Electronic – Can you reduce audio noise by decreasing input impedance with a resistive load

audioimpedanceinput-impedancenoise

First asked this over at sound stackexchange but they pointed me over here as a better place for it.

A general question but inspired by a specific example so framed as such.

I have powered studio monitors attached to a computer's unbalanced headphone port, and there is some amount of underlying noise, either from the computer's DAC or induced along the unbalanced cable or some combination. Overall not particularly surprising. With headphones plugged in instead, I don't hear the same noise at all.

My thought is that the likely cause of this difference is that the headphones, being a relatively low impedance load (64 ohm in this case) act to short out the noise so it's not heard. Basically the noise, wherever it's coming from, isn't strong enough to drive the headphones' drivers enough to be audible.

When the powered speakers are plugged in instead, they have a very high input impedance that then goes into their amplifiers (10k ohm). Now the same noise can drive the input easily, and is then amplified and played through the speaker.

So, the question is, if I were to put a resistor in parallel with the speaker input, or even plug the headphones in in parallel for that matter, would this decrease or eliminate the noise? Am I thinking about some part of this wrong?

thanks!

Best Answer

Three possibilities here:

  1. Random electron-movement noise: a 10,000 ohm resistor in 10,000 Hertz bandwidth, ignoring any A_weighted responses, will provide 12 nanoVoltRMS * sqrt(10,000 Hz) == 1.2 microVolts RMS noise; whereas a 62 ohm resistor will provide 100 nanoVolts RMS.
  2. your wiring may pick up RFI of AM, FM TV, cellphone energy and the powered-speakers have opamp inputs that non-linearly respond to that RFI/EMI and rectify it and downconvert it to within audio range and you hear that; cure is better cables, or RFI filters added at inputs to the powered-speaker.
  3. As mentioned, powered systems almost always have switch-mode power supplies for efficiency and low heat generation, but the switch-mode transformers still have primary and secondary windings with some capacitance between the two windings. The energy coupled between those windings will, per the assistance of mother nature, explore ALL possible paths back to the high dV/dT switching transistors in the supply; one of those paths will be your cable into the speaker.
Related Topic