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Re: Please explain impedance matching for amps&cabs


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Posted by Bob Gollihur (68.46.141.53) on June 02, 2002 at 19:05:52:

In Reply to: Please explain impedance matching for amps&cabs posted by schmoo on June 02, 2002 at 14:33:25:

: 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?


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