Grounding an audio amplifier - correct signal references

Grounding is a catch-all label for a number of different topics, which IMHO is confusing. The topics are:

  • Electrical safety;
  • EMI protection;
  • Current routing (especially power currents, e.g. power supply or speaker connection);
  • Correct signal reference points (a.k.a. "signal grounds").

In the last three posts I talked about the first three topics.

Finally, reference points for input signals, aka "signal grounds". Each input signal is a voltage, and as such is between two points - say "signal" and "reference". The reference is frequently called "signal ground" in the context of a single-ended connection. The problem arises when your signal source (preamp or volume control) has a different signal ground from that of a power amplifier - and it always does. The voltage difference between signal grounds leads to current flowing between them when connected. The voltage drop of that current on the impedance of the interconnecting wire/ cable adds to the signal as noise, buzz, hum or distortion.

Power amplifiers usually have their own local signal reference, connected somewhere to the reference ("power ground") end of the load, as this is almost always required for stability. When the power ground is connected to other "grounds" (e.g. the midpoint of the capacitor bank in the power supply) with wires carrying return supply current, the signal ground of the power amplifier will be different from those other grounds. This is where people talk about "dirty ground" (one very different from some other ground point) and "quiet ground" (one not much different from some other ground point).

Ways to deal with it: balanced, differential, floating inputs everywhere, including inside the box between PCBs; separate power supplies for each power amplifier (aka dual mono; may be impractical for multichannel due to cost/size/weight considerations); ground sensing output for the signal source (as implemented in our balanced volume controls).

Workarounds: THICK short wires connecting amplifiers to power supplies in an attempt to bring various ground potentials closer; something called "ground loop breaking resistor" or GLBR to reduce the effect of different ground potential.

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