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Class D Switching Power Amplifiers and Power D/A conversion

IcePower 300ASC vs 125ASX2
IcePower 300ASC vs 125ASX2
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Old 21st March 2019, 03:22 AM   #1
tengizk is offline tengizk  United States
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Default IcePower 300ASC vs 125ASX2

To my astonishment, 300ASC seems to be less forgiving than 125ASX2 when driving Martin Logan electrostatic speakers. I mean I tried to switch to 2 x 300ASC from stereo 125ASX2 and it does not work - 300ASCs (both of them!) go into protection mode when the music is loud enough, but 125ASX2 does not when playing the same tracks on the same loudness (verified with an SPL meter) and the power is well below 100 wpc as far as I can tell.

Since my speakers have powered bass section, it's something in the ES panel that makes 300ASCs freak out.

I carefully checked the amps spec - I am not seeing why it happens, 300ASC overall is obviously more capable amplifier than a single channel of 125ASX2, except the maximum current draw - 20 amps vs 30 amps for 125ASX2. Which does not make sense to me since both models are supposed to work fine with 3 ohm loads. The max load capacitance is also similar. Both have soft clipping.

What am I missing here?

Thanks!
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Old 21st March 2019, 10:56 AM   #2
chebum is offline chebum  Poland
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IcePower 300ASC vs 125ASX2
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Originally Posted by tengizk View Post
What am I missing here?

Output impedance. Output impedance of 300ASC is growing rapidly towards high frequencies (x400 higher @ 20Khz than @ 50Hz). Your speakers impedance is falling towards high frequencies. This both means current draw radically increases towards high frequencies. Hence protection mode for 300ASC.
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Old 21st March 2019, 08:33 PM   #3
e_fortier is offline e_fortier  Canada
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IcePower 300ASC vs 125ASX2
I believe you nailed it but seems to be about x100 (not x400)

About 0.006 ohm up to 300Hz and climbing to 0.7 ohm at 20Khz.

I love my 300AS1/300A1 combo but I don’t have electrostatic speakers.

BR
Eric
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Last edited by e_fortier; 21st March 2019 at 08:36 PM.
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Old 22nd March 2019, 01:48 AM   #4
tengizk is offline tengizk  United States
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Thank you!

So the difference is in the steepness of the incline? 125ASX2 shows output impedance going from ~.02 at 500Hz to ~.3 at 20k. In other words the maximum output impedance figure is not dramatically different from that of 300ASC (~0.7).

How does it exactly lead to the observed behavior? I mean, it was not a signal generator output at 10k+ that triggered it, it was music with the typical rapid decline in the power spectral density towards the higher frequencies above the voice range.

But, it's a moot point anyway - regardless of the reason there is no fix for this in my particular case. Will ned to re-purpose the 300ASC modules for something else.
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Old 24th March 2019, 10:35 PM   #5
kevinh is offline kevinh  United States
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Electrostatic speakers present a capacitive load to the amp.
So lower impedance at higher frequency, greater phase angle.
Guessing that particular ICE module in unhappy with a low impedance capacitive load
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Old 25th March 2019, 08:43 PM   #6
tengizk is offline tengizk  United States
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Originally Posted by kevinh View Post
Electrostatic speakers present a capacitive load to the amp.
So lower impedance at higher frequency, greater phase angle.
Guessing that particular ICE module in unhappy with a low impedance capacitive load
Yes, I realize that. I can trigger the protection mode when driving my ESLs with any amplifier that I have (both class D or AB) with a sine wave as long as the frequency/level of the signal is high enough, but it's only the 300asc that freaks out when I play music that other amplifiers handle just fine. I just don't see it in the specs what exactly is the reason.

Last edited by tengizk; 25th March 2019 at 08:46 PM.
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