Transistor mic preamp

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I built a mic preamp just for fun out of scrap electronics. I built it to be used with an electret mic capsule and my PC soundcard. This is my first electronics build and I am amazed that it even works. Here is the schematics of what I built
Simple%20Electret%20Mic%20Preamp.GIF


The volume is quite low. I can only hear when I speak very close to the mic. I have set the line level in my soundcard to max already. Is the volume low because I used a BC549 transistor instead of the original ?
 
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A BC549 should be ideal I would have thought. Why don't you check the voltages on the circuit and make sure they are reasonable (as a way of proving its basically OK)

You should ideally see around one half of the supply voltage on the collector.
 
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Its a very basic circuit tbh. The gain should be sky high at around 200 and not that well defined and repeatable because it depends a lot on the exact transistor specification. If the collector volts is low (and your sure the transistor is fitted correctly) then try increasing the 100k resistor, perhaps to as high as 1 or 2meg ohm.
 
Which mic are you using? The noise difference may just be due to the gain difference.


The mic I bought seems like a clone of panasonic wm61a. If I can get the mic preamp to have more gain I can try using it as a measurement mic ( just for relative measurements) by plugging it into the line in.

I found another 2 mic preamp circuits. Will these have higher gain ?

An externally hosted image should be here but it was not working when we last tested it.


ECM Mic Preamplifier

323-54587fe3b8.gif
 
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Both those are much better because they don't depend on the transistor characteristics to operate correctly. In the one transistor amp you will find that changing the transistor changes the performance.

These show all three. Note the scale of input voltage and output voltage. Another big plus point is that the two transistor circuits have a much higher input impedance which means that they don't load the mic capsule so much (and reduce its output voltage).
 

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The mic I bought seems like a clone of panasonic wm61a. If I can get the mic preamp to have more gain I can try using it as a measurement mic ( just for relative measurements) by plugging it into the line in.

I found another 2 mic preamp circuits. Will these have higher gain ?

An externally hosted image should be here but it was not working when we last tested it.


ECM Mic Preamplifier

323-54587fe3b8.gif


Comparing these 2 circuits there are some differences. The 2nd circuit has R3 and R4 instead of just 1 resistor. The 2nd circuit also has C3 connected to emitter of transistor 2. It also has extra resistors R7 and R8. Anyone know what is the reason for these extra stuff ?

In the first circuit what is the purpose of 100uF and 220uF capacitors near the bottom ? Are these also coupling capacitors ?
 
I built the Ecm mic preamp and its not working.
An externally hosted image should be here but it was not working when we last tested it.


Voltage across Vcc and ground becomes 3.5V after connecting power to the circuit. Vcc is 10V if not connected to circuit. I don't understand wat is going on. I checked the breadboard for connection errors. I did not find any. I placed red lead to Emitter of Q2 and black lead of multimeter to ground. I get 372mV. I am going nuts trying to make it work.
 
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Firstly, you need to be sure the supply is OK and can supply the current needed which is very small, just a few milliamps. You can check that aspect by connecting a 1k across the supply. If the supply holds up then your problem is in the preamp construction...

... so the only possible component that could pull the voltage down is the 220uf cap across the supply. Check its polarity ! If that is OK then you have a physical construction error because no fault in the circuitry could draw the current needed to load the supply that much.
 
Here is an Electret Condenser Microphone – ECM preamplifier circuit based on ultra low noise BC650C transistors. Due to difficulty of finding BC650C, instead of them, as you can see in the schematic, you can use BC549C transistors.

The circuit is self stabilizing and sets its Q-point at approximately half of the supply voltage at the emitter of Q2 transistor so this feature allows us to get maximum output voltage swing and highest dynamic range.

The 1kΩ resistor limits the current flowing through the microphone. It must be replaced by a 2.2kΩ resistor if the supply voltage is above 12V. You can omit this component if you are using a dynamic microphone instead of electret condenser microphone.

The design provides very high overall signal to noise ratio (SNR) and low overall noise output. A 100μF decoupling capacitor is connected across the emitter resistor of Q1 so it realizes a maximum gain for this stage. The second stage based on Q2 is direct coupled to minimize phase shift effects and achieve a flat output response from 20Hz to 100kHz.
 
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