On Class D Amplifiers Measurements

[LTig]

Yesterday, https://www.stereophile.com/content/devialet-expert-140-pro-integrated-amplifier-measurements


and the measurements... pfff but $6490

I agree, the noise floor looks horrible. But 120 mV ultrasonic noise would result in 3.6 mW power dissipation in a 4 Ohm load. No problem at all in my view even for tweeters with very low max power..

 

[maty]

But SNR-A at 1 watt 71 dB and...

At least here the Polish THD+N graph is coincident with Stereophile.

 

[maty]

Today, a subjective review about Hypex NC500 and tweeter / HF.

https://www.diyaudio.com/forums/vendor-s-bazaar/281361-hypex-ncore-nc500-build-post5995318.html

Btw, I use NC500 mono amps to drive woofers (B&W 800di) and they sound fantastic. Great full range too but I slightly prefer the treble from my blameless class AB amp. Subjectively it's just a bit sweeter and more open and extended...

 

[SIY]

Today, a subjective review about Hypex NC500 and tweeter / HF.

https://www.diyaudio.com/forums/vendor-s-bazaar/281361-hypex-ncore-nc500-build-post5995318.html

Subjective is NOT the same as "totally uncontrolled with peeking." This is the latter and absolutely worthless.

 

[LTig]

Today, a subjective review about Hypex NC500 and tweeter / HF.

https://www.diyaudio.com/forums/vendor-s-bazaar/281361-hypex-ncore-nc500-build-post5995318.html

Uh huh.

 

[pma]

One more question regarding NC400 and high capacitance load, like some ESL's mentioned in other threads. This is Hypex NC400 loaded with 6.8uF capacitor, no input signal. The amplifier oscillates at base frequency of about 52kHz - probably in interaction with its output filter. Output swing of oscillations is 3.4V1p-p. Is it OK, no problem for electrostats?

When it comes to step response with the capacitive load mentioned, it looks like this then:

(This record is measured with a LPF filter, so the carrier frequency is not visible and oscillation amplitude is reduced)

 

[SIY]

Presumably, the electrostatics have a transformer?

(1.2V RMS not being a big deal notwithstanding)

 

[March Audio]

One more question regarding NC400 and high capacitance load, like some ESL's mentioned in other threads. This is Hypex NC400 loaded with 6.8uF capacitor, no input signal. The amplifier oscillates at base frequency of about 52kHz - probably in interaction with its output filter. Output swing of oscillations is 3.4V1p-p. Is it OK, no problem for electrostats?

View attachment 41755

When it comes to step response with the capacitive load mentioned, it looks like this then:
View attachment 41756
(This record is measured with a LPF filter, so the carrier frequency is not visible and oscillation amplitude is reduced)

ESLs are not a purely capacitive load at 6.8 uf

 

[SIY]

Esls are not a purely capacitive load at 6.8 uf

No kidding.

 

[March Audio]

No kidding.

PMA knows better, I don't know what he's up to.

I wonder what happens to many class AB amps if you put a 6.8 uF cap across the output?

 

[SIY]

PMA knows better, I don't know what he's up to.

I wonder what happens to many class AB amps if you put a 6.8 uF cap across the output?

I often check that with a 2.2uF cap. Some are fine, some break into oscillation.

 

[March Audio]

I often check that with a 2.2uF cap. Some are fine, some break into oscillation.

Precisely. Trying to make out this is a class d issue is just BS.

 

[SIY]

Precisely. Trying to make out this is a class d issue is just BS.

Sorta. Pavel is an extremely smart guy whom I respect a LOT, and in his mind, he's digging for potential failure modes. And of course, there are always failure modes in real-world systems, but I'm skeptical of their relevance for the intended purpose.

 

[March Audio]

Sorta. Pavel is an extremely smart guy whom I respect a LOT, and in his mind, he's digging for potential failure modes. And of course, there are always failure modes in real-world systems, but I'm skeptical of their relevance for the intended purpose.

I know he is, I have come across his work previously, which is why I find this all a bit odd. There is no speaker load that's just a pure high capacitance, he knows an ESL will have an inductive transformer in series plus some resitance.

If you place that cap across an AB amp it will almost certainly break that too. All amps can go unstable into highly capacitive loads. So what are we learning?

 

[SIY]

If you place that cap across an AB amp it will almost certainly break that too. So what are we learning?

“Doctor, it hurts every time I do this.”
“Don’t do that.”

 

[March Audio]

I know this is just one example, but this is the equivalent circuit of 2 quads

 

[Blumlein 88]

This gets pretty far toward a cap at 1.5 ohm and negative 80 degrees around 20k. Never bothered the class D Wyred4Sound amp that I have.

 

[DonH56]

Clarification: My comments on square-wave (slew) testing and stability are generic. I barely noticed they were in a thread on class D amplifiers (did not notice, actually, or pay attention). It is the sort of testing I would apply to any amplifier of any class. Nor should my comments be used to infer anything about any particular amplifier (I do not recall mentioning any specific amps but I do not remember every post).

Added due to email claiming I was anti-class D, which is not the case and took me quite off-guard.

 

[pma]

More on complex load and square wave stability tests in
https://www.audiosciencereview.com/...ad-for-power-amplifier-torture-testing.10298/

Maybe March Audio might replicate that easy test on some of his class D modules?

 

[March Audio]

Clarification: My comments on square-wave (slew) testing and stability are generic. I barely noticed they were in a thread on class D amplifiers (did not notice, actually, or pay attention). It is the sort of testing I would apply to any amplifier of any class. Nor should my comments be used to infer anything about any particular amplifier (I do not recall mentioning any specific amps but I do not remember every post).

Added due to email claiming I was anti-class D, which is not the case and took me quite off-guard.

Really? Someone did that? To me the context of your comments was obvious, ie generic.