Lets really look at this 4 vs. 8 ohm thing

[dohanc]

Well I have a schematic here from a pro amp that's about 5 years old which has an impedence switch for using 4 ohm loads. There is actually a 4 ohm and 8 ohm tap coming off the transformer. Both taps goto the impedence switch where you you select the high voltage 8 ohm or lower voltage 4 ohm rail. This then goes to the bridge rectifiers and onto the power stage.

It is interesting to note that this switch was only used on their largest amp in the series and they replaced it with their current models which are even bigger than the old ones and none of the new ones have an impedence switch.

The impedence switch makes sense. It is so you don't pull to much current from your amp. (Remember audio amps are voltage amps, they will try to deliver whatever current the load wants).

As for power differences, if the impedence switches receivers are using are the same as the one I have described (which I assume they are) then the power across the transformer is always equal. I can convince you of that with some basic transformer theory, but maybe for a later post .

So, Crashguy, your article about it actually reducing the power by 66% is interesting. I can barely come up with a good reason for this. I would be very interested to follow up on this.

 

[toquemon]

Well, that stupid button was working fine with an old Marantz receiver that i bought just after i retired my older Onkyo amp. When i used the "8 ohm" switch with my old Cerwin-Vegas 280 SE (96 db efficency, but with 2 Ohm dips) there was a continue "shutting off" phenomena that wasn't happening with my very old Onkyo receiver. I don't like Marantz ever since. I just had it for 2 weeks and changed it for another Onkyo mid-fi receiver and never had the problem again.

With a Nakamichi AV10 (the flagship receiver that cost me a lot of money) i had a worst problem, the thing just died, repaired it only for seeing it dying again three months later (the famous 3 month guarantee period in repaired units).

The moral of this is that a "well-recognized" brand does no insure you perfection, especially when this "well-recognized" brand has been bought by a mass-market-profitable-hungry brand like LG, Phillips, Samsung, Sony or whatever.
What happens is the opposite of the car industry, at least Volkswagen is using in their "consumer products" the technological advances that brings Audi, Porsche and Lamborghini. In audio the mass-market brands use the "name" of their "well-respected" brands to cheat the public.

That's why i stayed with Yamaha and the old Onkyo stuff, at least they make all their parts and musical instruments; the president of Onkyo is a well recognized honest person and the people of Yamaha had played a guitar at least once on their lifes, so they remember how it sounds.

Sorry about the English, it is not my primary language.

 

[PENG]

As for power differences, if the impedence switches receivers are using are the same as the one I have described (which I assume they are) then the power across the transformer is always equal. I can convince you of that with some basic transformer theory, but maybe for a later post .

So, Crashguy, your article about it actually reducing the power by 66% is interesting. I can barely come up with a good reason for this. I would be very interested to follow up on this.

If you have the schematic you must be right for that amp. However, I had a Sony that specify almost 20% less power in the 4 ohm position. It depends on how much the voltage is reduced in the 4 ohm position (transformer tap in your amp's case), you could have the same, or less power with such a switch. Again, assuming it is a voltage (tap) selection type.

So crashguy could be correct if he was referring to an amp that reduces the voltage more to make sure the current drawn by the lower impedance (we are talking continuous rating here right?) load will not damage any components.

I think the bottom line is, if the amp is rated for 100W continuous into an 8 ohm load, you should get at least 50W into a 4 ohm load with that switch flipped to the 4 ohm position.

A good quality amp should give you close to the same rating in either switch position.

Most separates will not need that switch at all, and should give 50 to 70%, or in rare cases, close to 100% more power into a 4 ohm load.

In terms of peak, or short duration power rating, even receivers such as Denon, Onkyo, Pioneer, HK, and Yamaha will give you close to double or more power (at say 1% THD) into 4 ohms in one or two channel operation, without "that" switch. Based on HT/S&V's recent lab measurements, I was surpised to see that the HKAVR630 did not fare as well as Denon & Pioneer in its 4 ohm capability. However, I remember their flag ship models, the AVR7X00 did exceptionally well.

 

[WorkerBee]

Denon is what I've played with for the last 12 years, Polk's being 4 and the Klipsch being 8 ohm.
My #345-45 watter has powered 4 ohm rt55i's hours on end with never a problem.
My #685-100 watter has powered the "A" speakers a set of rt55i's "and" at the same time on "B" a set of Polk rt5's daisy chained to a set of Klipsch rsx5's. Recievers maybe a little warm sometime but never shut down as I have read with some guy's.

 

[crashguy]

Theoretically, it would make sense for the 4 ohm "silly switch" to reduce the voltage to the rails by 50%, as a 4 ohm load is 50% less resistence as compared to an 8 ohm load. The problem is, it depends on the design engineer that's building the thing. Does he/she decide that a great # of speakers rated at "4 ohms" (which is kinda a S.W.A.G. anyway (Scientific Wild *** Guess)) or an average, dip closer to 2 ohms over much of the range, therefore the 4 ohm setting of the switch reduces voltage to the rails by more like 60 or 70%??

My preference would be for companies to build the amps to be able to cope with a 4 ohm load (likely cost/size prohibitive in a 6 or 7 channel HT receiver), rather than installing a switch to reduce the available output power by half or more. We paid for those watts, don't take em away!!! If you buy a Heavy Duty truck to pull a heavy trailer, does it make sense for the automaker to tell you, "this truck has 300 HP....unless you hook a trailer behind it, then it's only 150 HP"?? The power should be there when it's needed.

I guess by the relatively small # of people who posted saying they had shutdown or burn out problems, this is not much of an issue anyway.

I appreciate everyones input and point of view, even if you are from North Carolina. ( gregz - your avatar cracks me up)

 

[Votrax]

Most Sony receivers impedance switch drops the rail voltage from +/-50VDC to +/-30VDC. That's more than enough voltage to provide 100W into 4 ohms. Of course that's not all channels driven.

 

[dohanc]

Crashguy - Good point. The rails could really be anything. I'm planning on getting a Yamaha Rx-v750 or 1500 in the next week or two. When I get it, I'll go probe around and check out some of the transfomer voltage.

 

[gregz]

I guess by the relatively small # of people who posted saying they had shutdown or burn out problems, this is not much of an issue anyway.

I remember a couple years back I helped out a church with their "PA" system that kept shutting down. They had an old home stereo receiver trying to drive about 8 speakers. I divided up the speakers into series/parallel connections so that the receiver would see 8 ohms, and the shutdown problems went away. Really, after the abuse that thing took, it was amazing it wasn't even damaged...

My Onkyo shorted out when I lived in New Mexico and a mutant bug managed to crawl into the finals and end its miserable life. The Onkyo came back after the bug remnants dried.

I guess some of these things are pretty durn durable.

I appreciate everyones input and point of view, even if you are from North Carolina. ( gregz - your avatar cracks me up

Y'all are just jeaolous you don't live here too.