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HELP advise PLEASE - Focal Sopra 2 Speakers - I want a tube amp but " Min 2.6ohm @ 104Hz "

HELP PLEASE advise bought a pair of Focal Sopra 2 Speakers that I absolutely love.

I want to use an integrated tube amp with them but the measurements worry me even though many Sopra owners say they happily use tubes with these speakers.  


HERE IS THE ISSUE:

  From HiFi News September 2015, lists Minimum impedance is around 2.6 ohms at 104Hz. Combined
with high impedance phase angles at low frequencies this results in a minimum EPDR (equivalent peak dissipation
resistance) of 1.1ohm at 85Hz – challenging to the partnering amplifier.

Can I used an integrated tube amp and hope for success? 


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From HiFi News September 2015, minimum impedance is around 2.6 ohms at 104Hz. So the amplifier should be capable of approximately doubling its 8 ohm output power into at least 4 ohms and preferably again into 2 ohms.
Min 2.6ohm @ 104Hz. Focal claims 91dB sensitivity for the Sopra No 2 but our measured pink noise figure of 88.2dB falls almost 3dB short of this. Low impedance is a factor in achieving this figure: Focal specifies a minimum impedance of 3.1ohm (which would indicate a 4ohm nominal rating rather than the specified 8ohm) but we measured a dip to 2.6ohm at 104Hz. Combined with high impedance phase angles at low frequencies this
results in a minimum EPDR (equivalent peak dissipation resistance) of 1.1ohm at 85Hz – challenging to the partnering amplifier. The forward frequency response [Graph 1, below], measured on the tweeter axis, is essentially flat in trend with just a mild presence band dip.




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128x128Ag insider logo xs@2xfsmithjack
audiokinesis
2,724 posts
 
Erik is correct.  I design speakers to be tube-amp-friendly so I am familiar with the issue he brings up. 

You might look into Paul Speltz's "Zero" Autoformer, which is made for exactly this situation.  The Zero Autoformer can effectively and elegantly raise the impedance of the load your tube amp sees. 

Duke 


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fsmithjack OP
712 posts
 
Good stuff - I thought I might be overthinking it. 
Also, I didn’t know the dip could make a tube amp sound louder. Glad I asked. Appreciate the info. I read these measurement can be tough on an amp but didn’t know what that meant in real world use.
Do you guys think a reasonably priced used integrated tube amp like Like a VAC 160i or Octave Audio V110SE or even a value one like Ayon Spirit III will work ok? These are all in my used price range of around $4kish. 
I don’t want to tube Pre end SS amp for no other reason then cost of 2 boxes and another IC’s and PC’s is more then I have to spend. Just downsized my speakers and love the Sopra’s so hoping I get lucky with a $4k ish used Integrated amp. These are all around that rough $4k range. 
mofojo
2,184 posts
 
+1 tube preamp. 
ohlala
1,234 posts
 
The VAC has assorted output taps, so that is nice in your situation. I would experiment with a SS amp as another data point/ reference if practical.  
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fsmithjack OP
712 posts
 
Been thinking about SS and tube preamp. I wasn’t but a few of you guys mentioned the idea and it has started to stick with me. I want the Linear power and control from SS but now need that tubes sound. The hybrid integrated I’m now borrowing is so good it’s tough. Wish I could afford it but I can’t. What do you think of GamuT D-200 MKIII and a Alnic L4000 combo. The transformer coupled output on the Alnic provide nice lower output impedance and Gamut is a little tuby on its own so was thinking good match for Focal Sopra 2? Think this would be better then a hybrid like a Mac 352?
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