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enter image description here

In the circuit shown above, I am unclear how Q1 connects to Q2+Q3. I understand Q1 is part of a common base amplifier and Q2 and Q3 form a direct coupled amplifier.

The circuit is described at

http://www.zen22142.zen.co.uk/Circuits/Audio/dyn_mic/dyn_mic.htm

My problem is in reading the Schematics/picture. Specifically, how are the 2.2uf, 4.7 uf , and 47k ohm componnets connected in the center of the picture? Thanks

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  • \$\begingroup\$ Wrongly. The 2.2U is reverse biased. \$\endgroup\$ Mar 24 '20 at 15:20
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The 47k component appears to be a potentiometer, using a somewhat nonstandard symbol. The center/side connection going to the capacitor is the wiper.

The capacitors are being used as DC blocking capacitors, preventing any DC link between the stages of the amplifier and decoupling the microphone's bias voltage from the signal. As @BrianDrummond and @analogsystemsrf point out, the 2.2 μF capacitor is connected the wrong way around.

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The two capacitors are DC blocks, conducting the (AC) signal while blocking the DC so that different circuits can work at different bias levels. The pot is just a volume control.

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The 2.2uf is WRONGLY installed. god eye for catching that, Jeffrey.

Since the VDD is Positive, the base of Q2 will be about 0.6 volts above ground, thus the 2.2uf cap needs its (+) terminal to the base.

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Unfortunately, during the power-on transient as VDD quickly rises to +12 volts, the top pin of the volume control will briefly spike to +6 volts. this is caused by the huge capacitor on base of Q1, which delays Q1 turnon/.

What to do?

I'd add a clamping diode from wiper to GND, the diode "bar" cathode wired to gnd.

This guarantees the cap never has more than 0.1v to 0.2v reverse bias, and that occurring ony during power up transient.

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That 2.2uF is connected the wrong way around. The +ve should be towards the higher voltage level.

With no signal from the mic all three terminals of the pot are at 0V. With a mic output signal the signal at the top of the pot wiggles about 0V and the wiper signal also wiggles about 0V but is a reduced amplitude version of the signal at the top of the pot.

The capacitors are blocking the dc bias levels of the two stages either side.

This is a classic way of implementing a volume control and avoids having any dc voltage across the pot which could cause a scratchy sound when the pot is operated. Although that symbol appears to be a preset rather than a potentiometer.

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