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On Class D Amplifiers Measurements
- Thread starter pma
- Start date
Except for one little thing, the 1701 (which I also use) is a great piece of gear. The little thing: noisy fan.
This isn't something I would worry about, especially since almost everything I play is 44.1kHz sample rate. But still I think it is very interesting that you can show this. I assume if you play a 1V sine wave you don't see those artifacts? In that case you can show that the point that Benchmark tries to make about tweeters demodulating Class D switching noise does not seem to be an issue here, correct?
DualTriode
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Hello All,
If you are speaking of little things, there are at least 2. There is fan noise from the APX1701 and from the APx555 audio analyzer.
You and @amirm should have a Q&A about fan noise here on ASR.
I have sorted headphone measurements by using a Topping headphone amplifier and a GRAS 12AA microphone power supply & conditioner. I use the linen closet stuffed with blankets and pillows across the hall separated by two closed doors for isolation of the headphone test jig from the APx555 fan noise.
For the fun of it I will set up a couple of different tweeters on the bench across the room, a couple of different amplifiers, put in my ear plugs and run some plots. We will see Class D IM and or driver distortion or not.
Thanks DT
20kHz square wave? That won’t happen with our usual bandwidth limited sources . But it’s nice to test the extremes of our tech ,also useful if you design this stuff.
I view it as an academic exercise, interesting for curiosity only, but that has no use for actual music listening.
I was reading the May 2022 edition of Stereophile and while reading the review of the Japanese flagship Luxman M-10X stereo power amp (20k usd) it reminded me of this topic, in particular this part quoted above.Sorry I am back again with this thread. It was the Amir's review
Audiophonics HPA-S400ET Review (Stereo Amplifier)
This is a review and detailed measurements of the Audiophonics HPA-S400ET. It was sent to me by the company after member request and costs 1 490,00 € (US $1,638). I must say this is one of the best packaging of class D amplifiers (Purifi in this case) that I have seen! It is slick and has...
and especially this measurement
that has shown that even the best current class D module, Purifi 1ET400, suffers from quite severe high frequency non-linearity. Thank you @amirm for performing this kind of @45kHz BW distortion vs. power test as a rule. I have decided to accept it as my standard test method as well and I have tested 3 of my amplifiers under same test conditions, the comparative result I have posted here in this thread:
This beauty runs on class AB (with 12W into class A bias). Let's see how it measures with frequency:
This is 20 volts (P = U^2/R = 20^2/4 = 100 watts). I can read THD+N at 4 ohms in 15 kHz is around 0.03% = - 70 dB.
It is significantly higher than the Purifi 1ET400A from Audiophonics above, which measures about -88 dB.
Or at 5kHz, Luxman THD+N is about 0.013% = -78 dB.
Audiophonics Purifi is -99 dB.
Power the Luxman M-10X reaches 350W into 4 ohms (THD+N 1%) while the Audiophonics 1ET400A does 367W in the same conditions, so they are equal here.
What about high frequency intermodulation then?
From the Purifi 1ET400A datasheet we don't have the exact same conditions, but taking a worst case, 200W into 4 ohms:
Luxman IMD skirts are about -98 dB while Purifi's (worst case!) around -112 dB.
So I guess the extra 18.4k usd buys a very, very nice chassis and meters, along with real pride of ownership. Also bragging rights about owning a Japanese flagship class AB amp and laughing about class D toys. But better measurements and fidelity to the input signal? Sorry, not the case here. Even in high frequencies.
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"Yeah, you are cherry picking. The Luxmann M-10X does not measure that well but there are other class AB amps that do better in high frequencies" one could say.
Alright, let's take the top measuring McIntosh MC462 then.
Big amp with those nice blue eyes staring at you!
Power is now P = U^2/R = 200 watts.
THD+N at 15kHz: McIntosh MC462 0.006% = -84 dB, Audiophonics Purifi -82 dB, so tied.
Lowering to 5kHz: McIntosh MC462 0.003% = -90 dB, Audiophonics Purifi about -98 dB.
McIntosh MC462 costs 9k usd, but at least has double the power at 720W into 4 ohms (THD+N 1%).
IMD skirts tells a similar story, with the McIntosh MC462 around -100 dB and Purifi -112 dB (worst case!).
Still one of the best class AB matches or slightly loses to the best class D in high frequencies.
Alright, let's take the top measuring McIntosh MC462 then.
Big amp with those nice blue eyes staring at you!
Power is now P = U^2/R = 200 watts.
THD+N at 15kHz: McIntosh MC462 0.006% = -84 dB, Audiophonics Purifi -82 dB, so tied.
Lowering to 5kHz: McIntosh MC462 0.003% = -90 dB, Audiophonics Purifi about -98 dB.
McIntosh MC462 costs 9k usd, but at least has double the power at 720W into 4 ohms (THD+N 1%).
IMD skirts tells a similar story, with the McIntosh MC462 around -100 dB and Purifi -112 dB (worst case!).
Still one of the best class AB matches or slightly loses to the best class D in high frequencies.
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I have the May issue of AudioXpress in my hot little hands. I think that @orchardaudio linked to a google drive PDF of it. It is the power amp for sure.
Here is the link for just my portion:I have the May issue of AudioXpress in my hot little hands. I think that @orchardaudio linked to a google drive PDF of it. It is the power amp for sure.
Thanks folks.
My pre-amp is (probably) not state-of-the-ASR-art, but I do like it. And unusually for me, I feel a sense of excitement at what it, with this power amp, could sound like. It's a feeling I have had only a few times in over 30 years of hifi, music listening and audiophilia nervosa, a period which has seen hundreds of pieces of equipment pass through my hands. I've learned to trust that feeling.
eliash
Senior Member
Nice, regarding the (imho unnecessary and distressing) mains noise, looks like magnetic pickup from the power converter (due to the following harmonics). Some (~700°C) tempered 0.5mm flat steel sheet metal parts around the ps and the amp modules may help...at least helped in one of my dense conventional audio designs...must not even be a "watertight shielding"...
DualTriode
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Now here it is months later.
Today I just received my AP AUX 040 filter. The box is still outside by the front door.
If there was another brand besides Audio Precision. They are all working remotely still. Getting return calls and emails answered is difficult.
Thanks DT
Today I just received my AP AUX 040 filter. The box is still outside by the front door.
If there was another brand besides Audio Precision. They are all working remotely still. Getting return calls and emails answered is difficult.
Thanks DT
So 300kHz-ish carrier at about 400mV, seems about right.
eliash
Senior Member
restorer-john
Grand Contributor
...looks like a longwave broadcast...hopefully sounding good...
I need a decent outdoor antenna. Radio Pavel, NC-252 was really noisy and unlistenable. Definitely not a Class A station.
- Thread Starter
- #640
This class D investigation has been an interesting journey, resulting in my deeper return to linear amplifiers. Some will say that HF pollution in tweeter will be inaudible, that limited bandwidth is OK, who cares. Maybe. No solid proof ever given.
These things are slow, yet. The square response in post #636 can be made nicer, of course, with the use of analog filter. What we get? This:
Nice, smooth, right? Maybe. True record? No, filtered one. Fast? No. Good slew rate? No. Severely bandlimited? Yes.
Another argument - HF carrier residuals will not get into the speaker because of the naturally inductive cable+speaker impedance. Really? Below is the record of speaker current when driven from 5kHz square from NC252MP output.
I would not say that the speaker current is free of 400kHz carrier. Any serious investigation of speaker harmonic and intermodulation distortion at acoustic side, let's say a comparison between AHB2 and Purifi driving the same set of speakers?
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