Did "...Nelson Pass explicitly recommend it over the other comparably priced amps in the line." for the Thiel CS 2.4's? Thiel suggests 100-400 Watts for the 4 Ohm nominal 3 Ohm minimum 87 dB/2.8V/m CS 2.4's. |
Please forgive me for not answering your direct question, but I can't help but wonder if Cmalak is onto something. You might get more satisfaction from putting the money towards a higher powered amplifier. |
Cal3713, I don't read that as Nelson Pass specifically recommending the XA30.5 over the rest of the comparably priced Pass line for the Thiel 2.4's, only as a recommendation below 20 Watts. Seems like a highly qualified recommendation for a speaker that's manufacturer recommends a minimum of 100 Watts. I will grant you that the XA30.5 does seem to behave more like a very high biased 100 Watt "double down" Class AB amp, than a 30 Watt limited pure Class A amp. Perhaps capable of 75 Watts of Class A power into 3 Ohms before sliding into another 225 Watts of Class B power into 3 Ohms? On the other hand, I'll hazard a guess that the X250.5 is probably biased in such a manner as to be able to produce about 18 Class A Watts into 3 Ohms before sliding into about 730 Class B Watts into 3 Ohms? If you have a smaller room and/or don't listen very loud, the XA30.5 could very well work very well for you. |
The impedance and phase angle would be challenging for most tube amps. Getting a tube amp with enough power to handle the load can get expensive. A tube pre that matches the input of the amp could work nicely. |
Cal3713, went back and read the entire thread. I still don't see a direct recommendation from Nelson Pass for the XA30.5 for the Thiel CS 2.4's. The OP does seem to vastly prefer the XA30.5 to the X250.5, but with the caveat of not having to play loud in a large room. The thread also points out that the Thiel Cs 2.4's spend much of their time below 4 Ohms. Aparently others there have had similar conjecture as I had re: output Class bias.
...But enough of that. I think the a Pass pre would be just the ticket. What ever you choose, I'd suggest using a fully balanced pre. |
Cal3713, the XA30.5 is quite a bit different than typically rated 30 Watt amplifiers, though I think I would choose a different Pass amp, it could very well work very well for you with your Thiel CS 2.4's. |
Shsohis, despite it's (confusing?) nomenclature and marketed specs, the Pass Labs XA30.5 behaves more like a very high biased 100 Watt high current Class AB amp than a 30 Watt limited Class A amp. While perhaps at minimum power needs of the Thiels, it should be adequate in a smaller room and/or with less ultimate volume requirements, and sound quite nice indeed. Others might feel the need for more power with these speakers.
Cal3713, sorry this has gone so off topic, but I hope you can understand that on some level it was bound to happen, and with the best of intentions. |
I don't take that much stock in the meters. In fact, I wish they would do away with them, and lower the prices accordingly. Keep in mind when you start to go out of Class A, and the volume demands more power, the power needs tends to increase exponentially, and rather fast too. |
Tomcy6, actually Stereophile measured 130, 195, 332 Watts into 8, 4, 2 Ohms respectively, before clipping (as defined by 1% THD). If one were to use the 2 Ohm measure and work up, it would start to look more like an 80 Watt high current Class AB amp. Since the OP speakers don't drop to 2 Ohms, I thought we could be a little more generous with a 100 Watt high current Class AB designation. All in all, still rather close to Thiel's minimum recommended power. |
Nothing personal, just for the record.:-) |
Are you limiting your considerations to tube pres? |
I can see advantages for either ss or tubes, but the Pass input impedance and preferred use of balanced inputs might make for limited tube choices. I think the a Pass pre is a no brainer. Tube options include Sonic Frontiers Line 3, and though I'm less familiar with them, the Atmasphere pre's look like suitable candidates. |
FWIW, some years ago David A. Wilson writing in the Absolute Sound, suggested that a low output impedance DC coupled pre into a low input impedance DC coupled input impedance amp could offer the best Xc factor. |
Yes, many if not most tube amps actually loose power into lower impedance's, and to be fair, most ss amps loose power into higher impedance's (the ss McIntosh, when using the appropriate taps, appears to be an exception to both). |
Not to suggest that good bass it the number one priority, but IME without good solid bass response the Thiels can sound a bit tipped up. I think the part of the appeal of the Thiels, is due to the outstanding balance of all paramaters, rather than any specific one. FWIW, I wouldn't completely ignore the bass, it can overlap into other areas of appreciation. |
I'm not surprised, but at least you absolutely know for yourself, nothing beats that. If you like Nelson's Class A offerings, but your power requirements and budget only allow for Nelson's Class AB amps, you might want to consider his older Threshold S series amps. They were biased to stay into Class A for the first 20% into their 8 Ohm rating before sliding into Class B, however when they double power down into lower impedance's i.e. 4 Ohms, the biased is halved as well. Still this more Class A bias than the Pass X series, and even with the cost of new caps, re-bias etc., less expensive too. Just a thought. |
As popular as the Aleph's were/are, they weren't my favorite Pass amps (though I do love the industrial pragmatic look). FWIW, I found them lacking in dynamics. Keep in mind that you wouldn't be getting much more power into 4 Ohms than you currently are with the XA-30.5. Pass is to be commended for suggesting you buy an amp, whether from them or someone else, for your speakers, rather than switching your speakers. That kind of refreshing integrity is priceless. With that said, and within your budget considerations, the Pass X250.5 seems like very sound advice. FWIW, that's what I would have suggested you started with from the beginning. I suspect you'll get a good price for your otherwise very fine XA-30.5 from someone who truly understands the amp, and has the appropriate speakers/room to appreciate it. It might have been a little costly, but I imagine that the journey has become somewhat interesting and educational? At the risk of appearing redundant, don't be afraid to consider an older refurbished Threshold. They don't pop up all that often (people tend to hang on to them), but you might be surprised by the high quality value they offer. You can probably resell it at it's cost to you in short time if you don't care for it, the only penalty being it's admittedly not inexpensive shipping costs. |
Thiel has used a variety of amps at shows over the years. When I questioned them about it, they admitted that they didn't always use their preferred amps, that often times it was about what the local dealers provided, or about trying to demonstrate that though not necessarily optimal, their speakers could be used with a variety of amps, including some less expensive ones in an effort to promote sales to an audience that might otherwise be scarred off by the thought of specific amp requirements. They admitted that they would have preferred to show their speakers off to their best ability, but due to logistics or other agendas, they didn't always do so. Though not specifically said; I suspect the smart business strategy of maintaining some give and take as well as just plain diplomacy with various manufactures might have come into play as well. |
I'm not really sure how the Mac autoformers would work with Thiels. For that matter, I'm not really sure I completely understand how the autoformers work at all with the ss Macs. Wouldn't the dB W's go down with the speakers impedance demands, just as they're demanding more? If so, might that make the speakers sound bright? Or would the rather contained impedance swing of the Thiels negate that issue? Other than to keep heat down, what's the point? Wouldn't proper heat sinks deal with that? Would someone please explain it to me? |
A Chinese amp with an unusual tube, that will put out less power than the amp you currently find to inadequately power the speakers your driving? Good luck with that! |
Does that mean it would provide the same power even if the speaker is demanding more power? Let's keep in mind, that it isn't power alone that the Thiels are asking for. |
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? |
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, I'm not sure that such sustained power delivery is actually "more linear/constant". Perhaps I'm mistaken, but I would assume as the impedance drops so does the sensitivity, therefore the speakers would need extra power to compensate. As such, I would think that the ability to "double down" would actually cause less variances in the audible bandwidth. I believe Thiels typically demanding low impedance is at least in part due to efforts to keep a steady fairly constant impedance. I think this steady load is why some report success with Thiels and some (usually high powered) tube amps. It appears to me that the recent use of concentric drivers in Thiels newer designs, though having a lot of pluses, seems to have introduced more difficult phase angles with even lower impedance (though with somewhat higher sensitivity ratings) than their older designs with more conventional drivers, making tube amp choices more challenging. |
Thanks Al. Interesting how in the example you provided the dBW's go up and then down with impedance halving. Some how I get the sense that McIntosh just fell back on some of the technology they previously used with their tube amps, and that it's a bit of a cost cutting approach, to avoid spending on expensive heat sinks, etc. Interesting that in the decades following Mac's introduction of ss amps, very few (any?) seem to have followed this with approach. |
It occurred to me that back when Mac first started building ss amps, the transistors of the time were not nearly as durable as they are today, and perhaps to maintain reliability they used autoformers to keep them from getting stressed. Speakers of the time were probably higher impedance loads designed for the tube amps that were prevalent at the time. Using the autoformers with the those high impedance speakers also permitted them run more power into them. |
