I'll join you on the scenic route
Dear Halon 451,
I can not be certain of what is behind being able to change the impedance of that Yamaha unit from 8 to 6 ohms.
The problem is we don't know the circuit topography. May be there is something unusual about it? However I find it hard to fathom that it is worth the trouble given the impedance curves of most speakers. I have been on the Yamaha site, but as is usual these days, there is no information proffered that gives a clue as to how the unit actually works, and the engineering details
This seems a budget unit, so it would not be unreasonable to assume that the power amps use planar transistors, or an integrated circuit version of same. In a unit like this you could safely assume class B biasing.
If that is so, increasing the supply voltage when driving a nominal 8 ohm load would allow for a marginally increased power output into the 8 ohm load. The only reason for changing it for a six ohm load would be to limit output current to protect the output devices. The problem with that idea is that, if the output stages are that close to the destruct point, then you could have a situation were a customer would set it to 8 ohm and then connect speakers of lower impedance. This I would have thought would be inclined to result in a service nightmare.
These days there is usually a circuit to limit the rail voltage if the output current becomes excessive. This of course drives the amp to clipping, and hard, but protects the output devices. I did note that this unit has outputs for A & B speakers. You can select A or B but not both. So may be the output devices are near the knuckle. If that is so, I would think it would be wise for an owner to leave it on the six ohm setting unless he could be certain that the speakers impedance does not drop below six ohms at any frequency. Many if not the majority of speakers that are nominally 8 ohm will.
I have to make a confession that I have never owned a Far Eastern receiver of any stripe. Obviously I'm somewhat familiar with the breed. I do know that as a group they tend to not perform optimally into low impedance loads.
This is a little unfortunate, especially as there are now many speakers on the market that are 2 1/2 way designs. By that I mean speakers with two bass mids, one above the other. The top unit reproduces mid and bass, but the lower unit is shelved at a much lower frequency, usually first order, to splice to the step response and diffraction loss of the upper driver. The lower unit also reinforces the bass. This on the whole is a good design philosophy and I have used it more than once or twice myself. In fact I believe all narrow fronted speakers should have step and diffraction compensation provided in some fashion. The point of this is that pretty much all 2 1/2 ways will drop to around 4 ohm impedance at some point, even if they are nominally 8 ohm. If the amp is current limited it results in a little less than optimal results.
Just a quick word to Reaper 360, who got us into more than enough trouble by posing the question. If you do purchase another receiver, you do not want to purchase one that has its greatest power output into 8 ohms. You want to find one that will drive four ohm loads, or less, comfortably. Then you know that it has ample power and current reserve. If your speakers are nominally 8 ohm, as long as your receiver will deliver adequate power into an 8 ohm load everything is fine. If it provides twice the power output into a four ohm load well and good. That is evidence of well designed and robust power supply and output stages.
