ICEpower asx2 line - the practical THD performance - diyAudio
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Old 16th August 2011, 02:23 AM   #1
tengizk is offline tengizk  United States
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Default ICEpower asx2 line - the practical THD performance

Gentlemen,

Has anyone done the THD measurements for 125asx2 (or any other model in the asx2 line)?

My measurements do not confirm the spec'd performance characteristics of the 50asx2 when driving a 4 ohm load. Specifically, the THD is up to 3-5 times higher than that specified in the datasheet. The measurement tool I use is very crude, though (I use REW), but I did confirm that when driving 8 ohm load the measured TDH+N is very close to the numbers in the spec (less than 0.1% in the full audio range @ 20 watts into 8 ohm)

The actual numbers are around 0.1%-0.5% @ 40 watts into 4 ohm load (one channel driven), the worst result is in the 400Hz - 4 kHz range, but for the lower or higher frequencies it does fall below 0.1%. The THD is dominated by odd harmonics, with the 3-rd being the most prominent, but I can easily see up to the 9-nth, which it within less than 20 DB of the 3-rd depending on the frequency. I made sure that the amp does not go into clipping, of course, and use the ballast resistors with no inductance.

As Ive mentioned before, my measuring gig is very crude I feed 1/20th of the output of the amplifier to the regular PC audio card and then visualize the result with REW, which also serves as the signal generator.

Direct signal path from the PC audio card out to the line input is more or less decent and for the full audio range provides the THD+N less than 0.005%, so I do not think that the PC card is to blame. I realize that with the D class amplifier, the built-in anti-aliasing filter might be not enough, but in this case the folded carrier frequency with the harmonics would be all over the place and it would be a heck of a chance for them to align with the fundamental and harmonics of the test signal. Besides, when moving the test frequency one can easily see that harmonics follow, so its the actual harmonics and have nothing to do with the carrier.

So, has anyone else done any THD+N measurements for the ICEpower asx2 line? If so, can you share the findings?

Thanks!
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Old 16th August 2011, 03:10 AM   #2
Pano is offline Pano  United States
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Good question. I did a few last year, but did not save the results. All I remember seeing was H3, nothing much else. But I tested into 8 ohms.

The input impedance of the amp is a bit low and drops with frequency. Did you double check the distortion at the input? Of course that should change with the output load as yours did. But I'd check anyway.
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Old 16th August 2011, 06:28 AM   #3
tengizk is offline tengizk  United States
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Hi Pano. Yes, I made sure that the frequency response is more or less flat in the audible range - there is only a slight roll off above 15k down to -6 db or so @ 20k.

Another discovery that I made concerns the stability of the actual technical specs for different modules of the same model. I have a couple of 50asx2 and the THD performance is noticeably different between them. The numbers that I gave in the initial post are for one that is better. The other module has THD numbers approximately 50% worse give or take.

Don't get me wrong though - these are excellent plug and play devices that would make a beautiful desktop amplifier for small 8+ ohm monitors. But I was hoping that I could use them to drive my bi-wire able ESL speakers. 40 watts per transducer or 160 watts in total would be plenty for my listening room.

Now I am not so sure if getting 50asx2-s was such a good idea - my current setup (I have a Marantz NR1602 - up until recently it was NR1601) provides quality amplification (>50 watts per channel with less than 0.08% for the full audio range - I have actually measured it) which, apparently, is better than a couple of 50asx2 can do - despite the power stage of NR1602 is rated for 6 ohm load minimum, it works just fine driving the ESL panels with 1.2 ohms of impedance @ 20k when fed with music, of course. Full power test signal @ 10k or higher triggers the overcurrent protection, but it should never be the case for normal listening.

Basically, now I am considering getting 125asx2 or even 250asx2, but before I go ahead and buy one it would be nice to know if the THD performance for those is really what the spec says. Although with my power requirement (50-80 watts per channel) - seems like either would have plenty of headroom power wise.
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Old 17th August 2011, 01:05 AM   #4
Pano is offline Pano  United States
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I was very happy with the 125asx2 that I used until I dropped and broke it. And the bigger Icepower modules I've used sound just like that - bigger. I think there is something nice about the close coupling of the PSU and amp. It seems to do good things for the bass and dynamics.
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Old 17th August 2011, 03:30 AM   #5
trevmar is offline trevmar  United States
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I am doing some testing of various chips/amps right now and I am finding two things which increase distortion into 4 ohms, the circuitry limiting the switching current in the output FETs, and saturation of the output inductors. I have attached a typical set of saturation graphs, from quite a good Sagami inductor range. As you can see, the inductance of a typical 7G14A-200M falls rapidly beyond the knee. This would lead to clipping, and odd harmonics. The current is higher into 4ohms, I**2 = P / R

I am sure the ICE designer took all this into account in his/her original design, but one cannot be so sure that a purchasing manager might not judge two toroid cores as identical just because they are the same size and color
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Old 17th August 2011, 06:28 AM   #6
tengizk is offline tengizk  United States
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Thans for the responses, but it seems like it was a measurement error. I don't know what happened, but when I repeated the experiments (this time multiple times) I actually see that the THD numbers are even better than what is specified in the data sheet.

Namely, at 40 watts (12.6 Vrms) with one or both channels driven into 4 ohm load the THD+N never goes above 0.03% in the full audible range. For the lower frequencies (1k and down) it actually falls below 0.01%.

One thing that I've changed is the sampling frequency of the sound card from 48k to 44.1k. I also tried another program (ARTA) that allows the sampling rate up to 192k.

The bottom line is that now I am consistently getting much better TDH readings. The harmonics are well below 75-80dB relative to the fundamental of the test signal.
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