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Q: Stupid Amp Question
Scott Gardner wrote:
On Thu, 11 Dec 2003 21:08:55 GMT, (Evil) wrote: Here's a basic amp question for all the experts here. Say a stereo amp is rated 100wpc. It has 2 speaker out posts, though, and a switch for "Speaker A, Speaker B" (or both). it's a 2channel amp so if hooked up to 2 speakers, does it then power at 50wpc for all 4 speakers or is it in parallel so still powers at 100wpc? This is something you see in home stereo more than car stereo. If the amp is rated at 100 Wpc into two channels, then that's what it will deliver into your two "A" speakers. If you have it power the "B" speakers at the same time, then not all four will be getting 100 watts each. Without knowing how the 2nd set of speakers is wired into the circuit, it's impossible to tell just how many watts each of the four speakers will get. In the majority, this is incorrect. Regardless of what the "rating" of the amp is, all amplifiers will deliver different amounts of power to a load depending upon the *impedance* of the load. Assuming it is a nominal 100 watts RMS per channel amplifier - which is *different* than the tag on the back of typical consumer gear that lists the power draw from the *wall* socket - it is rated at this power into some load impedance. The standard load impedance for *most* amplifiers is 8 ohms. This doesn't mean that your amp was rated into 8 ohms. So, if the amp puts 100 watts RMS into 8 ohms it will TRY to double that power into *half* the impedance, or 200 watts @ 4 ohms. Since most amplifiers have internal losses, it probably will fall short of *double* power into half the impedance. This *assumes* that the amp *can* double its power, which requires that the output voltage remain the same, and the current *doubles*. This requires two important things: 1) that the power supply can deliver the extra current and 2) that the output section can deliver the power and dissipate the extra heat. Not all amps can do this. As the impedance drops *below* 4 ohms, this becomes increasingly demanding and many amps will not perform well at all. Since you are talking about running TWO sets of speakers, you have to expect that the *nominal* impedance will be that of the two loads in parallel - half. IF they were resistors, then it would be exactly the same as two resistors in parallel, and if they were both 8 ohms then the result would be 4 ohms. BUT, if they are both *less* than 8 ohms in reality, and *not equal*, then the parallel combination will be *less* than 4 ohms and not exactly half of 8. Here's the rub - unless both pairs of speakers are *identical* (same model, no differences) running them in parallel is a spectacularly *bad* idea. This is because speakers are *not* resistive loads at all, and the combination of the two in parallel can and will produce unpredicatable variations in impedance and phase angle. In the most extreme case, like in the SR/PA applications where they are run at or near their power limits, this can cause *one* driver to "hog" power and get fried. Even in home use, you can have problems with the amp running out of headroom, over heating, and unpredicatable variations in frequency response. Also, i understand that most people say that speakers sound better when bi-wired. What is the best way to bi-wire? My B&W801s have 2 sets of speakers posts with a shunt that connects them. I am connecting my amp to one set and it is concurrently powering both drivers. I have seen bi-wire cable that goes from 1 set of speaker terminals on an amp and splits to 2 sets of speaker connectors. it had a writeup regarding the type of wire for each and how some "frequency" transmits better across one of the conductors than others. SO is it best to (a) cable from amp to speakers and use the speakers' shunt to bi-wire (b) use bi-wire cabling (c) connect separate cable from each of the amp's speaker out terminals. Does getting a separate identical amp and wiring that to the separate driver offer any performance upgrade to (a-c)? thx in advance -goro- If you do scenario "a", then you're not bi-wiring. Using the shunt across the two sets of terminals is for when you *don't" want to bi-wire. Correct. "Bi-wiring" is one of those "audiophile" things that some people claim makes a "huge" difference in the sound, but no one can ever reliably tell the difference between a bi-wired system and a conventionally-wired system in double-blind tests. This is not a fact, it's the poster's opinion - but there are measureable differences between a bi-wired system and a jumpered system if for no other reason than you are hanging two sets of capactive and inductive cables off an amp vs. one. Some people like what they subjectively percieve as the result of doing this. If it makes no difference to you, then it doesn't matter. All bi-wiring means is that you're sending the exact same signal from your amp output terminals to two different speakers, using two different pairs of wires. Electrically, you're connecting the two divers together in parallel, but rather than using the shunt at the back of the speaker cabinet, your running separate wires to each driver and terminating them at the same place at the amplifier. The theory is that by running separate wires to each driver (usually a tweeter and a sub), each wire pair would somehow "optimize" itself for the particular driver it was powering. Also, the "delicate treble signals" wouldn't have to ride the same cable as those "slow, clumsy bass signals". I don't know who chatacterizes things this way. But, clearly the use of a very inductive cable on the tweeter would have an effect, while not effecting a connection to say a midrange driver made through a not inductive cable... You can see what I think about bi-wiring - I think it's a lot of audiophile hocus-pocus that hasn't been able to stand up to scrutiny. Just one one wire pair from you amp to your speaker cabinet, and use the shunt to parallel the two drivers. Or not. As for getting a separate amp and using it to power some of the drivers, that's "bi-amping", not "bi-wiring". and it does have some benefits, most noticeably being more power. Actually that is not accurate. The power available to the load *might* be the same or less or more depending upon the power of the amp itself, the sensitivity of the driver being driven and the impedance of the load when separated, as compared to combined. What you will likely get is effective *headroom* in some cases, assuming a x-over at the *input* of the amps, and that the amps are not driven full range. The headroom benefit in the majority coming from reducing the maximum "duty cycle" (power vs. bandwidth vs time) that is required of a given amp. In practice, this may or may not have an audible effect or even a technically beneficial result - much depends upon the specifics of the amps and the load. Scott Gardner _-_-bear -- _-_- BEAR Labs - Custom Audio Equipment, Cables, Mods, Repairs - http://www.bearlabs.com |
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