How to replace a 50 ohm speaker with 25 ohm speaker?

Question

I'm repairing an electronic device at home. Actually it has a non functional speaker. So I've to change it. This speaker is a 50 Ohm, 0.2 Watt. But I didn't find this one on the market, what I could find is a 25 Ohm, 0.3 Watt.

I searched on the net, I found that replacing a 50 Ohm speaker with 25 Ohm may cause some damage.

The system seems that is designed to work with 50 Ohm speaker. So to meet this condition, can I connect the 25 Ohm speaker in the system and add 25 Ohm independent resistor (25+25=50)?

Answer 1

Yes, you can replace the 50Ω speaker with a 25Ω speaker and a 25Ω resistor in series. This will be at the cost of a bit of additional distortion and most certainly lower sound volume.

A bit of additional distortion because the impedance of the speaker is not exactly 25Ω over the entire range of frequencies, whereas the resistor will have a constant impedance, whatever the frequency. So it means that, depending on the frequency, the speaker will see a slightly varying RMS voltage across it.

Lower sound volume because, instead of 100% of the power provided by the amp being delivered to a speaker, we now have 50% transformed into heat and 50% delivered to the speaker. But this is not accounting for the speakers efficiency, which can greatly vary from one speaker model to another, so it could be compensated somewhat.

But I guess both of these matters are not really relevant in your case. Given the speaker power, it doesn't seem to be a hi-fi device (and if it's a toy, you'll certainly be happy it become quieter than before - I have myself added some series resistors on the speakers of a few of my son's toys).

Additional info: Why replacing a 50Ω speaker by a 25Ω speaker may cause some damage? Because the amp is built for a fixed gain and therefore fixed voltage level output. If you lower the speaker impedance, it will then have to deliver a higher current to feed the speaker (for the same voltage level - Ohm's law). So at some point, you may exceed its current rating. It is most likely not dangerous for the new speaker (provided it is rated appropriately), but can be dangerous for the amp in the device.

Not that if you choose a speaker with a higher impedance than the original one, you don't risk exceeding the amp ratings, so you're safe without resistor. But you're still subject to the lower volume drawback because the speaker will see less current through it.

Also, the power rating of the new speaker must be considered: in your case, you're safe because it is greater than the original one (0.3W vs 0.2W). And actually, because the new speaker will see half the power of the original one, you would be safe down to 0.1W.

Answer 2

A series resistor is the easiest and cheapest method, but if the new speaker is not loud enough when used in this way there are a couple of other options:

Add a buffer amplifier to increase the current available from the device's own amplifier

Based on the specification of the original speaker an LM386 might be a good choice, as long as the device's supply voltage is suitable. The 'minimum parts' circuit from the LM386 datasheet has a gain of 20, so you'd need to use a voltage divider to reduce the signal to a suitable level before feeding it to the LM386.

Alternatively you can find off-the-shelf amplifier modules or 'breakout boards' from electronics suppliers or on eBay - just check the supply voltage is compatible.

You'd need to identify points in the existing circuit to take the amplifier's positive and negative supplies from; just check that you take the positive supply from after the device's on/off switch.

This option will draw extra current from the device's power supply, if only in the tens of milliamps, so it's possible that the power supply could be overloaded by the extra current if it's designed without much headroom.

Use a transformer

Transformers can be used to change the level of audio signals just as they can for AC power. Valve (tube) audio amplifiers normally use a transformer in their outputs, for example.

Unless you can find a 50 to 25 ohm transformer, or close enough equivalent, off the shelf, then you'd have to make one yourself - I've never done this but there seems to be information on the web if you search. However you might be able to use a small mains transformer for the purpose, if you're prepared to experiment. Since you don't need the input and output to be isolated, you should be able to use a centre-tapped winding, or two identical windings connected as centre-tapped, as an auto-transformer. I don't know whether the primary or secondary side of a mains transformer would be more suitable but if you use the secondary, be aware that high voltages will be created in the primary!

You would probably want to try this out using the new speaker and some kind of test gear to confirm that it presents the right impedance to the source before installing it in your device.