Decoupling/interstage capacitors

[tazwolf]

Hope this is not too silly a question.

What do decoupling/interstage capacitors do? They are placed between different gain stages in circuits and if I am correct on the input and outputs of circuits. the current example (but its the same in a lot of ircuits) I am looking at is the Hot harmonics schematic where there is an input cap then a gain stage then another cap then another gain stage (the inverters) and then another cap on the output before the volume pot - what do these caps do? How do they decouple one stage from another?

/Tazwolf

[will]

Hi Tazwolf,

Coupling capacitors perform 2 main tasks.

Coupling capacitors block dc voltages from passing from one stage to another. Most gain stages amplify the signal but are biased with a certain voltage/current. This bias voltage usually needs to be removed at the end of each gain stage. Any voltage dropping across a resistor creates noise, this is why they are often used to decouple volume or gain controls.  

Another reason to use coupling capacitors is to filter out low frequencies. Clipping the lowest frequencies often sounds non-musical. The capacitor provides a 6 db/octave filter depending on the capacitance value of the capacitor and the impedance (resistance) of the next stage.

Regards,
Will

[R.G.]

Watch the "De-"s.

DEcoupling caps remove signals from conductors. DEcoupling caps are used on power supply lines to DE couple successive stages, prevent feedback through the power supply, etc.

COUPLING capacitors are in series between stages to couple the AC signal from one to the next, while blocking any different DC voltage levels.

[bwanasonic]

COUPLING capacitors are in series between stages to couple the AC signal from one to the next, while blocking any different DC voltage levels.

And these are the ones where capacitor mojo makes a difference?  Just starting to figure this out myself.

Kerry M

[tazwolf]

It was coupling capacitors I was referring to.
Thanks for your info it's cleared things up a lot as Bwanasonic says
it is often specified on schematics or parts list that these coupling caps be film and not electyrolytics so I was wondering what it was with these caps and there use that was so demanding.

thnaks again!