LM386: LoVolt power amps and loudness

[Steben]

How loud can you go with 0.5W?

Are the LM387, LM388 and LM389 still in production?
these are of the same family of the 386 but with higher output power.
Anybody who knows how these go into overdrive?

[Mark Hammer]

For many years now, the rule of thumb in the audio industry has been that - all other things being equal (such as speaker, power supply, gain in preamp stage, etc.) - one needs a tenfold increase in power to produce a twofold increase in perceived loudness.

That implies that 5 watts delivered in the exact same circumstances as 500 milliwatts will sound about twice as loud.

It also implies that 20 watts will sound about twice as loud as watts.

If you make an effort to use ALL the headroom available in those little 8-pin wonders (by providing them with a big input signal), provide them with an adequate power supply (I like an 8-pack of penlight myself), and use a reasonably efficient speaker in a cabinet that accentuates its volume (e.g., closed-back front-ported), you will be pleasantly surprised at just exactly how easy it will be to make those around you pound on the walls and scream "Turn it DOWN!  I'm trying to watch my show!!".

You can probably get LM387/88/89 chips somewhere, but they are really and truly intended for the vinyl turntable market; the 387 being a dual preamp chip for magnetic cartridges, and the 389 being an all-in-one solution for kids' mono turntables.

The LM377 dual 2W power amp chip does not exist anymore but is subbed for by the LM1877 which *does* exist.  This is essentially two LM380's on a 14-pin dip.  The 380 is also widely available.  I put together a 380-based portable amp for a trip I recently took and it held up admirably.  Plenty loud.

[Ansil]

lm380n8  my current favorite.

[Brian Marshall]

the speakers you choose, and the way the cabinet is built makes a load of difference as well.  If you can get your load below 8 ohms, and the chip can handle it that would be your best bet.

all in all the more surface area you have moving the louder it will be.

[Mike Burgundy]

Speakers are often rated (if you buy something by the pro's instead of cannibalising PC-speaker sets - which can yield good results btw!) for dB at 1m, 1W in a situation resembling an "infinite baffle". This means the speaker behaves as it would when it wasn't mounted in a box, but in just a baffle of infinite size. Some manufacturers do spec a box with this measurement.
Music instrument speakers are mostly made for "bang for the buck" - as efficient as possible, whereas hifi speakers sacrifice efficiency for linearity or whatever sonic effect the designer wants. This means that music instrument speakers are often *loud* compared to hifi (one of the reasons your 50W 1x12" tubeamp blows away your 2x100W stereo system).
A Celeestion Greenback does 97dB@1W,1m,  a modern one does 100, I have an Eminence that does 103, some go even higher than that.
What does this mean? Well, as stated, doubling power only means an increase in percieved loudness of 3dB.
So, basically, a 100dB rated speaker at 1W will be as loud as a 103 rated one, at 0.5W.
Is that loud? Well, yes. Concert levels are up to 110 - you won't get that, but it's about half as loud as that with these speakers. Turn up the TV loud, and ,say, you've got 70dB. A Really loud argument might *just* reach that, normal conversation is at 50dB. This might give you a ballpark.
Enclosures do help produce more bang, and those computer speakers I mentioned? The woofers often are nice and effecient in the guitar range... hih

[Steben]

Would it be possible to switch between power supplies?
(5V-9V-12V-...)
This in/decreases the max power of the chip, no?

Should it harm the overdrive behaviour?

[vdm]

this idea is possible, but from my experience the overdrive simply from overdriving the chip is not the greatest distortion around.

A designated distortion pedal before would be better than pushing it too hard with a booster.

I would suggest instead of the switchable power supply just run it at 12V or whatever voltage you want and use a master volume beforehand.

trent

[Mark Hammer]

The power a chip (or any amp for that matter) can deliver is a function of how much current the circuitry can tolerate passing to THAT speaker load at THAT supply voltage.  Bear in mind that all amplifiers really do is vary how much current from the power supply is fed to the load.

You will find, when you look through datasheets for any of the little 5, 8, or 14-pin wonders, that they will very often show charts for power attainable with load X over a variety of supply voltages.  Often, the plotted values on the graph will "run out" for one speaker impedance before they "run out" for another.  This is because the amount of current the chip has to pass at that load impedance and that supply voltage simply exceeds the sort of heat the chip can tolerate and the output transistors on the chip stop behaving like transistors and behave like fuses - i.e., they burn up.

This is the long way of saying that you CAN often tinker with supply voltage as a sort of "effect" (though it won't be all that huge an effect), but you need to be careful about whether the chip can tolerate what it is you intend to do.  After all, you don't want one of those "effects" to be dead silence while you wait for another chip in the mail.

Learn a bit more about the options for suitable heat sinking on small devices and THEN explore your supply voltage options.