Hey Guys, long time member, first post but still a newbie. Here's the info then the question. My Lucent 8410 work phone has an 45 ohm speaker inside which is not very loud on high volume (speaker mode). I want to use an amplified external speaker set for greater volume. My plan is to install a 3.5mm female jack on the back of the phone and use an amplified 8 ohm computer speaker set. My question is .... since the 8 ohm computer speaker set is amplified, will the mismatched impedance 45/8 damage the phone amp. My thought is that since the external amp will be driving the speakers, the phone amplifier won't be over driven? Am I on track or way off base? Thanks :O)

 

A crude way to "safe" this would be to place a 39 ohm resistor in series with the jack.
The internal amplifier will "see" 47 ohms and the speakers will get 15db less input voltage than the original 45 ohm speaker, then amplify it, hopefully enough.
Crude, but safe. Worth a try?

 

Use a 2.4:1 output transformer?

 

A crude way to "safe" this would be to place a 39 ohm resistor in series with the jack.
The internal amplifier will "see" 47 ohms and the speakers will get 15db less input voltage than the original 45 ohm speaker, then amplify it, hopefully enough.
Crude, but safe. Worth a try?

Thanks for the quick reply. Luckily, I have a spare phone to use to test with. I'll give the 39 ohm resistor a try as well as any other suggestions and post the results next week.

 

Use a 2.4:1 output transformer?

Thanks Joeyd999. Did a quick search for the transformer but couldn't find a 2.4:1 transformer. Any suggestions?

 

Thanks Joeyd999. Did a quick search for the transformer but couldn't find a 2.4:1 transformer. Any suggestions?

Once upon a time, one would find a suitable core and wind their own.

I don't do audio, but there are those here that do. Hopefully, they will chime in.

 

A crude way to "safe" this would be to place a 39 ohm resistor in series with the jack.
The internal amplifier will "see" 47 ohms and the speakers will get 15db less input voltage than the original 45 ohm speaker, then amplify it, hopefully enough.
Crude, but safe. Worth a try?

The OP wants to use amplified computer speakers. The input impedence is likely in the 10k ohm range. There should be no trouble to connect directly to the phone. This amplified speakers are intended to connect a motherboard with no problems.

No, my concern would be grinding issues. The phone likely uses a class D full bridge amplifier. That means no direct ground in the speaker and a ground loop waiting to happen.

@Sandpiper
Check the chips near the speaker section of the pcb. Google their Datasheets to see if they are linear audio amplifiers or a class D amp.

 

grinding issues.

I think you meant, "grounding issues".

The OP wants to use amplified computer speakers. The input impedance is likely in the 10k ohm range.

D'oh. I made the same mistake the OP did. I was fooled by the OP stating the speakers were 8 ohms when anybody should know the amplifier input is not 8 ohms.

 

Missed the amp part, also.

 

View attachment 115992
I think you meant, "grounding issues".

D'oh. I made the same mistake the OP did. I was fooled by the OP stating the speakers were 8 ohms when anybody should know the amplifier input is not 8 ohms.

Ground loops sound the same as grinding, but, yes, I did mean grounding.

 

At least he isn't going to hurt anything by doing what I said in my first post.

 

will the mismatched impedance 45/8 damage the phone amp.

no. the input impedance of your amplifier speakers is likely over 10K. I would put a small serial resistor on the input line, like 110ohm - 1K range - value is not critical, just to provide some protection against a potential short.

 

Just a brief update. Computer speakers worked great. Nice clear sound with plenty of volume. Didn't have a lot of time to test due to a full schedule but will give further updates soon. Thank you for all the replies, much appreciated.