Matched impedance for Audio?

Discussion in 'General Electronics Chat' started by blah2222, Dec 6, 2012.

  1. blah2222

    Thread Starter Distinguished Member

    May 3, 2010
    Hi just a quick question regarding input/output impedances for audio circuitry.

    It seems like voltage transfer is pretty important as in the case of a preamp at least as you are taking say an op-amp or JFET with a much larger input impedance as compared to the output impedance of the guitar.

    For audio at least, when would you care about power transfer through matching input/output impedances?

    Through reading "Electronic Projects for Musicians" by Craig Anderton, he states in the Appendix that most often matching impedance for maximum power transfer is desired but voltage transfer is becoming more important and a rule of thumb is to have an output impedance 1/10th of what the input impedance being fed is.

    Wondering what your thoughts are.

  2. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    Impedance matching is important, though feeding a low impedance into a high impedance is more acceptable than vice versa.

    The problem with low voltage/low current (aka low power) feeding a high impedance input (JFET), will allow noise to have more of an impact in relation to the signal.

    In power amplifiers, a low output impedance is required, as the load is 4-8Ω, and the power levels completely drown any extraneous noise.

    For low level signals, transformers will introduce more noise than the impedance matching effect would help.

    Higher pre-amp voltages allow for a wider signal to noise margin, effectively increasing power for the same amount of current (Watts=Volts * Amps).

    There are audio experts here that can refine the answer better, but keep in mind that "audio expert" is an entirely different, real engineer type, not an "audiophile" who is subjective and tends to base ideas on urban legend. :)
  3. nsaspook

    AAC Fanatic!

    Aug 27, 2009
    Impedance matching is important when you have audio lines that are very long. We had dry telephone lines that were many miles long for voice datalinks when I was a Navy TCF tech. At these lengths the characteristic impedance of the cables becomes important. We had racks of buffer amps and matching networks to deliver a clean signal to the distant end receivers.
    Last edited: Dec 6, 2012
  4. crutschow


    Mar 14, 2008
    Not sure I understand that last statement. The noise pickup characteristics of a short (with respect to the wavelength of the audio frequency) audio line are mainly determined by the lowest impedance point in the line. If the line is driven by a low impedance source, then a high impedance load, such as a JFET, will have little effect on the noise pickup.
  5. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    You are correct, I didn't have audio frequencies in mind when I wrote that.

    --ETA: One of the audio pros is here now! :D