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Re: Please explain impedance matching for amps&cabs

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Posted by JT Murphy ( on June 03, 2002 at 12:51:13:

In Reply to: Re: Please explain impedance matching for amps&cabs posted by François Blais on June 02, 2002 at 19:36:37:

: : : Hi-
: : : I really don't understand the relationship between amplifier wattage and cabinet impedance. Can some explain or recommend a site that explains this from the ground up. Honestly, whoever has the time, can you just break it down for me- a full exegesis? And there are others like me out there, so please post to the message board. Thanks! schmoo

: : Most sites get pretty technical, so I'll take a shot at it in simpler terms at the sacrifice of some deeper analyses.

: : First, tube amps (not preamps, but true tube amplifiers) are different. They use transformers to couple the amp's output to the speakers. The output transformers usually have a tap (a spot in the wire winding) for 4 and 8, and possibly 16 ohm loads, so the output wattage is relatively consistent.

: : Solid state amps don't use transformers, and the speaker load is directly coupled to the output stage -- just like on the input side, the speakers and the instrument actually become part of the circuit, which affects how the system performs-- which accounts for the different wattage ratings at different impedances.

: : These amps are generally rated at the most common impedances, 4 and 8 ohms, and the difference in the output voltage is affected by the load (impedance) that the speaker(s) present. They'll also usually indicate a minimum impedance, because an impedance lower than the specified rating will make the output stage work harder (hotter) than it is spec'd for, and the stress can fry components. So it's important to observe the minimum impedance spec.

: : Does that help?

: In addition to Bob's explanations:
: Impedance is important in the amp-speaker relationship for power transfer.
: Maximum power transfer occurs when the load impedance equals the amp output impedance.
: Usually, amp's specs sheets give the power rating at various load impedances.
: It is also good to know that a speaker cab impedance is not constant. It varies according to frequency.
: So a 8 ohms cab may have a 50 ohms real impedance at another frequency.
: (I think that in practice, the impedance is maximum at the cab's resonance frequency)
: When the cab impedance raises, the amp's output power decreases.

Let me add to the above by saying: amps don't have impeadences, cabinets do. When an amp's specs give its power (say 200 watts) at X ohms (say 8 ohms) it means the amp puts out 200 watts when presented with an 8 ohm load. The idea is similar to the word "impeadance:" higher ohms (impeadance) "impeade" (reduce) your amp's power. So, for an amp which is rated at 200 watts at 8 ohms and 300 watts at 4 ohms, an 8 ohm cabinet lets your amp put out 200 watts while a 4 ohm cabinet lets your amp put out 300 watts. Get it? (If you don't its not your fault because amp companies like to confuse things by using the 4 ohm or 2 ohm rating to advertise the cabinet. i.e. the Acoustic Image Clarus is advertsied at 300 watts. But, when driving an 8 ohm load, it only puts out a paltry 120 watts or so. It is "allowed" to put out its full 300 watts only when driving a 2 ohm load (now how many of you Clarus owners have 2 ohm speakers or carry around two 4 ohm cabinets?!?))

Now, the next idea comes up when you combine two 8 ohm cabinets and you get 4 ohms, two 4 ohm cabinets and you get 2 ohms. Don't worry about why, just remember it.

Next, cabinets have rated power maximums. Say my 8 ohm cabinet can handle 200 watts. If my amp puts out 200 watts at 8 ohms, we're in business. But what happens if I plug in two cabinets each rated at 8 ohms and 200 watts maximum. Now I have a 4 ohm load allowing my amp to put out 300 watts. But don't worry, the two cabinets equally share the 300 watts (150 each) well below their rated maximums of 200 watts each.

Make sense?


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