QuestionHello again, and thank you for your kind attention and expertise.
Here's the part I don't understand; you wrote "If you are worried about the loading stress on the amp, choose the 8 ohm setting." Should I be worried about the loading stress on the amp at 4 ohms? How do you decide that? Considering the tube amplifier I described, with 80 watts RMS per channel, is there a reason to worry about the loading stress, and how do you figure something like that out???? I hope this doesn't sound stupid to you, but when I am setting up a system, this is the type of question I never know how to answer.
Thanks again,
Bob
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The text above is a follow-up to ...
-----Question-----
Hi,
I am setting up a new audio system. I want to go all vacuum tube, and see if it really sounds ?better ?different etc from solid state.
I am going to use a vacuum tube amp that is rated at 80 watts RMS per channel. The amp has output impedance connectors for 4 ohm and for 8 ohm. The speakers I am using are a set of Swan Diva 6.2's. These are rated at SIX ohms impedance.
Firstly, although not at all stupid in the world of physics and electronics, I have never truly understood the "impedance" matching of speakers to amps. In this set up, should I use the 4 ohm amp outputs or the 8 ohm outputs? The speaker dealer says: "Try both and see what you like better; it won't hurt the speakers". Somehow, I can't believe that is right either.
Thanks for your help and opinion!!
Bob
-----Answer-----
The speaker dealer is right. And, I will tell you why.
The speaker impedance is a function of frequency. The manufacturer rarely provides the consumer the actual impedance curves. In reality the impedance can vary from well below the rated value to several times the rated value over the frequency range.
There are two main concerns - among many - for impedance. First, for most efficient transfer of energy amplifier to speaker the impedance should be optimally matched. Secondly, their is most danger to the amp when the impedance of the speaker goes too low and places a severe load that can cause burnouts.
Some loudspeaker manufacturers cheat a little by rating it higher than they should; then, in reality you are putting more power than its nominal power and it gives a better, but fake, sensitivity score. So, a speaker company may rate it at 8 ohms when in reality it is a 6 ohm speaker and at is dip just above low end resonance will drop down to, say, 4 or 5 ohms. For an amp rating of 8 ohms, that may be too low for its normal operation.
There was once a very famous speaker rated by just about everyone as a good sound that would blow the fuses on an amplifier whenever it played a certain sustained bass note on a Telarc recording. It was uses as thee definitive test signal for that speaker in checking if the amp could properly drive it - an many of them couldn't because this 8 ohm rated speaker had a dip in the impedance curve in the low frequency region that dropped down to about 3 ohms!
In any event, you can put an ohmmeter on the speaker and measure the dc resistance of the bass driver and judge from there. Normally, the rated impedance will be around 20% more than the dc resistance.
See the figures in this URL to get an idea of what a speaker impedance curve can measure out to be:
http://www.churchsoundcheck.com/imp1.html
So, because of the variable nature of the impedance curve, you should try both connections and take the one that sounds best to you. If you are worried about the loading stress on the amp, choose the 8 ohm setting.
In reality, I suspect that you will not hear any difference: Or maybe very little, and then only in the output level by 1 or 2 db.
Have fun,
C
AnswerThe lower the impedance of the speaker system, the higher the power stress is on the amplifier. With tube amps it depends alot on whether it is transformer coupled or transformerless, such as the Futterman design.
With 80 watts average continuous (there is no such thing as rms power; that is a misnomer and I don't understand why a smart manufacturer would use such improper terminology) you will be bumping up against saturation power frequently and thus pushing the average power up over time with heavy music, so you should worry a little about stress and distortion.
I tried to find the impedance curve of the Swan Diva 6.2 that you have but was unable to do so. However, the rated impedance is 6 ohms, therefore, we can assume that at the bass region from 50 to 100 Hz where so much music is located, the impedance will drop down to 4 ohms easily and maybe lower. If the two drivers are connected in parallel then you could have impedances as low as 3 ohms.
To be safe, if it was my system I would connect the speakers to the 8 ohm outputs. That is my personal opinion.
C
