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Old 27th December 2016, 11:34 AM   #1
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Join Date: May 2013
Location: Raftle
Default Phone Mic Input Bias voltage effect

Lenovo vibe k5 plus have bias voltage 1.7V
Xiaomi note 4 have bias voltage 2.7V

Anyone know what effect of that ? for single transistor mic pre-amplifier.
because with lenovo recording have high pitch noise and distorted

Last edited by Ghild; 27th December 2016 at 11:44 AM.
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Old 30th December 2016, 12:24 AM   #2
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Join Date: Nov 2004
Location: Germany
Have you also determined input impedance for the two (i.e. the bias resistor)? I would suspect it's something like 2.2 kOhms for the Xiaomi and 1 kOhms for the Lenovo.

1.7 V is kinda low though. Depending on what value bias resistor they used and what the FET in the capsule is like, this could be too low. Or did you buy your own mic preamp? Bipolar transistors also vary in how well they work at very low voltages. As a rule of thumb, types with low rated Vce fare better, so you may want even to go with classic 2N5087/89 instead of BC547/557 and the like. Mark Johnson had a thread over in Solid State fairly recently where he looked at the "knee" voltage in a current source application.
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