# Yet another impedence question!?!?



## Stratin2traynor (Sep 27, 2006)

I have an Epi Valve Jr head with speaker out that supports 4, 8 or 16 ohm. Right now I have a 16 ohm 2 x 12 cab. What difference if any would it make to wire the cab for 4 ohm or have an 8 ohm cab with the same type of speakers??? Just curious, thanks.


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## Wild Bill (May 3, 2006)

Stratin2traynor said:


> I have an Epi Valve Jr head with speaker out that supports 4, 8 or 16 ohm. Right now I have a 16 ohm 2 x 12 cab. What difference if any would it make to wire the cab for 4 ohm or have an 8 ohm cab with the same type of speakers??? Just curious, thanks.


It should not make any difference! As long as things are set the same at both ends so that a 4 ohm output drives a 4 ohm cab or whatever then the output tubes see the same plate load. That's WHY there are different taps on the output transformer.

There are those who disagree but they tend to be the same guys who need a book to figure out how to put a plug on a lamp.

They always seem to have something to sell you, 'though.

If you are using a LONG speaker cord to feed the cab (like 50 feet or more) you will lose less power if you use the 16 ohm configuration. That's because of a bit of physics described by Ohm's Law. The wire in the cord and the load of the speakers acts like two resistors in series. You want to keep the ratio between them as high as possible. If you have 16 ohms for the speaker and 1 ohm for the wire then the wire will drop 1/16 the power. If you have 4 ohms and 1 ohm then the wire will lose 1/4 the power.

I pulled the number for the wire out of my butt! Actually, if you're using at least 16 gauge or larger wire then the resistance of the wire is nowhere near as high as 1 ohm for 50 feet. It's much lower. Still, you do lose a bit of power. The resistance of the wire will only waste power as heat. It will NOT change the tone!

16 ohms became a standard in Britain back when concerts and long wires were just starting out. Amplifiers weren't as powerful and they often had to connect a LOT of cabinets for the PA! In that kind of situation the losses were no longer trivial so they went for 16 ohm speaker loads.

In an office building that might have speakers all over the place they use to this day a different distribution system. The amp has a "70 volt" output, which is actually around 500 ohms. This is fed on all the connecting wires and there is a small stepdown transformer at each speaker to convert down to 8 ohms. This is far more efficient.

Some guys trying to sell you something and/or pass themselves off as some kind of hifi guru will use these last points in their "technobabble" to get you to buy their cables or whatever. Don't listen to them. As I said, with the typical length of cable from your amp to the cab the losses are trivial. Use whatever configuration is easiest for you.

:food-smiley-004:


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## Stratin2traynor (Sep 27, 2006)

Thanks WildBill. I didn't think it would make much of a difference. Gotta get cracking on another cab now...


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