LM386 max input spec is 400mV?

If you change the pot position fast enough, or the pot doesn't have perfect contact the whole time, yes, it is a bad thing. It slaps the output of the amplifier around.

A much better way (if one end of the pot is at ground, and there is DC across its resistance), if you can arrange it, is to put the cap at the signal input end of the pot, and connect the wiper directly to the LM386. That way, moving the wiper does not change the average voltage going in to the amplifier.

Why are you using such a miserable device ?

Graham

Well, because it provides less than 0.3% THD, has an idle current of less than 5mA, provides 5 volt p-p signal into 8 ohms using a 7.2 volt lithium battery, and the gain is internally set to 20 so that minimal extra parts are required.

Do you have a better solution?

-Bill

LM4877 More output power, better THD and PSRR.

And you can get a 386 in single quantities just about anywhere. If you can't extract it from all kinds of consumer equipment that is using them already (lots of modems that nobody wants, for instance, will have a 386).

If he needed a better solution, as in better specs, likely he'd be pursuing it. But I'd say the 386 is still valid.

Michael

One end of the pot is at ground and the wiper connects directly to the LM386 input. The other end of the pot connects to a capacitor which is tied to the signal riding on a DC offset. So there is no DC voltage across the pot. My concern was when power is applied and the wioer of the pot is set at maximum (connected to the cap), a DC voltage will be momentarially applied to the input as the capacitor charges during power-on. It would just be a short spike of a few milliseconds.

The gain is 20, and I need 5 volts p-p out, so the normal input should not exceed 150mV peak.

Do you think a Schottky diode could be used from ground to the input to limit the signal to around 300mV? Or is that necessary?

-Bill

Yes. There will be a power up transient. If you use a capacitor on the bypass terminal, it will also produce an output transient at power up. If you couple the speaker to the output with a capacitor, and the other end of the speaker connects to one side of the supply, you will get a power up transient. (You can eliminate most of that by putting two equal capacitors in series across the supply, and tie the speaker between the middle node and the output) You might get the combination of these transients to partially cancel if you work at it.

Since the +-0.4 input volts is the absolute maximum, this is probably a good idea. I can see how the -0.4 volts might cause problems, but I can't understand how exceeding +0.4 volts does anything except saturate the output.

Wow ! You knocked me sideways. Is it really truly that BAD ?

What's the significance of lithium ?

You mean an extra 1/4W resistor ? Do you *want* or *need* a gain of 20 ?

Any modern power amp IC.

Graham

Because clots keep designing the useless pices of s*it into gear because they're incapable of looking beyond the ends of their noses to the open page of a modern databook.

The LM386 is *notorious* for ultrasonic oscillation btw. It's noisy as hell and distorts like a good 'un.

Graham

You don't get 0.3% THD at 5Vp-p output, the THD is over 10% at that output level. Look at the graph in the datasheet, the THD shoots up above 0.2watt (about 1.2v in 8ohm).

The LM386 is a horrid amplifier.

Bob

There is some confusion in your question. "you tie the input of the pot" to what? The signal cannot be 5V DC, signals are AC by definition. Please clarify what you mean, even post a circuit diagram.

You tie the input (of the 386) to a pot (the wiper). One side of the pot is at ground and the other side is tied to a cap, The other side of the cap connects to the input signal which is sitting at a positive voltage.

Very standard setup. How else would you do it?

-Bill