LM386 power amp IC question

I found an old LM386 chip in one of my piles of junk, and I'm thinking of putting it into use.

All the schematics in the data sheet use an idiotic, large output coupling capacitor between the chip and the speaker, which just makes me cringe.

I'm thinking that since the IC references the output at halfway between its power rails, and since I'm using a virtual reference in the rest of my circuit at V/2, can't I just tie the speaker's return to the virtual ground?

There is a chance that the two voltages may differ, causing some current to flow at quiescence, and any such current should be kept well below what the device can deliver.

Any other gotchas with this IC?

Thanks.

Reply to
Kaz Kylheku
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"Kaz Kylheku"

** No reason to cringe exists.

You must be an audiophool.

** ROTFL.

Jus use the electro cap.

.... Phil

Reply to
Phil Allison

what's going to happen if you feed the LM386 output current into the "virtual ground"? a virtual ground is usually an (inverting) input termial, not an output terminal.

you might be able to work it using a center tapped battery supply.

--
?? 100% natural

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Reply to
Jasen Betts

Dig further and find a second one. Then feed one 180degrees out of phase with the first one, and connect your speaker across the outputs of the two amplifiers. No more output capacitor, more power, and no Rube Goldberg.

Of course, it was never much of a "power amplifier", just enough for a lot of things that needed a bit of power to a speaker. Nowadays, one can find all kinds of more recent amplifier ICs in all kinds of devices, some likely do have the out of phase second amplifier built in so there's no coupling capacitor on the output.

One might find these on modem boards or even soundcards, though those may still be 386s. "Computer speakers", I just brought home two sets from the garbage this week, likely have them. Car radios will. Anything with a speaker is a suspect for donating a more recent audio amplifier IC.

Michael

Reply to
Michael Black

It's a crappy chip, but it'll make a noise on a speaker.

It used to be popular with the amateur radio crowd, so I bought a bunch before I knew any better.

That "1/2 VCC" is going to be very approximate -- use the cap.

I've noticed that unless it is very well bypassed it'll tend to have a high-frequency oscillation (several hundred kHz or a few MHz, I can't remember which) on the audio. Some portion of the amplifier is obviously saturating, because it makes the audio distort.

--
www.wescottdesign.com
Reply to
Tim Wescott

I don't see why it wouldn't work - providing the virtual reference is "stiff" enough to handle it. Presumably this reference is a power op amp output, voltage follower, referenced to 1/2 Vcc? Stiff to the tune of at least a 1/2 amp or so?

But there's no good reason not to use a coupling cap - all transistor stereos did that at one time (60's and 70's).

I've used both varieties of integrated stereo amp (bipolar and single supply) as their opposite number at one time or another.

Reply to
default

No, don't phase-shift your signal. Just feed the inversion of the signal to the other input. There should be no appreciable phase shift, just an inversion of polarity.

With a sinusoid, a 180 degree phase shift may _look_ exactly the same as a polarity inversion, but they are by no means the same. To illustrate this, just use an asymmetrical signal on your input, like a pulse train.

Hope This Helps! Rich

Reply to
Rich Grise

Just use two of them in a bridge amp.

Have Fun! Rich

Reply to
Rich Grise

Ruminating on this theme: A couple of equal resistors from Vcc to ground with large electrolytics bypassing them should also make a stiff virtual reference.

Stupid way to do things, and not net gain, but it should "work."

Familiar with Nelson Pass's super simple, single mosfet amps? He calls it the Zen Amplifier. He uses a cap to keep DC off the speaker, but several people have taken it further to use it push pull (bridged amplifiers) to eliminate the cap or power voltage followers to eliminate the cap.

But a LM386 is not high fidelity, so there's no point in killing yourself to get rid of the cap.

Reply to
default

You mean like a 386 just sitting there in a static form, properly bypassed for audio?

Michael

Reply to
Michael Black

Put a small cap across the feedback resistor. Limit the amplifier's bandwidth to 20kHz (2x what you care about), or so.

Reply to
krw

The LM386 was/is a great chip for what it was designed for which was low cost audio at *modest* output levels. It is not HiFi. Using the cap will not degrade the sound in anyway if the cap is of suitable size. The cap will in fact provide some protection to the speaker if things go pear shaped.

Reply to
David Eather

I thought we are talking about coupling to the speaker - the purpose of the large cap in series with the speaker lead.

Virtual references do work - you use a voltage follower instead of just a divider because it has the ability to respond to a change in current without changing the voltage. But in something like an audio speaker signal it has to be able to handle the few milliamps the 386 can throw at it. Most op amps have low current outputs and would probably not work well.

Your bridged amplifier suggestion is equally valid - lots of car audio equipment uses that as a way to drive more power into a load from the limited 12 volts they have to work with.

Reply to
default

Ah yes. Some audio power IC's do this bridging trick internally. Thanks for the suggestion.

Actually, I already have a capacitance there: the two capacitors (with balancing resistors) forming a divider which establish the reference voltage.

So if the speaker is based off the reference voltage, there is AC coupling to ground already.

I just bumped these caps up to 100uF each, which makes 200uF (since we can consider them to be parallel, looking from the p.o.v. of AC at the middle reference.)

This is good: use the caps we already have, avoding the introduction of redundant ones.

Also it occurs to me that if I had a second LM386, I could use it simply to generate the middle reference voltage.

This is similar to the out-of-phase trick except that the voltage of the second amp does not move opposite to the signal. The advantage is that this reference voltage can be used by the entire circuit, allowing the capacitor divider to be removed.

Reply to
Kaz Kylheku

Yes it is, but this is not a hi-fi application. I've looked at lots of datasheets for possible alternatives, such as the RC4580.

Next project, maybe.

Right. Since the 1/2 VCC has caps to either power rail, that is more or less taken care of.

I will watch for that. I do have the 0.05 uF cap + 10 ohm resistor at the output to pull high frequencies down, right from the datasheet example.

Reply to
Kaz Kylheku

Of course what Michael is talking about is a polarity reversal.

This is often called "180 degrees out of phase" by electronic hobbyists. It's a kind of jargon.

I know, it makes me cringe too, but as long as everyone understands that what is being discussed is a polarity reversal, it isn't an issue.

Reply to
Kaz Kylheku

I think that this protection is taken care of by the voltage divider with bypass caps.

The divider has enough resistance such that any DC flowing through the speaker to the V/2 reference is going to be quite limited.

(Of course there could be a fault in that V/2 network somewhere, and the speaker is exposed to that.)

Ah well, I have some airline headphones here that can be sacrificed for a smoke test, and before that, I can measure DC current at the output terminals.

Reply to
Kaz Kylheku

The ones I've used simply have an inverting stage driving the other leg. Nothing fancy at all.

Be careful of any noise on this reference. We've generally used an active filter to drive the reference.

But your driving impedance into the reference is very low.

What's your filter corner?

Yes, but you'd be better off driving the load differentially.

You still have a pretty low impedance driving that reference. That can cause all sorts of grief up the line. You have feedback paths all over the place.

Reply to
krw

"Kaz Kylheku"

** Our smug friend here is about to discover what the term "motor boating " refers to .

.... Phil

Reply to
Phil Allison

Consider:

Your weird biasing arrangement capacitively couples to the speaker - so you do not get the benefit you wanted by avoiding the standard circuit configuration - so on that front it is an outright failure.

In addition to this you introduce a whole bunch of problematic biasing arrangements which solve *no* problems and introduce others - so that is a outright failure.

Your new arrangement needs more board space and more components to work and at *best* it will be only as good as the standard configuration - so that is a bust too.

Just use the standard configuration and use the extra power supply cap from the other configuration as the speaker coupling cap.

Reply to
David Eather

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