Build a mic preamp that can be powered by a 9V battery or a simple regulated source. In many occasions, we have an amplifier already built with its preamplifier and tones. For some reason, we need to add a microphone input. A specific case is in the jukebox amplifiers. Nowadays, the use of karaoke in the same video clip is becoming increasingly popular. It is at that moment that it becomes necessary to add a microphone preamp. Another need is that of the musicians, who need to amplify their voice, and in many cases an electro-acoustic guitar. This time we present a preamplifier for a microphone that can be powered by a 9- volt battery and even a simple 24- volt source.volts. You can amplify a common microphone or an electroacoustic guitar. It also can mix your signal, with another signal coming from a line preamplifier with tones.
– 1 of 0.01 μF.
– 1 of 0.1 μF.
– 1 of 4700 ρF.
– 1 of 0.047 ρF.
– Two electrolytic one μF.
– Two electrolytic (tent) of 1 μF.
-Two 2N5089 semiconductors.
– 2 of 1 MΩ.
– 2 of 47 KΩ.
– 2 of 2.2 KΩ.
– 2 of 100 KΩ.
– 1 of 4.7 KΩ.
– 1 of 22 KΩ.
– 1 of 1 KΩ.
– Two in 250K
– 1 log of 250K.
– One line of 10K.
– 9V power supply.
– One double inverter key switch.
– Two 6.5mm jacks (one mono and one stereo)
– One-click of the battery.
– One transformer jack.
What is our preamplifier?
This pre-amplifier is quite simple and consists of two transistors and four parallel controls, to be able to change the intensity of the volume, the treble, the bass, and the gain.
The pre-amplifier, we can use it as a buffer at the beginning of a series of elements and special effects devices for our guitar or simply, place it before our plant to obtain a truly powerful and spectacular sound.
The integrated circuit TL071
The main component of this preamplifier is the integrated circuit TL071. The TL071 is a low-noise J-FET operational (junction field-effect transistor) amplifier. It is connected in inverter configuration. This means that the signal enters through pin 2, which is the inverting input. The other input or pin 3 (not inverter), is used to make a voltage divider or pedestal, which allows connecting the TL071 with simple voltage. That is, we do not require a symmetric font. This is a great advantage in case we want to feed it with batteries. The TL071 can be replaced by the TL081 or by the famous 741.
The gain of the preamplifier is determined by two resistors ( R1 ) of 1K and ( R2 ) of 100K. The preamp has a gain of 100, which is the result of dividing 100K /1K. If you want to increase the gain, it is only necessary to change the resistance of 100K for a higher one, for example, 200K. Keep in mind that the more profit, the more noise there may be. So just change the resistance if the volume of the microphone or the electroacoustic guitar is very low. The maximum limit at which we can raise the feedback resistance is one megohm. In case of needing to lower the gain, my advice is to raise the resistance of 1K. For example to10K. It’s a matter of trying.
Some recommendations to take into account when making this circuit
1) If you are going to make the printed circuit with the ironing method, be sure to use the drawing that is in the mirror mode. Otherwise, the will be upside down and you will lose the job.
2) Place all the components as shown in the component mask that is delivered in the PDF file at the end of this article.
3) Check very well that the tracks do not have shorts between them and that the components are placed with the correct polarity.
4) Weld well and wash the printed circuit with thinner and a toothbrush. Dry the form very well before connecting it to the power supply.
The microphone input jack
To facilitate the assembly work and avoid possible noise filtering, we have placed a monophonic spider jack in the microphone input. Note that the contact pins are on the right side, looking at the front jack. We make this clarification since in some electronic stores they sell jacks with the pins, on the contrary, that is, they come from the left side and this jack does not work for this project. If you only get the left jack, you must disarm it and reverse the pins.
If you only get stereo jack, you should remove the pin from the center and you’re done.
If you do not get a spider-type jack, you can use an ordinary jack, and remove two cables from the card. A cable is a positive input, which is the one that goes to the 2.2 of the capacitor and the other would be land ( GND).
The power input is simple
The power input is simple. That is to say that only positive and earth are used. This facilitates power with batteries, which can be from 9V, up to 24V DC.
Every operational circuit is normally connected with the asymmetric source. This is done to achieve a point of rest the output of zero ( 0 ) volts.
This resting point can be achieved by making the integrated one believe that it has an asymmetric source, but using a simple source, we will explain this briefly.
Normally an operation such as TL071 or 741 will be powered with a positive voltage on pin 6, the negative voltage on pin 4, and ground on pin 3.
To be able to use a simple source, pin four is connected to ground and a voltage divider is made, also known as a pedestal that is connected to pin 3, known as the non-inverting input. The divider consists of two 10K resistors ( R3 and R4 ). One resistance goes from a positive voltage to pin 3 and the other resistance goes from ground to pin 3. At that moment if we measure the voltage present on pin 6, we will have Vcc 1/2. It means that if we feed the preamplifier with 12 volts, we will have 6 volts on pin 6.and we will have -6 volts on pin 4 — this using pin three as earth. We hope that this explanation will help you when you want to connect any operational with a simple source. A clear example of this way of connecting the operational can be observed studying the pedals for electric guitar. A 9-volt battery usually powers these.
We always recommend using connectors in all ports of our circuits. This, to facilitate the connection or disconnection, in case of a breakdown or maintenance.
Both monoblock amplifiers and stereo have placed a connector MOLEXof 2.54mm three pins. This one gives the signal in the two pins of the ends and the earth is in the middle. If you plan to connect the preamplifier to a mono amplifier, just take the signal from either end and the center ground. And if the idea is to connect it to a stereo amplifier, the signal from each pin of the ends is sent to each input ( Land R), and thus we will have the sound in the two outputs to speakers.
Signal input of another preamplifier and adder
Another great advantage of this preamplifier is its easy adaptation in any amplifier. It has an adder formed by two 20k resistors, which are responsible for mixing the microphone signal, with the two signals L and R coming from the line preamplifier with tones that are used or our control of tones and volume controlled by voltage.
In our PDF file, you will find several connection diagrams that will give you different ideas of application forms of this preamplifier.
We hope that this project will please everyone.
NOTE: ALL OUR PROJECTS ARE TESTED BEFORE THEY ARE PUBLISHED. Read this article and study the diagram until you understand how the circuit works. Do not put it together just to put it together. When the operation of a device is clear, there will be no obstacles when constructing it.