Reduce audio amplifier output power [closed]

I need some help to adjust the volume of the amplifier. It is a 2.5 W amplifier and it has to be connected to a 2 W speaker. I want to reduce the gain of the amplifier in order to avoid damage to the speaker. How can I do it? Or can I just connect the amplifier to the speaker without anything between them?

• Don't bother. You don't have to match amp and speaker power so precisely. Just turn the volume down a little if it sounds nasty (distorted). – user_1818839 Jan 23 '18 at 17:37
• If you feel you absolutely must, you can use a resistor network as a voltage divider to cut max power by 20%. One would need to know the nominal impedance of amp output and speaker to spec the values, though. – Hot Licks Jan 23 '18 at 22:56

Don't bother. 2.5 W is unlikely to hurt a 2 W speaker. It will just cause distortion. Somewhere in your audio chain there must surely be a volume control. Turn down the volume to the point where the sound from the speaker isn't distorted.

The 2.5 W rating of a power amp is what it can put out, given the right signal and load. Put in lower volume, and you get lower volume out, which also means less power out.

• Thanks for the attention. The problem is that I have an audio board amplifier: adafruit.com/product/2130 and I can't modify the volume. So can I use it without any potentiometer between the speaker and the amplifier? – Antonio Iannacci Jan 23 '18 at 15:22
• @Ant: If you need to add a volume control somewhere, do it at the input of the amplifier, not the output. – Olin Lathrop Jan 23 '18 at 15:26
• What's the sense of adding a control at the input of the amplifier? I need something to reduce the power from the amplifier to the speaker. – Antonio Iannacci Jan 23 '18 at 15:53
• Which is exactly what reducing the input to the amplifier will do. – Colin Jan 23 '18 at 15:55
• I just followed the link to the product you have, it says "There's even a volume trim pot so you can adjust the volume on the board down from the default 24dB gain." That is the solution to your problem. Turn it down a bit. – Colin Jan 23 '18 at 16:06

Your assumption that a 2.5 W amplifier can damage a 2 W speaker is false.

You can just use them together and it will work just fine.

The power rating of speakers is "weird" anyway as it depends on frequency and how the speaker is used (in a case, air-tight case or no case at all).

When the speaker is about to damaged you will hear it as you will be overdriving the speaker and it will distort. That's not pleasant to hear. Then just lower the volume.

Now that you've said what the product is you're using https://www.adafruit.com/product/2130 in a comment on Olin's answer, you can see from the product page it has a trim pot for adjusting the gain.

The page says "There's even a volume trim pot so you can adjust the volume on the board down from the default 24dB gain." This seems like a direct answer to your question of how to reduce the gain of your amplifier.

• I can't read the schematic so don't know here that trim pot is. If it is at the input it will NOT prevent the amplifier to reach it's maximum gain as that also depends on the amplitude of the input signal. – Oldfart Jan 23 '18 at 17:03
• I've not looked at the schematic either, but it would be odd for the description to mention reducing gain if it was just attenuating the input. Either way it sounds like it would solve the op's problem. – Colin Jan 23 '18 at 17:28
• No it does not. Simplified: the gain is the amount the signal increases. If you half the gain and then apply in input signal of double the amplitude you are back where you started. Thus your output signal is function of the gain and the input signal. Without knowing anything about the input signal you don't know what gain to set to make it safe. – Oldfart Jan 23 '18 at 17:35
• Yes, that is obvious. The op wishes to decrease the signal coming out of the amplifier, whether that is achieved by attenuating the input, or reducing the gain the overall result is the same. – Colin Jan 23 '18 at 18:11

Adjusting the gain is not so easy. You could put a voltage divider at the input but that works only if you know the maximum voltage coming in and the sensitivity of your amplifier.

Much simpler would be to add an extra resistor in series with your speaker with a value of ~1/10 of your speaker impedance. Thus an 8 ohm speaker would require a ~0.8 ohm series resistor. (Nearest E12 value is 0.82 Ohm)

The resistor should be rated for 0.5 watts and will get warm. (Or hot if the heat can't go away).

Corrected after re-calculation because of comment from Olin Lathrop.

• Your math is off. You want a resistor that is about 11% of the speaker resistance to drop the power from 2.5 without resistor to 2.0 W with resistor. That would be about 850 mOhm for a 8 Ohm speaker. This is assuming the amplifier output is a constant voltage source. – Olin Lathrop Jan 23 '18 at 15:30
• @OlinLathrop so you are suggesting to insert a resistor of 850mOhm in series between the amplifier and the speaker? – Antonio Iannacci Jan 23 '18 at 15:54
• @Ant: No. Read my answer. – Olin Lathrop Jan 23 '18 at 15:55
• Ok so I'll just use it without inserting anything between the amplifier and the speaker, right? @OlinLathrop – Antonio Iannacci Jan 23 '18 at 16:01

In the audio production world, it is said that you need amplifiers with a power level above the nominal power level of a speaker. A beefier, similar quality amp will reproduce transients with better fidelity, at the cost of possibly accelerated wear to the speaker element itself.

On the other hand, when using an under-powered amp, while it will "preserve" the life of the speaker, the reproduced signal will be of lower quality.

Using an amplifier several times the power rating of the speakers it is driving is not an issue, as long as the impedance is properly matched: the amplifier should never drive a load that is lower (in ohms) than its own rating, while most amps will happily drive a higher resistance load (not necessarily without some audio distortion).

A limiter (an audio device) can limit the signal sent to a power amplifier in order to protect a speaker. Common processes involve either signal ducking (turning down the volume at predetermined rates) or clipping of the signal itself (soft or hard) thereby preventing the speakers from being overdriven, but such processes are not reliable and always involve audio distortion.