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Re: Please explain impedance matching for amps&cabs

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Posted by François Blais ( on June 02, 2002 at 19:36:37:

In Reply to: Re: Please explain impedance matching for amps&cabs posted by Bob Gollihur on June 02, 2002 at 19:05:52:

: : 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.

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