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

Looking for recommendations for a client.

Please - submissions not from hearsay, but from actual user experience - Thanks!

Pair of Lumin AMP in bridge mode.

Luxman M900u. Stable to 1ohm.


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Well my Purifi amp has an output impedance of 67 microOhms and, per specs, is fine with 2 Ohm loads, delivering 450 watts

Accordingly Damping factor at 2 Ohms = 29,851, which poor if you like flabby bass

Originally Posted by Jim Smith

Looking for recommendations for a client.

Please - submissions not from hearsay, but from actual user experience - Thanks!

Whats the load Jim and desired listening levels ...?

Originally Posted by Jim Smith

Looking for recommendations for a client.

Please - submissions not from hearsay, but from actual user experience - Thanks!

Ah ... so client endorsements are not hearsay: I'm informed.

Originally Posted by Feanor

Well my Purifi amp has an output impedance of 67 microOhms and, per specs, is fine with 2 Ohm loads, delivering 450 watts

Accordingly Damping factor at 2 Ohms = 29,851, which poor if you like flabby bass

Doesn't have enuff heatsinks , outputs or PSU to sink current into a 2 ohm or lower loads , most Class D are really designed for 8 ohm loads with a low z of 4 ohm being optimum.


Regards

Originally Posted by a.wayne

Doesn't have enuff heatsinks , outputs or PSU to sink current into a 2 ohm or lower loads , most Class D are really designed for 8 ohm loads with a low z of 4 ohm being optimum.

Regards

In your OP you didn't give examples of speakers with "very" low impedance combined with very low efficiency (sensitivity). In practical terms, what portion of widely available consumer speakers meet these criteria?

I'd say there aren't very many. Meanwhile class D amps like Purifi, Hypex, and ICEpower typically lead with their 4 Ohm rates and are quite capable of driving such loads in normal consumer practice. Some unusual situation might justify fans for better ventilation but such a resort is not emphasized by the makers.

If you really want to drive very low impedance to high levels and/or with very insensitivity speaker, you ought to look at professional amps by Crown, QSC, et al.; most of these are designed for these circumstances. E.g. the QSC RMX5050A will deliver 2500 wpc into 2 Ohms and will do so all day.

Of course, these amps don't have much audiophile cachet, but they do the job. (If you want euphonic distortion, you can can put a tube preamp upstream.)

Speakers which meet such a low efficiency criteria is for a different debate , the focus right now is on amps which do meet the criteria necessary to drive such transducers..!

BTW i have tried most of those class D amps you listed at low Z they do not meet ( not speculating as you are ) the criteria , well not if Proper Timbre and natural sound is important they will work but mostly thinking where the Bel canto Black or Merrill’s element Class D approach may do the trick , Pricey and may not handle the thermal load , which is pretty much.


Regards

PS: Tried both the high powered QSC and Crown, their fans run on High constantly to handle the thermal load and not high fidelity sounding at all ..!

Originally Posted by a.wayne

Speakers which meet such a low efficiency criteria is for a different debate , the focus right now is on amps which do meet the criteria necessary to drive such transducers..!

BTW i have tried most of those class D amps you listed at low Z they do not meet ( not speculating as you are ) the criteria , well not if Proper Timbre and natural sound is important they will work but mostly thinking where the Bel canto Black or Merrill’s element Class D approach may do the trick , Pricey and may not handle the thermal load , which is pretty much.


Regards

PS: Tried both the high powered QSC and Crown, their fans run on High constantly to handle the thermal load and not high fidelity sounding at all ..!

So fair enough. Without specifying speakers that meet the criteria the question is ... uhmm ... academic.

Then too what's "proper timbre and natural sound" is, in my 50 years of hi-fi listening, highly subjective. A majority of audiophiles prefer euphonic to accurate, but not all.

Everything in audio is euphonic and to preference, it’s why so many colorful choices ..




Regards

You should read that report carefully ..

W

Originally Posted by TheOctopus

I did and I owned a pair of MC501’s for years. Thermal stability was never an issue. Bench testing at 1/3 rated power continuous makes many amplifiers heat up. McIntosh shuts down to prevent failure of components under thermal stress which is smart.

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..

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

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.

Originally Posted by TheOctopus

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.

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.

Originally Posted by a.wayne

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 ..!

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. ..."

Originally Posted by Jim Smith

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.

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.

Originally Posted by Feanor

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. ..."

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



Regards

Originally Posted by Feanor

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.

Euphonic ..!


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

Originally Posted by Jim Smith

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.

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


Regards

Originally Posted by a.wayne

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

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

Originally Posted by u-sound

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!!

So get a boring speaker like most ....

Originally Posted by a.wayne

Euphonic ..!

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

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.

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..



Regards

Originally Posted by Jim Smith

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.

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

Originally Posted by Jim Smith

Looking for recommendations for a client.

Please - submissions not from hearsay, but from actual user experience - Thanks!

Jim - I bought a pair of low efficient, low impedence Martin Logan electrostatic 13A speakers a couple of years ago, but after using Avantgarde for 15 years, I had no suitable amplifier. Looking at sources such as Martin Logan Owners forum, I went for the Sanders Magtech amplifier. Sanders build electrostatic speakers and found a dearth of suitable amps, so he designed his own with those particular features. It was a good amp for this type of speaker, but sadly the speakers never sounded good in my difficult room. Both the ML and Sanders were sold and I'm back with Avantgarde. My GamuT D200 also did well with the MLs. Peter

I heard the Sanders amps were built by either ATI or Coda, I can't remember which. I personally wouldn't use Coda for a bass amp but ATI are beasts. I'm sure Sanders were built to their spec though. The Coda use no feedback, they are high current but don't have the definition I like in the low end. I wish I had more time with the ATI in my system, I have never heard an amp with that much low end power at any price. A friend brought his over for a quick listen back when I had Revel F52's.