Question about audio amp output load

Discussion in 'General Electronics Chat' started by sstbrg, Aug 19, 2009.

  1. sstbrg

    Thread Starter Active Member

    Nov 29, 2008
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    0
    Hi, I've got a dev kit with an audio chip and an amplifier normally driving an 8 ohm speaker.
    My question is: will it cause problems if I connect it to a 150 ohm load instead? Also, how does an amplifier reacts to change in load impedance?

    As far as I know, the amplifier is class D.
     
  2. JDT

    Well-Known Member

    Feb 12, 2009
    658
    85
    Should not cause problems but the output power will be reduced.

    Class D means that it uses Pulse Width Modulation instead of linear control. This gives greater efficiency. Often used on high power amplifiers to reduce heat generation and on small headphone amplifiers (in MP3 players) to get better battery life.

    The output is filtered before being fed to headphones or speakers. With 150 ohms instead of 8 ohms, this filtering may not be quite so good. But you probably won't notice.
     
  3. sstbrg

    Thread Starter Active Member

    Nov 29, 2008
    54
    0
    Thanks for the quick reply.

    You're saying I shuoldn't notice any change in volume?


    If that's important, I'm using Nuvoton ISD151xx's SPK output. Instead of directly connecting it to a speaker, I'm connecting it to another system, which according to its specifications is a 150 ohm load on anything connected to its input.

    I'm not so good with analog electronics, as you've probably noticed :rolleyes:
     
  4. JDT

    Well-Known Member

    Feb 12, 2009
    658
    85
    I looked at the data sheet for this chip. It has a "Bridge" connected output designed for connecting to headphones or a small speaker.

    [​IMG]

    This means that as SPK+ "goes up", SPK- "goes down". The speaker sees the difference between the two signals. It does this to get more voltage swing - and more output power from a low voltage supply. With no signal, both outputs will be at about half supply voltage.

    To couple this into another bit of equipment, I would suggest you use a circuit like this:-
    [​IMG]
    The resistor gives the load it needs and the capacitor removes the half- supply-volts DC offset. The total audio voltage will also be halved but you should be able to amplify this in the next bit of equipment.

    Without the capacitor you risk damaging something.
     
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