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Anonymous | I know this is probably a dumb question and its setup correctly however I have a onkyo tx8511 and on the back its got the 4ohm(8ohm a+b) / 8+ohm selector. This should be on the 8ohm for speakers with an 8 ohm impedence correct? or is the 4 ohm side better, just will allow the reciever to run anything 4+ ohms. |
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Silver Member Username: GmanPost Number: 201 Registered: 12-2003 | It should be on 8 ohms for 8 ohm speakers and 4 ohms for 4 ohm speakers. If it was better on 4 ohms for 8 ohm speakers they would have no settings at all. A 4 ohm setting allows the amp to push out more power against less resistance without causing clipping, overheating, or worse. 8 ohm speakers, while taking more power to play as loud as 4 ohm speakers at the same efficiencies (say they are both 88 db's),---8 ohm speakers are a much easier load for an amp to drive. The ohm ratings on speakers are a rough average of the electrical resistance in the audible frequencies---for instance, on an 8 ohm speaker the ohms can vary from 4 to 24, but average around 8. On a 4 ohm speaker it can range to 2 or even less which makes the speaker get closer to the wire resistance which can cause driving problems. It can get real dicey on long wire runs, unless you get extraordinary thick wire. Unless you are nutty over a particular 4 ohm speaker, I normally recommend getting 6 or 8 ohm speakers. Your choice of receivers you can use expands greatly. I see the Onkyo 8511 is a stereo receiver. It is much easier for manufacturers to hold their amp costs down with 2-channel amps. So you are getting a far superior amp in this $300 receiver than one would get in a $300 surround, nevermind the extra chipsets and complexities of AV surround receivers. |
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Silver Member Username: GeekboyPost Number: 198 Registered: 12-2003 | Anonymous/Gregory: Just to add on to what Gregory wrote. The A+B switch on the receiver serves two functions. First, if you have a single set of 4ohm speakers connected to either A or B, then you set the switch to 4ohm. Second, if you have a single set of 8ohm speakers connected to either A or B, then you set the switch to 8ohm. This was Gregory's response to you. But, if you have two pairs of 8ohm speakers connected with one pair on A and the other on B, you set the switch to 4ohms (this is because the A+B speakers are connected in parallel which, by definition, halves the impedence (8x8)/(8+8) = (64)/15) = 4ohms --Formula: (R1xR2)/(R1+R2). You do not ever want to have 2 sets of 4ohm speakers connected (one on the A terminals and one on the B terminals) as the receiver will see a nominal load of 2ohms (4x4)/(4+4) = (16)/(8) = 2ohms. This will almost assuredly put the receiver into Protection. This answers your question if you actually have 2 pair of 8ohm speakers connected (to the A and B terminals). Gregory has already answered your general question. |
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Bronze Member Username: DmeisterPost Number: 32 Registered: 12-2003 | And actually the 4/8 Ohm switch is generally a current *limiting* switch to keep the receiver from pushing through more juice than what the power supply can actually handle. If the receiver were truly capable of delivering stable power into 4 ohms, it wouldn't use such a switch. |
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Silver Member Username: GeekboyPost Number: 199 Registered: 12-2003 | Darryl: do they do that with just a current limiting resistor? |
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Bronze Member Username: DmeisterPost Number: 33 Registered: 12-2003 | Here is a brief discussion of these switches: Audioholics FAQ |