[Solved] What's the best speaker-driver when 5v USB powered?

My project has a small internal speaker. I've been using a LM386 chip to drive the speaker since this chip is cheap and so popular.

However, there isn't much sound available. With no load (no speaker) the chip will output a 4-volt (peak-to-peak) sine wave without much distortion. But with the speaker loading the circuit, the voltage is reduced to 2 volts p-p. Increasing the volume above 2 volts just clips the top and bottom of the waveform into a square wave. I think the chip must have this limitation built-in to avoid overheating.

But is there a better way to power the speaker from my USB cable voltage source? A way that still would not pull too much on the 5V source (which must remain stable -- and well filtered -- to power other analog and digital components as well)?

I'm using an 8-ohm speaker, (measured dynamically). Is that best?

Look on eBay for a PAM8403-based amp.

They can push quite a lot of power :D

// Per.

jcallen: If you want your speaker to sound twice as loud, you would have to increase power by 10db, a factor of 10 to 1. Sorry, but thats how our ear pans work. :confused:

Wrong. You would have to increase voltage by 10dBV. You would increase power by 3dB, a factor of 2 to 1.

CosmickGold: I've been using a LM386 chip to drive the speaker since this chip is cheap and so popular. I'm using an 8-ohm speaker, (measured dynamically). Is that best?

The LM386 is so popular because it has been around for a long time. I stopped using them 20 years ago. There are now better technologies available.

Class-D amplifiers are much more efficient, so small amps usually don't need a heatsink.

Power to the speaker depends on voltage swing and speaker impedance. Volume depends on power and speaker efficiency.

Voltage swing on the speaker can be doubled with two amps in bridge, with the speaker in between. Twice the voltage is four times the power. Drop the speaker impedance to 4ohm, and you have 8x the power you had before.

This PAM8403 board might be ok. You can see it's bridged, because both speaker terminals are "hot". http://www.ebay.com/itm/5pcs-PAM8403-2X3W-Mini-Audio-Class-D-amplifier-board-2-5-5V-input-/171907237052?hash=item28067844bc

Just measure the resistance of the speaker, and add 10-20%. Dynamic impedance is very frequency dependent, and usually lowest at ~500hz. Leo..

Wawa: Power to the speaker depends on voltage swing and speaker impedance. Volume depends on power and speaker efficiency.

Voltage swing on the speaker can be doubled with two amps in bridge, with the speaker in between. Twice the voltage is four times the power. Drop the speaker impedance to 4ohm, and you have 8x the power you had before.

So the speaker would sound 8 times as loud? WOW :D

Did

jcallen: So the speaker would sound 8 times as loud? WOW :D

Did't say that. "power" and "volume" are two different things. Leo..

How much louder [u]would[/u] it sound? If I'm putting 1 watt rms into my 8 ohm speaker and then increased power to 2 watts rms, how much louder would it sound?

A normal USB port is limited t0 500 ma, so thats a maximum power of 2.5 watts. To get more you will have to provide a hi power USB port, or a powered USB hub.

Twice the power is +3db. Noticeable, but not twice the volume. Leo..

mauried: A normal USB port is limited t0 500 ma, so thats a maximum power of 2.5 watts. To get more you will have to provide a hi power USB port, or a powered USB hub.

True, but a music signal peaks 10-20db higher than average, so a 5-10watt amp with buffer caps on the power rail might be perfectly happy with that. Leo..

Wawa:
Twice the power is +3db. Noticeable, but not twice the volume.
Leo…

Thanks Leo, thats what I learned 55 years ago when that new fangled “Stereo” first came out, at the dawn of the transistor age.

Zapro: Look on eBay for a PAM8403-based amp.

I ordered 10 of them for 7 bucks! It looks like the perfect answer. I need only one channel instead of two, but for such a low price, who cares. :)

Wawa: The LM386 is so popular because it has been around for a long time.... Class-D amplifiers are much more efficient.... Voltage swing on the speaker can be doubled with two amps in bridge, with the speaker in between. Twice the voltage is four times the power. Dynamic impedance is very frequency dependent, and usually lowest at ~500hz. Leo..

Thank you for a lot of valuable information (and in only a few words). I didn't even know about "Class-D Amplifiers". But just now read that they "operate as electronic switches, instead of as linear gain devices". Which means they are constantly switching between no resistance, and no current; resulting in a controlled total current without ever having "resistance" to eat up part of the power; it all goes to the speaker instead! Sounds great! :)

You're idea about using two amplifiers to achieve twice the voltage is a great one, I almost tried it with two LM386 amps. But then realized they are clearly clamping the current to a particular level, not the voltage. And since all current would be passing through both amps in series, the total amperage through the speaker (and thus power) would remain the same. However, I wonder about connecting two LM386's parallel to each other, and to the same end of the speaker. Then they could supply twice the total current (and so four times the power), each then able to produce that 4 volts I saw with no speaker.

mauried: A normal USB port is limited t0 500 ma, so thats a maximum power of 2.5 watts. To get more you will have to provide a hi power USB port, or a powered USB hub.

Very helpful to know! I'll stick with the 2.5 watts for the sake of my customers and their USB ports.

===================

Thank all of you who answered. You've supplied a wealth of information, and I'm taking in every word. I've ordered the new amplifiers as suggested. So my question is now fully answered!

The chips you have ordered already have two amps in bridge (per channel). With a 5volt supply, you will be getting ~9volt peak/peak from the amp. That's 4.5volt peak into 8ohms = 2.5watt peak power. You can always lower power consumption by using a resistor in series with the speaker. Your LM386, with 1volt peak, would have produced 125mW peak power on 5volt. Leo..

Zapro: Look on eBay for a PAM8403-based amp.

They can push quite a lot of power :D

// Per.

They arrived today, and I was quick to install a PAM8403in my project, replacing the old LM386.

WOW! What a difference!

That's all the power I could possibly need for my project, and better fidelity as well. (Also no more feedback oscillations, etc.)

And Wawa, thanks for the in-dept description That helps too.