A high-end digital amplifier for everyone

[kiwone]

Thanks for your answer and thanks to inform me that you speak also in french but in this forum I think it would be rude ton in Moliere's language but, maybe, if I can explain some wishes I'll write you an email in french.

do you thin if it's possible to built this nearly famous Multichannel DPA as good as a Bel Canto ( if you know this one) and if it possible to built this amplifier just with monaural module amplifier like in very High End analog amplifier? I don't know if it's better but it just like this we could built an amplifier with 1,2,....,8 channel and choose the power of the different module ( different power for the subwoofer channel.)

 

[kiwone]

Hello everybody

I put a little "up" to this thread because it is very important for me and I want to know more about this project and built this High End Digital Power Amp.

So please do not let die this thread, I need all of you!!!

Thanks

 

[magnus]

Volume control and AC3 ideas

Hi,

I'm following this thread as I have been waiting for something like this.

I would think a lot of people would be interested in a 5.1 setup. Now, the thought that pops up in my head is you could do the following:

1.
Get a 5.1 decoder that will output digital signals (S/PDIF or I2S I suppose) for all channels. Quite likely this is a "hacked" receiver based on the fact that it is hard/impossible to get a license from Dolby for home use. I know for a fact that this has already been acomplished at avsforum but I can't find the thread just now. In any case, it can be done. Another alternative is a completely homebuilt decoder which is certianly possible but highly doubtful IMO.

2.
It would then be feasible to distribute these signals to the speakers via, say, Cat5 cable. The reason for using Cat5 would be so that you could send volume information as well, embedded in the stream data. I'm no I2S expert so tell me if I'm totally wrong here. The reason for doing this would be to have an "analogue" volume control. You would send a 16 bit word that would set the rail voltage and thus the volume of the amp. The reason for doing this would be to avoid a digital volume control.

3.
Also on the wishlist is to have a stereo amp with digital x-over filtering done in the FPGA. This way you could do away with the lumped x-over filters (save for a cap on the tweeter for safety perhaps). Great idea, no?

Comments? Feasible?

I hope so.

/Magnus

 

[kevyjo]

Forgive me, I'm way out of my league here, but I find this whole discussion very intriguing, and I'd like to contribute my envisonment of "the ultimate audio/home theater system", but that takes pages and pages, I thought it might be innapropriate, or unworkable to type such a long thread response. I've thought for years that the adversarial relationship between the disperate camps of audiophilia/home theater enthusiasm is stilly, and the ultimate evolution of audio would bring everything together in a no compromizes package. I am not tech savy enough to understand the tech details about this subject yet, but it seems that DAX ultimate focus is universalizm, which I find very intriguing.

 

[DAXgroup]

DAX website updated

Hi everyone,

We have updated the DAX website with the documentation for the platform and the current output stage. The documentation being a little raw, we decided to include our senior design project report, describing the fundamentals of the project. Keep in mind that the platform undertook multiple changes since then, so the report is not necessarly reflecting the current hardware and software implementation.

Regards,
DAX Group

 

[kevyjo]

I cant seem to open the new links, get an error. One thing I'd like to know, you have talked about the flexiblity/programmability of this platform; could it also incorporate speaker dynamics, and incorporate a digital crossover? I know this is a big leap, but I figured this development stage would be a place to throw out such possiblilities. Like i said, my understading of this subject is very limited, im sort of looking "outside in", but think of the possibilities: If an audio/HT system had an interactive "brain" that incorporated speaker dynamics, you could take advantage of the dramatic improvement in fidelity, imaging and dynamics that active crossovers/bi/triamplification can get you. If this system could indeed be smart, it would also open up the possiblity of digital processing automatically compensating for specific speaker driver shortcommings. Say you have a modest set of bookshelf speakers. The 'brain' sends a customized signal to the woofer, and another to the tweeter, optimized for the performance of the driver. The brain would be programmed with the drivers characteristics, and would know what its ultimate limitations were, so they would never overload and never distort. At low/moderate listening levels, a bookshelf speaker may be capable of 20hz bass extension if the drivers were driven to their limits, but not beyond. as listening levels increased, the brain would automatically cut the signal at the lowest bass levels as to not overdrive the woofers. putting all of this together, you could have a system of modest size that could have pretty much shocking dynamics and fidelity. What do ya think?

 

[nowater]

I agree with kevyo -- a "brain" allowing DSP and digital crossover is a great idea, and brings many more people's interest to the project. Maybe based on several TAS3103 chips, allowing numerous channels or active driver crossovers to the output.

And an option for the output module to be power DACs or ordinary DACs.

Grant

 

[Jaka Racman]

Hi,

before hijacking this thread into " A high-end digital multichannel amplifier with digital crossover and speaker- room equalisation and triamplification and everything else anyone comes up with including kitchensink ", would anybody please at least read article which has been up for at least 24h and understand what still has to be done before this becomes basic "A high-end digital AMPLIFIER".

In line with that my proposal would be to go with multiphase output stage. This is the quotation from classd.org :

As explained in the Sharp section, driving a power stage with DSD is about the least intelligent way to make a high power audio signal. Philips work their way around the problem by operating 8 (or any power of two) small power stages which are paralleled using common-mode coils. Each of the 8 power stages is assigned 1/8th of the DSD data volume which means that each bit is represented for 8 clock cycles. One could also say that the DSD signal is passed through a rectangular FIR filter 8 taps long outputting 9-level data. The result is that each output stage switches at a fraction of the DSD rate, resulting in lower switching losses and lower distortion. As the output stages are small, switching speed is also much higher than in an equivalent single power stage, resulting in even lower distortion.

As such this topology is a very original and effective implementation of the multiphase power stage first proposed by B&O.
The modulation index problem is addressed by doubling the length of the FIR filter. When it has 16 taps, a 17-level output signal is produced of which only the middle 9 actually occur due to the 50% modulation limit of DSD. These 9 levels map onto the 9 levels of the power stage, allowing 100% modulation.

I have some idea how to implement this, but being an analog guy I will have to simulate this before in spice which will take me some (lot of) time.

Best regards,

Jaka Racman

BTW, DAX, I am really impressed by the originality and quality of the design.

 

[phase_accurate]

I find the CLASSD.ORG homepage quite interesting and I have also been discussing things with the owner some time ago.

But I don't know where he takes his conclusion from that you could drive a DS output stage only to 50 % of its output voltage capability. When I simulated a DS amplifier it was perfectly possible to drive it up to almost 100%.
I am of course aware that an ordinary DS modulator would deliver an idle-output signal that is consisting more or less of half the clock frequency.
As this can be rather high, switching losses might be high as well (apart from other RF related problems). OTOH conduction losses (while idling) in the output transistors are smaller at a higher switching frequency, for a given output filter. So there is always a tradeoff.
With DSP preprocessing it should be possible to generate a DS signal that doesn't switch with too high a frequency while retaining good resolution (the bit-flipping technique is one of them).

According to a book about DS -modulators and -converters, any asymmerty in the output stage's behaviour (of a one-bit converter !) would result in ONLY an offset voltage. And this definitely NOT bad news.

Regards

Charles

 

[tiroth]

I'm out of my depth here, but is there an issue with power supply pumping in a power DS output stage, if such an offset exists?

 

[phase_accurate]

I'm out of my depth here, but is there an issue with power supply pumping in a power DS output stage, if such an offset exists?

Yes there is if it is a half-bridge. For a full-bridge there won't be a power supply pumping problem.

I didn't want to say that it is not necessary to address this problem but that the asymmetry should theoretically not give ANY OTHER problems than offset.

Regards

Charles

 

[kevyjo]

Im sorry, I didnt' realize I was hijacking this thread, from DAX original post it appears to be an open ended topic. I realize there are many technical issues that still need to be worked out. I guess I dont see the harm in me asking questions about it's POTENTIAL capabilities in the audio decoding realm, to me it would seem a topic central to the units possible appeal/application. Given the offerings already commercially available, a 50watt purely digital amplifier might not be enough to garner a broad enthusiasm if it is not presented with novel capabilities. I've read over the tech papers and found little that discusses the units conceivable capabilities on these topics, so I was just looking for some embellishment. I realize the tech issues of its basic function are paramount, but I see no reason why we cant look furthur down the road at the same time working on the issues at hand.

 

[nowater]

....I guess I dont see the harm in me asking questions about it's POTENTIAL capabilities in the audio decoding realm, to me it would seem a topic central to the units possible appeal/application. Given the offerings already commercially available, a 50watt purely digital amplifier might not be enough to garner a broad enthusiasm if it is not presented with novel capabilities....

I must agree 100% with kevyjo on this. Anyway, who is getting excited about replacing a well-performing line level DAC with a poor-performing "power DAC" power amp with less than 40W into 8 ohms and more than 0.1% distortion at 10W, rising sharply in the treble? Not me.

The DAX group's original message described the project as "a complete digital audio amplification development platform". They referred to the hardware as "being universal and allows development of different digital amplification schemes. We separately developed a modulation platform and an output stage board so that they are independently upgradeable."

The output stage may be a critical part of the project, but not THE critical part. The project is the platform (see DAX quote above), and the platform is the platform, if you'll excuse me for stating of the obvious.

I am much more excited about what can be done with the "platform" part of the project, and ensuring a lot of flexibility in the platform, so it can handle DSP, filtering, multiple outputs, delays. And like I said before, a line level DAC output stage board is a great idea for those who want better performance from their power amp. I can't see the point in a project that ONLY allows a power DAC, given their limitations.

Grant

 

[Jaka Racman]

Hi nowater,

and ensuring a lot of flexibility in the platform, so it can handle DSP, filtering, multiple outputs, delays

have you ever considered a career in marketing? I think you have great potential there.

best regards,

Jaka Racman

 

[Petter]

[snip]
The output stage may be a critical part of the project, but not THE critical part. The project is the platform (see DAX quote above), and the platform is the platform, if you'll excuse me for stating of the obvious.
[snip]
Grant

I beg to differ:

In the short run, this project will succeed of fail almost purely on the quality of the output stage.

In the long run, provided the preceding point is well executed, platform capabilities will likely be an important differentiator.

Petter

 

[Nathan Lewis]

My comments

It looks like there is a lot of interest here. I for one am interested! Pretty good to get a tube guy interested.

Personally, I would need a PC board with SMD components pre-mounted. How much would this cost for a run of 20? Of 100? What's the digital input?

My guess is the biggest determinant of sound quality is going to be power supply. After all, the amp is just a high-frequency switch connecting the speakers and power supply. You'd need a PS that is capable of being switched on an off, seeing alternating infinite and near-zero impedances, without oscillating, sagging, or ringing in some fashion.

There are commercial digital in-digital out crossovers available. Combined with a few of these amps, the Holy Grail of multi-amping may finally be at hand.

Good point, how are you dealing with attenuation (volume)?

 

[jackinnj]

while we are on the topic

to see some of the best literature in the field -- go to the USPatent and Trademark Office www.uspto.gov -- advanced search engine -- enter the following search parameters:

ttl/Class AND ttl/D

when you go through the filings on this (or any other topic) you will find a commonality of citings -- either in other seminal patents or in the engineering literature.

 

[DAXgroup]

Im sorry, I didnt' realize I was hijacking this thread, from DAX original post it appears to be an open ended topic. I realize there are many technical issues that still need to be worked out. I guess I dont see the harm in me asking questions about it's POTENTIAL capabilities in the audio decoding realm, to me it would seem a topic central to the units possible appeal/application.

For now, we concentrate on the digital AMPLIFICATION part. This is where the real engineering challenge is (for us anyway). We want to play with different modulation scheme, experiment with new ways of adding feedback to such a system etc...Designing digital crossovers should be a no brainer, given the tools that already exist. Using the platform to drive a line level DAC is also easy to do. We already have great ideas for the future of the project. Many of them connect with the suggestions people are bringing. The platform is indeed very flexible. Nothing keeps you from using it as a digital signal processor for FIR filtering, digital crossover, room correction and what not. But as we said, at this moment, we want to complete the "high-end digital amplfier" part of our project. Once we have that, we will be able to start building crazy things that will really bring the project to a new level.

In line with that my proposal would be to go with multiphase output stage.

We are not quite sure we fully understand what multiphase output stage is? We don't inderstand exactly how the one-bit stream is 'splitted' in 2^n stream (before or after modulation) and how it is 'remixed' in the analog world. The PRF of our 'modified' sigma delta modulator barely exceeds 350 kHz, so it's not really an issue. Can you give more details on that? It sounds really interesting and we would like to learn more.

Given the offerings already commercially available, a 50watt purely digital amplifier might not be enough to garner a broad enthusiasm if it is not presented with novel capabilities.

I must agree 100% with kevyjo on this. Anyway, who is getting excited about replacing a well-performing line level DAC with a poor-performing "power DAC" power amp with less than 40W into 8 ohms and more than 0.1% distortion at 10W, rising sharply in the treble? Not me.

Without being rude, we put to much efforts in that project to accept negative comments stating this amplifier cannot compare to existing products. What are your motivations for describing our DAX as a "poor-performing power DAC"? Please.

40W is maybe not much to drive a classical speaker, but for a lot of people, it is more than sufficient. However, if we exploit the idea of driving each way separetely (tweeter, middle and possibly sub) with three output stages, we have something much more powerful in the end. In addition, since the channel separation would be done digitally, this resolves the phase problem introduced by the passive cross-over. Furthermore, nothing in the current platform bounds us to use this actual output stage...if you are unhappy with 40W, it is really easy to build your own output stage (check the IRF website, they even have complete application notes) and crank up your supply voltage to 120V. We also think the sonic quality of our amplifier should not be attacked based on a few specifications and a large amount of misconceptions. The DAX as it is right now is already great realization by itself. In addition, it sounds great even if there's still some work to do. How many amps out there are showing 1% THD but people are ready to sell their house to buy one. Not so far from this thread there are a large amount of people spending hundred's of dollars building pass clones. These class-A amplifiers have been recognize to sound great, however if you look at the specifications, you'll discover these numbers are really not impressive (Aleph-3: 0.2% THD at 30W in 8 ohms; 1% THD at 60W in 4 ohms). It's a matter of defining if you're building an amp to show numbers to your friends or if you are ready to make compromising and realizing the REAL benefits of a given system. Whether some people want it or not, audio is an evolving field. We cannot stick at building tubes or class-A amplifiers forever and hope it will still be the state of the art. New technologies are there to replace the previous ones. The only way we can achieve this is by WORKING on these new solutions.

I think it will be important to share our vision in order to have the project growing here. If what people want right away is the new big thing in a shiny aluminum case, we will come back in 2 years with the final product. However, we tough we could do that work together. This project can bring a lot to people who are willing to learn about digital audio, and people who knows about digital audio can bring a lot to the project.

Regards,

DAX group

 

[Petter]

We are not quite sure we fully understand what multiphase output stage is? We don't inderstand exactly how the one-bit stream is 'splitted' in 2^n stream (before or after modulation) and how it is 'remixed' in the analog world. The PRF of our 'modified' sigma delta modulator barely exceeds 350 kHz, so it's not really an issue. Can you give more details on that? It sounds really interesting and we would like to learn more.
DAX group

I think I understood the prior posting but will allow the author to elaborate.

You might feel more enligtened if you peruse another application http://www.linear.com/prod/datasheet?datasheet=935&product_family=power which is a related method of providing higher effective frequency and longer on-times through a poly-phase approach.

 

[kevyjo]

DAX, please don't interpret my comments as negative, I did not mean them as such. The little I can honestly understand about the unit seems fundamentally groundbreaking to me, my comments were more "political" in nature; that it may go unnoticed by many people because it just doesnt' sound exciting enough or they don't understand its real capablities (the fate of too many groundbreaking things in the past have suffered). The reason why I talked about the fully interactive speaker system is because, as you said, a 40w amp may be not powerful enough for some applications, but might be uniquely suitable for an active speaker system, which frankly is a very underexplored realm of audio, especially to the degree I talked about ealier. I will leave this thread alone though, at this point I dont' have the knowlege to contribute to the refinement of its output stage, which I understand where you need help at this point.