Text2Speech amplifier circuit query

[t0m]

I have a couple of questions about the LM386 amplifier circuit for the webbotlib speech module.

1. What does the diode in this circuit do?

2. Is R4 supposed to protect to the master chip from current surges, or something like that? It seems to me that the 10 ohms will cut the power available to the speaker. Is there another way to protect the master chip without suffering this disadvantage?

Thanks.

[Webbot]

The circuit is very similar to those in the manufacturer datasheet http://www.ti.com/lit/ds/symlink/lm386.pdf - note this shows the same 10 ohm resistor which, along with the capacitor, acts as a filter.

The diode is there to cope with you connecting the supply the wrong way around.

[Soeren]

Hi,

[...] this shows the same 10 ohm resistor which, along with the capacitor, acts as a filter.

I think you're talking about R1(?) (in your schematic), while t0m asked about R4 (still in your schematic), which isn't in any app note, as pin 6 is shown without any connections (V+ implied), except for a 50nF supply filter in the AM radio amp.

Unless the supply carries a lot of noise/transients, there's no reason for R4 (and C5 can be a lot smaller). Motors, servos, relays etc. may warrant using the R4/C5 if the supply isn't handled well at the sources of noise though.