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Classe Sigma AMP5 Multichannel Amplifier Review

amirm

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#1
This is a review and detailed measurements of the five channel Classe Sigma AMP5. It was kindly sent to me by a member and originally cost US $5,000. I see one on ebay for $1,900 right now.

This is one heavy amplifier for a class D architecture:

Classe Sigma AMP5 Five-Channel Digital Amplifier Multichannel Amplifier Class D Digital Review.jpg


At first I thought it had a linear power supply and its transformer was the reason behind the weight. Alas, looking into other reviews and pictures inside, it uses a switching power supply. So all the weight is from the enclosure and likely the very thick front plate.

At first I was hoping that the glossy front where the logo is, is an LCD display with VU meters and such. But there is not. It is just for looks. I must say as high-end products go, it is not that attractive to me relative to the high cost of the enclosure. It has sharp corners which is not nice.

The back shows quality selection of parts:

Classe Sigma AMP5 Five-Channel Digital Amplifier Multichannel Amplifier Class D Digital Back P...jpg


I like the staggered speaker binding posts which makes it easier to plug things in. XLR connections is what I like to see in an amp in this price range and we have them. This amp was mated to their processor of the time which I suspect is discontinued longer than this amp.

Amplifier Measurements
I tested two channels (Ch1 and Ch2) using XLR input. Here is our dashboard:

Classe Sigma AMP5 Five-Channel Digital Amplifier Audio Measurements.png


Rather high second and third harmonics dominate causing the overall ranking to be middle of the road:
best multichannel high-end amplifier review.png


Nice to see precise channel matching though.

Frequency response showed unexpected results:

Classe Sigma AMP5 Five-Channel Digital Amplifier Frequency Response Audio Measurements.png


Researching I found out there is an analog to digital converter which explains the sharp cut off. I thought that meant DSP functions but there is none. Instead, the digital signal is used as part of a protection circuit. I can see that as a parallel path but not as the main path compromising the response this way. This amp came out in 2015 so it is not like we didn't have higher sample rates available then to at least go up to 48 kHz and get us flat audible response.

Signal to noise ratio is middle of the road as well:

Classe Sigma AMP5 Five-Channel Digital Amplifier SNR Audio Measurements.png


Crosstalk is "OK" but should be better:
Classe Sigma AMP5 Five-Channel Digital Amplifier Crosstalk Audio Measurements.png


Power vs distortion shows odd response at 4 ohm which gets worse at 8 ohm (see later):

Classe Sigma AMP5 Five-Channel Digital Amplifier Power into 4 ohm Audio Measurements.png


Very strange noise response at the start and the multiple sources of distortion take over, each causing their own non-linearities.

On the positive front, we have healthy amount of power. Not much more is available though in burst mode (typical of class D amps with their regulated power supplies):

Classe Sigma AMP5 Five-Channel Digital Amplifier Max and Peak Power into 4 ohm Audio Measureme...png


Here is power into 8 ohm:

Classe Sigma AMP5 Five-Channel Digital Amplifier Power into 8 ohm Audio Measurements.png


That step function is quite bizarre.

Worst measurements came when bandwidth was increased to 45 kHz and frequency sweeps were used:
Classe Sigma AMP5 Five-Channel Digital Amplifier Power vers Frequency vs Distortion and noise ...png


No way this amplifier is so well behaved. Instead I think there is high amplitude noise that is masking any distortion present (until clipping). We can see that if we perform a wideband frequency analysis:

Classe Sigma AMP5 Five-Channel Digital Amplifier 1 kHz FFT Audio Measurements.png


We see sharp increase in noise right above audible band. This is either caused by noise shaping in the ADC, the Amp or both and explains the previous graph.

There is naturally switching spikes (near 400 kHz) but I am disappointed that its level is just 8 dB or so below our fundamental. I like to see 20 dB or more attenuation.

Conclusions
This is a custom Class D design and switching power supply from Classe. It clearly shows deficiencies of the design and creating a market for Hypex to come and show how class D is done. Digitizing the input is not a good idea if there is no user benefit to it and there is none here.

We could get depressed about the above or look at copious amount of power this amplifier produces while remaining cool (a bit above room temp in testing). Since lack of power is the #1 reason amps sound bad, I am going to put enough emphasis on this as to not give it a failing grade.

Overall, I am neutral on the Classe Amp5. At current sale prices, you have about $400 per channel for 400+ watts of power. It may be hard to match this with Hypex amp so you decide if it is worth it or not.

------------
As always, questions, comments, recommendations, etc. are welcome.

Have not been to the garden for a week as I have been busy with our septic tank issues. Nor can we preserve anything because we can't use any water in the house. So here is a picture at the start of our harvest season:
Pears Corn and Squash.jpg


This is the first year we had good success growing corn. Had so much that we couldn't eat it all and some became starchy.

You see our most prized and delicious pears in there too. They turn that pink/red color and are the most delicate, juicy and sweet pears you have had. Alas, the tree produces at least a couple of hundred and they go from being barely ripe to overripe and useless in a week or two. We ate and canned a bunch. So will be enjoying them in winter.

About to spend a ton more money on the forum so donations are appreciated using: https://www.audiosciencereview.com/forum/index.php?threads/how-to-support-audio-science-review.8150/
 
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LTig

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#6
That's the matching amp to my Sigma SSP AVP which has XLR outputs for L+R and RCA outputs for the Center and Surrounds. Strange that the amp digitizes at 48 kHz samplerate since the SSP digitizes analog signals at 96 kHz.
 
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amirm

amirm

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Thread Starter #7
That's the matching amp to my Sigma SSP AVP which has XLR outputs for L+R and RCA outputs for the Center and Surrounds. Strange that the amp digitizes at 48 kHz samplerate since the SSP digitizes analog signals at 96 kHz.
I think it is 44.1 actually.
 

Vasr

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#10
Seems like all the Crown amp fans that want a multi-channel amp would flock to this. No fan noise would be icing on the cake. ;)
 

stunta

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#14
I think these reviews will get more people into gardening than buying the audio gear. That is a good thing I suppose.

Disappointing from a company like Classe. They used to be known for solid engineering.
 

Tom C

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#17
Classe has known some particularly dark days. Cut ‘em a break! At least they were tryin’! Which is more than you can say for some.
 
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#20
Very optimized.
Looks like each channel amp is just pair of 5-legged output devices under the MAX10 clip on heatsink. And pair of capacitors, at least they are 105 C rated.
I'd say it's barely hifi because in D class all this must interfere as hell. No separation, no shielding, switching PSU in same enclosure.
I prefer fully modular old-school power amps, with separate trafo windings if possible.
 

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