If you try to look for a good and simple schematic of an audio preamplifier, which can be easily assembled, you will realize how difficult is to find it. To fill this gap, we propose 4 small audio preamplifiers that besides their simplicity, they exhibit excellent sound characteristics. Although each preamplifier uses only 2 transistors, it exhibits excellent sound quality.
Each preamplifier is able to boost a weak audio signal at an adequate level, for driving the input of a final amplifier. All the projects in this article were assembled and tested, so we can guarantee that their characteristics correspond to those declared.
Since all the schematics presented here are monophonics (support a single audio channel), you may wish to build two identical circuits to use them for a stereo source. Taking into consideration the simplicity of the circuit boards and their low cost, this will not be a problem.
The circuit schematic for the first audio preamplifier is presented in fig 1. The preamplifier uses only two NPN transistors. Both transistors are BC172. You may also use BC547 instead, or another equivalent type as well.
The technical features of first preamplifier are summarized as follows:
Nomimal voltage: 12V
Current consumption: 1.2mA
Voltage gain: 50-55
Maximum input signal level: 150 mV p/p
Maximum output signal level: 8 V p/p
Frequency response: 20 Hz to 200 KHz
Although the recommended supply voltage is 12 volts, the preamplifier can be powered as well from any voltage source ranging from 9 volts to 15 volts. If you choose to use 9-volt, the maximum input signal amplitude will be limited to 120 mV p/p. Significant distortion may be present, if you exceed this limit.
It worth’s to remind that in order to convert the value of a voltage expressed in volts peak / peak in a voltage expressed in volts (rms), you must divide the p/p value into the fixed number 2.82. Thus, 150 mV p/p are equivalent to:
150/ 2.82 = 53 mV rms
The maximum output voltage of 8 volts p / p corresponds to an rms value of:
8/ 2.82 = 2.8 V rms
In fig.1, we have also included the nominal bias voltages at certain points of the circuit (given a power supply voltage of 12V). Since there are tolerances at components values, do not worry if you measure slightly different voltages than those presented in figure 1.
To simplify the assembly procedure, we have designed an appropriate small printed circuit board. You may easily etch and drill this small PCB presented in figure 2, and you may follow the assembly guide of figure 3 for soldering the components.
When placed on the printed circuit board, transistors TR1-TR2 must be placed with the flat side of their body pointing to the left, as shown in Figure 3. Remember also to place correctly the electrolytic capacitors according to the correct polarity.
Another simple audio preamplifier is shown on figure 4. This specific design uses also the same transistors used in the previous design, but differs from the classic pattern of a transistor preamplifier, because the base of the second transistor (see TR2) is directly connected to the collector of the first transistor and the signal output is provided from the emitter of the final transistor, rather from its collector.
This preamplifier is capable to accept at its input, much stronger signals than the previous design. It is capable to accept as much as 2 volts peak / peak at its input –which is about 0.7 volts rms.
The technical features of this specific preamplifier are summarized as follows:
Nomimal voltage: 12V
Current consumption: 1.5mA
Voltage gain: 4.8
Maximum input signal level: 2V p / p
Maximum output signal level: 9.6 V p / p
Frequency response: 10 Hz to 900 KHz
The recommended supply voltage is 12V, but the audio pre-amplifier of figure 4 can be as well powered from voltages within the range of 9 to 15 volts. The circuit is extremely simple and you may easily assemble it using the small PCB presented in figure 5.
The third schematic of an audio preamplifier is presented in figure 7. It has the advantage of being an adjustable-gain amplifier, and its absolute voltage gain may vary from about 10 to 33. The gain can be adjusted from R4 potentiometer.
In this schematic, the base of the second transistor (see TR2) is directly connected to the collector of the first transistor (TR1), and the output is taken from the collector of TR2, via C4 capacitor.
Setting R4 trimmer to its maximum resistance value will set the amplification to its maximum value (about 33); turning it to its minimum value, will set the amplification to its minimum value (about 10). It is obvious that setting the trimmer halfway you’ll get about the half of the maximum amplification.
The technical features of the preamplifier of figure 7 are summarized as follows:
Recommended power supply: 12V
Current consumption: 0.8mA
Voltage gain: adjustable from 10 to 33
Maximum input signal level: 0.3 to 0.8V p / p (for maximum and minimum gain, respectively)
Maximum output signal level: 9.6 V p / p
Frequency response: 10 Hz to 800 KHz
To build this preamplifier, use the recommended PCB of figure 8 and follow assembly instructions of figure 9. When placing the transistors, make sure for the flat side of their body to point to the left, as clearly stated in Figure 9.
As well as the previous designs, the preamplifier of figure 7 can be also powered from voltages within the range of 9 to 15 volts.
Finally, we present a fourth design of an audio preamplifier. The specific circuit schematic is shown in figure 10, and it is suitable to amplify very weak audio signals. It has very high gain and uses a PNP and an NPN transistor.
For the PNP transistor, you may use BC.213, BC.308, BC.328 or another equivalent type. For the NPN, you may use BC.172, BC.547 or other equivalent types.
The technical features of the 4th preamplifier can be summarized as follows:
Recommended power supply: 12V
Current consumption: 1.2mA
Absolute voltage gain: 115
Maximum input signal level: 70 mV p / p
Maximum output signal level: 8 V p / p
Frequency response: 20 Hz to 200 KHz
Although the recommended supply voltage is 12 volts, the preamplifier can be powered as well from a voltage source ranging from 9 volts to 15 volts.
If you choose to use 9-volt, the maximum input signal amplitude will be limited to 50 mV p/p, otherwise the output will be distorted.
To build the preamplifier, use the recommended PCB of figure 11 and follow assembly instructions of figure 12. When placing TR1, marked as BC213, BC308 or BC328, turn the flat side of its body to the right, and when placing TR2, marked as BC172, BC547, turn the flat side of its body to the left.
To avoid failures follow these guidelines:
- When connecting the 12 V power supply, be careful not to reverse polarity (do not reverse the negative wire with the positive one), because you will damage the transistors.
- Use shielded cable for the input signal, as well for the output signal and connect wire-shield to ground (the negative power terminal).
All the circuits presented here, have low output impedance and you do not have to worry if they will match to the input impedance of a final amplifier. They will be fine to boost the input of a 20K input impedance amplifier; they will be fine as well to drive a 50K, or a 100K input impedance.
