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In Reply to: RE: Output Impedance Question??? posted by ecline6 on February 27, 2014 at 19:13:21
The taps are there to transform the speaker impedance towards the output tubes.
The best tap to use really depends on many factors, one of them is your personal preference.
Technically an 8 Ohm speaker system is best connected to the 8 Ohm tap, but if the speakers exhibit large impedance dips
it will probably sound and perform better on a 4 Ohm tap.
Experiment and use the one that you prefer.
It is quite typical that a 4 Ohm speaker will not sound better on an 8 Ohm tap, but this is not always the case.
An 8 Ohm speaker on a 4 Ohm tap will increase the impedance that the output tubes are driving.
This can translate sonically into a more pleasing to the ear experience but a side effect is less output power.
Output tubes are by design (hopefully so) made to drive a specific impedance load value.
This is a static calculation determined by many simultaneous parameters.
If an output tube stage is designed to drive a 2.5K Primary and one connects a 4 Ohm speaker to an 8 Ohm Tap
cutting the reflected impedance by half towards the power tubes(s) means
that the tube now sees 1.25K Ohms.
Whether this makes better sound or not may really depend on your ears.
Mathematically it is not the correct solution.
But music is a dynamic thing and when things are in motion and not operating at fixed values throughout a given range
pleasing results may not always be intuitively or mathematically correct.
Follow Ups:
Thanks for the responses so far. I have a much better understanding of what is going on now.
I know which tap I prefer but I still wonder how I could determine/ measure which one is which. Is there a way to measure the tap at the speaker outputs???
You can measure with a multimeter. Reading the Ohms scale, the 4 Ohm tap to ground will read slightly lower than the 8 Ohm tap. (not much difference, probably a half Ohm difference).
You can also measure the audio output (AC Volts)at the taps. With no load, inject a constant level input source (a tone or pink noise). Keep the level VERY LOW, around one volt. (you can damage an output transformer by driving it to high levels unloaded). The 8 Ohm tap will be outputing slightly more voltage than the 4 Ohm tap.
Thanks for the tips. I did measure the taps and there was a slight impedance difference just as I was told. I am now 99% sure I know which tap is which.The more I listen though the more I find both taps sound good. But, my favorite did turn out to be the 4 ohm tap with the 4 ohm speaker.
Edits: 02/28/14
Nice report thanks for the update.
Will a true 4 ohm speaker sound louder from the 4 ohm tap or will the 8 ohm tap still sound louder at a lower to moderate volume???
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