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Hello,

I have recently bought the MK136 Super Stereo Ear kit from Velleman and it works perfectly, but I have some questions about the schematics (see: http://www.vellemanusa.com/downloads/0/ ... _mk136.pdf):

How does the kit work? What funtions do the different components have? I get the fact that there are two microphones and the signals from these microphones are seperatly amplified by the opamp. The amplified signal goes to the stereo jack output. But what functions do the different components have (resistors, capacitors etc)? Are they being used as a filter? What type of filter? Butterworth filter? High pass filter/low pass filter?

What type of microphones are used? Are these capacitive microphones?

Does the potmeter influence the amplification factor?

It would be nice to have some answers to these questions. Any knowledge on the subject is welcome!

Thanks in advance!

Sorry, circuit descriptions are not available.
Should not be a problem, as this is a classic op-amp circuit. I'm sure if you Google you will find similar circuits, with description.

Hi this is quite easy.

Okay, some time ago but anyway.

You should see the left/right as two amps. have a look at the left channel.
The mic has a "ghost" voltage via R2, and the sound signal is fed through the C3.
Then a part of the signal is taken from RV1A and fed via c4 to the opamp.
r5/r6 set's the + input to half of the Suppy voltage. This is a must for opamps because they are direct current coupled.
If not, the your opamp will clipp immediatly. In fact this is also needed for the - input, but is done via the amplification devider.
The - input is the amplification-divider. R12/R15 gives the amplification.
C11 suppresse oscilations, C9 is a low frequentie- high passthroug filter against R12 ( ~ 16Hz)
R8 keeps the current in case of short circuit at low level. C7 keep the DC away from the headphone and R13 make it possible to load/unload C7 when no headphone is connected.

Rv1 regulates the signal but not the amplification.

Hope this helps. its is a very basic one, but works perfekt, like almost all opamps.