Decrease 10 ohm speaker to 4ohm?

[Rob_Feature]

I know this is a really unorthodox question...but here's what I've got:

I'm taking an old transistor radio that looks cool, gutting it, and putting bluetooth inside. Pretty common project but note that I'm not using the transistor radio's amplifier...I didn't feel like figuring out the right way to wire it..I'm just using the cheap-o amp that comes with a bluetooth speaker. So I bought a $5 bluetooth speaker, pulled out the module which expects a tiny 4 ohm speaker, and cut the speaker wires.

I want to use the speaker that's already mounted in the transistor radio. Only problem is that speaker is rated at 10ohms.

Is there a simple way to lower the impedance of the 10 ohm speaker down to the 4 ohms that the module expects? I hooked it up directly at 10 ohm, thinking it should just be quieter (if I'm thinking correctly, a higher ohm speaker never will damage the amp, it should just require more power to drive it, right?)...it worked when hooked up, but distorted at a pretty low volume.

So, any simple options here? Some way of wiring it or adding simple components to lower the resistance to the expected 4ohms? Please note this certainly isn't an audiophile project...I just wanted to make a cool looking cheap BT speaker. So simplicity is the key here! Thanks!

 

[62caddy]

In a nutshell, no and any attempts to do so would only make matters worse.

You're correct that no harm will come from using a higher impedance driver than the amplifier is designed for; the higher the impedance, the easier on the amplifier. However the maximum output will be restricted somewhat, reducing the maximum loudness by several dB perhaps.

 

[Rob_Feature]

However the maximum output will be restricted somewhat, reducing the maximum loudness by several dB perhaps.

Thanks for confirming my thinking. I guess what prompted this post and is puzzling me a bit, is why does the speaker sound distorted at anything over about 25% volume? It seems to me that since the speaker has the high impedance, I should be able to crank it all the way up, and possibly then it's still not loud enough. Yet, it has a fair amount of volume at 25% (given what it is) but then starts to distort.

Granted...it coudl be that this is just a totally crappy 50 year old speaker hooked up to a crappy $5 bluetooth module . I'll probably chalk it up to that (maybe replacing the speaker) if there's no technical reason that should be happening.

 

[62caddy]

Granted...it coudl be that this is just a totally crappy 50 year old speaker hooked up to a crappy $5 bluetooth module . I'll probably chalk it up to that...

I think you're probably on the right track there.

 

[Rob_Feature]

Ok, cool, thanks. Yeah, i pulled the old 10ohm speaker, and I (very luckily) had 2 8ohm clock radio speakers...wired those in parallel and crammed em in the case. Sounds as good as it's going to I think (which...is decent for the $7 in total I spent on this project). Here it is...even has stupid lights that glow from inside (that were on the bluetooth pcb). The leather case is the coolest part of this thing.

 

[RTally]

Most audio amplifiers work by amplifying voltage applied to a load. The maximum power is related to the maximum voltage that the amplifier can apply.

Power to the speaker is inversely related to the voltage applied to the speaker. P = V*V / R (Power equals voltage squared divided by resistance).

In your case, your inexpensive bluetooth amp has a limited voltage rail. So when applying a reasonable voltage level to a 10 ohm speaker, the speaker will be receive 2-1/2 times less power than the 4 ohm speaker. That is about a 4dB difference, assuming that the two drivers have equal sensitivity, which is unlikely.

The simplest solution is to find an appropriately dimensioned 4 ohm speaker for the radio that has equal sensitivity to the original 4 ohm driver. Alternatively, use an amplifier that can drive the 10 ohm driver to the volume level you desire.

 

[RTally]

Seems like you posted a solution at the same time I posted.

Glad you got it working to your satisfaction.