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Old 29th April 2009, 11:34 AM   #1
Hylle is offline Hylle  Denmark
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Default Speakers for tube amps

Do you have experinces with this. is it better to have a 8 ohm impedance for tube amps in general, than a 4 ohm with a bit higher sensitivity?
I guess the 8 ohm needs less current and will have a better damping factor.

I guess to really know, you need the same speaker in both impedances!
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Old 29th April 2009, 03:17 PM   #2
rknize is offline rknize  United States
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I would say the impedance (with the right OPT of course) get trumped by sensitivity. Of course, what tubes like the most is a flat impedance curve, but this information is harder to come by. Speaker cables become more important with 4 ohm speakers.
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Old 29th April 2009, 03:49 PM   #3
kuroguy is offline kuroguy  United States
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yup, the speaker impedance will affect the selection of the output transformer, as the impedance of the speaker will have an effect on the reflected impedance the output stage sees. Many output transformers have multiple secondary taps for different impedance speakers. Sensativity of the speaker is the important number for a tube amp.
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Old 29th April 2009, 04:04 PM   #4
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What kind of tube amp? Some are more sensitive to cross-overs than others...

When a speaker says 4 or 8 ohms you will note that it is called "Nominal Impedance". It is far from a single number.

A flat impedance -- both magnitude & phase -- is generally desirable.

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Old 29th April 2009, 04:28 PM   #5
pointy is offline pointy  United Kingdom
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if you only have an amp output of 20watts you will not be able to drive most modern 8 or 4ohm speakers.

you will need to match the impedance of the output transformer to that of the speakers if it is not already matched

this may mean working on the transformer which you will need to discharge before any work on it
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Old 29th April 2009, 04:32 PM   #6
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Quote:
Originally posted by pointy
if you only have an amp output of 20watts you will not be able to drive most modern 8 or 4ohm speakers.
That is a huge -- and inaccurate -- generalization. Many modern loudspeakers have been designed for low power tube amplifiers -- especially if you are diying your own.

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Old 29th April 2009, 04:42 PM   #7
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Default Some things to keep in mind.

There is no such thing as a truly flat impedance curve or at least I've never seen one. They will always have a resonance and some climb with frequency and some do even weird things. Magnepans climb and have built in Fletcher Munsen loudness curve. If the amp is Class A power is delivered to the load by the load so it's totally impedance dependent. Keep cable length to a minimum. Mono blocks are best to sit right at the speaker with only a few inches of cable. Long cables do weird things because of capacitance/inductance and loss. With tube amps you want a speaker with at least 96 db measured 1 meter with 1 watt input signal. Most speaker manufacturers don't rate their speakers this way but they should. They used to but don't anymore. Look for Klipsch LaScalla, Belle, Heresy, Cornwall, K-horn and in Altec Model 19, Valencia and the studio monitors A7-500, 844A, A8,604E and in EV Sentry IVB or Gillum G3. Ray
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Old 29th April 2009, 04:50 PM   #8
Jeb-D. is offline Jeb-D.  United States
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Quote:
With tube amps you want a speaker with at least 96 db measured 1 meter with 1 watt input signal.


Power rating of the amp doesn't matter?
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Old 29th April 2009, 05:09 PM   #9
kuroguy is offline kuroguy  United States
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Quote:
Originally posted by pointy
if you only have an amp output of 20watts you will not be able to drive most modern 8 or 4ohm speakers.
Yup, not really correct. In fact, I'll go as far aas to say that statement is just plain wrong. My tube amp can drive most modern speakers and it only puts out 5 watts. I even took it to a speaker store and tried it out with most of the speakers in the store. It worked well with all the speakers they had and drove all of them to a suitable volume level, and that was in a listening room that was much bigger than my listening room at home.

Quote:
Originally posted by pointy you will need to match the impedance of the output transformer to that of the speakers if it is not already matched
Actually, if you already have the output transformer, your choice is only to match the impedance of the speaker to that of the amplifier. Regardless, an unmatched set of speakers and output transformers will lead to higher distortion and less power transferred to the speakers. Whether or not this is actually a problem depends on the particular setup, but a few ohms in either direction isn't usually a problem.


Quote:
Originally posted by pointy this may mean working on the transformer which you will need to discharge before any work on it
Um, unless you want to disassemble the transformer and start removing or adding windings (good luck on that) there is nothing to work on. also, transformers don't store energy, so there isn't anything to discharge with respect to the transformer. Maybe you meant to discharge the capacitors in the amplifier, of course, that is something completely different that discharging a transformer (whatever that is).
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Old 29th April 2009, 05:12 PM   #10
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Default Power rating

The power rating should be root means square (RMS) not music power or any other rating. With these small box, infinite baffle acoustic suspension, low sensitivity 85db/1watt/1meter or less, yes probably the amp power does matter. The problem there is the amp rums out of steam or winds up clipping on deep bass, bottoming out the woofer excurssion and causing distortion out the wazoo. You never want the amp to even approach clipping, that's why you want a very efficient speaker. If the amp is clipping then it is internally hemoraging distortion through the roof. If the speaker has a decent enough magnet structure and 10,000 gauss or more in the air gap it will have a high sensitivity. The problem is that manufacturers want to skimp and sell cheap speakers.
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