Preamplifiers

I started this project after reading an article in Stereophile magazine called "Aunt Corey's Buffered Passive Preamplifier" by Cory Greenberg (Stereophile, November 1991, pg. 91). My version can be both a great headphone driver and preamp. The 4 ohm output of the buffers will drive headphones without reaching their maximum output current. The idea with buffers is that with unity gain you should be minimizing any distortion added to your signal. And trust me, the idea works.

The headphone amplifier with the natural crossfeed filter published on HeadWize in fact was my first DIY-project. I built this device because I was unsatisfied with the sound reproduction via the headphone-socket of my CD-player. But, as things go, I started to like constructing and decided to design and build some power amplifiers also. Having finished these, they sounded so good that I also made a new matching preamplifier with integrated headphone-amp.

The microphone has high sensitivity in the audio range, but in the ultrasonic range, the sensitivity decreases rapidly. The receiver is very sensitive. To prevent overdriving and feedback due to the high sensitivity of the microphone in the audio range, the gain of the amplifier should be low in the audio range and increase with frequency in the ultrasonic range.

The 2N3392 transistor is a low-noise type in a TO-92 housing and can be replaced by a NTE199 or ECG199. Potentiometer R5 of 100K is a linear type with an on/off switch attached. The value of C1 may need to be between 0.05μF and 0.1μF (47nF/100nF). Experiment with the value for best performance.

This preamplifier has a low output impedance, and is designed to drive long cables, allowing you to listen to a remote music source without having to buy expensive screened cables. The very low output impedance of around 16 ohms at 1KHz, makes it possible to use ordinary bell wire,loudspeaker or alarm cable for connection. The preamplifier must be placed near the remote music source, for example a CD player. The cable is then run to a remote location where you want to listen. The output of this preamp has a gain of slightly less than one, so an external amplifier must be used to drive loudspeakers.

Afterwards the Modular preamplifier presentation good is I present also a simpler circuit in the manufacture but very good quality. And in this circuit the philosophy that follow is: simple is also better. As you will see in the circuit, it was used only that a Volume potentiometer. Even the Tape Monitor choice, is considered more enough seldom in the use, remaining useful only in those who they allocate and insist to use 3 heads cassette players and tape open reel. Otherwise can used also this input as LINE, after does not differ in nothing from the other inputs or be suppressed. All the components that constitute the circuit should be top quality. The preamplifier gave sound very good and neutral.

This preamplifier has a low output impedance, and is designed to drive long cables, allowing you to listen to a remote music source without having to buy expensive screened cables. The very low output impedance of around 16 ohms at 1KHz, makes it possible to use ordinary bell wire,loudspeaker or alarm cable for connection.

This easy circuit provides good gain to weak audio signals. Use it in front of an RF oscillator to make an RF transmiter that is very sensitive to sound.

The circuit is so simple that there is little to say about it, but it gives very good results. Tr1 is operated in grounded base mode with input to its emitter to give low impedance input. The values shown give correct operation from 9v. If operating from other voltages you may wish to alter the 1K8 resistor to give symmetrical clipping at high levels. If you wish to switch the speaker (to use the same speaker as a speaker and as a mic, then you should arrange the switching to short the speaker out before disconnecting it. This is because the d.c. through Tr1 also flows through the speaker so removing the speaker will upset the d.c. conditions and cause a large thump in the speaker. You could of course fit a 1K resistor in the emitter and use a 100µF capacitor to couple the speaker.

The circuit is a preamplifier with digital regulation intensity of sound. He is separated in three departments. In first schematic (Fig.1), is found the circuit of control of electronic potesometer. The control become from two pressing switches. The S1 (UP), that put up the intensity of sound and the S2 (DOWN), on the contrary, that lower the intensity, checking them two oscillator follow a bistable multivibrator R-S (ICIC-D), with that determined the direction of counter IC3.. The gate IC2B ensure deactivate the counter in the limits of region of regulation. With the S4 we have the possibility selecting the level of sound, in which it will come back after RESET or when we give supply in the circuit. This operation is very useful when we select null level of sound, then with switch S3 we have the possibility of going to situation MUTE, to rapid nihilism of sound.