Non-inductive resistor in parallel for ohms match?

PatriotBadger

Senior Member
Messages
1,818
Kind of a strange situation, but here goes.

I have a SS combo amp rated for 60w @ 3 ohms, with a 3 ohm speaker (thanks Vox, you weirdos). I want to perform a speaker swap but want to maintain as close to full power output as I can. The speaker I want to use, a Cannabis Rex, only comes in 8 and 16 ohms.

Any reason I wouldn't want to use a non-inductive 8 ohm 100w resistor in parallel with the 8 ohm C-Rex to get a 4-ohm load? It will rate the power down a bit of course, but with the C-Rex being a 50-watt speaker I suspect it will work out just fine.

Thoughts?

http://www.parts-express.com/8-ohm-100w-non-inductive-dummy-load-resistor--019-020
 

darkfenriz

Member
Messages
207
I cannot think of any point in burning extra watts in an extra resistor - the watts delivered to the speaker is the only ones that matter and it won't increase by the addition of a resistor.
The only things you'd increase that way is your electricity bill and chances of amp getting overheated...
 

PatriotBadger

Senior Member
Messages
1,818
So if I understand correctly, you're saying that output watts are divided by the number of elements in the load, in other words the speaker would only 'see' 30-ish watts of the generated output power anyway? Keep in mind that the amp is designed to deliver 60 watts @ 3 ohms, so there isn't any overheating danger running it @ 4 ohms.
 

darkfenriz

Member
Messages
207
Correct, you won't increase the watts to the speaker without decreasing its impedance. There are not many options for a decent 4ohm speaker, G12T100 comes to my mind.

On a side note I wouldn't trust any solid-state class-AB power specs these days, especially from major manufacturers.
Weird impedance figures or power consumption ratings magically lower than rated power should raise eyebrows.
 

Kyle B

Member
Messages
5,074
Mathematically, you're looking at it backwards. Don't fret that....everybody does at first.

The power is not controlled by the source, it's controlled by the load. In your case, the speaker. There isn't always 60W looking for a place to go

If your amp is rated 60W at 3 ohm, that means it can drive (from ohms law) P=V^2/R or V=sqrt(P/l*R) = sqrt(60*3)= 13.4v

It will put out 13.4V (max) regardless if you have no speaker, or if you have several speakers connected. That 13.4V on an 8 ohm speaker yields V^2/R = 13.4*13.4/8= 22W

So no matter what other things you do, the absolute maximum power you can get from an 8 ohm speaker is 22W.

If you want more audio power, get a 4 ohm speaker.

Adding the parallel resistor might make sense if it were a tube amp. Since it's not, the 8 ohm load is a non issue.

To firm your understanding, consider the output wattage if no speaker were connected. That would be infinite impedance. In that case....P=V^2/R, P=0. Makes sense, right? No speaker, no load, no current, no power.
 
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Jeff Gehring

Gold Supporting Member
Messages
6,202
If you want to use an 8 ohm C-Rex speaker, and want the spec'd load (or at least pretty close to it) to get the most audio bang, add a 2nd 8 ohm C-Rex (in an extension cab) in parallel. That way you are turning most of your available audio power into SPL and not wasting it heating the room (as you would if you had instead paralleled the speaker with a resistor)...
 




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