30
SETTING THE BIAS
In setting the bias of your Cortado MKII, you are simply adjusting the amount of nominal current that
is passing through the FETs, Q1 and Q2. The more current that passes through the FETs, the more gain
the circuitry will have on the signal from the piezo disc. But on the other hand, the more gain the circuit
has, the more the small differences in the matched FETs will be magnified. The result is less common-
mode rejection and thus a higher noise floor. So we are seeking a balance between output volume and
noise rejection. We found these FETs work well with the bias set in the range of .1mA to .5mA through
each FET.
Follow these steps to set the bias on your Cortado MKII:
1. First, you need to supply power to the Cortado by plugging the XLR jack into a mixing console or
other phantom power source (such as the Espresso, by Zeppelin Design Labs).
97
2. With phantom power applied to the circuit, use your multimeter to measure the DC voltage
across TP (test point, marked red on Figure 3) and ground (98.99). Make sure that the solder
joints on the PCB don’t short against the tin container. A convenient ground point to put your
meter probe is through the hole in the bottom of the container (100).
98
99
100
