ClassD amps and their performance

D Louth

class D has been with us for many years now but it's been 'updated' with better faster devices. The technology is the same. There are real fundemental problems with it if you believe that we 'need' extended high frequency linearity. While the recording world are pushing for 24-30bits and 192-300khz sample rates class D manages a low overall bandwidth due mainly to switching noise and a 'horrendous' zoble/hf filter. Most class d amplifiers barely manage a 50khz bandwidth. The roll of comes in very early as well.

where do we want to go with audio... up or down?

 

http://www.dailymail.co.uk/news/art...e-table-power-use-Camerons-green-crusade.html

 

Slightly (ok significantly) off topic but this beating up of domestic users in the name of the "environment" is just a political game.
The main abusers of the environment are industry and large companies.

Anyway, enjoying my Pathos TT till the thought police visit....

 

Hi RobM I must confess to not being as aware as you are about the length of time that Class D has been around and it may be that what you are saying is why they don't sound as good as amplifiers that use different technology.

I would like to think that everything we do in this hobby/passion is towards better sound but lets be honest the forces of apathy are ranged against us . I am not an advocate for any type of technology (i freely admit to liking valves)but if something else offered me an improvement i would not dismiss it . Not every thing that measures poorly(valve amps) or is bandwidth limited sounds poor but my experience of Class D has not been entirely good. However others who have written on this thread have not had bad but good results with many makes of Class D. We are all different,we hear differently,have different rooms etc.

The number of people who are into audio is small and ,regardless of technology used we and the industry need to wise up and get at least some of the rest of the population on board or the future is not looking good. A very small customer base is trying to keep the manufactures and dealers etc in a job,allowing them to make a living and have the cash flow to pay for research to develop better sounding kit etc . Increasingly these companies are forced into other areas to stay alive (multiroom etc) and that is not about sound quality.

We all help to pay for the future of our hobby ,the rest of the world for the most part cares not a jot about it(sadly) and could not care about sampling rates and bandwidth and sound quality hence the success of the i pod and down loading sub mp3 quality music (the height of quality is a polo-phonic ring tone on a phone)
I have heard people listening to music on phones . Now i am not against portable music,however in away overexposure to music turns it into a lowest common-denominator that people don't care how they hear it;very sad.
We need to find away to help people re-engage with music and then appreciate how it sounds and is reproduced ,any ideas ?

Hi Baudrillard ,thank you read the item ,very scarry .Maybe my imagining isn't so far of from happening .

Regards D Louth 77

 

Thanks for the article, really needed that.How about we find a use for politicians ?Fertiliser maybe.Sorry off thread and not greatly imprssed by our politicians nowadays.
I did at some point consider the Firstwatt stuff but it doesnt produce much power and I think DLouth is right .What I am looking for is a 300b that can produce 50watts or similar.I have looked at the Musical Reference amps and they may be followed up but I will try some listening to class d as I wont really have an opinion until I hear them.
I did look at the Butler audio stuff it has a good name,thats their bog standard stuff not their rolls royce stuff and I know someone who uses them and is very impressed. I really like my speakers but I do feel the 300b is just stretched too far at loud levels or complex pieces.

 

Sorry Dodd Audio is another amp that interested me including there pre amp.

 

D Louth

I do think there is a place for all technology out there BUT it seems that some of the things that come out are only released because the manufacturer is trying to find a place for it. Class D definitely has a place in this world. It runs cool, is small and you get sound out of it. The problem with it is that unlike 'normal' bi-polar class A or AB types the transisitors in class D are only 'on' for half the time. Meaning there is a 'dead-time' beween positive and negative going waveforms. This is where the problems lye. Although the dead-time has been reduced significantly over the years, it does not stop the need for a huge output coil and RF stopping components on it's output.... the extremely high frequency switching speed between the two transistors creates large amounts of RF. RF when rectified in an amplifiers input stage turns into DC! More problems!

At this end of audio we are trying to get the precious signal from source to speaker in a form which has the least amount of 'modification'. The fact that there is a 'dead-time' damages the signal enough. In practise class D is not compatible with high resolution audio as it's bandwidth is just too low. More interestingly how many companies at this end of audio are behind the technology? It's cheaper to manufacture to start with so you would think that more companies would jump on the bandwagon.

I have used the hypex modules (which about 80% of class D manufacturers use) and they are terrible but cheap. See www.hypex.nl The specs are VERY common with this type of technology. -3dB down at 50Khz but the drop starts at about 18Khz. I don't think we should be going down that route for high end audio. Great for marketing but not this end of audio. There is nothing wrong with what we have been using for the last 40 years... it just needs perfecting, a bit like turntables.

 

Am I missing out on all that music at 60khz??

Correct me if I'm wrong, but Naim are a proponent of bandwith limiting...

 

Hi robM

I don,t disagree with what you have to say,though i am not qualified to agree or disagree with your facts on the technology behind Class D.

The end result of High-End sound ,should as rob says be High-End sound but as i said over page ,most people don't care about that except us and manufacturers have responded to that and are using Class D to fill a role in these type of products and as rob says cost is a major factor in Class D usage.

Hopefully a market will still exist for designers and companies who share our values and will continue to refine existing technology and who knows produce something new that surpasses all previous attempts at a new standard .

As i also said over page i have not had any brilliant results with Class D so please don't think that i am an advocate for the technology , i am not but if it works for you ? Then i would be cautious and not to critical until i could hear the situation for myself. Sometimes what we think might not work does and sometimes the opposite works (when it should not) That is both the fun and frustration in this hobby.

Hi Colin2040 i have only read about Dodd Audio and not heard it ,sorry i can't help ibut f you find something that does all that valves do well please let me and everyone else about it. however my gut feeling is that your a valve guy and once a valve guy maybe always a valve guy?

Regards D Louth 77

 

Hi Anthony .

Yes Naim do seam to follow this ,all though they have changed the sound a bit in the last few years . However i still don,t like the sound myself. There are some good things in Naims designs but for me it sounds wrong,just in the way that Class D sounds wrong to some but not others . There are many happy Naim Owners but i could never be one ,but does that make them wrong or me right ,its just that i don't like the sound.

D Louth 77

 

Naim's amplifier bandwidth tops 30Khz, not much below 32hz either lol!
There are a few implementations of the Hypex modules which are very goo indeed, again not using the module as the amplifier stage maybe the key possibly
We have no issues with bandwidth in the designs we have used, purely impliment the sampled frerquency correctly, a proper power supply and design an output stage that works with the method you have chosen. This all does seem to have a beneficial effect.
Still its all in the ear of the listen nothing more.

 

I don't think it's worth worrying about the output filter as long as it's implemented properly. Best remove them from the crossover's of your speaker if they're that bad!

"At this end of audio we are trying to get the precious signal from source to speaker in a form which has the least amount of 'modification'"

A full digital amp can arguably said to have the shortest signal path possible. Oh, and it's got no feedback. If you use a CD player from its analogue output you've already got a longer signal path than an all digital class D amp.

As with any amp, the specs don't lie. Class D provides high fidelity.

As WM said though, you buy an amp cos you like the sound.

 

Do you use a CD player? According to your criteria they're a waste of time then. Limited bandwith you see..

Higher sampling rates are being used for the improved impulse response according to the people who make the stuff. There is no musical info at 384khz for goodness sake!

SACD has horrendous out of band noise figures as a result of noise shaping, no different from Class D in principle.

As far as I am aware, real world dynamic range doesn't exceed 20 bits for any kit. Things may have improved while I wasn't looking..

 

review of the BC e300i thingy in this months Stereophile. Kinda so-so on the bench, 2db of droop by 20k, HF response modulated by speaker impedence.... etc etc..

not exactly stellar linearity by any standards,.

 

This is all nteresting stuff ,and I will really need to review digital,before any purchase. Has anyone had great success with class a ss,I have tried Monarchy dont think they matched my speakers too well,what about the Usher 1.5 plenty of power and appears to get good reviews.Distributor near me as well so I could try that as an option.Maybe I need to stick to valves but nmore power..was it the glow master that got good reviews recently?

 

A friend of mine has a glowmaster , he likes it, perhaps i could give you his number and you could discuss it. Keith.

 

Thanks coops I'll keep it mind,wont be changing to soon as holiday just about to commence but if you dont mind I'll get in touch with you once I have secured funds and have made some decisions.

 

Colin Hi look forward to it, regards Keith.

 

Lyngdorf Class D is great!

Like Antony, I have been enjoying my Lyngdorf TDA 2200/Lyngdorf CD 1/Dali Helicon 400 set-up now for nearly two years. It sounded good from the off, but goes on getting better and better. Before choosing this system, I listened to about 15 other systems of comparable price (all the big names), and this one just blew them all away.

I like the 'all digital' approach, and the Class D sound is clear and un-fatiguing. Some reviews have criticised the Lyngdorf timing, but my system gets my foot tapping whatever I play through it (except Classical, with which it is a trifle un-involving, but that might be just my taste). Accuracy and imaging are also excellent to my ears.

You need to leave it on all the time, though. It takes a couple of hours to warm up! No problem since quiescent power consumption is very low.

 

I think you'll find that class D is not actually digital. The control switching is digital in your amp. There are basically tww types:
1. digitally controlled class D
2. analog controlled class D

If you want a digital amp because you like 'digital sound' buy a class T amp.