Question about schematics

Other Hardware General Electronics
1

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/minikits/manuals/manual_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!

2

Going thru 1 path:
R1 provides a DC level if nothing connected. Also powers the microphone.
C1 lets only AC pass thru.
RV1B is voltage divider for volume control.
C2 only lets AC onto junction of R3/R4, which set opamp input at 1/2 supply voltage.
C10 is power supply decoupling.
R14/R10 set the gain of the op amp.
C12 provides frequencuy stabilzation (no oscillation).
C8 - helps keep opamp pin2 at DC level.
R11 gives the output a minimal load if no headphones are present
R7 - what value is 8E2?
C5 only lets AC out to the headphones.

Looks like basic non-inverting amplifier to me, gain (1 + 47/1 = 48) only, with couple parts added for stability.

3

R1/R2 provides a power and load to the microphone (electret).
C1/C3 DC blocking capacitors for microphone.
RV1A/B is volume control.
C2/C4/C5/C7/C8/C9 necessary due OPA single power line. Could be omitted with dual +/- power supply.
C10 is power supply decoupling.
R14/R10 & R15/R12 set the gain of the op amp.
C12/R14 & C11/R15 form Low Pass Filter, help prevent HF oscillation, cut off 1 / 2 x 3.14 x R x C = ~ 34 kHz.
R11/R13 keep C5/C7 fully charged, prevent loud "click" when head phone jack connected.
R7/R8 - output short circuit protection.

4

Thanks for the quick response! I have some more questions:

  1. Why does the microphone get powered through R1? Why isn't there a straight connection between the power source and the microphone?
  2. C1 blocks out the DC component. Why is there another DC blocker (C2) after the potmeter and another DC blocker after the opamp (C5)?
  3. R3 and R4 are voltage dividers which set opamp input at 1/2 supply voltage. Why? Does this have something to do with the opamp range?
    (opamp datasheet: Texas Instruments 0q263do8iq991qftdprcsxqcjsky datasheet pdf)
  4. What is the difference between C6 and C10? Are they both power decouplers and why do you need both of them?
  5. Lowpass filter C12 and R14 block out high frequencies. Why do I want that to happen?
  6. Why is C8 used? I don't get it...
  7. Why would there be a "click" if R11 and C5 are not present ans what exactly is this "click"?
  8. What does the resistor value 8E2 mean?

I really want to understand how this device works. Any help is welcome! Thanks!

5
  1. Why does the microphone get powered through R1? Why isn't there a straight connection between the power source and the microphone?

Electret mic has a JFET transistor inside as first stage amplifier, which require a resistor as a load for normal operation. If you asking why they didn't put resistor inside, I can only guess:
a). During that time (~50 years ago) when mic was design there was no surface mount components, and regular resistors were so big, they could not fit inside :smiley:
b). To keep 2 wire, instead of 3 ( power, signal , ground ), so make mic design compatible with connectors of existing equipment on the market (2 wire electrodynamic mic for example) .

  1. C1 blocks out the DC component. Why is there another DC blocker (C2) after the potmeter and another DC blocker after the opamp (C5)?
    Because potentials are all different. Pot connected to the ground, so Vp=0. Mic output Vm=~3V. OPA input Vo = 1/2 Vpower.

  2. R3 and R4 are voltage dividers which set opamp input at 1/2 supply voltage. Why? Does this have something to do with the opamp range?
    R3 and R4 create artificial ground, spliting power in half. In this case regular ground became negative V power rail for OPA, in other words it converts single +Voriginal power supply to +- 1/2 V dual power supply.

  3. What is the difference between C6 and C10? Are they both power decouplers and why do you need both of them?
    They have different characteristics due different technology - electrolytic and ceramic. There is no cap on a market that could be equally efficient in very wide frequency range. ( tenths Hz to hundredth Mhz ).

  4. Lowpass filter C12 and R14 block out high frequencies. Why do I want that to happen?
    It was explain already, to prevent oscillation. And: to decrease a noise from unwanted spectral components, plus decrease a noise generated in R14 itself, decrease susceptibility to RF pickup.

  5. Why is C8 used? I don't get it...
    Read 3). Ground "seen" by OPA is -V power rail, you can't connect R10 to it any more.

  6. Why would there be a "click" if R11 and C5 are not present ans what exactly is this "click"?
    C5 must be present all the time , again because output of OPA is at artificial ground potential, and regular ground -1/2 V power.
    "Click" is a loud sound generated in headphones by rush current charging cap C5 to 1/2 V.

What does the resistor value 8E2 mean?
8.2 OHm.

6

What does the resistor value 8E2 mean?
8.2 OHm.

I've seen 8R2 mean 8.2 ohms, but not using a E, could it mean 800 ohms as in 8 x 10e2?

7

Probably a typo, If you look at #1 in assembly step instructions, it says (Grey, Red, Gold), 82 x 0.1 = 8.2