Atma-Sphere Class D Mono blocks

[Dogberry]

Dr. Olive has claimed to have proven that listeners prefer lower distortion. You proffer that people like distortion. You can understand why I find those positions at odds.

I expect there is a right amount of distortion, making sense of both propositions. Nobody likes obvious distortion, and just a little of the correct sort adds something pleasing, even if inaccurate. Twenty years ago I had a pair of Audio Electronic Supply SE-811 monoblocks and they were extraordinarily musical, although inexpensive and I'm sure it was because of the 'just right' distortion.

 

[Gregadd]

I do not take issue with the notion that less distortion is preferable. The issue is how do you eliminate distortion. Are you taking something with it? For example, at least one amp allows you to choose between negative feedback and low distortion. If I chose low feedback, it does not mean I love distortion. IMO it means NFB does something I don't like.

 

[DonH50]

I do not take issue with the notion that less distortion is preferable. The issue is how do you eliminate distortion. Are you taking something with it? For example, at least one amp allows you to choose between negative feedback and low distortion. If I chose low feedback, it does not mean I love distortion. IMO it means NFB does something I don't like.

The last statement may well be true, but discovering exactly the cause can be tricky. A lot of people point at NFB like it is some sort of boogieman leading to all sounds of sonic issues, but properly implemented it is transparent. TIM is a real problem with feedback, along with instability and such, but for the vast majority of components those are well understood and solved long ago. NFB should improve performance, reduce drift, and all the positive things it is noted for. I suspect NFB gets a bum rap, but then again in the audio world objectively better performance, decades ago the holy grail, is today often treated with contempt and disdain.

Changing feedback not only changes distortion, it changes the spectral response of the distortion, along with a number of other parameters like frequency response, output impedance, noise floor, etc. If NFB in an amp creates a flatter and more extended frequency response, and removing it changes the response such that perhaps bass is raised a little and highs are rolled off a bit because bandwidth is lower, the output impedance is higher, and thus your speakers are sounding a little different, you or I may prefer that sound. You could implement the changes in other ways than reducing NFB, but the average audiophile is not able to do that -- it is up to the component designer. Instead we pick and choose components that work together to sound best to us, and how much of that is due to the amount of NFB, lower or higher distortion, or anything else, we neither know nor care. At least I don't; I just want the (my) system to sound good to me.

 

[Gregadd]

I just chose NFB as an example.anu think of NFB as a panacea.. it is sort of like a non carbohydraye diet.
If less is good .one must be better


There position is to use more not less
I just get turned off when people say audiophiles love distortion.

 

[Atmasphere]

I do not take issue with the notion that less distortion is preferable. The issue is how do you eliminate distortion. Are you taking something with it? For example, at least one amp allows you to choose between negative feedback and low distortion. If I chose low feedback, it does not mean I love distortion. IMO it means NFB does something I don't like.

Feedback in other fields of endeavor is known as Control Theory. It works, if applied correctly. That's rare in audio...

Feedback has two issues that must be overcome in a successful design. The first is that the feedback must not become distorted prior to doing its job (which is to mix with the incoming signal so as to create an accurate correction). In most amps made in the last 75 years, the feedback signal hasn't been treated with this sort of respect, so the feedback generates extra higher ordered harmonics that are unpleasant. Norman Crowhurst wrote about this problem in the late 1950s and 20 years later Peter Baxandall did the same.

The second problem is a lack of Gain Bandwidth Product in the circuit. If insufficient for the gain of the circuit closed loop, distortion will rise on a 6dB slope when the GBP limit is reached (in many amps that might happen at only 1KHz) and likely faster at higher frequencies, since feedback is being reduced on a converse slope. This means higher ordered harmonics can be quite a lot higher than the THD number might suggest.

If these problems are addressed in the design, the chances of that design being more musical are vastly increased.

Changing feedback not only changes distortion, it changes the spectral response of the distortion

If feedback is properly applied, it will simply reduce distortion without adding harmonics, so it does not always result in a change of the spectral response.

I've found this is a very hard fact for audiophiles to swallow. I was there myself. We never put feedback in our OTLs simply because I didn't feel we could satisfy the requirements of the factors I mentioned above. Class D changed all that because high GBP values are so easy to obtain.

If you get the right distortion signature it really doesn't matter what created it. Put another way, if you have a tube amp and a solid state amp with exactly the same distortion signature they will sound the same as long as there are no FR errors (which will happen if the output impedance is an issue) to give one away in a comparison.

At that point you have to ask yourself why tubes? If it doesn't matter how you arrived at a benign distortion signature, why pay for the more expensive devices, put up with the heat, coupling the tubes to a load and replacement issues? So far I've only found one really cogent answer, which is tubes can drive the high impedance of some ESLs much better than solid state, which loses significant power into higher impedances.

 

[Gregadd]

I admire your candor. Carried to its' logical conclusion. Why bother with those speakers?

 

[Atmasphere]

I admire your candor. Carried to its' logical conclusion. Why bother with those speakers?

ESLs have powered diaphragms- the electrostatic principle. When you apply a signal, the motive force does not sag like it does in a permanent magnet speaker. So you get more accurate movement of the diaphragm; the 'speed' of electrostats that so many talk about. They can be really convincing, and IMO the Sound Lab ESLs might be one of the top 4 speakers made in the world today.

Field coils are the conventional driver equivalent. Field coils didn't go away in the 40s and 50s because they were inferior, they went away because they were less expensive. Alnico is often considered the best magnet material, not because its stronger (its the weakest) but because it can be focused properly in the voice coil gap and also sags the least when current is applied to the voice coil.

 

[Gregadd]

Top 4? That means there are three alternatives.
I am juts messing with you Ralph. I hope you never stop making tubes.

 

[DonH50]

If feedback is properly applied, it will simply reduce distortion without adding harmonics, so it does not always result in a change of the spectral response.

Well, it sort of has to change following the loop gain bandwidth and all that jazz, but the change will (should) be inaudible... Distortion should be in the mud. I suppose I should have said "potentially" or "audibly", spoke (wrote) too loosely for a technical crowd.

I've found this is a very hard fact for audiophiles to swallow. I was there myself. We never put feedback in our OTLs simply because I didn't feel we could satisfy the requirements of the factors I mentioned above. Class D changed all that because high GBP values are so easy to obtain.

Yah. A lot of angst comes about IME/IMO from poor execution, largely driven by inadequate GBW (sorry, GBP in your nomenclature), a problem solved long ago. Class D amps have incredible GBD by design and topology, partly because self-oscillating designs get as much GBD as possible.

If you get the right distortion signature it really doesn't matter what created it. Put another way, if you have a tube amp and a solid state amp with exactly the same distortion signature they will sound the same as long as there are no FR errors (which will happen if the output impedance is an issue) to give one away in a comparison.

At that point you have to ask yourself why tubes? If it doesn't matter how you arrived at a benign distortion signature, why pay for the more expensive devices, put up with the heat, coupling the tubes to a load and replacement issues? So far I've only found one really cogent answer, which is tubes can drive the high impedance of some ESLs much better than solid state, which loses significant power into higher impedances.

Not sure this was directed at me? FWIWFM (not much), I agree with all of it, including the neat ability of a tube design to couple better to ESLs. Probably sacrilegious, but Futterman did as much to hurt as he did to advance OTL designs IMO, and did not necessarily target the right speakers.

 

[Atmasphere]

@DonH50 +1

 

[D.Duttilleux]

Imo, there is an overarching and decisive aspect in the discussion about amplifier distortion: signal integrity.
In a basic digital only chain this concerns the output stage of the (preferably balanced) source (e.g. a DAC), the amplifier, and last but not least: the complex non-linear interaction between amplifier and speakers.

In this context the attached paper is interesting, though no specific information about the amplifier is provided.

 

[Solypsa]

In a digital only system the dac and amp could indeed be one device...

 

[Atmasphere]

In a digital only system the dac and amp could indeed be one device...

You would have to do a conversion from digital to the actual switching output. Most class D amps these days switch at 500KHz or higher so you can see that you would to convert from the digital words to a modulated pulse width stream at the switching frequency.

 

[DonH50]

There have been several attempts at a "power DAC" but AFAIK none of them performed well or proved successful for driving high power into speakers. They have been restricted to low-power applications. They may look superficially similar, but there are actually a lot of differences (devils in the details) between a delta-sigma DAC and a typical class D amplifier circuit.

You could of course incorporate a DAC into the amplifier chassis, and include additional digital processing, like a number of professional amplifiers. And probably consumer, but I have not tracked those -- I think we consumers tend to want a straight-forward power amp and put all the processing into the preamp or processor.

 

[D.Duttilleux]

Imo, there is an overarching and decisive aspect in the discussion about amplifier distortion: signal integrity.
In a basic digital only chain this concerns the output stage of the (preferably balanced) source (e.g. a DAC), the amplifier, and last but not least: the complex non-linear interaction between amplifier and speakers.

In this context the attached paper is interesting, though no specific information about the amplifier is provided.

The funny thing is that the term signal integrity occurred to me after reading a long article about recording and reproduction techniques, in which a distinction was made between analog and digital.
Simply put, analog signal transmission is chaotic, while this is not or less the case with digital, since the original signal is reconstructed.

Signal integrity takes into account both the frequency and the time domain and this reminded me of the clock technology that M. Croese developed (based on 1920s science at Siemens).

 

[Dogberry]

Ralph, do you have opinions about your tube OTL amps (or, indeed, your Class D amps) with Quad 2905 ESLs?

Chris

 

[Atmasphere]

Ralph, do you have opinions about your tube OTL amps (or, indeed, your Class D amps) with Quad 2905 ESLs?

Chris

We have customers running the ELS57s with our class D that seem to do alright. I suspect the class D will work better on the 2905 since that speaker's impedance curve is a bit more tame. Our M-60 drives it pretty well from customer feedback.

 

[D.Duttilleux]

Not sure if this has been posted before:

 

[Nuforce]

I have found a dealer here in the UK who has a pair for home audition so I'll arrange that closer to the summer. I still remain convinced they won't sound as nice as my MA-1s.

My magnepan ribbon tweeters will tell me the truth as these are what these amps drive and they are extremely revealing.

 

[PYP]

I still remain convinced they won't sound as nice as my MA-1s.

do you think that could be a factor in your in-home evaluation?