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Tubelab Discussion and support of Tubelab products, prototypes and experiments

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Old 25th February 2013, 04:33 PM   #1
the trooper is offline the trooper  United States
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Default Output transformer question

I'm building a tubelab sse, I'm at the point where I'm ready to order transformers. I checked the ohms on my speakers and got 6.9
My question is on the secondary impedance, would I be better off ordering a 6 ohm or 8 ohm?
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Old 26th February 2013, 10:52 PM   #2
amandarae is offline amandarae  United States
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You check it by measuring it or reading on the label? When you measure, you are reading the DCR (DC resistance) not the impedance of the driver. Your 6.9 sounds right for an 8 ohm impedance driver.

If I were you, I will order an 8 ohm secondary tap for the OPT.

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Old 27th February 2013, 01:21 AM   #3
rknize is offline rknize  United States
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I would look at the label on the speaker to find out what the impedance is. ~6 ohms DCR is roughly what to expect from an 8-ohm driver at rest, however the actual impedance of the speaker may be lower if there is a complicated crossover network.

What speaker is it?
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Old 27th February 2013, 06:25 PM   #4
the trooper is offline the trooper  United States
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Yes I measured the speaker with a meter. I have been reading up on impedance from other posts, lots of info on this forum. Should have done that first I guess. Looks like 8ohm on the secondary would be the way to go
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Old 2nd March 2013, 12:39 AM   #5
boywonder is offline boywonder  United States
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Trooper: To clarify, you are measuring DC resistance with your meter. Your meter has an internal battery to apply a small DC current to the speaker (or resistor or whatever) to give you a resistance value at DC (abbreviated DCR).

Impedance is this resistance at various AC frequencies, not at DC. Speaker driver voice coils are inductors, and if you have multiway speakers the crossovers have inductors, resistors and caps. The inductor's and capacitor's resistances change with frequency. Resistors "resistance" does change with frequency.

So a typical speaker has an "impedance plot" of resistance vs freq. Some are fairly flat across the frequency spectrum and others are not. Flat impedance speakers usually play well with no-feedback tube amps since no feedback amp's output impedance is relatively high.

As mentioned, typical 8 ohm impedance speakers usually measure around 6-7 ohms DCR.
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