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Posts: 29958
Location: SW United States
Joined: November 2, 2000
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“I hope you guys can help me out with a basic electronics question. I recently put new drivers in a set of old speakers I have, when I asked the gentlemen who sold me the drivers if their average impedance was closer to 4 or 8 ohms, he told me closer to 4. The reason I asked was because my amp (Dared VP-20) has both 4 and 8 ohm taps. When I run the 8 ohm taps, I get maybe slightly more volume and the power supply seems to run warmer. 4 ohm taps, the amp stays a bit cooler but the sound is slightly quieter and maybe a little less punchy.” Usually when I hear a description of the sound being “punchy”, it’s not necessarily a good thing- I often find a presentation of such description lacking in “microdynamics”........... “I think I have the heat thing figured out, the speakers a probably something like 6ish ohms overall, taxing the 8 ohm tap a bit and not really stressing the 4 ohm one. It's the sound level that has me perplexed, shouldn't the 4 ohm tap be the ‘louder’ of the two?” No. The taps are to maximize power to drive a given impedance, which for tube amps in most cases is roughly equal watts. A 4-ohm load draws more current than an 8-ohm load, so the voltage would run lower to enable greater current capability (lower source impedance) into the ideal load. Using the power equation P = V2 / Z, rearranging the equation to V = sqrt( P * Z ) [P=power, V=output voltage, Z=speaker impedance], for a 50-watt amp driving an 8-ohm load, the output voltage would be 20 vrms, for a 50-watt amp driving a 4-ohm load, the output voltage would be roughly 14 vrms. So generally speaking, the speaker will see a lower voltage from the 4 ohm tap, hence the speaker won’t play as loud. The tradeoff, however, is since the source impedance from the 8 ohm tap is higher, there will be greater frequency response aberrations if the speaker’s impedance curve varies greatly with frequency. Distortion is likely to be higher out of the 8-ohm tap as well, due to excessive current draw on the part of the speaker. “And in the end, for playing at moderate to mildly loud volume levels, would you just end up going with the tap that sounds the best?” I'd say yes......... Unless the amp runs uncomfortably hot using the 8-ohm tap.
 
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