I assume the answer is yes but underpowered.  thought I would ask

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Not necessarily. 1/2 W is plenty to drive car speakers. Check with Wes Carl, Wayfinder, and several others here who build amps using that and similar kits, with all kinds of speakers, including car audio types.

There is little question that a speaker made specifically for guitars is more likely to work.  However, I've re-purposed all kinds of speakers with success-- and failure.  Some don't work well at all, others work far better than expected.   I just got through making an amp out of a Fischer large box speaker.  I had two of them but on one the cabinet was ruined, and on the other one of the dual speakers was ruined.  So I pulled apart the ruined cabinet and salvaged the speaker to replace the broke speaker in the other cabinet.  I added an amp head to the mix and now I have an amp with two 10" speakers that can get loud enough to make the floor vibrate.

The secret to it:   if one doesn't know electronics (and I don't)... try it and see if it works.  You do need to understand ohm matching-- the amp has to match the speaker rating.  But beyond that, some stuff works, some doesn't, and the only way to find out is to test it.   I've heard several people say car speakers work fine. But of course, that depends on the car speaker and the sound you're looking for.

Most car speakers are 4ohm and most guitar amps are 8 ohm. 

A 4ohm speaker will be damaged quickly hooked up to a 8ohm amp, but you can wire 2 4ohm speakers in series(+ of speaker 1 to amp +, - of speaker 1 to + of speaker 2, - of speaker 2 to amp -) to work with a 8ohm amp.

Car speakers will not last long though with a guitar amp because they aren't made for that kind of use.

if the 4 ohm speaker is of sufficient wattage then it wont suffer any damage. what CAN happen is that a 4 ohm load on the output of an amp can overload the output transistors and burn them out. basically less resistance more current flows , exceeds transistor capacity. dead amp. the only time you will blow speakers is if you are trying to put 100w through a 10 w speaker. then it gonna die!

Timothy, I almost followed all of that, but one question:   when you say you can overload the output transistors, are you speaking of trying to run a 4 ohm speaker on an 8 ohm amp, or what situation?

yes. amps are usually designed with an output load in mind. that can usually be 4 ohms to 16 ohms. a speaker is a dynamic load. what it is is a coil of wire suspended by a cone in a circular groove  in a permanent magnet. what power the speaker can handle depends on how strong the magnet is. the gauge and number of winds in the coil and the stiffness of the suspension. the part of the speaker that connects to the rim and allows movement. 

assume you have a 5W amp module. the most obvious thing to do is to try to match it to a 5W speaker.If you only have a 10W speaker then you are not going to hurt the speaker or the amp you just wont get full efficiency out of the amp.

Now lets use that 5W amp to drive a 50W speaker. we are now putting only 1/10th of the power that the speaker can handle. you will hear sound but there wont be any punch to the sound. the movement of the speaker cone is called excursion , how far back and forward the speaker cone can move. with only 1/10th of the power the cone wont move much at all.

go to the other extreme and put a 1/2 W speaker on a 5 W amp, you are now putting 10 times the power into the speaker than it was designed to handle. the speaker cone will try to move beyond the limits that it was designed to and the suspension will fail. blow out the speaker. unless the added current and voltage through the coil doesnt burn it out first. 

now that was all assuming the amp and speakers were the same  that is 8 ohm load and 8 ohm output amp.

with load mismatch the amp controls the voltage the only thing that can change is the amount of current . To horribly oversimplify this if you have half the load you get twice the current. by ohms law twice the current is twice the power developed assuming the voltage stays the same. so now your 4 ohm speaker is trying to pull 10 W out of you amp. 

Valve outputs are slightly different because they have a transformer as the load and the speaker is on the secondary.



That is very helpful! Thank you!

Double applause Timothy.  You should be a teacher.  That is the first time someone has gotten through to me in plain English, how a speaker works and how to blow one (technically speaking).  I'm actually copying that to a text file for future reference.

I know! Right!

Damage can also occur to the speaker coil from ohm mismatch. One of the parts will be damaged for sure.

But the real damage to speaker coils comes from using a instrument that has a high level output in a small dedicated range of sound that's used with a speaker that's designed for a lower intensity wide range of sound for stereo audio output.

You can use car stereo speakers, but it will damage them a little at a time and eventually rag them out and make them sound terrible. If you use a guitar speaker, that will not occur.

Could be true  But I have some I built 7-8 years ago that are still working well  I do agree a guitar speaker is build for that purpose  But I dont seem to have any problems with car or boat speakers  And lots cheaper 


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