lm 386 and output impedance

[swt]

Hi. i have built a talk box using the 386 as the small amp, with a fet pushing it for more distortion and level, but i can't get 4 ohms drivers around here. i only have a 16 ohms. can i add a resistor in parallel to get a little more power from the ic?. remember it's just for talk box use, i don't care too much about the tone, and variations in impedance-resistance-inductance, just a small volume boost. thanks a lot.

[petemoore]

  Adding resistors which produce heat instead of sound to the speaker circuit will divert output to heat, reducing volume at best/only.
  Why not just connect the 16ohm speaker to the amp? You won't get as much volume as a 4 ohm because the amp isn't loaded as much, maybe the volume control allows you to 'get the chip to run hot' anyway though.

[Peter Snowberg]

A parallel resistor will not help.

If you can't get a 4 ohm driver, then think about using the LM386 to drive a pair of power transistors (bipolar or MOSFET) which will then drive the speaker. That way you can use more voltage to compensate for the higher resistance.

A better solution would be to use a larger amp chip like the LM3886 and supply it with enough voltage to get the output you require.

[R.G.]

Ohm's law to the rescue again!

The LM386 is both voltage and power limited. It can only put out so many volts, determined by its power supply. When you are using the max power supply, you run into its power dissipation limits.

But using a 16 versus a 4 ohm speaker means that you are votlage limited.

If you generate 1 watt into a 4 ohm speaker, the the voltage which does that is V = SQRT(P*R) = SQRT ( 4) =  2V rms. That same voltage into a 16 ohms speaker is P = V*V/R =2*2/16 = 0.25W. You can't use all the current available because you are voltage limited.

One way to help is to drive the speaker between two amplifiers fed out-of-phase signals - the so-called bridge connection.

This doubles the available voltage, and gets you twice the available power compared to what you had.

 

[Peter Snowberg]

See the R.O.G. Little Gem Mark II for a bridge connected LM386 setup.

[swt]

well thanks a lot for your kind and helpful replies!!. I'll try a new design with a tda 2030 and higher voltage supply, to see if that fill my needs. Or maybe higher voltage supply to the 386...mmm need to take a look at the datasheets...i'll let you know.