This thread is long since dead, but I thought I'd finish it off now that I've tried a Pass Labs X250.5 and compared it to my INT-30A.
The X250.5 had better bass, but in the end the 30A was just much more natural. Vocals (even on hip-hop) sounded more real, and the soundstage was better defined. So, after all the amp comparisons, I ended up keeping the INT-30A, despite the shortcomings. Although I did add a Thiel SmartSub to help put some additional bass in the system. |
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. |
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. |
Hi Paul,
I have no particular insight into that. But I note that the MC402 and MC501 are both described as double-balanced, meaning that each channel consists of two fully balanced amplifiers that are bridged together via the autoformer. I'd expect that a key factor in the choice of design approach was that the hardware required to implement a fully balanced amplifier that could provide the current necessary to support those kinds of high power levels into low impedance loads, using conventional approaches that don't use autoformers, would result in a large increase in their already very hefty size and weight, with cost increasing commensurately.
Best regards,
-- Al |
Al, do you think that McIntosh is on too something that other SS folks are not addressing? Obviously they have been doing this for a long time - is it a difference that makes a difference and all towards the positive, or a just a tradeoff like so many other things in audio design? |
Gentlemen, I believe that what the autoformers in the McIntosh amplifiers essentially do is that they allow the amp to maintain the very low output impedance of a solid state amp, while relieving the transistorized output circuits of the need to deliver progressively larger amounts of current into speakers having low nominal impedances. The autoformers apparently do that by causing the solid state output circuits of the amp (ahead of the autoformer) to see the same load impedance when an 8 ohm speaker is connected to the 8 ohm tap as when a 4 ohm load is connected to the 4 ohm tap, and as when a 2 ohm load is connected to the 2 ohm tap. Presumably that is accomplished by stepping down the voltage that is produced by the output circuits (ahead of the autoformer) such that the voltage to the 4 ohm tap is nominally 0.707 times the voltage at the 8 ohm tap, and the voltage at the 2 ohm tap is nominally 0.5 times the voltage at the 8 ohm tap. Based on P = Esquared/R, that will result in the same MAXIMUM power capability in all three situations, while at the same time eliminating the need for the output transistors to have to supply progressively larger amounts of current as the nominal load impedance is reduced. Thus the amps have the same maximum power ratings for 2, 4, and 8 ohm speakers, based on the presumption that the speaker will be connected to the corresponding output tap. For a GIVEN output tap, though, the variation of the amount of power that will be delivered into varying load impedances is, as usual, dependent on the output impedance of the amp, until the limits imposed by the amp's maximum power capability are reached. The output impedance of the circuit ahead of the autoformer is negligibly small, since it is solid state, and the voltage step down provided by the autoformer will make it even smaller, at least on the 4 and 2 ohm taps. (Transformers transform voltages in proportion to their turns ratio, while at the same time they transform impedance in proportion to the square of that turns ratio). Note that the MC402 and MC501 have specified damping factors of 40 or more, that number corresponding to an output impedance of 8/40 = 0.2 ohms, which is in solid state territory. So if a nominally 4 ohm speaker is connected to the 4 ohm tap, and its impedance drops to 2 ohms at some frequencies, for a given input voltage to the amplifier twice the amount of power will be delivered to the speaker at frequencies for which the speaker's impedance is 2 ohms, compared to frequencies for which it is 4 ohms. That does NOT, however, mean that that the amp's MAXIMUM power capability will double down. Current limitations and/or thermal limitations and/or limitations imposed by self-protection mechanisms will limit the MAXIMUM amount of power into 2 ohms to much less than twice the 4 ohm maximum power capability. Otherwise there would be no need for the autoformer. I note this comment in John Atkinson's measurements of the MC501: The 8 ohm tap was limited to 225W into 2 ohms (17.5dBW), for example, while the 4 ohm tap delivered 1000W (24dBW), the 2 ohm tap 630W (22dBW). So as long as the amp is operated within the limits of its ability to deliver power into given load impedances, it will behave like a solid state amplifier. Meaning that the tonal effects resulting from the interaction of its output impedance with variations of speaker impedance as a function of frequency will be similar to those of a typical solid state amp. However, the amp's MAXIMUM power capability will not double down, as it will in the case of many high quality solid state amps. Hope that clarifies more than it confuses. Best regards, -- Al |
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. |
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). |
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.
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Is it the case that many Tube amps don't provide more power as impedance dips? |
"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. |
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. |
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? |
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 |
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!
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Ha... well, I see you have a strong opinion on that! I thought I was comparing 60W (the XA-30.5 into 4Ohms) to 160W out of the EE. Still learning how to compare amp output though...
As for the Mac, all I read about was the response into varying loads from a single tap... so the way I read it, from any single output, they will double down in provided Watts as the speaker's impedance drops by half (i.e., goes 4 to 2 ohms). |
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. |
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! |
I thought the point of the autoformer was to provide the same watts into any load from a give tap? Load tolerant? |
Also, unsound... in a different thread, I read that the Mcintosh amps (at least the big ones) still double down as impedance drops. So basically, a 402 would give you 400W out of each of the different taps, but if you had a 4 Ohm load on the 8 Ohm tap you'd still pull 800W. It would just tax the amplifier a lot more than if you were attached to the 4 Ohm tap, where you'd be pulling 400W with the same exact load. That said, I don't understand the technology and am just regurgitating someone's anonymous internet post. |
After saying I wasn't interested in any tube amps, I did run across these guys and become interested... The eastern electric 160W mono-block tube amps:
http://www.stereomojo.com/EasternElectricM156TubeMonoblockAmplifiersReviewtm/EasternElectricM156TubeMonoblockAmplifiersReview.htm
They'd be cheaper than the 250.5 and provide a lot of power for tubes, and come with great reviews... |
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? |
Cal3713...if there is a way to audition the X250.5 before purchasing it (or maybe negotiating a time frame in which you can return the amp if it's not a good fit and just be out the shipping costs)?
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I'll just be buying used and hopefully reselling at a negligible loss if I don't like the combo (unless Reno gets a demo/used 250.5 in).
There are a number of McIntosh 402s up right now that are certainly interesting... also assuming that I could resell without too much loss there, so the purchase would be the demo.
I've been scared off of tubes. Everything technical about the match sounds bad, so the only real draw is the characteristic "tube sound." I'm also good at getting worried about non-issues, so I don't like the idea of having to worry about tube maintenance. |
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. |
I ran an X250 w/2.4's for a year or so & while they sounded & imaged fantastic on really well recorded/produced material they were really fatiguing on less than perfect material. I had a decent front end & all the recommended ancillaries so it wasn't that. Never got around to trying them w/a tube amp before I sold them & moved on. My guess is tubes are the synergistic combo |
Not to suggest that good bass is 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. Explains a lot of the stereotype, but also what some swear by Theil as among the best speakers ever made - got to have the right amp:) Agree with both those statements. It's funny that with my 250wpc MF SS integrated amp, I found the top end harsh and the overall sound a bit thin, whereas with my 110wpc ARC tube amp, I have a much more natural and neutral tonal balance without any portion of the frequency spectrum thrust forward or recessed. I think the power supply in the ARC amp was clearly superior to that of MF. Cal3713...if there is a way to audition the X250.5 before purchasing it (or maybe negotiating a time frame in which you can return the amp if it's not a good fit and just be out the shipping costs)? |
"without good solid bass response the Thiels can sound a bit tipped up." Explains a lot of the stereotype, but also what some swear by Theil as among the best speakers ever made - got to have the right amp:) |
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. |
Ohhh, and obviously the low end isn't my primary focus (hence even trying the XA30.5 to start with)... imaging is really the most important target as far as I'm concerned. |
The original owner of my 2.4s used a McIntosh MC-402... which he certainly loved (I think he was still going to use it with a new pair of 3.7s).
Wish folks had had a warmer response to the idea of the Aleph 2s, that would have saved over 1k over the 250.5. You all convinced me, however, that I'd probably end up reselling them sooner than later.
Thanks for the input. Despite being fairly sold on the 250.5, I am open to trying some other brands, so the other suggestions are welcome. |
Yes, I was mighty impressed with the ARC VT100, and I assume the Ref 110 would be better yet. ARC seems to also work well with Maggies - so difficult loads and tube amps that do seem to work well. I have never heard the combination, nor heard others comment on it, but it would seem the that the stereotypical sonic flavors of Thiel and MAC (6600 integrated would do) would make for a pretty good combination - detailed neutrality with smoothness, power and autoformers for handling impedance swings. Again, never tried it, but would be curious. |
Pubul57...Thiel has shown its speakers often with Bryston and Simaudio amplification at shows int he last 2-3 years. But again, as Unsound has pointed out, this could be more a function of local dealers, etc...I drive my 2.4s with ARC Ref 110 and love the combination. I am probably leaving a little bit of performance (dynamics and lw end extension) on the table but what I gain in terms of tone/timbre, separation, transparency/resolution, and 3D dimensionality is quite nice. I initially drove the 2.4s with Musical Fidelity's A5 integrated amp (250wpc into 8ohms and 500wpc into 4ohms) but the top end was a bit jarring and resulted in listener fatigue. I also tried out Simaudio P7/W7 (separates) and the i7 (integrated) for about a week each and liked the Simaudio/Thiel combo. |
I bet the Belles MB-200 Monoblocks up for sale right now would drive the heck out of them. |
Yup, there is that side of it too:) |
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. |
Owned the Aleph 3, 2, and J - and loved them, but I would describe them as amps that do best with speakers that are tube friendly - highish and smooth impedance - non are real current monsters, which is what I think a speaker like the Thiels or Maggies need.
Just out of curiosity, does anyone know what amplifiers Theil has used at shows the past 5 - 10 years? Likely a good match. I actually heard 3.6s with ARC VT100s a few years back and that sounded pretty darn good, so even a well designed tube amp with enough power should work. |
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. |
What do you folks think about something like a set of Aleph 2s? Quite a bit cheaper than the X250.5, not as much power, but at 200W into 4 ohms, they should be a better mate to the 2.4s than the XA-30.5...
I did mention briefly the possibility of going to something older like the Aleph 0 to the Pass folks and they pretty unequivocally said to do the 250.5... just because the older designs have so much more distortion. |
I have been using a Bel Canto REF1000M with my Thiel CS3.7s and the 2.4SEs before that.
I have heard the Electrocompaniet Nemos and they sound incredible with the Thiels.
I am a bit curious how the Plinius SA-Reference would sound with them. That delivers 100W of Class A into 8 ohms in stereo mode. |
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. |
Cmalak, all part of the learning process for OP, and sometimes you just have to hear things for yourself to know for sure. The XA30.5 is a magical amp, but not magic, the Thiels simply need more power to play their best. It is a conondrum, I often consider 20-30 watts Class A amps since I think they sound great with the right loads (Luxman for example)but I always fear I'm forcing the power issue and I should know better, even on my easy to drive and relatively sensitive Merlins. If I owned Maggies or Thiels, I simply would not consider driving them with the undoubtedly wonderful Pass XA30.5 and other "low" wattage SS amps, as much as I would like to since those amps sound so darn good at their power rating - but for those speakers give me a 200 watt MAC! |
Cal3713...thanks for the update. That was my concern in my post above. I have seen X350.5 often paired with the 2.4s to good effect. I am sure the X250.5 will give you plenty of power and current delivery to drive the Thiels with ease. Good luck with the journey and enjoy it. |
This thread's been dead for a while, but I thought I would chime in with some conclusions now that I've lived with the XA-30.5 and Thiel 2.4s for a while. Anyway, on these speakers the amp is spectacular, beautiful, quiet, delicate, with great imaging... on quiet, delicate music. As the music gets more complex and ads greater bass requirements, it just can't keep up. Certain disks and movies with that complexity (e.g., rap & hip-hop) just sound flat and uninteresting.
I've discussed the issues with some professionals... Reno-Hifi suggested that the XA-60.5 might ameliorate the problem, Pass themselves suggested (in this order) the XA-100.5, X350.5, and then X250.5. In talking to Pass I even mentioned selling the speakers and getting another well imaging speaker that mated better with the XA-30.5. They said to keep the Thiels... even with the implicit understanding that that might mean going with a different companies' amps.
In any case, I think I will probably try a 250.5 next (the only suggestion within my budget) and see what happens from their. Unfortunately the first pre-amp solution I tried was using an INT-30A, which is a nice one-box solution to my prior pre-amp issues (and cost efficient... just $1200 more than the 30.5). I just added an external phono... the Chinese Jasmine LP2 MKII (after trying a pretty poor sounding PS Audio GCPH). Anyway, I'd like to have done it right the first time, but the journey's been educational. And I guess shortly I'll still be looking for a pre to match to a Pass amp. Keep your eyes open for a mint INT-30A going up in the next couple months. |
The Atma-sphere MP3 would match up very well with the XA30.5, although it also worked very well single-ended with the Joule LA-150 MKII and Signature edition, both of which can be set to a lower output impedance. If the 2.4 is anything like the 2.3 and 3.6s I think the combination of the rich tones of the Joule would do wonders with a speaker as neutral as the Thiel, a little warmth would not be a bad thing. The combo sounded pretty darn good with my Merlin VSMs which are also not particularly forgiving speakers. You would not have suckage with either the Atma-sphere or Joule preamp mated to your Pass amp. |
Modwright LS 100 with built-in phono stage would be a possibility. It is a true value, as are all of Dan Wright's designs. |
Have you looked at the Thor ta2000 listed right now? I think he has it at around 2900 but you may be able to get it at close to your budget. I just bought the ta1000 and it is great. Great new owners of Thor to take care of any maintenance or upgrades you may be interested in. |
I totally agree with Unsound
I have the Thiel 2.4's and they need lotsa amperes to start to sing .. in fact I dive them with two Parasound JC1's
Forget the ridiculous power of the Pass Labs XA-30.5 .. kinda splendid amp for Harbeth C7ES or Super HL5 but totally useless on the Thiel 2.4. |
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. |
For educational purposes, is there a reason that low output impedance is "good" besides the fact that that's what the Pass requires for system matching?
And I do like the features of the Sonic Frontiers Line 3 that I was reading about... |
It isn't balanced but the extremely low output impedance is part of what sold me on the kW preamp. And its other measurements and the monster transformer didn't hurt either... |
Thanks unsound. Despite it's auspicious start, this thread has ended up being useful, both for recommendation and educational purposes... |
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. |
