Power amplifiers that excel with very low impedance - and low efficiency loads

a.wayne said:

W

No they dont , you should stop winging it like this Serge, the 501 heatsinks are woefully inadequate and will have thermal issues on low Sensitivity loudspeakers (can use up to 8 times the avg watts for normal listening) and no the 1/3 power level pre conditioning is very rarely ever failed by competently designed amps with proper SOA parameters ..!

Yes very rare to see a thermal failure like that and worse in such a short order ..



Regards

PS: The 1/3 power preconditioning test became order due to the woefully designed High output SS amps of the time , specifically Phase Linear. The very popular power unit by Bob Carver had a pension for failing into a fireball and was woefully heat sinked and no SOA to talk of..

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Besides my own pair of 501s, I have a friend who also owns a pair and uses them with Sonus Faber Stradivari speakers. I was running my pair with Guarneri Homages which were not overly demanding. Not once did our amps shut down in a decade.

Then there is at least a few dozen more guys over on that other forum and no one has every had any issues....

McIntosh amps are legendary in their own right and with over 50 years of history and experience under their belt, the company knows what it is doing when it comes to amps. The Autoformers are there to impedance match the load to the amp section closer and you get the 500w of power whether your speakers are 8, 4 or 2 ohms. Try that with your own amp...

I don't own 501s anymore, I've had all kinds of gear come and go but I would not hesitate to come back to McIntosh if one day I started getting nostalgic about the amps with big blue meters.

D'Agostino S250 - small form factor - will drive anything and looks perty

XV-1 said:

D'Agostino S250 - small form factor - will drive anything and looks perty

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D'Agostino amps will definitely deliver. I also enjoyed the Momentum Monos with 1600w into 2 ohms. Talk about excelling with low impedance. I had them driving the Wilson Alexia and while that speaker did see some dips below 2 ohms if I recall, the amps never broke a sweat or got hot. Of course the price for the pair... well, we know how that goes in audio. Let's just say I paid less for a BMW 540i back in the day.... Nuts.

Jim, you haven't said what specific speaker you're trying to drive here. If I'm reading between the lines, or taking it to the extreme of your stated requirements, I'd be tempted to think you're working with Apogee Scintillas with their infamous 0.6-1.0 ohm resistance... Back in the day the big Krells were the amp of choice for those. Today I'd be thinking of something like a Pass X600.8.

Now, I've never tried to drive anything with that low of impedance and low sensitivity in real life - even my Focals don't drop below 2 ohms, and they're reasonably efficient too - so my thoughts here fall firmly in your forbidden "hearsay" category. But the engineering physics suggest that the key for driving really low impedance is 1) hoping you have a mostly stable resistive speaker load rather than inductive/capacitive, 2) plenty of cooling capacity on the amp, and 3) don't want to listen too terribly loud.

TheOctopus said:

Besides my own pair of 501s, I have a friend who also owns a pair and uses them with Sonus Faber Stradivari speakers. I was running my pair with Guarneri Homages which were not overly demanding. Not once did our amps shut down in a decade.

Then there is at least a few dozen more guys over on that other forum and no one has every had any issues....

McIntosh amps are legendary in their own right and with over 50 years of history and experience under their belt, the company knows what it is doing when it comes to amps. The Autoformers are there to impedance match the load to the amp section closer and you get the 500w of power whether your speakers are 8, 4 or 2 ohms. Try that with your own amp...

I don't own 501s anymore, I've had all kinds of gear come and go but I would not hesitate to come back to McIntosh if one day I started getting nostalgic about the amps with big blue meters.

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Owned many myself as far back as 1977 , pretty well informed on the brand and i don't doubt your claims on reliability etc , still doesn't cover for the very poor showing as tested by JA ..!

That was pretty disappointing for such an expensive amplifier ..!


Regards

Thanks for all of your replies!

The speakers are a fairly new brand, from outside the USA.

While my client doesn't want to be involved with pointing out publicly an issue about which he is concerned, and perhaps causing a bad outcome for the new manufacturer, I can say that this is the bass driver in a multi-way high end speaker.

The driver's impedance is 1.4 ohms.

He has been thinking of using direct access to only that driver with an appropriate amplifier. The remainder of the sound (maybe 80%) from that speaker is quite nice, it's only problematic with the inefficient/low impedance bass driver in action.

a.wayne said:

Owned many myself as far back as 1977 , pretty well informed on the brand and i don't doubt your claims on reliability etc , still doesn't cover for the very poor showing {of the Mac MC501} as tested by JA ..!

That was pretty disappointing for such an expensive amplifier ..!

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I don't see where that's coming from: JA's measurements for the MC501 see excellent and he said as much.

Specifically JA said ...

Jim Smith said:

Thanks for all of your replies!

The speakers are a fairly new brand, from outside the USA.

While my client doesn't want to be involved with pointing out publicly an issue about which he is concerned, and perhaps causing a bad outcome for the new manufacturer, I can say that this is the bass driver in a multi-way high end speaker.

The driver's impedance is 1.4 ohms.

He has been thinking of using direct access to only that driver with an appropriate amplifier. The remainder of the sound (maybe 80%) from that speaker is quite nice, it's only problematic with the inefficient/low impedance bass driver in action.

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Since the offending impedance pertains only the bass driver, then I'd wonder and it be driven by a separate amp, (bi-amped). In that case perhaps a "pro" amp could used, (as suggested earlier). A more euphonic amp can be used to the higher frequency drivers.

Feanor said:

I don't see where that's coming from: JA's measurements for the MC501 see excellent and he said as much.

Specifically JA said ...

"The McIntosh's maximum output power depended on the output tap chosen and the load, but when the tap was matched to the load, the amplifier easily exceeded its specified 500W at our 1% THD definition of clipping. ..."​

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Read thru again its there and glaring ...! As discussed by both Serge and myself ..



Regards

Feanor said:

Since the offending impedance pertains only the bass driver, then I'd wonder and it be driven by a separate amp, (bi-amped). In that case perhaps a "pro" amp could used, (as suggested earlier). A more euphonic amp can be used to the higher frequency drivers.

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Euphonic ..!


So you are suggesting biamping with two class D amps for Proper Euphonics ..!

Jim Smith said:

Thanks for all of your replies!

The speakers are a fairly new brand, from outside the USA.

While my client doesn't want to be involved with pointing out publicly an issue about which he is concerned, and perhaps causing a bad outcome for the new manufacturer, I can say that this is the bass driver in a multi-way high end speaker.

The driver's impedance is 1.4 ohms.

He has been thinking of using direct access to only that driver with an appropriate amplifier. The remainder of the sound (maybe 80%) from that speaker is quite nice, it's only problematic with the inefficient/low impedance bass driver in action.

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Is his main amps tubes or SS , does he want one amp for everything or active Bi amping..?


Regards

Jim Smith said:

He has been thinking of using direct access to only that driver with an appropriate amplifier. The remainder of the sound (maybe 80%) from that speaker is quite nice, it's only problematic with the inefficient/low impedance bass driver in action.

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too much of a concept, skip it!
even in the rare case of a harmonic outcome, it would be very inefficient cashwise.
again, skip it!!

a.wayne said:

Read thru again its and glaring ...! As discussed by both Serge and myself ..

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Well yes, sure: the amp shuts down after only 5 minutes at 1/3 power which isn't ideal. As Atkinson says, the heatsinks are small for an amp of that power so maybe that's the reason, (or maybe the temperature sensors are miscalibrated).

But as Atkinson implies, continuous running with sinewaves isn't going to happen in a home environment, or any environment relevant to audiophile listening. Some applications might need you to run continuously at very high power but that's what "professional" amps are for.

Ought a $8200+/pair amp to have larger heatsinks? That's a consumer call; McIntosh's judgement that is that their target buyer would rather have somewhat small, lighter amp that is nonetheless able to produce rate power under realistic listening conditions.

Bill ,,

SOA and temperature drift ( lack of temp stability) is very important to the sound of an amplifier bias stabilization. This amp will be in full class B mode as it gets hot and this will increase distortion and reduce sonics , not because some don't or cant hear it , doesn't mean it’s not there , some of us do require some class A bias to enjoy our music ..!



Its this reason for the preconditioning to get the amp up to operating temp then measure , this was not possible with this Mac , so JA was being political and disingenuous with his comments , understandable in todays zeitgeist ..!

Stereophile 25+yrs ago under Archibald would have canned them on the spot and out the door..


Regards

u-sound said:

too much of a concept, skip it!
even in the rare case of a harmonic outcome, it would be very inefficient cashwise.
again, skip it!!

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So get a boring speaker like most ....

a.wayne said:

Euphonic ..!

So you are suggesting biamping with two class D amps for Proper Euphonics ..!

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Not at all. My choice might be class D amps, somebody else's might be a Atma-Shere MA-1's

Thanks again for all of your replies. It looks to be an exceedingly difficult issue for him to overcome, but at least there have been some interesting suggestions.

a.wayne said:

Read thru again its there and glaring ...! As discussed by both Serge and myself ..



Regards

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Wasn’t a problem with listening to music then, with the 501 and McIntosh did not address that which is not a problem for real world applications in the future generations.

Here is a more recent review of a McIntosh stereo amp with auto formers.Read the review and notice how many times the word “superb” comes up…

“Before doing any testing, I preconditioned the MC462 by running its 8 ohm output at one-third power for 30 minutes into 8 ohms—thermally the worst case for an amplifier with a class-AB or class-B output stage. At the end of that time the heatsinks were very hot, at 158.6°F (70.3°C). I usually precondition amplifiers for an hour, but I was concerned that the MC462 would get even hotter.”

“McIntosh specifies the MC462 as being able to deliver 450Wpc (26.5dBW) into a load matched to the nominal output Autoformer tap. With clipping defined as being when the THD+noise reaches 1%, fig.6 indicates that the MC462 exceeded its specification even with both channels driven, its 8 ohm output clipping at 516Wpc into 8 ohms (27.1dBW). The trace in this graph stops at 1%, as that is when the amplifier's protection was triggered. Into 4 ohms (fig.7), the McIntosh's 8 ohm output clipped at 720Wpc (25.6dBW). It's fair to note that I don't hold the wall voltage constant for this test; with both channels clipping into 4 ohms, the wall voltage had dropped from 121 to 115.4V. The MC462's 2 ohm output delivered 190Wpc (22.8dBW) with both channels driven into 8 ohms at 1% THD+N, 298Wpc with both channels driven into 4 ohms (21.7dBW, fig.8), and 536W (21.3dBW) with one channel driven into 2 ohms (fig.9).”


Summing up the McIntosh MC462's measured performance is easy: It is an extraordinarily well-engineered, exceptionally powerful amplifier.—John Atkinson




McIntosh Laboratory MC462 power amplifier Measurements | Stereophile.com

Serge you don't even pretend to understand or care , hahaha ..


Im now beginning to understand how such a stalwart company is laughed upon in the modern hi-end era and considered junk by many..

:disbelief:

Regards

Jim Smith said:

Thanks again for all of your replies. It looks to be an exceedingly difficult issue for him to overcome, but at least there have been some interesting suggestions.

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I don't think its remotely difficult Jim , but you already know that .. :thumbsup: