Why is the LM386N-4 power rated @ 32Ω load, when the other variants are rated @ 8Ω? How can I find an appropriate speaker?

I'm trying to build an LM386 amplifier, but when looking at the datasheet to select the appropriate chip, I noticed the LM386N-4's power rating is based on a 32$\Omega$ load, while the -1 and -3 variants are all rated based on an 8$\Omega$ load. Why is that variant rated on a different load?

The only speakers I can find at my chosen suppliers (DigiKey or Mouser, which seem to be the only two that have all the other parts I'm looking for) that also will fit the enclosure I'm using and have power ratings within the LM386's range are all 8$\Omega$ speakers -- how do I figure out what the power output of the -4 variant would be with an 8$\Omega$ load, and would I need to do anything to make it work?

Lastly, I'm trying to get the most power possible, so the 1W of the -4 variant is of course appealing... but I note that the -4, despite having a minimum supply voltage of 5V is also rated at 1W based on $V_s$ of 16V. Since I plan on using an existing 6V supply, will I get less actual power from the -4 variant than I would by using the -3 (whose 700mW power rating is based on $V_s$=9V)? My instinct says the power would be something like $\frac{6V}{16V}\times1000mW=375mW$, versus $\frac{6V}{9V}\times700mW\approx467mW$.

What confuses me further is that the performance characteristics graphs (which I can barely make heads or tails of in the first place) don't seem to be different between any of the variants. Are they really all the same?

• On 6V I'd expect the N-1, N-3 and N-4 to all do the same. – Brian Drummond Sep 30 '16 at 16:37
• 32Ω is typical for headphones, so it sounds like the -4 variant is targeted for them. – tcrosley Sep 30 '16 at 16:48

1 Answer

The -4 only has a higher Vs rating. So to increase power both Vs and R load must be increased to limit THD to 10%.

They are all output current limited due to the design's very efficient low idle current.

Low quiescent current drain: 4mA

Your choice using this is to get 1W and 10% THD max is to use a string of 4 speakers in series.(4x8 Ohms)

A more powerful chip would have a higher idle current, not desirable for batteries.

• Because the speakers I'm looking at are rectangular, I can probably fit 2 in the enclosure, or maybe three if I get creative... how can I figure out the power output and THD with a $16\Omega$ or $24\Omega$ load? I'm not worried about idle current, I'm using a wall wart (and 100uF and 0.1uF caps on the supply input) – Doktor J Sep 30 '16 at 16:50
• Imax^2*R so 24/32=2/3W – Tony Stewart Sunnyskyguy EE75 Sep 30 '16 at 16:51