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How to read digital LCR meter

audioguru

Well-Known Member
Most Helpful Member
I am glad to hear that it works. I used an electret mic from a smashed old cell phone. Lots of toys and wireless phones use electret mics that you could use.
 

gary350

Well-Known Member
My new Electret mics arrived to day I put one on the FM transmitter. Sound still clips terrible. I tested the voltage on the board batteries are 9v the other side of the 5v regulator reads 5.2v. There are some harmonic frequencies I finally got a very good signal tuned into 106.4MHz but sound is terrible. When I push the radio button to search for stations it stops on 106.4 When I talk into the mic i can barely understand the words on the radio.
 

audioguru

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Most Helpful Member
Gary, I think you built my FM transmitter circuit. Mine sounds extremely clear on any FM radio with no clipping even when the sounds are loud.
I simulated the mic preamp circuit with an input level that is 11 times a normal conversation level and the output shows no clipping.
1) Maybe the cheap Chinese mics are no good.
2) Maybe the mic is connected backwards, its metal case should connect to 0V.
3) Maybe the transistor is bad or a resistor has the wrong value.
4) Maybe the radio is cheap and is overloaded by the strong transmitted signal. My cheap clock radio becomes overloaded by my FM transmitter and causes reception all across the dial.

You said, "There are some harmonic frequencies, the batteries are 9V" but there are no harmonic frequencies and there should be only one 9V battery.
 

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audioguru

Well-Known Member
Most Helpful Member
I simulated the mic preamp with its transistor connected upside down. It has severe distortion and its output level is much lower than before.
 

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jjw

Member
My new Electret mics arrived to day I put one on the FM transmitter. Sound still clips terrible. I tested the voltage on the board batteries are 9v the other side of the 5v regulator reads 5.2v. There are some harmonic frequencies I finally got a very good signal tuned into 106.4MHz but sound is terrible. When I push the radio button to search for stations it stops on 106.4 When I talk into the mic i can barely understand the words on the radio.
Which circuit did you build?
The circuit in msg#1 is wrongly biased and has too much gain.
 

gary350

Well-Known Member
Many years ago I built a 2 transistor amplifier it clipped very bad. I solved the problem by putting a volume control between the 2 transistors. I turned the volume to the input of the second transistor down until clipping stopped.

I build circuit on message 19
 

audioguru

Well-Known Member
Most Helpful Member
An audio amplifier using more than one transistor has negative feedback to set the gain and reduce the distortion. Then a volume control cannot be used between the transistors so the volume control is at the input. A 50k log volume control can be added to the transmitter preamp as attached.

You built my FM transmitter circuit and its mic preamp is one transistor, Q1. I simulated Q1 in my post #64 with a loud input level and its output shows no clipping. My FM transmitter sounds perfect with no distortion so something is wrong with yours. You said the supply from the regulator feeding the Q1 circuit is 5.2V which is fine. Measure the DC voltage at the collector of Q1 when there are no sounds feeding the mic, it should be about 2VDC as shown in my simulation.

Did you use good quality 2N3904 transistors or another part number, or cheeeep Chinese copies?
 

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