help me understand output coupling caps

[Top Top]

I was building an oscillator for a toy I made for my niece for Christmas based on a 40106 hex schmidt trigger. The basic schematic I was basing it on from the internet was designed to be used with a line out.

On the breadboard, I originally had it going to a jack and into an amp and it was working fine that way, with a very strong output (it was making noise even with the input on the amp turned all the way down.

I hooked it up to a little 32 ohm speaker (salvaged from another toy) that I planned on using in the toy, and got nothing.

So on a whim, because I have seen it in other circuits, I put a .01uF cap in series from the output of the oscillator to the speaker, and sure enough, I got signal through the speaker, loud enough to use in the toy.

I think, and correct me if I am wrong, this type of cap is used on many audio circuits for both the input and output. Why is it needed?

[GibsonGM]

Hi Top,

An output cap, and also the input cap, is generally there to block DC voltages from going into the amp/circuit following the one in question.  DC makes a very annoying bzzzzzz when it goes to a speaker, amp or what have you.  Can mess up the bias on a stage following, too. 

For instance, if you have a distortion pedal that uses DC to bias and operate an opamp, transistor, etc., there will be an AC signal riding on a DC level up to that output cap. That's how devices amplify.  Since capacitors "like" AC and show a high reactance to DC ("don't like"), they will pass the AC signal along and stop the DC.....this is known as a blocking capacitor.

ALSO, and not unimportantly, active devices such as tubes, transistors, opamps and what have you exhibit what is known as the Miller Effect.  They have a capacitance associated with their terminals.  So, a signal can be attenuated at high frequencies by this capacitance unless it is dealt with.  Usually that is done with a series resistor on the input, which forms a filter with that input capacitance and 'boosts' the highs.   An input cap works in conjunction with this input filtering, setting the 'frequencies of interest' for the particular device. 
There is a ton of info on the net about this, look up "input cap", "output cap", "Miller Effect", etc etc.
But in a nutshell, your output cap is blocking DC and letting that nice, pure AC signal thru so the speaker can do its job the way it is supposed to!
Hope that helps,

Mike

[anchovie]

Input and output caps remove any DC component from the signal so you are left with the AC audio waveform referenced to ground. The positive part of the waveform makes the speaker move one way and the negative the other, making the air vibrate and converting the electrical signal to sound.

Your amp will send a ground-referenced signal to the speaker, hence you get something audible. Direct to the little 32-ohm driver, the DC part of your oscillator (half the supply voltage, I'd expect) was probably high enough to push the cone all the way in one direction, hence it doesn't get wobbled by the AC part!

[wavley]

ALSO, and not unimportantly, active devices such as tubes, transistors, opamps and what have you exhibit what is known as the Miller Effect.  They have a capacitance associated with their terminals.  So, a signal can be attenuated at high frequencies by this capacitance unless it is dealt with.  Usually that is done with a series resistor on the input, which forms a filter with that input capacitance and 'boosts' the highs.   An input cap works in conjunction with this input filtering, setting the 'frequencies of interest' for the particular device. 
There is a ton of info on the net about this, look up "input cap", "output cap", "Miller Effect", etc etc.
But in a nutshell, your output cap is blocking DC and letting that nice, pure AC signal thru so the speaker can do its job the way it is supposed to!
Hope that helps,

Mike

I've recently had experience with this and an oscillation, moved the grid resistor to the tube socket and the oscillation stopped.