AC or DC Coupled Preamplifier or Amplifier

 

 

Audiophiles sometimes read a description of an amplifier or preamp as being "AC coupled" or "DC coupled."  We'll describe what that means.  

Quickly, a DC coupled amplifier has no lower limit of frequency response.  It goes all the way down to 0 Hz, as opposed to 20 Hz. 

AC vs. DC

Alternating Current means that the incoming voltage is going above and below 0.  This is how we transmit musical signals, and also how the voltage from a wall outlet works. 

DC is direct current.  Imagine any battery in your home.  It  is DC because the voltage does not switch directions.  The (+) terminal is the positive, the  (-) is the negative and that's that!

AC vs. DC Coupling in an Amplifier

In the drawing at the top we have an output stage for a preamp, followed by a capacitor, followed by a resistor which is simulating the amplifier's input impedance.  To make it simple, this is an AC coupled circuit because the capacitor is blocking any possibility of DC making it to the amplifier input. 

Ideally the preamplifier's idle voltage, or offset is 0V.  That is, with no music, the connection between the transistors is nearly always zero, when we ignore noise which is AC anyway. 

Coupling capacitors can be at the output of a preamplifier and/or the input of an amplifier. When there are no coupling capacitors, and the amplifier and preamp are free to pass a steady 1V signal we call that DC coupled. 

The Problem With DC

For DC coupled devices, when DC is present at the inputs of an amplifier then that voltage will make it through to the speakers.  Since the normal gain for amplifiers is around 20x, a 0.1V DC offset would translate to 2V at the amplifier outputs, shy of 1 Watt for an 8 Ohm load.  

Fortunately this rarely happens as modern DC coupled devices usually have some circuitry to compensate for DC offest. 

The worst case scenarios however are when using tube preamps with degraded output capacitors which no longer perfectly block DC.  That is, they leak DC. When attached to a DC coupled amp this could cause speaker drivers to become ballistic or smoke woofer coils. 

The Problem With AC

Many great sounding amplifiers are AC coupled.  They have a capacitor at the inputs, and won't pass a DC voltage.  A single cap is of course a 6 dB/octave high pass filter.  The value of the capacitor is chosen such that it has almost zero impact at 20 Hz. 

The problem is however we are introducing yet another part in series with the sound, and the quality of these caps may interfere with the sound.  

 

Which is better? 

We would not recommend buyers decide on one vs the other based on theoretical sonic differences but instead consider the actual sound quality and potential risks in matching equipment.