Zetex DDFA

[Jon Lord]

Without having studied this thread and Zetex/Diodes available information in detail, there is a point, which was probably not considered:

Slew Induced Distortion. (Whatever the significance of this type of distortion is).

Any of today's average linear (non time- or level discrete) power amplifier is quite inimmune to that, but there are a class of amplifiers (higher order loops, call it NDFL, Sigma Delta, PID or whatever, which are more or less equivalent and some FF error correction schemes as well), which can fail to handle such pathological signal conditions correctly.

At least for a TIM100 signal and at least much worse as a test with static sines within the audible range would suggest ...

Before we hear the argument, that these are all artifical signals, it is a well known fact, that e.g. vinyl playback includes strong pulse noises outside the audible band, which can make such designs to "lose countenance" and reveal the disadvantages of aggressive error correction.

So guys, is there any hint, insight, information, that besides the nice THD and DNR and bit resolution figures, the Zetex design (with the feedback from the power stage) behaves well under the above mentioned artificial and real world conditions ?

As good as a well-behaved "bread-and-butter-blameless" Class A(B) design, providing about the same static linearity figures and the same idle power of 50W per channel (NAD M2) ?

[iand]

Originally Posted by Jon Lord

Without having studied this thread and Zetex/Diodes available information in detail, there is a point, which was probably not considered:

Slew Induced Distortion. (Whatever the significance of this type of distortion is).

Any of today's average linear (non time- or level discrete) power amplifier is quite inimmune to that, but there are a class of amplifiers (higher order loops, call it NDFL, Sigma Delta, PID or whatever, which are more or less equivalent and some FF error correction schemes as well), which can fail to handle such pathological signal conditions correctly.

At least for a TIM100 signal and at least much worse as a test with static sines within the audible range would suggest ...

Before we hear the argument, that these are all artifical signals, it is a well known fact, that e.g. vinyl playback includes strong pulse noises outside the audible band, which can make such designs to "lose countenance" and reveal the disadvantages of aggressive error correction.

So guys, is there any hint, insight, information, that besides the nice THD and DNR and bit resolution figures, the Zetex design (with the feedback from the power stage) behaves well under the above mentioned artificial and real world conditions ?

As good as a well-behaved "bread-and-butter-blameless" Class A(B) design, providing about the same static linearity figures and the same idle power of 50W per channel (NAD M2) ?

Any class-D amp with bandwidth limiting in the front end is pretty much immune to any "slew-induced distortion" by construction, there's just no mechanism for it to happen.

This would certainly apply to the DDFA as well; any amplifier with the kind of ridiculously low 19/20kHz IMD figures that this has is extremely unlikely to have any slew-rate problems anyway.

[Jon Lord]

Ian:

You may be right, that this design isn't prone to "hard" IMD effects by design in the basic modulator without feedback and power stage "prediction", but with the "Digital Feedback" from the power stage applied this is certainly not the case.

Please take a look at the following link:

NAD M2 Direct Digital integrated amplifier Measurements | Stereophile.com

The 19/20 kHz IMD is somewhere around 0.04%, which is ridicoulously high.

Please consider, that these figures are two orders of magnitude away from any recent/average DAC performance.

Please take a look at the high order uneven IMD products !
I never saw a recent linear amp/or DAC having such broad IMD harmonics.
From what I see here I would expect a TIM100 in the range of 0.1% from this amp at the same power, which is very high.

Many Japanese linear amplifiers of the mid 60ies were already better than this.

The NAD M2 Master Series Amp is very likely the best commercial implementation of Zetex DDFA available, so I would expect the best possible performance ...

Please also look at the idle dissipation of the M2, which is unbelievable 50 W per channel.

So again: Why choose this proprietary technology over a cheap "blameless Class A(B)" + DAC ? Besides the nice marketing blah ...

[darkfenriz]

Proof of concept?

[iand]

Originally Posted by Jon Lord

Ian:

You may be right, that this design isn't prone to "hard" IMD effects by design in the basic modulator without feedback and power stage "prediction", but with the "Digital Feedback" from the power stage applied this is certainly not the case.

Please take a look at the following link:

NAD M2 Direct Digital integrated amplifier Measurements | Stereophile.com

The 19/20 kHz IMD is somewhere around 0.04%, which is ridicoulously high.

Please consider, that these figures are two orders of magnitude away from any recent/average DAC performance.

Please take a look at the high order uneven IMD products !
I never saw a recent linear amp/or DAC having such broad IMD harmonics.
From what I see here I would expect a TIM100 in the range of 0.1% from this amp at the same power, which is very high.

Many Japanese linear amplifiers of the mid 60ies were already better than this.

The NAD M2 Master Series Amp is very likely the best commercial implementation of Zetex DDFA available, so I would expect the best possible performance ...

Please also look at the idle dissipation of the M2, which is unbelievable 50 W per channel.

So again: Why choose this proprietary technology over a cheap "blameless Class A(B)" + DAC ? Besides the nice marketing blah ...

This has all been discussed before -- if you look at the DDFA as just a power amp it seems complex, but the real point is to replace the complete signal chain from digital audio in to speaker out with performance similar to a high-end DAC -- not just IMD, but low-level linearity. Many functions such as eq/crossover/limiting/driver protection can be done a lot better in the digital domain than in analogue.

The "digital feedback" from the power stage -- which is actually a noise-shaping ADC clocked at 108MHz -- is probably the best way of closing the feedback loop with very low latency, to allow effective noise shaping and distortion reduction over the audio band.

I suspect the real objections to this type of approach come from those who don't understand how it all works, or don't like the thought of everything being done digitally with no room for analogue tweaking, or it all being proprietary, or all of the above. For those with no such axes to grind, the reviews of the M2 suggest that the design did achieve its objectives.

Anyone who thinks 0.04% IMD at 19/20kHz is "ridiculously high" should ask themselves what kind of speaker they plan to use for listening, never mind the distortion thresholds of the ear ;-)

[darkfenriz]

Whether 0.04% of IMD is hearable can be questionable, but there's no denying there are many better amps in that aspect, including class d ones.

[nicoch58]

Sorry but specifications are useless....

Is like a class AB pushpull with FB ie 0.0000 HD can sound better then SE class A

and don't thinks that class D sound better then tube amp...is cheaper and sound good but not better...of course you must add the right speakers.
is better 100db speakers with 1Watt tube amp then 85db with 200watt is you ask me...Open baffle...

[TheShaman]

Originally Posted by nicoch58

Sorry but specifications are useless....

Is like a class AB pushpull with FB ie 0.0000 HD can sound better then SE class A

and don't thinks that class D sound better then tube amp...is cheaper and sound good but not better...of course you must add the right speakers.
is better 100db speakers with 1Watt tube amp then 85db with 200watt is you ask me...Open baffle...

dBs and Watts you mention in your last sentence are specifications too.
So I guess only the specifications we don't understand are useless?

[nicoch58]

yes ,but you know what I means ....

specifications is just to know how work not how sound.... if I'm not wrong speakers have 3 to 5% HD...
PS
interesting is that HD is at rated db...ie better have 100db speakers

[nicoch58]

Now for a digital system having a power amp with real USB async input is fantastic news !
no more dac ,i/v output stage ,cable ,pot,pre....

Hopes are that price will go down and tech do better ....
One day will be a four channel with dps xover and we can build all DIY speaker we wants.....(2ways)