mic pre amp in line... please help!!

[aq_mishu]

guys,

I need an assistance... stucked in here...

well, I have a radio shack headset+mic. I have modified it to work with my Radios... the first thing i did is i added a DIN5 connector... then I intended it to be used with a kenwood uhf walkie talkie... the pinouts are fine... perfect and correct... I made the exact companion for the DIN5 connector... it works... problem is elsewhere... it has a mic muff made of foam... yeah... surely a good thing... i am not interested to remove it... the electret mic is severely directional... thus i need to remove the muff and put it totally close to my mouth... still it's not so clear like direct without headset... thus other part can not hear me clearly...

now, i have checked and found several mic pre-amp made of transistor or op-amp... each of those are fine, except that first, it needs a vcc. I can get that from the radio set... (as the pin has a vcc port too). this requires to be also tied to mic as well... done... then suddenly discovered that the vcc is internally tied to the mic pin of the radio's pcb. thus it's creating a loop

any ideas on how to solve it?? because i need inline mic amp... but my mic pin of radio and headphone's mic is getting all in the same track...

 

[Audioguru]

Add a 330nF film capacitor between the output of the mic preamp and the radio mic input signal/bias wire that blocks the DC. Then add DC to the mic directly through a 10k ohms resistor.

 

[aq_mishu]

https://3.bp.blogspot.com/-Qh80slYe...AM/ScocJcsfLzk/s1600/3-9v+audio+amplifier.jpg

this is what i used... now, I have used 2n2222 for the transistor except that all other remained same...

also note: there are though 3 pins, but only two are used, and the dc bias is internally connected so far i guess. it's a kenwood tk3107 uhf tcvr.


Mishu!

 

[Audioguru]

Since the electret mic has 3 wires then you need to connect it properly. One wire (red?) is for the positive power supply voltage, another wire (white?) is the output that connects to an added 330nF film capacitor in series with the mic input of the radio. Connect the bare (black?) shield wire to the radio input ground.
When it is connected properly then it will have LOTS of output signal and you probably do not need the preamp transistor.

 

[aq_mishu]

man, it's a radio shack headset with mic. I connected it to a DIN5 pin.

It originally has one 3.5mm stereo and one 2.5mm stereo jack. the TCVR from KENWOOD (TK3107). Now, I have checked for the mic part... though it has both DC and mic pins, both are internally shorted. Also in TCVR, same, both are internally shorted.

That's making the thing hard... (i had other ear mic that works great... probably impedance mismatch of the radioshack headset for mic. But thus i need to amplify. )

 

[kam1787]

mmmmmmmmmmmmmmm?

the mic needs 3 wires; V+, output, and ground. If there is no V+, then no-one will hear you! If the equipment you are plugging into do not supply the V+, then you have to arrange it so they do!

 

[Audioguru]

The OP said the headset mic worked, but at a low level and the sound is not clear (distorted?).

The MIC pin of the radio has DC on it so maybe it is from the resistor that feeds power to the Jfet in a 2-wires electret mic. If the power is fed to a dynamic mic then it might cause low level and distortion.

We still do not know if the mic in the headset is a dynamic type or an electret type with 2-wires or 3-wires.

 

[aq_mishu]

i failed to open the case... it's radioshack's race-scanner's headset...

well, I found only two pin went in to the mic... like david clark. impedance is somewhere 250ohm.

the mic pin and v+ is shorted... I tested with meter. (in headset, i have though pins for v+ and mic out, both found shorted internally).

yes!! distorted when with the amp. without the amp, volume is too too too low. but when I blow, i get the sound... but when i talk, others barely hear me...

Mishu~

 

[Audioguru]

An electret mic by itself is about 1500 ohms. Your 250 ohm mic is probably a dynamic mic with a voice coil and magnet and MUST NOT be powered.
Connect a capacitor in series with it to block the DC power to it but the capacitor will pass sound frequencies.

Did you measure the resistance of the mic that came with the radio?

The extremely simple amplifier has a very low value for its input capacitor that cuts most low voice frequencies. It should be at least 3.3uF.
Also the transistor is hopelessly biased wrong with a 100k resistor causing severe clipping. Use a 1.2M resistor. With the low impedance dynamic mic its gain is much too high causing pretty bad distortion.

I simulated it:

 

[aq_mishu]

the first thing i'm gonna do is figuring it whether dynamic or electret... when i hooked the bias wire, it sounded a "tap"... probably dynamic one...

will measure resistance and also will add a cap as prescribed.

btw, what will be a nominal voice voltages without bias from a dynamic mic?? i'll also check in oscilloscope if needed...

 

[Audioguru]

At a distance of 10cm from your mouth when you talk in a normal conversation level the output of a dynamic or electret mic will average about 10mV. A headset mic is closer so its level will be higher, I have never measured one.
An ordinary digital multimeter measures the 50Hz or 60Hz mains frequency accurately but most guys cannot talk at such a low frequency. The lowest I can talk is 80Hz. The numbers on a digital multimeter will be bouncing up and down in a blur anyway. An oscilloscope shows peak-to-peak voltages that will be 14.1mV from a hand-held mic.

 

[aq_mishu]

It's a dynamic mic so far... And got no chance to work, but has a plan to decouple it with a 47uF cap to try.