you are picking up RF, this VLZ MICpre is the solution.
VLZ MICpre is exactly the same as the MICpre60
except for the RF filter at the mic input. To adhere
to the Very Low Impedance (VLZ) concept, a Common
Mode Choke is used. Together with capacitors, they
form a second order low pass filter that effectively
inputs are Balanced Mic and Line. Output is Balanced
too. The microphone gain pot is replaced by a selection
of gain settings. Simply use a jumper to select a
gain to match the microphone in use. This is not as
convenient as using a rotary pot but the benefit is
superior performance. The gain ranges from 38dB to
63dB in five settings.
Noise at 60dB Gain
photo above left is the selector for the gain. Position
1 is the highest gain (63dB). Postion 5 is the lowest
at 38dB. When in use, select the gain to match your
source be it a microphone or a keyboard.
of the main concerns with a 63dB amplification is
noise. The photo on the right is with the Gain set
at 63dB. As can be seen, the noise level is only +/-
2mV. This is superimposed over my oscilloscope floor
noise which is at +/- 0.8mV. All inputs are shorted
to ground for this measurement.
VLZ Micpre Schematic
schematic above is the full VLZ MICpre. Right at the
Mic input is a Common Mode Choke followed by two capacitors.
This is the RF filter which is rarely seen in mixers.
mic gain stage consists of 4 low noise transistors
configured as Sziklai pairs, commonly known as Complementary
Feedback Pairs. Sziklais are noted for their high
impedances, somewhat similar to Darlingtons.
the two Sziklai pairs are the pcb headers in place
of a potentiometer. Only one gain setting can be selected
with a jumper. The output is Balanced with a 10dB
gain at maximum. This is equivalent to the three red
leds lighting up above the 0dB in the mixer. In use,
the output should be adjusted for unity gain for optimal
power the VLZ MICpre, check out our PSU15REG.
It is equipped with voltage regulators for +15V and
MIC Gain Settings
Balanced Mic and Line Inputs
Electronically Balanced Output
Input Impedance = 1.2K ohms
Line Input Impedance = 10K ohms
= 8.0" X 2.2" (204mm x 56mm)
Double Sided | FR4