VLZ MICpre

If you are picking up RF, this VLZ MICpre is the solution.

The 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 rejects RF.

The 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.

Gain Selector
2.0mV Noise at 60dB Gain

The 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.

One 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

The 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.

The 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.

Between 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 performance.

To power the VLZ MICpre, check out our PSU15REG. It is equipped with voltage regulators for +15V and -15V.

Specfications

MIC Gain Settings
1=63dB
2=58dB
3=49dB
4=43dB
5=38dB

Electronically Balanced Mic and Line Inputs
Electronically Balanced Output

Mic Input Impedance = 1.2K ohms
Line Input Impedance = 10K ohms

PCB = 8.0" X 2.2" (204mm x 56mm)
Double Sided | FR4

VLZ MICpre
Kit
US$ 85.50

VLZ MICpre
Pcb
US$ 44.50

$40 Minimum Order on Check Out
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60 Downes Street | Calais | ME 04619 | USA