How do I know if my amplifier can run a speaker

For audio applications, it matters to have a strong signal in the current when it comes to driving a speaker since 10mA at 5V which may be sufficient for a digital IC cannot provide sufficient power to run a speaker.

For audio coming from a DAC, first one needs low pass filter. This can be accomplished using stages of butterworth filter with cutoff frequency of 20KHz, right?

But how should this signal be amplified before being fed to the speaker? Is a common collector amplifier sufficient or is a unit gain feedback o-amp stage for current buffering preferred? Speakers have quite small load of usually 8ohm, so the source impedance to drive them shall have to be lower I assume?

A loudspeaker has a strong resonance. It causes sounds to be "boomy". Therefore modern audio amplifiers use a fairly high open-loop voltage gain and a lot of negative feedback to reduce the gain to a useable amount, reduce distortion a lot, increase the bandwidth and reduce the output impedance a lot. The output impedance of a typical audio amplifier is 0.04 ohms or less so that it damps the resonances of a speaker very well.

It is easy to calculate how much voltage and how much current an amplifier must provide to a speaker. If you want a maximum of 10W into an 8 ohm speaker then the RMS voltage is 8.94V then the peak voltage is 12.64V and the peak-to-peak voltage is double at 25.3V. If the peak voltage is 12.4V then the peak current is 12.64V/8 ohms= 1.58A.

Basically you need a X 10 voltage amplifier followed by a current amplifier able to deliver more then .1 A rms. The larger the current the greater the power output. For ordinary room listening you should aim for 3W for a party you might need 100W. The best thing is to buy an audio amp chip, they cost less then the cost of the individual components. The power supply will determine the cost and the power output, so decide on how much you want to spend on the PSU, then you can work out what voltage would work, say 12V at 1A, then select an audio chip to suit this voltage.
Frank

With a 12V supply, an amplifier might produce a maximum of 9V p-p into an 8 ohm speaker which is 3.18V RMS which produces only 1.26W into 8 ohms. Depending on the design, the output level and power might be less.

Hi,

There are audio amplifiers you can buy in a hifi shop.
There are plenty of ready made speaker amplifier modules. (ebay)
There are plenty audio amplifier ICs, with datasheets and schematics in it.

Additionally there are different amplifier techniques. Mostely analog, but there are digital ones also.

Klaus

Actually, my question is about opamp ICs. How do I know that the one I have can drive a speaker? I mean they have 8 or 4 ohm impedance right?

Op amps only have a low current output, say 10 mA. So they will only put very little power into a loudspeaker. .01 X .01 X 8 = .8 mW, enough for sensitive head phones, but a waste of time for a loudspeaker. As said you need AMPs to drive a loudspeaker, so a chip designed for this is the best way to go.
Frank

Hi,

electronics has to do with U and I.

And thats the way to go.
What U and I do you need?

Yes, I mean "you", because a speaker can be silent (needing only microamperes) or it can be loud (needing more than 10A).
(For sure it also depends on what speaker it is. Electomagnetic? Piezo? --> almost capacitive! Earphone?...)

Then look at the OPAMP´s datasheet in it´s "recommended operation conditions".
* Max. output current will be listed
* Max output voltage will be listed (maybe in a chart)
* Max supply voltage will be listed.
* Some even list a "minimum load impedance"

"You" have to take care not to overload the OPAMP.

Klaus

OK, I do not know what different type of speakers exist. I shall read on that first. In my case I shall be using the small speaker, the kind present in children's battery operated toys, since the speaker only needs to make some sound effects and not play loud music.

The speaker in a child's battery operated toy could be a real speaker with a magnet and a moving coil of wire that measures 8 ohms. In needs to be driven with a fairly high current that is not available from an opamp. Or it can be a piezo squeaker (not a real speaker because it does not produce low sound frequencies) that measures about 1000 ohms. Almost any opamp will drive a piezo squeaker.

These children's toys only use a .1W loudspeaker. Still too much for an op amp, just look around for the lowest audio amp chip. There are some 1W ones around. Then design your power supply to suit it (6V?, 9V?, 12V?).
Frank