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I am thinking about buying a pair of PSB bookshelf speakers with an impedance of 6 ohms.
The amp I want to use to drive them has an impedance switch on the back: 4 ohms or 8 ohms.
Which setting would you choose?
Sorry if this seems dumb -- I have been reading posts about impedance here and elsewhere and I don't really get it.
Thanks.
Follow Ups:
so that the Amplifier can pass the rigorous (and largely irrelavant) continous power test with out blowing up. For music, even for 4 ohm speakers use the 8 ohm setting. This optimizes you clipping point.
There is an exception to this, however: If your loudspeaker has a extreme reactivity - greater than 45 degree in the lower frequencies you should set to 4 Ohm (even with a 6 Ohm speaker)
Note: For McIntosh Autoformer or Tube Amp Transformers with multiple taps - it's more relavent - to match the impedance for optimum power transfer efficiency - or to just listen to what sounds good.
Three most important things in Audio reproduction: Keep the noise levels low, the power high and the room diffuse.
Stereophile said there was a 45 degree phase shift in the lower frequency region where the impedance also dipped to four ohms...
Maybe he *should* use the "LOW" switch, or use "HIGH" with caution...
Thanks for chiming in. And thanks for explaining what the switch actually does... it limits current by reducing rail voltage... makes sense!
Cheers,
Presto
These usually use a dual tap transformer and a switch. The other way is to actually have a curent limiting circuit employed on the signal - it keeps the rail voltages the same but limits the signal voltage swings. Realy cheap.
Three most important things in Audio reproduction: Keep the noise levels low, the power high and the room diffuse.
The vast majority of those switches simply lower the transformer voltage a bit so things don't get too hot and blow up.
Leave it on 4Ω and never have to worry about it.
Ah yes, the big "IMPEDANCE" switch. Now, don't QUOTE me on this...
But these impedances "switches" on amps do not TYPICALLY envoke some magical reconfiguration / bridging modes of amplifier output transistors to optimize for load impedance. I have heard it SAID that on some amps (especially HT receivers) that the switch simply engages a sort of current limiting circuit to allow the amp to drive the load, with some sonic penalty aka dynamic range compression, while preventing the amp from bursting into flames or melting into a puddle.
In fact, the existence of these switches does not instill confidence in me that the amp is very stable at lower impedances at all. Conversely, even some (admittedly top shelf) home theatre receivers can drive loads as low as four ohms with NO switch. Some PA amps are stable at 8, 4 and even 2 ohms - but many are fan cooled, so it's not a fair comparison to an amp that relies only on convection cooling with heat sinks.
So here is what you do. Give us the make and model of the speakers, and make and model of the amp. We can look at what size "cahonas" this amp has, and what the impedance plot of this speaker looks like if a camp like, say, Stereophile magazine has reviewed them.
Sorry for the long winded "non answer" but "buy them and turn it up slowly and see what happens" is something I do all the time and don't recommend to others! ;)
Cheers,
Presto
The speakers are PSB Image B6.
The amp is a Yamaha A-S500.
Guess my question is which setting will do the best job of driving the speakers?
Looking at the Stereophile review these speakers dip to 4 ohms in a couple of spots despite their 6 ohm rating. This can be expected in some cases with "nominal" loads. Impedance is not a "number" - it's a frequency dependent phenomenon, aka a graph.
Your speakers are 6ohm so according to the manual you can set the switch to the "HIGH" impedance position.
If you get amp overheating or distortion, you should try the "LOW" setting.
This amp is rated down to TWO OHMS which is amazing for an amp in this price range. Due to it being only a 2 channel amp (weighing in at 26 pounds) they had room for wonderfully large heatsinks in there.
Survey Says: Set it to HIGH and give her a go! Risk to the amp is almost non-existant.
Cheers,
Presto
Hmm. I'm doing some searches and it looks like the consensus is to use the 8 ohms setting.
What about for a Harbeth C7 which is at 6 ohm? Which setting is best, 4 or 8?
Any 6 ohm or higher rated speaker, run on the HIGH imedance switch.
Any 4 ohm speaker run on the LOW impedance switch.
If a 6 ohm speaker gives you problems (overheating, etc.) then just switch to the LOW setting.
Your amp will be fine. If you want to experiment, run your 6 ohm speakers on each switch setting and pick the one that sounds the best.
In this case, Elizabeth was right! ;)
Cheers,
Presto
Hey Presto I post on several forums and it is always great to see someone who is willing to go above and beyond to research specs and help people out.
The world of forums needs more guys like you
Bravo!
Hey Prosound!
I notice you jump in a lot with help and advice too. The nice thing about so many people seeing the same question is that we can formulate what amounts to the best "collective response". Plus we all learn in a collective way too... it's a win win. When we're all playing well with others, that is! ;)
Cheers,
Presto
Great - thanks a lot.
Learned some things from all of the responses.
Jeffery
If your amp is a ss one then, as Presto said, in all likelihood the switch merely introduces a current limiter.
As long as you don't push the amp hard it should not make a blind bit of difference and 8ohm will be fine.
If the amp is a tube job impedance settings are important but I could not answer your question as I don't use them.
I have 4-and 8-ohm speakers, run pretty stout 70's receivers and have no problems whatsoever when swapping speakers in and out (just that I never mix pairs, common sense, and never mix impedances on any one amp, again common sense). Just my experience here, YMMV.
Freedom is the right to discipline yourself.
.
If the amp is shutting off because it is overheating...you can't use your ears anymore! ;)
Although this can affect sound, it's really more about the amps ability to handle the heat of the higher current due to the lower impedance.
Edit: Except this time you're correct! ;) He can probably use either setting safely.
Cheers,
Presto
Edits: 03/15/12
Wow this really sounds great....POOF puff of smoke..what the heck just happened?
My ears said it was good!
ROFL
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