The Audio Buddy from M-Audio is a favorite cheap and cheerful mic preamp and direct box. Its performance is good for the very low price, but there are known issues. I took mine apart to see what’s inside.

We have another 4560 used as an oscillator and parallel buffer generating a clipped sine wave rail to rail at about 27.5 KHz. This feeds a voltage doubler. Theory says 30V (-15 to +15) X 2 above the -15V rail = 45V, which is within the 48V ± 4V standard for phantom power. However...

We now have something like this, a single stage charge pump. Under test with a couple of MXL 991 mics, which are pretty current-hungry, we find:

                 Before         After

No load    41.2V         42.5V

1 mic        29.6V         35.0V

2 mics       24.4V         30.8V

We could improve droop further by reducing the 1K resistor and the 100 ohm resistors, but this is pretty good. Cutting the resistors would increase noise on the mic power, and though the frequency is ultrasonic, it might mix with non-linear elements and make audible birdies.

Based on a NJM4580L dual op-amp, the input stage is the same as found in most low-cost gear. Op-amp with gain of 100X (40dB), capacitor coupled to the mic input, and 1K input resistors each side, giving a 2K balanced input impedance.

The non-standard bit is the phantom power. Instead of 48V through 6.8K resistors, there’s 40V through 4.7K. More on this later, since phantom power is one of the known issues with this device. The manufacturer doesn’t claim 48V, just that there is phantom power, and it works with most mics.

The guitar / instrument input is a simple buffer with a gain of 10X (20dB), and an input impedance of 100K. The jack is shorting on the tip to keep noise out when not in use. The ring is connected to the mic preamp, so it’s shorted out when a guitar is plugged in.