Protecting your amp and guitar from dc... why?

[ham_phist]

Hey all, just a quick question. I've noticed in my reading that the caps right after the input and right before the outputs are to block dc voltage. Why is that necessary?  I know a guitars signal has both positive and negative cycles, making it ac (right?), but is dc harmful in any way?

Thanks to everyone for being cool!

[Ciaran Haslett]

A speaker reproduces the AC voltage of a guitar signal by travelling forwards and backwards (I.e. Alternating between positive swing and negative swing).  This creates compression waves in the air and hence, sound.

DC is a constant positive swing.  The speaker would be in a instant state of positive excursion. If it can't go back to rest...then no compression wave was generated hence, no sound.

[Frank_NH]

"...but is dc harmful in any way?"

Well, 100V DC can be quite dangerous...

I think your real question is why all the blocking/coupling caps on our pedals.  The main answer is biasing.  Think of a signal in your pedal consisting of a steady part (DC) on top of which is a fluctuating part (AC), which can go + or - (e.g.  a +/- 1V sine wave signal).  It's the AC part we want to manipulate (i.e. amplify, distort, ...).

In an amplifier (or overdrive box), the signal entering a transistor or op amp stage often needs to be biased so that the DC part is not zero but rather at a prescribed "bias" level (e.g. 4.5V in a tube screamer).  This is because if it were at ground (0 V) and the stage operates at a positive supply relative to ground (e.g. 0 to 9V), the negative part of the signal would have no place to go and would be clipped at 0V.  Hence we want the signal to be "biased" to that it can swing equally positive or negative without running into the top (9V) or bottom (0V) of the permitted voltage range.  For an op amp, this DC bias is often half the supply voltage or 4.5V for a 9V battery.  For other amplifiers (FET and BJT transistors) it may be something else.  In fact. for JFET amplifiers, the bias voltage is 0V, which is set by a large resistor from input to ground, which is why you don't see blocking caps in some JFET circuits (for example, some of the Runoffgroove "amp-sim" circuits).

So you can see that to set the bias point for your AC signal, you need to make sure it comes in without any DC - hence the need for the blocking capacitor.  And likewise, you may need blocking caps at the output of an amp stage so that the subsequent stage(s) can be biased as needed.  In some cases, there may not be any need for an interstage blocking cap (but you may see one anyway).  It doesn't harm anything, but it does reduce the signal level a little (as does any passive component), which is why passive tone circuits reduce your output level.

Besides all of the above, there's this from sound.westhost.com:

"The purpose of a coupling cap is to pass the wanted audio (AC) signal, while blocking any DC from preceding stages or source components. DC will cause pots to become noisy (scratching noises when operated), and cause relatively loud clicks when (if) muting relays or similar are used. Since DC carries no audio information, there is no reason to allow it through your audio system. Some power amps will misbehave very badly if DC is present, and even small DC offsets into the speakers (anything above ~500mV) displaces the cone from its central position, and increases distortion. There is also a small static power dissipation - 1V DC across a 4 Ohm loudspeaker causes a constant static dissipation of 250mW. Not much, but the cone displacement can be much greater than you might expect."

More here:

http://sound.westhost.com/articles/coupling-caps.htm

[induction]

Also, a pedal without an input cap can cause DC current to run through your pickups, which can cause volume loss and change the frequency response (it attenuates treble, usually).

Lack of (or malfunctioning) input or output caps can also reverse-bias the blocking caps in other pedals in your chain, which can damage them if they are polar (electrolytics, tantalums). I've never seen it happen, but it's possible.

[ξεναγος νεκροπολης]

hey guys...everyday i'm learning a little something here.
thanks Ciaran Haslett, Frank and induction.
i never had to wοnder about such a question as ham phist made here so...
the only thing i knew about AC or DC was AC/DC...

[ham_phist]

Indeed, thank you all!