[Syan25]

I think what I read was referring to the impedance mismatching of say a source component to an amp -  I was reading it in reference to Burson Audio Buffer. There is mentioned that impedance mismatching can cause cables to sound different.
 
http://www.thehifijournal.com//full-reviews.php?2010_08_26_15_10_00_a5fad393b53cf64df61d5a4b68b940c2
 
It would be interesting to know more about that though

 

[BlackbeardBen]

Quote:

I think what I read was referring to the impedance mismatching of say a source component to an amp -  I was reading it in reference to Burson Audio Buffer. There is mentioned that impedance mismatching can cause cables to sound different.
 
http://www.thehifijournal.com//full-reviews.php?2010_08_26_15_10_00_a5fad393b53cf64df61d5a4b68b940c2
 
It would be interesting to know more about that though

I'm still skeptical, because the cable impedance (again, assuming sufficient gauge cables) is still going to be tiny in comparison with the load (amp) impedance, whether we're talking about impedance bridging or impedance matching (but obviously much more so when talking about impedance bridging).
 
The only time I've heard of verifiable difference in interconnect cables is for phonographs, which have a much lower signal than normal line level. That's why you need a dedicated phono amp or input on your receiver/preamp.  I've heard, but can't substantiate, that RFI can occasionally be a problem on unshielded cables.  But that's unrelated to the cable impedance - just to the very low power of the signal and the ability of the interconnect to act as an antenna.

 

[Syan25]

That makes sense to me. 

 

[JerryLove]

Quote:

You said length changes affect delay, but in any sensible scientific test that would be the first controlled variable considered, nothing else withstanding. By right if the lengths/cross sectional area and any other variables affecting resistivity are kept equal and all the cable needs to do is to transfer an electrical signal from point a to point b then there should be no differences in any cable regardless of the sensitivity of the instrument, isn't it? Don't get me wrong, I myself am a believer in cable upgrades for purely aesthetic purposes only (DIY ^^) but I have yet to find anything substantive to prove my POV right (and vice versa, most obviously). Has anyone done a comparative test of any kind with better measuring instruments than their golden ears?

 
I'm confused. A scientific test of what? The delay added by 950ft of wire would introduce a 1/1,000,000 second delay. The best a human ear could hope to catch would be about 1/1,000 delay. It had been asked what instruments could measure, and the answer is "a lot".
 
Now, what can we hear? We can here distortions in excess of 1%, so we would want to avoid wire with more than 5% of the resistance of the speaker they were connected to (this is the problem with low gauge wire).
 
We can hear delays over 1ms (though unless we are discussing bi-wiring with very different lengths wire: it's not clear to me that this would be relevant as a Left-Right difference), so we should avoid cable lengths that vary by more than 950,000 ft.
 
Actually: given the electrical loss that 180miles of cable would have, and the resistance of even very large wire over that distance: let's just avoid wire that long at all when it comes to speakers.
 

 

[BlackbeardBen]

Quote:

Quote:

You said length changes affect delay, but in any sensible scientific test that would be the first controlled variable considered, nothing else withstanding. By right if the lengths/cross sectional area and any other variables affecting resistivity are kept equal and all the cable needs to do is to transfer an electrical signal from point a to point b then there should be no differences in any cable regardless of the sensitivity of the instrument, isn't it? Don't get me wrong, I myself am a believer in cable upgrades for purely aesthetic purposes only (DIY ^^) but I have yet to find anything substantive to prove my POV right (and vice versa, most obviously). Has anyone done a comparative test of any kind with better measuring instruments than their golden ears?

 
I'm confused. A scientific test of what? The delay added by 950ft of wire would introduce a 1/1,000,000 second delay. The best a human ear could hope to catch would be about 1/1,000 delay. It had been asked what instruments could measure, and the answer is "a lot".
 
Now, what can we hear? We can here distortions in excess of 1%, so we would want to avoid wire with more than 5% of the resistance of the speaker they were connected to (this is the problem with low gauge wire).
 
We can hear delays over 1ms (though unless we are discussing bi-wiring with very different lengths wire: it's not clear to me that this would be relevant as a Left-Right difference), so we should avoid cable lengths that vary by more than 950,000 ft.
 
Actually: given the electrical loss that 180miles of cable would have, and the resistance of even very large wire over that distance: let's just avoid wire that long at all when it comes to speakers.
 

To compound things, a cable that long acts as a transmission line...
 
Interesting about the biamping/biwiring thought - I think the answer is something along the lines of, "Bass frequencies are so low that a 1 ms delay (or 1 ms too soon) is very small in comparison with the wavelength (on a decent set of floorstanders, a max of 200 Hz or so), so a 1 ms delay on both sides will have a negligible effect on human perception."  Oh, and that's not even accounting for the time that it takes the woofer cone to start moving.  Considering that some loudspeaker designs correct for the time delay in driver response of the woofers by staggering higher drivers to the rear (although not by enough to make much of a difference), a minor 1 ms delay in the woofer signal isn't necessarily a bad thing.  Thus, an early signal might be bad - but on the other hand the bass frequencies are more readily transmitted through the floor and furniture (where the speed of sound is higher) to the listener.  You don't hear of people complaining about feeling their bass before they hear it, do you?
 
Biwiring, now that's a whole 'nother can of worms...
 
Biamping, too, at least the passive kind.  It's funny how some of the naysayers (and I'm not talking two channels from a receiver, which is more or less nearly useless) don't understand the basic concept behind how joining two electrical circuits work.  There are people who say that passive biamping is useless because both amps are amplifying the whole signal - but they forget that the load presented to the amplifier matters just as much.  The upper range amplifier sees the high frequencies as being of low impedance, and the low frequencies - those below the crossover, which dissipates their energy into heat - as being of very high impedance.  Thus, it doesn't amplify the frequencies below the crossover to any significant degree (only what is dissipated through the crossover).  Vice versa, with the lower amplifier, it works the same.  So in terms of getting more power, it still makes sense to passively biamp.  Of course, there's other benefits to passively biamping - like gain control between the amps - and even greater benefits to active biamping (getting rid of the crossovers, for one).

 

[JerryLove]

I *would* be concerned about phase delay on a single speaker on two levels.
 
1) Something being some multiple of 180-degrees out-of-phase and causing cancelation.
2) Delays approaching 1ms between two drivers (let's set a not-quite-arbitrary delay at 0.5ms). For example, if the midrange and tweeter were 1ms apart, that's far enough for the later one to be perceived as an echo at the crossover point. In short: it would be bad.
 
My point being, however, that none of this is realistic in regards to cables. A properly low-impedance cable will transmit with far greater fidelity than the human ear can hear.

 

[BlackbeardBen]

I did a quick calculation to see how far sound travels in 1 ms - at sea level, a mere 13.5".  Now, some of the best loudspeakers ever made use staggered, separate bass towers that are normally located further than that behind the mid/high towers.
 
"Time alignment" (that's actually a registered trademark, but I'm referring to the generic concept) of the drivers would suggest that the woofer towers should be located in front - but the reverse is recommended (for the Infinity IRS).
 
"Time alignment" can be done in the crossovers (using inductors, I imagine), but I'm not sure what speakers, if any, use that.  And then we get back to the sound conducted through the floors/walls/furniture that I already mentioned.
 
Of course, Jerry, I'm just talking about traditional woofers in a three or more -way speaker; I can see midrange drivers posing an entirely different problem.  But none of them are an issue in the real world, as you mentioned.  Only reflections are going to be a whole 180 degrees out of phase; that is unless you've wired the top or bottom with the wrong polarity and not the other half of the speaker.
 
Anyway, how many people are biwiring or biamping (there's a whole 'nother potential area for time delays and such there), anyway?  It's not even an issue for headphones...

 

[JerryLove]

Quote:

I did a quick calculation to see how far sound travels in 1 ms - at sea level, a mere 13.5".  Now, some of the best loudspeakers ever made use staggered, separate bass towers that are normally located further than that behind the mid/high towers.

 In that case, a sound which came out of multiple towers would have a "room effect"-type echo (this, BTW, is one deliberate point of omni-polar speakers, which use the room to create these echos.
 
Where we *really* run into trouble is when the is <1ms difference and a different FR curve together (this is what happens when a speaker with poor off-axis performance gets first-order reflections to us within 1ms of the original sound.
 
This is not an issue with different sounds. If the bass drum arrives 2ms later than trombone, it makes no difference.
 

Anyway, how many people are biwiring or biamping (there's a whole 'nother potential area for time delays and such there), anyway?  It's not even an issue for headphones...

Neither is likely to do any harm. BiWiring does no good (unless your cable was mismatched to begin with), and passive bi-amping is of dubious usefulness.
 
and this is the section of head-fi that covers speakers

 

[BlackbeardBen]

Quote:

Quote:

/img/forum/go_quote.gif

I did a quick calculation to see how far sound travels in 1 ms - at sea level, a mere 13.5".  Now, some of the best loudspeakers ever made use staggered, separate bass towers that are normally located further than that behind the mid/high towers.

 In that case, a sound which came out of multiple towers would have a "room effect"-type echo (this, BTW, is one deliberate point of omni-polar speakers, which use the room to create these echos.
 
Where we *really* run into trouble is when the is <1ms difference and a different FR curve together (this is what happens when a speaker with poor off-axis performance gets first-order reflections to us within 1ms of the original sound.
 
This is not an issue with different sounds. If the bass drum arrives 2ms later than trombone, it makes no difference.
 

Anyway, how many people are biwiring or biamping (there's a whole 'nother potential area for time delays and such there), anyway?  It's not even an issue for headphones...

Neither is likely to do any harm. BiWiring does no good (unless your cable was mismatched to begin with), and passive bi-amping is of dubious usefulness.
 
and this is the section of head-fi that covers speakers

 
Ahem:

 
Quote:

Biwiring, now that's a whole 'nother can of worms...  
Biamping, too, at least the passive kind.  It's funny how some of the naysayers (and I'm not talking two channels from a receiver, which is more or less nearly useless) don't understand the basic concept behind how joining two electrical circuits work.  There are people who say that passive biamping is useless because both amps are amplifying the whole signal - but they forget that the load presented to the amplifier matters just as much.  The upper range amplifier sees the high frequencies as being of low impedance, and the low frequencies - those below the crossover, which dissipates their energy into heat - as being of very high impedance.  Thus, it doesn't amplify the frequencies below the crossover to any significant degree (only what is dissipated through the crossover).  Vice versa, with the lower amplifier, it works the same.  So in terms of getting more power, it still makes sense to passively biamp.  Of course, there's other benefits to passively biamping - like gain control between the amps - and even greater benefits to active biamping (getting rid of the crossovers, for one).

Passive biamping is not of dubious usefulness.  Once it's clear how an amplifier responds to a load (see above), it's easy to see in terms of available power how it still has benefits.  Like I said, there's also the ability to control the gain to each section of the speaker, and the ability to vary your amps.
 
You can come over to my place when I get back to the US in June/July and I can show you, if you'd like.  (I think I'm going to try to host a mini-meet.)  You'll also get the chance to hear speakers that the HD 800 and T1 could only dream of being even in the same league of, performance wise... 
 

 

[JerryLove]

 
Quote:

Originally Posted by BlackbeardBen /img/forum/go_quote.gif

Passive biamping is not of dubious usefulness.  Once it's clear how an amplifier responds to a load (see above), it's easy to see in terms of available power how it still has benefits.  Like I said, there's also the ability to control the gain to each section of the speaker, and the ability to vary your amps.
 
You can come over to my place when I get back to the US in June/July and I can show you, if you'd like.  (I think I'm going to try to host a mini-meet.)  You'll also get the chance to hear speakers that the HD 800 and T1 could only dream of being even in the same league of, performance wise... 
 

 
Of course passive bi-amping is of dubious usefulness.
 
Are we talking 2-way or 3-way? I'll assume 2 since we aren't mentioning tri-amping.
Where's the crossover? Given that and the music being listened to, on a second-to-second basis: what percentage of the power is being drawn by the tweeter? What does the ohm curve look like on the tweeter?
 
It's unlikely that your load is 50/50, so you won't see a 3db advantage to bi-amping. You'll see the heavier-load driver get the power the lighter-load driver was eating.
Or if the issue is the ohm load, that's almost entirely an issue from the lower driver. You add nothing there by splitting off the tweeter.

And once we leave the technical discussion that the *best case* is 3d (and real case isn't that much), we discover that in almost all cases a more powerful single amp is cheaper than two lesser amps used for bi-amping (and even that has assumed the best case dual-amp bi-amping. If you are just using a multi-channel amp to bi-amp, then you are also still dealing with the limits of the (smaller) shared power supply).
 
Gain control already assumes that the crossover is mis-set (and remember that using amps in passive bi-amp for gain control divides in two blocks, with no slope, at wherever the crossover point is). No, far better frequency control can be had at the EQ than with something so crude.
 
No. Passive bi-amping isn't entirely ineffective but, in the vast bulk of situations this discussion happens under, will have no signifigant advantage over having not bi-amp'd and will actually be inferior to having spent the same money on a single larger amps.

 

[BlackbeardBen]

Quote:

 
Quote:

Originally Posted by BlackbeardBen /img/forum/go_quote.gif

Passive biamping is not of dubious usefulness.  Once it's clear how an amplifier responds to a load (see above), it's easy to see in terms of available power how it still has benefits.  Like I said, there's also the ability to control the gain to each section of the speaker, and the ability to vary your amps.
 
You can come over to my place when I get back to the US in June/July and I can show you, if you'd like.  (I think I'm going to try to host a mini-meet.)  You'll also get the chance to hear speakers that the HD 800 and T1 could only dream of being even in the same league of, performance wise... 
 

 
Of course passive bi-amping is of dubious usefulness.
 
Are we talking 2-way or 3-way? I'll assume 2 since we aren't mentioning tri-amping.
Where's the crossover? Given that and the music being listened to, on a second-to-second basis: what percentage of the power is being drawn by the tweeter? What does the ohm curve look like on the tweeter?
 
It's unlikely that your load is 50/50, so you won't see a 3db advantage to bi-amping. You'll see the heavier-load driver get the power the lighter-load driver was eating.
Or if the issue is the ohm load, that's almost entirely an issue from the lower driver. You add nothing there by splitting off the tweeter.

And once we leave the technical discussion that the *best case* is 3d (and real case isn't that much), we discover that in almost all cases a more powerful single amp is cheaper than two lesser amps used for bi-amping (and even that has assumed the best case dual-amp bi-amping. If you are just using a multi-channel amp to bi-amp, then you are also still dealing with the limits of the (smaller) shared power supply).
 
Gain control already assumes that the crossover is mis-set (and remember that using amps in passive bi-amp for gain control divides in two blocks, with no slope, at wherever the crossover point is). No, far better frequency control can be had at the EQ than with something so crude.
 
No. Passive bi-amping isn't entirely ineffective but, in the vast bulk of situations this discussion happens under, will have no signifigant advantage over having not bi-amp'd and will actually be inferior to having spent the same money on a single larger amps.

 
The best case scenario for getting a twice as powerful single amp to run your speakers is 3 dB, too...

Okay, I'm talking the Infinity Renaissance 90, a four-way speaker, passively biamped two ways with the dual-coil woofer being powered by an Adcom GFA-555 and the mid-bass, mid, and tweeter driven by a Carver TFM-15CB.
 
First of all, find me an amp that's as powerful as the triple nickel (200 W/ch into 8 ohms, 325 W/ch into 4 ohms, very stable into 2 ohm loads (probably around 450 W/ch), and capable of handling 1 ohm transients) plus that Carver (100 W/ch into 8 ohms, 150 W/ch into 4 ohms) for less than the combined price I paid for them ($300)...
 
The Ren 90s are nominal 4 ohm speakers, and because of the dual voice coil for the woofer (that lowers the impedance at the woofer's resonance frequency), its impedance curve is close to flat at 4 ohms down to 20 Hz.  The mid-bass/mid/tweeter impedance rises as at higher frequencies, but I don't know exactly how much - there's no technical review for them online, although another speaker using the same planar midrange and tweeter (but different mid-bass) goes from just about 3 ohms at the mid-bass to mid transition (600 Hz) to just shy of 9 ohms at about 35 kHz.  I'm just going off the white paper Infinity put out for them and its small graph (of dubious accuracy) that only goes up to 2000 Hz.
 
Note that I've put the more powerful amp on bottom - that woofer eats current like there's no tomorrow, thanks to the dual coils.  It also puts out tighter bass than I thought was possible.  The Adcom drives the woofer absolutely flawlessly by itself. 
 
However, when I try to use just the Adcom to power the whole speaker, the bass just isn't there.  Hardly at all, anyway.  It's either a crossover issue (which I haven't had time to investigate for bad caps), or an issue where the Adcom just can't provide enough current to the woofer when it's driving the mids/highs to a given level - at low levels I would guess that's hardly the case.  It's not like I have a pair of 900 W into 4 ohms Krell monoblocks laying around to test that theory.   The Ren 90s are known for having light bass when underpowered.  Anyway, when it's hooked up just to the woofers, the Adcom has no problem whatsoever.
 
The Carver has no problem keeping up with the Adcom.  I don't know their respective gains, unfortunately.  However, for balanced highs/mids with the fixed output of the Adcom to the woofer, the gain controls on the Carver go to somwhere around 2 o'clock.  Like I said, gain control is one of the reasons to passively biamp, regardless of the other benefits or lack thereof.
 
I don't know how accurate they are, but the power meters on the Carver average around maybe -10 dB when playing at my limit of short-term hearing comfort.  Transients go higher, of course - big bass spikes, like in, say Angel by Massive Attack, go pretty high - but that's just energy dissipating into the crossover.  I've never seen the meters peg out at +3 dB, which is what I presume to be the safe maximum transient output (averaged over the time it takes the needle to move).  So - running out of power isn't close to being an issue for me with the Carver on top.  But, that's exactly what's important.
 
The planar midrange and tweeter drivers are extremely delicate (very thin Kapton diaphragms and aluminum traces) - bad clipping will destroy them almost immediately.  (This is one reason soft clipping tube amps were recommended by Infinity for running the top half of their speakers.)  The tweeter does have a self-resetting circuit breaker, which is good because they're usually the first driver damaged - but the midrange does not.  That's bad news if I were to run one amp for the whole speaker and the amp were to current clip the signal at midrange frequencies.  With separate amps, the current draw for the bass frequencies on the mids/tweeter amp is much lower thanks to the high impedance of the bass frequencies below the crossover - so the probability of current cliping is reduced to very small levels - not to mention that I can now monitor the amp's output with the meters.  The Adcom does have handy distortion level indicators that will light up as clipping begins to occur, however.
 
So I hope you understand what I'm getting at here.  There's more to it than just "MOAR POWER!".  Then again, we're not talking about the vast bulk of situations - we're talking about big vintage Infinitys and their insatiable appetite for power.

 

[JerryLove]

Quote:

Originally Posted by BlackbeardBen /img/forum/go_quote.gif
 
The best case scenario for getting a twice as powerful single amp to run your speakers is 3 dB, too...

 
When discussing wattage: it's not the best case, it's the only case. Twice the power will *always* be available, where in a bi-amp configuration, twice the power will usually not be available (unless the load is exactly 50/50 split). (obviously this assumes that all amps involved have power supplies capable of powering RMS at max wattage)
 
Quote:

First of all, find me an amp that's as powerful as the triple nickel (200 W/ch into 8 ohms, 325 W/ch into 4 ohms, very stable into 2 ohm loads (probably around 450 W/ch), and capable of handling 1 ohm transients) plus that Carver (100 W/ch into 8 ohms, 150 W/ch into 4 ohms) for less than the combined price I paid for them ($300)...

 
Since $300 is nowhere near retail for those two amps you must be discussing "price I found them for". In that case, my McIntosh, found at a garage sale for $150, has more power for less money.
 
And since the crossover on your speakers matches their sensitivity, you are now limited (in SPL) to whatever the weaker of the two amps can do. If your speakers are (say) 80db sensitive and 4 ohm, by using the Carver you've reduced your max SPL from about 104db (325W) to 101db (150W).
 
You've actually worsened your performance (from a wattage standpoint).
 
Now if you rip out your crossover we can have an entirely different discussion.

 

[BlackbeardBen]

Quote:

Quote:

Originally Posted by BlackbeardBen /img/forum/go_quote.gif
 
The best case scenario for getting a twice as powerful single amp to run your speakers is 3 dB, too...

 
When discussing wattage: it's not the best case, it's the only case. Twice the power will *always* be available, where in a bi-amp configuration, twice the power will usually not be available (unless the load is exactly 50/50 split). (obviously this assumes that all amps involved have power supplies capable of powering RMS at max wattage)
 
Quote:

First of all, find me an amp that's as powerful as the triple nickel (200 W/ch into 8 ohms, 325 W/ch into 4 ohms, very stable into 2 ohm loads (probably around 450 W/ch), and capable of handling 1 ohm transients) plus that Carver (100 W/ch into 8 ohms, 150 W/ch into 4 ohms) for less than the combined price I paid for them ($300)...

 
Since $300 is nowhere near retail for those two amps you must be discussing "price I found them for". In that case, my McIntosh, found at a garage sale for $150, has more power for less money.
 
And since the crossover on your speakers matches their sensitivity, you are now limited (in SPL) to whatever the weaker of the two amps can do. If your speakers are (say) 80db sensitive and 4 ohm, by using the Carver you've reduced your max SPL from about 104db (325W) to 101db (150W).
 
You've actually worsened your performance (from a wattage standpoint).
 
Now if you rip out your crossover we can have an entirely different discussion.

Yeah, I know 3 dB is the only case for doubling the power with a single amp.  I realize my wording wasn't right there.
 
Yes, that would be the "price I found them for" - and, yes, there are occasionally better deals to be had, if you spend long enough looking or get lucky enough (the two being intrinsically related of course).  What Mac do you have?  MC 2500?  MC 2600?  That's a heck of a deal, if it has more power than the Adcom and Carver together (465 W/ch RMS into 4 ohms - sorry, I was mistaken, the carver is 140 W/ch into 4 ohms, not 150).  The kind that some guys have wet dreams about...
 
Okay, I did some little math about the gains on the amps.  I said that around 2 o'clock (ranging 360 degrees from 6-6 o'clock) makes the Carver balanced with the Adcom.  The Adcom has a gain of 27 dB, and the Carver 39 dB (maximum).  So 39 dB /12 hours (max) * 8 hours = 26 dB.  Pretty damn close to balanced; 1 dB is barely perceptible.  So yes, it appears that the two halves are balanced in gain at that point.
 
But that doesn't explain how the Adcom alone puts out almost zero bass.  1 dB more gain on the highs was not the difference.  It was more like 20 dB or more, and at all levels, from very quiet to the limits of the amp.  It's not the terminal bridges - I checked them and also tried plugging the Adcom into the bottom terminals, and it resulted in the same performance.  It doesn't make sense that the Adcom was current limited, because then it wouldn't have any problems at low volumes (it did).
 
For the record, when listening to bass heavy music, with the Carver's gain set as I indicated, the Adcom's 1% distortion lights (at or just before clipping, well above the maximum rms power of the amp) light up before the Carver's meters indicate I'm approaching its transient maximum.  Just because this amp combo wouldn't allow me to play white noise as loud (as having a second 555 on top) or any given waveform above the bass/mid-bass crossover as loud (with just the single 555) doesn't mean that its practical available power isn't as high or higher (than the single amp) based on the energy content of the music being played.  Again, none of this precludes the benefits of having separate gain controls for the top an bottom, either.
 
The combined power of the amplifiers - minus the minor losses into the crossover - is still available.  (An active crossover and eliminating the bass/mid-bass crossover would only remove the small load of the crossover itself to the amplifiers, so the difference in power is quite small, notwithstanding other benefits.)  It's just not distributed evenly, which is exactly what Arnie Nudell and Gary Christie at Infinity recommended in the '70s, '80s, and early '90s for their speakers - a powerful solid state amp powering the woofers (or woofer columns), and less powerful Audio Research (or similar, but they used AR) tube amps for the midrange and tweeter planars.
 
I don't really want an AR (I wouldn't turn one down if it fell in my lap, like your Mac did) - in fact, what I really want is another original 555.  That'll give me exactly what I need to power a pair of RS-1Bs (or any other big Infinities) when I have the money and space for them.

 

[JerryLove]

Deals are, always, deals; and "the equipment I already got" has made you and me both create some... interesting configurations.
 
Looking at a purchasing decision, for less than the retail price of either I could get a more powerful Emotiva XPA-2.
 
More importantly, the XPA-2 costs about the same as a pair of UPA-2's, whose combined power is lower. I'd sooner buy a singe XPA-2 than a pair of UPA-2's.
 
As I'm sure you are aware: one cannot simply go by the max the gain dial will turn to in order to figure out where the limits of an amp are.

 

[BlackbeardBen]

Well, neither the GFA-555 (original version) or the Carvers have been available new for some time...  And yeah, I had them both when I got the speakers.  Previously I had gotten the 555 to power my Infinity Kappa 8s, which it does fine by itself.  The first of the three Carvers I've got (all the TFM-15CB) was to power the smaller RS-5s I inherited from my dad.  The second one was an accident (won two auctions I where I only meant to win one), and the third was just a deal I found at the local record store.  Honestly, I'm likely to get rid of the two that have iffy meters - I just don't need this many amps...
 
That Emotiva XPA-2 looks like a great amp at a great price - a modern version of the 555.  I must say I don't really like its looks though, the front panel looks like a computer component...
 
But considering that even at low levels the Ren 90s aren't properly balanced with the Adcom power them, I think there's something more to this than just the available current.