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Ohm Question

B

blown240

Dec 10, 2004
83
0
#1
If I have an amp that has to be run at 8 ohms and a speaker cab that is only 4 ohms. Cant I just run a 4 ohm resistor somewhere inline between the amp and cab to make the amp see 8 ohms?

THANKS!!!
 
#2
Yes, but it will eat half the power from your amp. So if you have a 500w at 8ohm amp, you're going to need a resistor rated for a bare minimum of 250w, preferably more.

A much better idea would be to rewire the cab for 8 or 16ohm operation. What's the make and model of the cab?
 
#5
That's gonna be one big honkin' resistor too. And it's going to get real HOT, so you'll need a cooling fan. Here's a pic of an 8-ohm, 250-watt resistor.

dale-nh250.jpg
 
#6
Nick Kay said:
Yes, but it will eat half the power from your amp. So if you have a 500w at 8ohm amp, you're going to need a resistor rated for a bare minimum of 250w, preferably more.

A much better idea would be to rewire the cab for 8 or 16ohm operation. What's the make and model of the cab?
Click to expand...

You can't rewire a 4-ohm cab to 8 ohms using the same speakers. Assuming the 4-ohm cab to be four 4-ohm speakers wired in series-parallel, it could be rewired to 16 ohms by wiring the speakers in series. If the existing cab has two drivers, it's probably two 8-ohm speakers wired in parallel, so you could rewire it in series to get 16 ohms, but again, there's no way to rewire it to 8 ohms.
 
#9
The amp is an old Fender 300ps that is 300 watts but needs 8 ohms. The cab is an Ampeg 8x10 with 32 ohm speakers all wired parellel, so its 4 ohms. I dont want to run the head at 4 ohms because I really dont want to accidently cook it.

The other option is to rewire the Ampeg cab so that it has 2 jacks each with 8 ohms. But then I could only play thru 4 of the speakers...
 
#10
I figured a way that I can get 7.53 ohms from my Ampeg 4 ohm 8x10 cab (eight 32 ohm speakers).

I can wire the top 4 speakers in parallel which will give me 8 ohms. The bottom 4 speakers will be wired in series giving me 128 ohms. Then the top and bottom will be wired in parallel bringig it down to 7.53 ohms.

there is also a way to wire it to 8.35 ohms and 12 ohms.
 
#11
blown240 said:
I figured a way that I can get 7.53 ohms from my Ampeg 4 ohm 8x10 cab (eight 32 ohm speakers).

I can wire the top 4 speakers in parallel which will give me 8 ohms. The bottom 4 speakers will be wired in series giving me 128 ohms. Then the top and bottom will be wired in parallel bringig it down to 7.53 ohms.

there is also a way to wire it to 8.35 ohms and 12 ohms.
Click to expand...


That will work, but keep in mind there will be a big difference in volume output between the set of 4 parallel speakers and the set of 4 series speakers. Are you sure that each speaker is 32 ohms? Not arguing, just asking. :smug:
 
#12
Check with Fender. They have a tendency to list the minimum impedance higher than needed. That amp may be safe to run at 4 ohm. For example, my Fender Bassman 400H is advertised with a minimum operating impedance of 4-ohms, but the manual states that it will run safely at 2-ohms without voiding their warranty. It wouldn’t hurt to check.
 
#14
blown240 said:
I figured a way that I can get 7.53 ohms from my Ampeg 4 ohm 8x10 cab (eight 32 ohm speakers).

I can wire the top 4 speakers in parallel which will give me 8 ohms. The bottom 4 speakers will be wired in series giving me 128 ohms.
Click to expand...
The top drivers will be receiving 94% of the amp's output and the lower drivers the remaining 6%. :rollno:
Tube amps are the opposite of SS, they are comfortable running at lower impedance loads than their nominal rating, and don't like higher impedance loads. Don't worry about it. ;)
 

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