the second transistor and is then rectified by the capacitor and diode combination that follows. The rectified sine wave now having only positive excursions is buffered by the third transistor and is then fed to the 74ls393. Counter IC which is configured as a divide by 256 counters. The output of the IC pin 6 and ground is connected to the frequency meter.

Audio Remote Security and Monitor

This is a audio security monitor circuit. This circuit is built by two IC, LM386 and TL071. This design is a simple design that can use this in the garden and listen for any unusual sounds, or maybe just wildlife noises. If you have a car parked in a remote location, the microphone will also pick up any sounds activity in this area. The cable may be visible or hidden, screened cable is not necessary and you can use bell wire or speaker cable if desired. In this figure is show a circuit and how the circuit done.

The work of this circuit is starting the power supply. This circuit used 12V as a standard power supply voltage, or a 12V car battery may be used. The circuit is in two halves, a remote microphone preamp, and an audio amplifier based around the National Semiconductor LM386 audio amplifier. The remote preamp uses an ECM microphone to monitor sound. A direct coupled 2 stage amplifier built around Q1 and Q2 amplify the weak microphone signal. Preset resistor R2 acts as a gain control, and C1 provides some high frequency roll off to the overall audio response. Q1 is run at a low collector current for a high signal to noise ratio, whilst Q2 collector is biased to around half the supply voltage for maximum dynamic range. The power supply for this preamp is fed via R10 and R6 from the 12V supply. C4 ensures that the preamp power supply is decoupled and no ac voltages are present on the power lines. The amplified audio output from Q2 collector is fed into the supply lines via C6 a 220uF capacitor. The output impedance of Q2 is low, hence the relatively high value of C6. C6 also has a second purpose of letting the output audio signals pass, whilst blocking the dc voltage of the power supply.

At the opposite end, C7 a 10uF capacitor, brings home the amplified audio to the listening location. The signal is first further amplifier by a x10 voltage gain amplified using the TL071. C8, a 22pF capacitor again rolls off some high frequency response above 100kHz. This is necessary as long wires may pick up a little radio interference. After amplification by the op-amp, the audio is finally passed to the LM386 audio amplifier. R14 acts as volume control. R13 and C12 prevent possible instability in the LM386 and are recommended by the manufacturer. Audio output is 1 watt into an 8 ohm loudspeaker.

120 VAC Dimmer Lamp Circuit

It is a simple design full wave phase control. This circuit is supplied by AC line. In this circuit is there the four diodes provide a full wave rectified voltage to the anode of a SCR. This is the figure of this circuit;


This is a principle work of the circuit. Two small signal transistors are connected in a switch configuration so that when the voltage on the 2.2uF capacitor reaches about 8 volts, the transistors will switch on and discharge the capacitor through the SCR gate causing it to begin conducting. The time delay from the beginning of each half cycle to the point where the SCR switches on is controlled by the 50K resistor which adjusts the time required for the 2uF capacitor to charge to 8 volts. As the resistance is reduced, the time is reduced and the SCR will conduct earlier during each half cycle which applies a greater average voltage across the load. With the resistance set to minimum the SCR will trigger when the voltage rises to about 40 volts or 15 degrees into the cycle. To compensate for component tolerances, the 15K resistor can be adjusted slightly so that the output voltage is near zero when the 50K pot is set to maximum.
Note; Don’t touch this circuit when it is connected with Voltage AC and Current AC.

25W Hi-Fi Power Amplifier Circuit

25 W Hi-Fi power amplifier circuit is a one of simple design that’s build by transistor NPN and PNP. The output devices are MJL4281A (NPN) and MJL4302A (PNP), and feature high bandwidth, excellent SOA (safe operating area), high linearity and high gain. Driver transistors are MJE15034 (NPN) and MJE15035 (PNP). All devices are rated at 350V, with the power transistors having 230W power dissipation and the drivers are 50W. The amp may also be operated at lower supply voltages for less power, but I do not recommend less than ±18V, which will provide around 15W into 8 ohms. This supply voltage (approximately) may be obtained by using a 15-0-15V transformer. This figure is show the circuit and operation of the circuit.


The supply voltage should be a maximum of ±25V. This supply is easily obtained from a 20-0-20V transformer. All resistors should be 1/4W or 1/2W 1% metal film for lowest noise, with the exception of R9, R10 and R15 which should be 1/2W types, and R13, R14 must be 5W wire wound. The bootstrap capacitor (C5) needs to be rated at least 25V, but the other electrolyc can be any voltage you have available. The potentiometer (VR1) must be a multi turn type, as the current setting is critical. Each of these amps will require a 0.25°C/W heat sink (very large). Consider using a fan or even water cooling to keep temperatures as low as possible.