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Topic: Speaker ohm. |
Michael Haselman
From:
St. Paul
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Posted 17 Nov 2003 7:29 am |
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| I just got 2 Celestion 15 ohm G12s. If I use the 16 ohm out of my Sovtek, they work fine. What would happen if I used the 8-ohm or 4-ohm outs? |
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David Deratany
From:
Cape Cod Massachusetts
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Posted 17 Nov 2003 1:37 pm
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If you hook them up in parallel ( + to + and - to -) you will end up with 7.5 ohms. Actualy impedance varies with frequency, so the stated figure is only an estimate/average. Close enough to 8 ohms. I think you should be using them at 8 ohms. If you were paralleling them at the 16 ohm tap, you're losing almost 50% of your power.. impedance mismatches waste power, big time.
I would NOT run them at 4 ohm.
15 ohm is a curious figure. I am not fmiliar with those speakers, but are you sure they don't say 16? |
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Michael Haselman
From:
St. Paul
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Posted 17 Nov 2003 1:51 pm
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They definitely say 15 ohms. I've done a google search and they are odd, but it's 15. Maybe it's a British anomaly.
------------------
Marrs D-10, Webb 6-14E |
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C Dixon
From:
Duluth, GA USA
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Posted 17 Nov 2003 2:00 pm
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Maximum power transfer is only when the source impedance (amp) is equal to the destination impedance (speaker).
If the destination impedance is not equal to the source impedance, there will be a loss of maximum power. It is perfectly OK to missmatch in the upward region. It can be risky to missmatch in the downward region.
IE, upward means the destination impedance is higher than the source impedance. However, remember that none of the above matters much; unless one is running the amp near or completely wide open. At lower levels than the maximum power output of a given amp, the currents are not high enough in most cases to cause damage.
For example consider the following:
1. Amp 8 ohms, speaker 16 ohms.. loss of power. No damage.
2. Amp 4 ohms, speaker 4 ohms..maximum power with no damage.
3. Amp 8 ohms, speaker 4 ohms...loss of powers, but damage may occur; although NOT likely in MOST cases.
4. Amp 8 ohms, speaker 2 ohms...loss of power, damage quite likely.
5. Amp 16 ohms, speaker 2 ohms...almost guaranteed damaged IF the amp is run wide open.
If you are concerned, then always err in the "upward" region.
Finally, impedance is measured (when it comes to amps and speakers) at 1KHZ. (1000 cycles per second) Which is near the "mid" region in musical instrument amplifiers.
carl[This message was edited by C Dixon on 17 November 2003 at 02:10 PM.] |
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Jon Light (deceased)
From:
Saugerties, NY
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Posted 17 Nov 2003 2:04 pm
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| Yeah--I've got a G12M-70 marked as 15ohm. |
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David Deratany
From:
Cape Cod Massachusetts
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Posted 17 Nov 2003 2:19 pm
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C. Dixon,
All true.
Another consideration is that solid state amplifiers usually have maximum power output at 2 or 4 ohms, half that at 8, and half that again at 16. Some aren't designed for 16 ohm loads at all. Coupling a 16 ohm load to a solid state amplifier that is happiest with a 2 or 4 ohm load would be a double whammy (highly technical term), power wise  . I can't think of any reason not to match them if you can.[This message was edited by David Deratany on 17 November 2003 at 02:21 PM.] |
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C Dixon
From:
Duluth, GA USA
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Posted 17 Nov 2003 4:17 pm
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David,
You are correct. Actually a SS amp does not have an impedance output as such; if they have no sign of an output transfomer. IE, the load IS totally the speaker; whereas in tube amps some of the load IS in the primary of the output transformer. This is one of the reasons that SS amps can deliver more power to the speaker than tube amps can everything else being equal. IE, some wattage is expended in the transformer itself and is never seen by the speaker(s).
The reason for the 2 ohm limit has nothing to do with matching per se; rather 2 ohms is approaching a dead short and in terms of output transistor currents, when you go lower than 2 ohms, the current being drawn by the output transistors rises exponentially and just about guarantees blowing them in short order.
carl |
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Murnel Babineaux
From:
Mermentau, Louisiana, USA
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Posted 27 Nov 2003 11:56 pm
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It is safe to run tube amps 100% from it's output load impedance.
in other words;
4 Ohm transformer = 2 to 8 ohms
8 Ohm transformer = 4 to 16 ohms
Be advised that the load impedance is directly reflected back to the primary side (tubes). Running them 100% out will slightly degrade the tone of your amp.
MB
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