If you have to ask …..

We’re discussing audio blocking capacitors; elements in the signal path that our music has to run through to get to our ears.  These capacitors block DC and let AC pass through – AC being the essential element of music – DC is needed for things to work but we do not want it in our signal path.

So, just how does a capacitor decide what is AC and what is DC?  Frequency.  If we remember that it is the alternating movement of our loudspeakers – in synch with the music – that creates sound in the first place, then we have to figure out where we draw the line between moving and not moving signal.  The output of a battery does not move – but the output of a microphone or phono cartridge does – so clearly we can say that a battery is DC and a phono cartridge is AC.  How do we differentiate between these two extremes and where does one draw the line?

For a capacitor to transfer music through it that musical signal must be moving back and forth at least a little.  If it is moving back and forth between + and – once per second that is enough for a capacitor to transfer this signal from its input to the output IF the capacitor is the right size.

Capacitors come in different sizes and they are rated in what we call farads – a measurement of capacitance – and for our purposes we use capacitors that are significantly less than a farad, typically a microfarad (one millionth of a farad).  The bigger the capacitor the lower the frequency of sound it will pass.

Are we happy with one cycle per second?  The reason I ask is that as capacitors get bigger they get more expensive, pass audio with less accuracy and take up a lot of real estate on a circuit board.  Here’s a case where bigger isn’t necessarily better – and if our goal is to keep the number of parts in the signal path to a minimum – and make sure the quality of the parts in that path is the best – then we need to make sure we actually want to pass 1 Hz.

You cannot hear 1Hz – but you can hear 20Hz and you can feel 16Hz and even below.  So if we’re designing a product with a blocking capacitor, we need to figure out how low we want to pass signal – and not go too low so as to cause problems.  As a designer I typically use a 10X rule of thumb.  If the lowest I want to hear is 20Hz, then I use a rolloff of 2Hz if there’s only one capacitor in the signal path.  That is, unless it’s a turntable preamp I am designing and then we have to be careful about letting unwanted table rumble through the system.

The take away here is that capacitors in the signal path need to be carefully noodled out by the designer.  The quality of the capacitor and its size are critical elements when designing a high end product.

Paul McGowan

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