I am unsure what is wrong with my amplifier / microphone coupling.
I have read many of the posts on stackexchange including the following:
- I am using low impedance headphones (~38Ω, Logitech)
- I have tried bridging the pins on the jack leaving the LM386 and it helps, but not enough
- I tried measuring the impedance on the mic - obviously incorrectly. But, initial values seem to start around the same (I think this is the problem?)
The amplifier circuit on my breadboard :
Based on this audio amplifier kit.
Datasheet here: http//www.jameco.com/Jameco/Products/ProdDS/151204.pdf
As far as I can tell, the circuit is working as designed (i.e. there is audible output from the circuit; gain adjustment seems to work, no smoke, etc)
So far, I only have one microphone to test it with. It is a "Boundary Business Microphone" from RadioShack.
As sold, there was a pendant about 18" from the condenser microphone which housed a small circuit and took a single LR44 (button cell, 1.5v) battery.
It is currently modified to take an LR6 (AA, 1.5v) battery. The diagram for the (modified) circuit is below:
I presume the reason for the inclusion of the pendant is the ~16ft long cable (which I have no desire to remove).
When I made the aforementioned modification, I was suspicious that the change from an LR44 to an LR6 (AA, 1.5v) would alter the behavior of the microphone. However, this has not been a problem when using this mic with a PC audio card (SB Audigy) or a handheld voice recorder. With this circuit, I can hear only faint input even when blowing hard onto the mic or tapping it with my finger.
Do I have an impedance mismatch due to two/too many pre-amp circuits?
If so, how do I fix it? If not, please point me in the right direction.
- Return the microphone circuit as near to the original operational parameters as possible: (for the sake of "knowing it is right")
- The LR44 has an internal resistance of 4.19Ω
- The LR6 has an internal resistance of 0.28Ω
- ^Values obtained here: http//www.g-holbeck.com/english/elteknik/indexri.htm
- The resistor in series with the battery on the circuit board is 2.2kΩ
- I should replace the 2.2kΩ resistor with a 2.6kΩ in the microphone circuit? Is it that easy?
And, since I will need to use this microphone with this circuit for now:
- Add a bypass to the microphone to decouple the pre-amp circuit in the pendant
Reduce the input voltage on the amplifier circuit
- Currently at 9v
- The datasheet for the circuit recommends 7 - 15v input.
- Lowering the voltage will reduce the output from the LM386?
Reduce the voltage supply to the microphone
- Seems like this will require adjustments to the bias voltage.
- Not sure I have a clue how to do this...
Something I have totally overlooked? I am a newb after all. Please forgive my ignorance.
Thanks for any help, I really want to understand this.
PS: Could only add two links, so some are intentionally broken above.
The circuit which I built is on the right below. I included the microphone pendant circuit in this diagram on the left. I had to draw this myself because the datasheet listed above explicitly claims copyright to its contents (so, no screenshots).
^^Right click, view image (in Firefox, IE at least) to see the full size.
I tried to embed the image other ways but EE.SE considers anything other than this way a 'link' that contributes to my two link maximum.