[GlorytheWiz825]

I'm new at DIY and electronics in general. I was looking at the schematic from tangentsoft and figured out the voltage gain is Vo = (R3+R4)/R3*Vin. So I know what the resistors are for. Now I have no idea what the capacitors are there for. Under DC conditions, aren't capacitors open circuits? So I'm generally confused on why the capacitors are there in the amplifier. Any insights would be helpful!

 

[MisterX]

Quote:

Under DC conditions, aren't capacitors open circuits?

Yup, that is the idea.

http://tangentsoft.net/audio/input-cap.html

 

[GlorytheWiz825]

Quote:

Originally Posted by MisterX /img/forum/go_quote.gif Yup, that is the idea. http://tangentsoft.net/audio/input-cap.html

Still a bit confused after reading that. So according to the article, the capacitor is there to strip off the DC offset "from an AC signal". The input is powered by a DC source, 9 V batteries, so why would there be an AC signal?

Thanks for the help!

 

[Denim]

That AC signal is the music. The DC offset is not at 0 volts (it wouldn't be an offset if it was), so caps are used to block the DC and allow the AC to pass through. Before the cap, the music rides on a DC level, so the mid-point (which should be at 0 volts) is actually higher. After the caps, the DC is no longer present and the mid-point is returned to 0 volts.

 

[Pars]

Actually, in a properly designed/implemented/functioning source device (iPod, CDP, etc.), there should be no DC offset, so the music waveform would be riding on a 0 Vdc level. In the real world, some devices (sources) do have a DC offset, which if allowed into the amplifier, is amplified just as the music waveform is, by the gain factor of the amp. At this point, it could be harmful to the headphones. Therefore input coupling caps, which block any DC present on from the source. Most commercial equipment contains output caps, so as long as they aren't leaky, etc., you should not see DC on the input (or at a very low level, say 1mV). Direct (DC) coupled equipment usually has circuitry or a design to limit or eliminate any DC component on its outputs (such as the DC servo in the Gilmore Dynalo/Dynahi) without resorting to caps in the signal path.

 

[GlorytheWiz825]

Thanks for the clarification guys. So the AC signal is from the music (the source), and not actually from the amplifier. And the input capacitor removes the DC offset, thus minimizes the risk of damaging the headphones. Looks like I've found a great forum for answering my many newbie questions. =)