Some years ago, I’ve had the need to build a microphone preamp. I was planning to buy a condenser microphone (Behringer C3), but had no way connect it to my PC soundcard.
At that time, I didn’t have a soundcard with balanced inputs and phantom power. So, I decided to exercise my electronics knowledge and build a microphone preamp from scratch, with these characteristics:
- Balanced input
- Ability to supply 48V to the mic through phantom power
- Unbalanced output
- Mains powered (127/220 V)
- Use readily available components (for me, at that moment)
For those who aren’t familiar with phantom power, Wikipedia provides some useful explanation.
The design is divided in two blocks: the power supply and the preamp itself.
For the power supply, I’ve decided to use a transformer with 12V secondary, as in my case it was readily available. Its design was based on a project available at Elliott Sound Products, with some modifications.
To provide 48V to the mic, it is needed to elevate the secondary voltage somehow. It is done by a voltage quadrupler/rectifier, followed by a linear regulator. The quadrupler/rectifier is able to provide approximately 60V DC to the linear regulator. This topology allows the use of inexpensive and easily available 12V transformers.
The linear regulator is based on a BD139 medium power NPN transistor and a 1N4749 Zener diode (with a Zener voltage of 24V). The trimpot must be adjusted until the output is at 48V.
48V is required for the mic, but to power the preamplifier, two Zener regulators were inserted, to provide 24V (positive supply to the opamps) and 12V (reference voltage to the amplifier).
Overall power dissipation at the linear stage is relatively low, and I haven’t seen the need to fit the BD139 transitor with a heatsink. As the preamp is not meant to be battery powered, low power consumption and high efficiency weren’t as important as simplicity and low noise operation.
The preamplifier itself is a differential amplifier followed by a inverting amplifier with adjustable gain. The opamp may be a NE5532 (as I used) or any other audio opamp. 1N4148 diodes are used as protection against ESD and wrong polarity.
For best performance, the resistors that comprise the differential amplifier should be of low tolerance, but I’ve built mine with 10% tolerance carbon resistors and hadn’t problems with noise.
Output level is adjusted through the potentiometer, which sets the inverting amplifier gain. It is recommended to use a audio taper (logarithmic) pot.
At the mic input, a female XLR connector is used, and care should be taken to wire it properly, or the microphone can be damaged. The diagram below represents the frontal view of the XLR connector. Some connectors have numbers identifying each pin.
- Pin 1 – Ground (cable shield);
- Pin 2 – Positive polarity terminal (aka “hot”);
- Pin 3 – Negative polarity terminal (aka “cold”);
With the unit properly mounted and powered, the voltages between pin 2/pin 3 and pin 1 should be approximately 48 V each one.
A 1/4″ or 1/8″ mono jack may be used as output connector. The output can be directly connected to a amplifier, a mixer or a soundcard.
The design can be optimized in many ways, but considering the compromise between simplicity and performance, this preamplifier has worked very well for me.
This project was designed to be used with balanced microphones with phantom power, or any compatible equipment. Its use with guitars, dynamic mics or any uncompatible instrument/equipment may cause damage.