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Impedence and amp settings

 
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magvike
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Joined: 02 Jun 2004
Posts: 26
Location: San Diego

PostPosted: Thu Dec 09, 2004 10:28 am    Post subject: Impedence and amp settings Reply with quote

Here is a little blurb from Mike Soldano from his Ask Mikey column. I though it was the other way around:
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Hey Amp Man!
I have an old Ampeg V4. It has a switch on the back that allows me to run 2, 4, or 8 ohms. I use a 4x12 cab at 8 ohms. I am planning on building a 2x12 cab. I have two extra 8 ohm speakers that I want to build it with. I'd like to get this 2x12 cab running at 8 ohms so that I could run it out of the Ext. speaker jack from my Ampeg. The only problem is, if I run the two 8 ohm speakers in series, I'm getting 16 ohms...if I run them parallel, I'm getting 4 ohms. Is there a way to run two 8 ohm speakers together and make the combination 8 ohms? Perhaps with a capacitor or a resistor? Thank you for your help. Jeff

Sorry to say, Jeff, but as you've already surmised, there is absolutely no way to wire two 8 ohm speakers together and still have an 8 ohm speaker load. It ain't gonna happen. I suggest you sell your two 8 ohm speakers and pick up a pair of 16 ohm speakers and wire them in parallel for your cabinet. I think you'd find that a pair of Eminence Legend 30s would sound real good with your V4, and they're very reasonably priced, as well. However, if you're intent on using the speakers you have, you could probably wire them either way and your V4 would most likely tolerate it. Here's what will happen. If you wire the cabinet to 4 ohms, your combined speaker load will be about 2.7 ohms. I'd set the amp for 4 ohms, since, in a mismatched impedance situation, it's always safer to have the load be less than what the amp is set at. Note that if you do this, the 2x12 cabinet will be getting most of the amp's power (and be louder), since it is the lower impedance cabinet. This could be too much power for the two speakers, and you may blow them. If you wire the cabinet to 16 ohms, your total load would be about 5.3 ohms. I'd leave the amp set on 8 ohms for this scenario, for the same reason I mentioned earlier. In this case, most of the sound (and power) will be routing through your 4x12 cabinet, and you won't be hearing much out of the 2x12 cabinet that you spent all that trouble building.
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Mike
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magvike
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Joined: 02 Jun 2004
Posts: 26
Location: San Diego

PostPosted: Thu Dec 09, 2004 11:12 am    Post subject: Reply with quote

And here is his reasoning:
_________________________________________________________
Hey Amp Man!
In your column, "Ask Mikey",in the most recent issue of Musicians Hotline magazine, you said the following in answering a question about impedance mismatching. "In a mismatched impedance situation, it's always safer to have the load be less than what the amp is set at". That's contrary to everything I've ever heard or read in the past. It's always been my understanding that in a mismatched impedance situation, that it's always safer if the load is GREATER that what the amp is set at. That's the setup I'm currently using, please set me straight so I don't blow my transformer.
Thanks, Dave

Well, Dave, I don't know who you've been listening to or what you've been reading, because they're just plain wrong. Here's how it works: If the load is lower than what the amp is set for, like using a 4 ohm cabinet with the amp set at 16 ohms, the power tubes will be worked harder and will run hotter. This, of course, will shorten the life of the power tubes. However, if the load is higher than the amp's setting, like using a 16 ohm cabinet with the amp set at 4 ohms, the voltages in the output transformer will be higher than normal. These excessively higher voltages increase the risk of arcing, which can destroy the output transformer and/or tube sockets. That's why running an amp with no load at all invariably ends up blowing the output transformer. I'd say tubes are easier to replace, wouldn't you?
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Interesting, eh?
Mike.
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Daved
Robben Connection


Joined: 08 Aug 2003
Posts: 943
Location: Terra Firma, Ether Sea

PostPosted: Tue Dec 21, 2004 9:02 am    Post subject: Reply with quote

magvike wrote:
And here is his reasoning:
.....if the load is higher than the amp's setting, like using a 16 ohm cabinet with the amp set at 4 ohms, the voltages in the output transformer will be higher than normal. These excessively higher voltages increase the risk of arcing, which can destroy the output transformer and/or tube sockets. That's why running an amp with no load at all invariably ends up blowing the output transformer......


Sounds a bit confusing & contradictory to me. First he says that a 16 ohm load on a 4 ohm amp will destroy the transformer, then he says that's why a 0 ohm load on an amp will destroy the transformer?!?! Isn't 0 ohms ("no load at all") a lesser load than what the amp would be set for, reinforcing that speaker impedance should NOT be less than what the amp is designed for?

As discussed in an earlier thread:
"The key thing to always remember when experimenting with impedance matching is that speaker impedance should NEVER be less than what the amp is rated for (if the amp is 16 ohm, don't configure your speakers to 8 or 4 or 2 ohms.... if the amp is rated at 8 ohms, don't configure your speakers to 4 or 2 ohms... etc.). Speaker impedance at LESS then what your amp is designed for can seriously damage the amp."

Here is a link to an informative site with interesting analogies to help explain impedance & why & how an excessively low speaker impedance can damage the amp (Particularly note the analogy about the water pump, near the bottom of the page):
http://www.bcae1.com/spkrmlti.htm
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( }:-Daved

"This boy's diseased with rhythm!" -Bing Crosby (Road To Rio, '49)
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