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My 50W, 8ohm amp can't drive my 86db, 4ohm speakers? WTF??

OK, I am only writing this tongue in cheek.  Recently my integrated amp stopped working so while I wait for it to get repaired I just tried to use my 10 year old Sonos ZP-100, rated at 50W, 8ohms to drive my Sonus Faber Guarneri Evolutions speakers.  I knew the results  would not be stellar but I just love listening to music everyday I was hoping the Sonos would get me by for a few weeks.  Sonus Faber speakers are notorious for being power hungry and my Evolutions did not disappoint in this regard.  These speakers are rated at 25-200W, 86db, 4ohm.  At low volume the sound is alright but even at moderate levels, around 70-75db I start to hear what sounds like a slight buzzing/distortion at the upper mid range.  Its not really obvious unless you are familiar with the song.  In looking at the review Stereophile did of my speakers, the impedance does indeed drop to 4 ohm at 200hz and 4000hz so what I am hearing makes sense.  I assume this is my Sonos clipping?  If so, I am surprised that it would clip at such low volumes.  I thought clipping occurs at higher volumes?  I knew the Sonos wouldnt be able to really drive my speakers but I did not expect it to sound so bad at such a low volume.  Oh well...I will keep counting the days until I get my amp back.
128x128tboooe
tboooe's avatar
tboooe OP
1,009 posts
 
Ha ha ha.  My ears are equally offended!  I hope to have my amp back in 2 weeks.  In the meantime I will occupy myself by buying a sub and more room treatments.  The audiophile spending machine waits for no one (or amp)! 
zufan's avatar
zufan
557 posts
 
Pair of Zu will fix that right quick  ;)
mofimadness's avatar
mofimadness
4,944 posts
 
"My 50W, 8ohm amp can’t drive my 86db, 4ohm speakers?"

Really...I’m shocked :-)
bombaywalla's avatar
bombaywalla
1,958 posts
 
tboooe,
I've written about this before but let me write this again - there are watts & then there are watts. All watts, IMO, are not created equal. What i'm talking about here is the power amp's ability to deliver load current. 
For example you could have a 50W/ch into 8 Ohms using a 200VA transformer. A 50W/ch amp most likely uses a 20VA secondary voltage in the power transformer. So, a 200VA power transformer can deliver 10A to 2 secondaries (stereo amp). Thus, each channel gets 5A max. You could have another 50W/ch power amp that uses a 400VA power transformer. The ability to source load current just doubled meaning that the power amp with a beefier power transformer will most likely drive a lower impedance speaker better.
And, this is the difference you are seeing with your Evolutions vs. your Sonos amps.
Yeah, you are right that the amp would most likely clip at higher power levels.
There are 2 types of clipping - voltage clipping when you turn up the volume & the voltage swing on the output stage exceeds the DC power rail of the power amp. And, the 2nd is current clipping when the power amp does not have sufficient current to deliver into a low(er) impedance & (current) clips.
I don't know what that buzzing sound it - maybe current clipping. It would depend no the size of the Sonos power transformer  which would give you an idea of the max steady-state current possible. Peak current will be higher.
Like a wrote above the 50W/ch channel amp probably has a 20VA secondary which tells me that the DC rail voltage is +/- 28.3VDC unloaded. It probably drops a few volts when there is a music signal passing thru the amp i.e. when the amp is delivering a load current. Let's say the rail voltage drops down to +/- 25VDC. So, if your program material has peak voltages exceeding this rail voltage at 70-75dB SPL then, yes, the amp will clip each time the music signal exceeds 25V. With the SF being 4 Ohms & 86dB efficient it does not look likely. Thus, it could be current clipping (power supply not robust enough).  
 
tboooe's avatar
tboooe OP
1,009 posts
 
Looks like my Accustic Arts integrated amp is repaired.  The parts that failed were:

1) $83.99 - 15V Regulators
2) $49.60 - Rectifier

Total bill with labor came out to around $500.  Ouch!  However, its a lot less than buying a new integrated amp.  Hopefully this repair will keep my amp chugging along for at least 2-3 more years!  Then I think I will step up to a tube integrated!

As I am no electrical engineer I am not sure what these parts do and if these are common failures in amps.  I am just happy to be able to listen to my system again soon!
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