low power consumption audio circuit design tips

Discussion in 'General Electronics Chat' started by rick5, May 15, 2009.

  1. rick5

    Thread Starter Member

    Jul 17, 2008
    Hi AAC forum,

    I've designed an audio circuit that I thought was going to be primarily operated via a DC power supply socket, but now think it will be more useful as a battery operated device.

    I've got my circuit fully modelled in a SPICE package and am looking to optimise power consumption throughtout to prolong the battery life. SPICE also allows me to look at the power consumption of all resistors and op amps, I guess this should be equal to the sum power drain of the whole circuit (ignoring reactive components like capacitors).

    Of course we all know P=VI, P=(V^2)/R and P=I^2R. I guess this exercise is all about trade offs.

    I can look at the whole circuit power drain at some nominal audio frequency of say 1KHz by just looking at the total current consumption and supply voltage.

    What happens though if I've got balanced supply rails of +9v 0v -9v. If I look at the current drain through the 9V rail, is the total power consumption that current x 9V or 18v?

    In audio circuits people tend to want to keep resistor values down to minimise Johnson noise, but that keeps currents high and therefore is a trade off with low power?

    Op amps with low quiescent currents are a good idea, I think. I'm open to using discrete transistors if they would help. I'd be grateful for any other tips to get me thinking on the right lines.

    Does anyone know of any equations for calculating useful battery life based on the cell voltage, mAH rating and chemical type? The thing about these mAH rating is that you don't seem to be able to do linear calculations with them.

    Eg. Suppose you have a device that draws 10mA and runs off a 1.2v AA rechargeable cell that's rated at 1000mAH. In theory it follows that the device would be supplied with the current it needs at the right voltage for 1000/10 = 100hours. Most books say that in practice it doesn't work like that, but nobody seems to give any equations. I was expecting to see some nice drain formula that I can plug into a Maths package and work it out.

    If I could mathematically model the voltage drop of the battery and the rise in it's internal resistance, I could design a low battery indicator around a voltage reference. In commercial products this must be a quite a common thing to want to do.
    Last edited: May 15, 2009
  2. PRS

    Well-Known Member

    Aug 24, 2008
    Post a schematic, you'll get lots of replies and advice. As it is we're shooting in the dark. Fet Op Amps are a must to keep current low. If you use discrete transistors use Fets.