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Pre-amp suggestions for Thiel 2.4/Pass Labs XA30.5

Looking to get a pre-amp to mate with my Thiel 2.4s and the Pass Labs XA-30.5. I've got about at $2500 limit and need a phono section (or need to to pick up a MM phono stage in addition... but still stay under the $2500 limit). I'm currently using a Classe CAP-151's pre-amp section, but it seems that it's getting outclassed a bit by the new additions. Any suggestions? I'm open to both tube and SS options.
cal3713
cal3713's avatar
cal3713 OP
1,421 posts
 
Here's a quote from a discussion about Mac autotransformers:

The output autoformers have different windings in them so the 2,4 & 8 ohm taps are like 3 different amps that are optimized for speakers with the corresponding 2,4 & 8 ohm resistances. If you have a MC501 rated at 500 watts and you hook up a 4 ohm speaker to the 8 ohm tap you will get the expected 1000 watt output (maybe a little less). This could cause the amp to go into protection though, it is usually best to match the speakers to the correct tap.

http://www.audiokarma.org/forums/archive/index.php/t-71322.html
unsound
7,344 posts
 
As you know the 60 W @ 4 Ohms of the XA30.5 was the Class A output, the XA-30.5 would actually put out another 130 or so Watts in Class B @ 4 ohms for a total of about 190 Watts @ 4 Ohms.
I really am a bit confounded by the ss Macs. Perhaps I'm mistaken, but I thought the use of autoformers prevented doubling down into halve impedance's?
unsound
7,344 posts
 
Good link! As far as I can tell from the discussion the ss Mac will only double down if the tap used is higher that the actual speaker demand, and that to preserve the amps stability that is to be avoided. To prevent the amp from being over stressed they use an autoformer with multiple taps, with the idea that one should use the tap that comes closest to the speakers load (I suppose that might be an issue with some speakers with wide impedance swings, the Thiels don't). When used as recommend the amp should not double down and therefore with lower impedance's would have lower dBW output. I could be wrong, but this would appear to be a high powered but not especially robust (in comparison to some other high end ss amps) amplifier design. If I'm mistaken, please anyone enlighten me.
pubul57
3,712 posts
 
"but I thought the use of autoformers prevented doubling down into halve impedance's?"

That what I thought, the point being that you would get more linear/constant power output throughout the bandwidth without variances caused by impedance swings.
cal3713's avatar
cal3713 OP
1,421 posts
 
As you know the 60 W @ 4 Ohms of the XA30.5 was the Class A output, the XA-30.5 would actually put out another 130 or so Watts in Class B @ 4 ohms for a total of about 190 Watts @ 4 Ohms.

~~~

Is it the case that many Tube amps don't provide more power as impedance dips?
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