DC filter capacitors

**broken link removed**

does anyone know why the capacitors (4.7uf) are needed in front of the transisitors???

i'm assuming they only allow ac or voice signals to pass into the transisitors, yes???

That's correct.

Thats right,

More precisly they act as coupling capacitors to prevent any dc voltage from the biasing ckt to pass through hence preventing a dc shift in the original signal.

Correct

A coupling capacitor only allows the ac sinals and blocks the dc signals.
the one you mentioned is also a coupling capacitor.

It ensures that the source and load do not effect the biasing conditions in the circuit.A similar capacitor is neede in the output to couple the load to the load resistor.

Given that electronics is a hobby I tend to work at a more basic level. The advice already offered is several levels up from where I am at.

I tend to think in terms of DC then work my way up to light (low frequency to high frequency). Then I think about the passive device in terms of resistance.

A resistor is just that and from DC to light it's resistance to the flow of electricity is relatively constant - I emphasis relative here.

A capacitor would conduct no DC like a resistor with infinite resistance but starts to conduct to a greater degree as frequency goes up.

An inductor behaves like an upside down capacitor - at DC it's like a short circuit and it conducts less and less as frequency goes up.

Note that all of this is very simple and basic. It ignores the storage of energy, resonance, etc. It doesn't explain why the component is there - as others have explained. It does help me think thru what might be happening in the circuit so that I can figure it out - sometimes.

"Coupling capacitors" as used in this application (they couple the signal to the next stage much like coupling railroad cars together) are also sometimes called "blocking capacitors" to describe their DC function -- blocking DC to prevent two different DC levels from getting into a big fight.

Dean