Audio Amp gain

Discussion in 'General Electronics Chat' started by Wendy, Oct 23, 2010.

  1. Wendy

    Thread Starter Moderator

    Mar 24, 2008
    OK, this is basically a spin off of another thread, and is something I've wondered about several times in my career.

    600Ω seems to be a magic number for impedance for audio amps.

    What is the average maximum gain you would need out of a audio amp circuit? I don't really have a handle how much signal is input into something like this, though I can easily translate power ratings to speakers.

    So what kinds of gain would you need to drive computer speakers, for example? I would be interested in other examples too.
  2. Audioguru


    Dec 20, 2007
    Hi Bill,
    I have seen very few amplifiers with an input impedance as low as 600 ohms. Maybe for the preamp of a dynamic microphone. Line level inputs are usually 10k ohms.

    Computer speakers already have a power amplifier built-in. Their input is line-level which has maximum RMS levels to about 500mV. Extra gain is used so that lower than normal signals play loud enough when the volume control is turned up. The amplifier might have a sensitivity of 100mV with the volume control at max.

    An elecret or dynamic microphone has an output of about 5mV speaking at a normal conversation level about 10cm away. The gain from a preamp to boost it to line level is 500mV/5mV= 100 but extra gain is usually added for faint sounds to be heard. Then the preamp will have a gain of 200 to 300.

    Most power amp ICs for cars have an output at clipping of 14W RMS at low distortion into a 4 ohm speaker with a 13.2V supply. The output swing is 7.5V RMS. They have voltage gains of from 30 to 100.
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    That 600 ohm figure seems to be the one for telco equipment line transformers.

    With audio amps, the idea is for a high input impedance (47K used to be the common value) and a very low output impedance.
  4. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    preamplifiers have the widest gain requirements, to output a "line level" signal no matter the input, such as phono cartridge or blu-ray player. The preamp handles the impedance matching and the volume control.

    Power amplifiers have a lower gain with less range, but a very low output impedance. Typically, they can only take a "line level" input from the preamp.

    Various systems do swap the volume and tone controls between preamp and power amp, however, especially in "all-in-one" systems such as a portable stereo/boom-box.

    --ETA: For 600 Ohms, I've only seen that in telco things, otherwise 300 or so for dynamic mics. Older Tube TV's and such did have an abundance of matching transformers, unsure why 600 was chosen.
  5. marshallf3

    Well-Known Member

    Jul 26, 2010
    600 is the common impedance of Low-Z stage microphones which operate over a shielded balance line.
  6. Ron H

    AAC Fanatic!

    Apr 14, 2005
    Here's the history of 600 ohm audio impedances, according to Bill Whitlock (see post #6):

    See post #8 also.
  7. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    Impedance can be any value so to speak. It solely depends on the type of amp.

    But for maximum power transfer, Impedance shud be matched or else power will be reflected back in to the source.

    Gain is also defined by the design and type of amp.
    An amp can have different stages of different gains.

    A preamp is mostly for high Voltage gain. A power output stage is designed for Maximum Power gain. To drive a Power output stage to it's rated output it should be driven by Voltage driver stage.

    So for an amp. High gain voltage is for preamp. High Current gain is handled by the output stage