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Most mfr's rate their amps at 8 ohms since the early 70's. Very few speakers, in spite of their nominal ratings, are truly 8 ohms impedance across the entire frequency range. Your amp will do it's best to deliver power regardless, so you don't really need to "switch" anything.
Ok. Thanks. So if I had 8 ohm speakers and wanted my amp to put out 100 w/channel (4 ohms) instead of 70 w/channel (8 ohms), I'd need different speakers.
That's correct. Some speakers have a single midbass driver with a 4 ohm nominal voice coil, while some other models with dual midbass drivers have 2 - 8 ohm drivers in parallel for a 4 ohm nominal impedance. If I were choosing between one manufacturer's speakers that offered two similar models, one at 8 ohm (with single driver) and one at 4 ohm (with dual drivers), I would personally be tempted to go with the dual-driver 4 ohm model (for more $, of course) for the following reasons: 1. To get a possible 50% - 100% more power out of the amplifier (depending on amplifier design)2. To get ~3dB more sensitivity from the speaker3. To get a little lower distortion from the speaker, since each driver's cone exursion is half as much. But I would first and foremost decide on a speaker purchase based on how much I liked it's sound, and not worry or care if it was a 4 ohm or 8 ohm speaker. Steve
Seeing how his amp is rated at 70wpc @ 8 ohms and only 100 wpc @ 4 ohms, his amp may not like driving a speaker that is nominally rated @ 4 ohms.
It would certainly depend on the "4 ohm" speaker. My GR Research Diluceo is a 4 ohm nominal speaker but only dips to 3.8 ohms minimum. Other 4 ohm speakers may have a significantly lower minimum impedance. Stereophile tested his amp, the Adcom GFA-535, and while it operated fine at 4 ohms, did blow fuses into a 2 ohm load. It is a modest amplifier though, and it's power supply is limited, as you pointed out. The 4 ohm speaker impedance is probably a moot point for now, because as he mentioned in a previous topic, Mass. Wine Guy recently acquired a pair of Silverline Minuet 8 ohm speakers and is pleased with them, and was asking about a more powerful amplifier recommendation in that topic. In this topic he is just trying to clarify his understanding of speaker and amplifier impedance. Steve
It's more complicated than just the impedance, isn't it, when phase angles are involved?I have a vague notion what this means but I'd love to hear a layman's explanation of phase angle.-Mike
Yes, speaker impedance is a very complicated animal. Usually speakers will have a large spike in their impedance at resonance frequency then slope downward, only to peak at high values as the frequency rises. The 8 ohm rating is sort of an average, but really is not the true characteristic of that speaker. As an example some 8 ohm speakers may actually dip down to 1 or 2 ohms at specific frequencies, and that is getting real close to a short circuit, which will raise hell with driver devices (tube or SS).Wayner
Wow. I'm overwhelmed. Ignorance (mine) is bliss. Given the 85 to 88 db sensitivity of my speakers, should I be looking at amps with a lot over 100 watts per channel, or would 100 to 150 probably be an improvement?
A typical pair of speakers of 87 dB sensitivity will produce 87 dB SPL at one meter. If you listen at 3 meters (about 10 feet) the SPL drops -9.7 dB to 77.3 dB SPL. To seriously listen you're going to want to get back to 87 dB or so. To make the math simpler, let's just round it up to needing 10 dB more power. OK, average SPL is now around 87 dB during certain musical passages, so you're using a measly 10 Watts per channel. No big deal - right? Well... but you're listening to a good digital source, so transient peaks come along with many plucked guitar notes and drum/cymbal strikes and what-not. Those can easily be 10dB higher than the average signal, even though they don't last for more than a millisecond. To keep from clipping them though, now you need 100 Watts/ch.That's not too big of a worry as a lot of guys have 100-watts or a bit more available from their amps. Uh-ohhh... what if that peak is only 3 dB louder (+13dB)? Now you need 200 watts/ch. At +16 dB you need 400 watts and at +20dB you need 1kilo-watt! Is a 20 dB peak "common"? Probably not common, but possible. The better the recording, the more probable it will be. Now... what if you "push" the volume a bit to where you're listening at 90 dB SPL or a bit higher at times... like in a real concert? All that headroom you "thought" you had is now long gone and now you're clipping the amp pretty frequently.But... those peaks are so fast and short in duration compared to the average power you're using, you don't "hear" any clipping. Only when you're clipping the *snot* out of the amp do you typically hear it as such, but if that happens you know it and turn down the volume. In lieu of such obvious clipping, what do you hear? You hear...
