Class-D amps - a different re view

Eleberoth2 - let me follow on this.

In point 3 he mentiones up to 2V of 50kHz frequency on the speaker cables - complete nonsenese - unless he talks about normal audio signal (my Rowland ha -3dB at 65kHz).

Point 4.
"Make the output impedance, a passive filter, variable with frequency and dependant on speaker loading."
I design electronics for 30 years but have hard time to understand how passive filter is variable with frequency. There are common mode chokes and capacitors on the output (zobel network) but they are present in most of other amps. This filter is set to about 65kHz (-3dB). Filter is within negative feedback and damping factor for Icepowe is about 4000. Early amps on Tact modules had output filter outside of feedback - but he makes general statements.

Point 5.
"Allow the amplifier to be marginally or completely unstable with high or open circuit output loading"
Complete nonsense - since switching amp is constantly unstable (cannot become more unstable - big advantage) being analog modulator/oscillator.

Point 6.
"Employ soft compressor clipping circuits prior to full power clipping"
Yes it does - (big advantage) designed in on purpose to protect tweeters from high energy during overdriving input. Many amps (like NAD) have this advantage

Point6.
"Employ high order negative feedback to improve in-band distortion figures"
Every amplifier does this either with global or bunch of local feedbacks. I know about multiple feedbacks in Icepower but suspect them to be shallow since output is pretty linear. I don't know how he knows about high order but Class D is different creature and has different feedbacks. He might be implying deep negative feeback - usually bad because of TIM distortions but there is no need for deep feedback and even if they put one in it cannot cause TIM. TIM comes when feedback cannot react fast enough and output transistors go to saturation getting charge trapped at their junctions becoming immune to following signal for a while. In Class D output transistors work differently (switching) and are respodning to time not voltage.

Point 7.
"Use feedback to provide numerically high damping factor at low frequencies and claim that this guarantees fine bass.(regardless of the interface to the loudspeaker)"
Isn't he implying by word "numerically" that this damping factor is not real and manufacturer is not honest. That is what I call manipulation. He makes impression of poor bass performance of Class D - this won't fly either. Enyone who listened to Class D amp knows tahat bass performance is its best feature.

Speaking od damping factor - my Icepower has 4000 and probably unnessesarily since inductor in series with woofers has obout 80mOhms and limits DF to 100. At 20kHz similar limitation is imposed by about 0.2uH imductance of the speaker cable. DF of 200 would be plenty.

There are 3 kinds of switching noise in Class D amp:
Carrier frequency - approx 0.5MHz
Switching spikes of output Mosfets - approx few ns
Switching power supply frequency approx 50kHz

Carrier has very little chance to get thru linear power supply with all capacitors then power transformer and finaly input chokes and capacitors (pi filter). It has even less chance to get thru regulated switching power supply (my Rowland).

Switching spikes have very little energy and are filtered on power supply as above, and on the speaker cables by means of common mode chokes and capacitors (zobel network).
Carrier in not completely filtered on speaker cables (about 1% left) but needs 525 fett of speaker cable to become 1/4 wave antena.

Switching power supplies are present in probably every modern TV set and are strictly regulated. Not only that my Rowland for instance has CE certification (part of which is measurement of radio emissions) but also Icpower module used inside (200ASC) has folowing info in the datasheet:
EMI conforms to: EN55013
EN55020
EN61000-2
EN61000-3
FCC part 15-B

Elberoth2 - There are few early amps with poorer filtering like previous EVOs (not Icepowers) from Bel Canto or some NuForce amps but he implies they have poor performance because of it or that the company performs some illegal acts designing the switcher.

His point 1. is practically same as ponit 2. It is not possible to push noise out without pushing in - it's the same thing. He just makes it sound worse.

Elberoth2 - jus shortly

9. "Use steep low pass filters to limit the upper high frequency range, partially negating the purpose of wider bandwidth, e.g. SACD, source material while resulting filter phase shifts may be audible in the working band"
Don't know what he means - Icepower has 65kHz bandwidth (-3dB) and typical phase shift at 20kHz.

10. "Have input circuits which are highly susceptible to higher frequency input signals including upper band noise shaper noise and DAC artefacts, and which then contribute to poor treble sound"
Why input circuit are more susceptible ti higher freaquency - I don't know.

11. "Have output circuits with poor high frequency resolution resulting in high levels of intermodulation products at the high frequency end of the spectrum"
Resolution is unlimited (analog) and IM distortions for my Rowland are 0.0005% at 10W 4 Ohm 14kHz/15kHz. How many other amps can do that?

12. "Have 'sampler' noise shaped noise floors which vary dynamically with the level frequency and complexity of the input signals"
I don't understand this language - what samplers is he talking about in analog modulator?

13. "Have comparatively small power supply reservoirs, in the light of their low frequency output current potential"
My Icepower being smallest of the series has peak current 11A. Larger 201 has 20A and largest 501 has about 50A.

14. "Have thermal dissipation limitations due to the small power module size which means that thermal dynamic variations are present in the performance with time"
Nonsense - my amp dissipates abot 5W and runs completely cold. Output switchers (Mosfets) have close to zero resistance when "ON" and switch within nanoseconds - not much power dissipated there (even no heatsinks).

15. "To protect the fragile output stages all kinds of pre-clip, and aggressive fold back protection regimes are included which are frequency dependant and are also programmed for duty cycle"
Again - tossing terms. Output is more robust than traditional amps having strong Mosfets in H-Bridge and all sorts of protection. Foldback protection is used only in linear regulated supplies (not used in amps for output power). Someone was testing Icepower at full power with music, sinewave and noise for many hours - found them to be exceptional. I can find it if you want.

16. "Operate at an equivalent sample rate which is insufficient for good resolution above 7kHz. DSD 1 bit pulse-width modulation operates at 2.4MHz, nearly ten times the rate used in Class D amplifiers"
It does not sample anything and has unlimited resolution at any frequency (it is analog). Has bandwith limit of 65kHz because of 0.5MHz carrier. 2.4MHz he mentiones or 2.8 MHz with SACD has different purpose (sampled system Nyquist - there is no DAC here)

17. "Deliver high constant DC voltages relative to local ground (up to 70V) at the output terminals (of course not between the +,- terminals) and hence the loudspeaker connections and cable"
DC voltage on output of my amp is under 1mV. What he means by "High Constant DC Voltage"? I don't know terms like that. This is garbage - believe me

Just one more comment to pnt. 17 - I said I cannot understand what he's saying. It looks like he things there is up to 70V of constant DC voltage on the speaker terminals???? Please help me to understand this!!
I don't know what bothers me more claim of 70V DC on the speaker terminals or terms like "Constant DC" (had impresion that DC is constant by definition)

Kijank, I want to thank you for taking the time to unmask this pretender, Cplloms.

I am just a listener. One critic of class D, and tube lover extraordinaire, told me recently he can hear class D sampling. When I questioned how, he used the example of upsampling DACs vs. non-upsampling. I had to agree with him on the difference of those two. I also had to tell him I cannot hear any similar sampling distortion in the ICE amp I am using, H2O.

From what I think I understand from what you said, ICE modules do not sample. Am I correct here? That would explain why I do not hear anything.

I can put my ear directly to my drivers and hear nothing but pure music. There is no noise.

Another question I'd like to put to you is whether you have found everything inserted into the best class D amps has to be as faultless as possible?

Thanks again for your input.