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

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.

Yup, there is that side of it too:)

I bet the Belles MB-200 Monoblocks up for sale right now would drive the heck out of them.

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.

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.

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.

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.

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.

"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 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)?

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

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...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.

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?

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...

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.

I thought the point of the autoformer was to provide the same watts into any load from a give tap? Load tolerant?

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.

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).

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

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.

"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.

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?

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).

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.

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

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?

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

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.

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.