A high-end digital amplifier for everyone

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A high-end digital amplifier for everyone or everyone working on a high-end digital amplifier

(See the complete document with images here.)

Hi,

We would first like to introduce ourselves to everyone. We are a small group of former and actual Electrical and Computer Engineering students that worked over the last two years on a digital audio amplification project. The reason of this post on the forum is that our work is going to its final stage and we are planning to release it to the DIY audio community. We believe that this forum (www.diyaudio.com) is simply the reference in that domain and this is why we are making our official announcement here.

What we developed is a complete digital audio amplification development platform and we invite you to read forward to understand all the benefits of our work. What we are about to release is not a conventional simple Class-D amplification project; this project won the 115th Audio Engineering Society convention student design competition last month in New York.

When we first started to work on digital audio amplification we considered the existing solutions. First, there was the conventional Class-D amplifier basically made of an analog audio input, a triangle wave signal, a comparator and a switching output. This solution while being simple and efficient did lack in performance and creativity. The input is still analog and the process itself suffers from gain-bandwidth limitations. On the second hand there was the direct PCM to 1-bitstream conversion in the digital domain. This concept really goes deep into the idea of bringing a digital audio signal to life. Considering the CD as the actual primary source of audio input, it was obvious that the idea of keeping the source in the digital domain in the process of amplification was the key. This is therefore on what we started to work.


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The people around this forum who know the most about digital audio amplification probably already know what big companies are offering. While these companies were developing their digital audio amplification solutions, we were doing the same. However, now that we have a functional digital audio amplifier, instead of taking our work and putting it into a plain integrated circuit, we are offering the DIY community the chance to work with us on this cutting edge technology.

The main idea behind digital amplification is the conversion of a pulse-code-modulated (PCM) digital signal to a 1-bit stream that can be amplified by a class-D analog output stage (power switch) to a high voltage digital code. This high voltage 1-bit pulse stream is then low-pass filtered in order to produce an amplified version of the original analog signal across the loudspeakers.

Our platform splits into two entities: the hardware and our reference design firmware. The hardware we put in place has the goal of 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 hardware and software have been designed to allow for the addition of various add-ons such as analog inputs, LCD, tone control, mp3 decoding etc.

Hardware:

a) Digital platform

The heart of the hardware platform is an FPGA (Field Programmable Gate Array). This device brings a lot of advantages on the table. It is re-programmable, it allows the integration of various independent blocks that have access to any of the numerous I/O pins, it operates at high clock speed, etc. Basically, with such a universal device, it is possible to implement the PCM to PDM converter, but also any digital signal processing blocks (equalizer, tone control, digital filters etc...). In addition, all the control functions can be implemented in the same device, no need for an additional micro-controller to add an LCD or an infrared receiver for example. Therefore, this means the FPGA can be used to implement just about anything you can imagine without any hardware change. In this regard, it is really a universal and evolutionary core for the project.

In order to make processing of the digital audio signal easier, two integrated circuits have been used. The first one, a digital audio decoder (Cirrus Logic, CS8416), is used to decode the S/PDIF or AES3 data stream and output the PCM audio data (e.g.: I²S format) for easy processing by the FPGA device. The second one, an asynchronous sample rate converter (TI - Burr Brown, SRC4193), makes it possible to support any audio sample rate (e.g.: 44.1 kHz, 48 kHz, 96 kHz, 192 kHz etc.) at a glance. These two ICs additionally offer practical features such as logarithmic attenuation, balance, etc. They have been selected because they are the two best devices available as of today.

As a matter of convenience, the platform offer digital input connectors (coax and optical), all the required power supply regulators and 79 I/O's from the FPGA distributed on header connectors.

Tremendous effort has been put into the design of the printed circuit boards. The modulator board requires a four layer board to allow for wide power and ground planes which drastically reduces inductance and provides well defined impedance for the high frequency noise. Particular attention was given to the FPGA decoupling and signal integrity.

b) Output stage

The main goal of the output stage is to reproduce the 1-bit code generated by the modulator at a higher voltage. A low pass filter is used to remove the high frequency content from the amplified PDM waveform before sending it to the speaker. The IC selected for the H bridge amplifier is the TAS5112 manufactured by Texas Instruments. It is one the most recent H-bridge IC designed for audio applications. It can deliver up to 50W of stereo power into 6-ohm speakers at very high efficiency (90%). It includes a logic interface (controlled by the FPGA), integrated bridge drivers, high efficiency MOSFET outputs, and all necessary protection circuitry.

As stated earlier, the output stage was built separately from the digital platform to increase flexibility. In fact, additional channel and different output power can be obtained by adding/changing the output stage, without changing the digital platform. The switching nature of the output stage circuit imposes very strict constraints on the layout and choice of external components. A two-layer board was made with proper use of ground planes to reduce EMI.

Firmware:

Having this hardware in place, we developed the FPGA reference design firmware. It mainly consists of a function control block, a digital audio receiver and a digital signal processing block.

The heart of the digital amplifier resides in the modulator, which converts the PCM audio data into a 1-bit stream capable of driving a power switch. The modulation scheme we have implemented is different from most of the existing PWM-based solutions; it is known as sigma-delta modulation (pulse-density modulation – PDM). Notwithstanding their high linearity and low clock frequency, sigma-delta modulators are generally not considered for digital power amplification because of their high pulse repetition frequency (PRF) and they encounter stability problems at high input amplitudes. Inspiring ourselves from work published by M.B. Sandlers in the journal of the Audio Engineering Society, we have designed a stable 7th order sigma-delta modulator with PRF reduction algorithm. Most of the design work was performed in Matlab/Simulink. We believe that the current design gives very high quality results. However, it might be interesting to compare with other modulation schemes.

In the end, what we have is a complete and functional, fully digital amplifier. What makes it really powerful is the fact that it is completely evolutionary. Anyone with this hardware could see its amplifier gain functionalities as more work is done on the firmware, a firmware in which all the efforts of the community could be invested. This is exactly why the DIY community makes the project so powerful. Releasing the firmware in open-source would allow putting all the efforts in the development of one single high-end digital amplifier. Furthermore, not only the developers could benefit from the evolution of the project, but any hobbyist interested in audio projects. Imagine having an amplifier that could sound better and have more features at every firmware update, an amplifier that any DIYer could own.


Today, we are making our official announcement to see what interest the DIY community could have in the project. We already have a few prototypes working and we think the project has reached a milestone. Unfortunately, we cannot support the project financially anymore. We must admit that while we are still very interested in adding new features and improving things, we start to lack manpower. In a near future, we will be able to release the technical manuals, but meanwhile we ask you to give feedback and visit our website for more information: www.digitalamplification.com. Feedback for us is very important in deciding whether or not we are going to release everything in the public domain. It would take us a tremendous efforts on our part to prepare this release (documentation, code organisation and commenting, CVS server, General Public License, etc.), so we want to be sure that the interest is there and that the project will live on.

Best Regards,
DAX Group
 
Hi,

Have looked around your doc. Nice work ;) But I am missing the feedback loop from the output in the documentation/pictures. Without this any pulse modulated power amp will suffer from power supply modulation and hence distortion related to this. Not to mention dead-time distortion. What about this?

Cheers
 
Hi,

well, first of all, I consider this a very interesting effort. Releasing a development into the DIY public is a strange thought, tough. I'm interested in seeing this evolving and am curious about it's potential in the hands of DIYers instead of "pros"...

Something I'd like to know clearly in the first place: Do you (DAX) consider releasing the firmware *only*? Or is it the whole project (including schematics, layouts and documentations) you want to "live on" in public?

Cause I can't imagine myself developing software for a company which earns money with selling the only hardware (as their property) that the software can work with...

ciao,
Sebastian.
 
Without this any pulse modulated power amp will suffer from power supply modulation and hence distortion related to this. Not to mention dead-time distortion. What about thi


Being basically a power-DAC* it wouldn't be possible to apply feedback from the output stage.
The TacT Millennium AMP (digital PWM) doesn't have NFB either, this means that such an amp is basically feasible but there are some very stringent requirements to be met by the PSU. This is also valid for the switching behaviour of the output stage. The problems are greatly reduced by the fact that those guys use a power stage that is designed with such applications in mind (the TI amps use the same working principle as the TacT amp!).

This is not the first SD-amplifier however (but the first with exactly this topology) to my knowledge) however: Sharp SM-SX100 and the Tripath amps are also SD amplifiers. The "Technische Universität" in Munich owns a patent for an SD amp as well, I don't know however if anyone makes commercial use of it:
http://l2.espacenet.com/espacenet/bnsviewer?CY=ch&LG=en&DB=EPD&PN=DE19619208&ID=DE++19619208A1+I+

For your project I wish you good luck.

Regards

Charles



* If one takes the meaning of "digital" serious then there will not be such a thing like a digital amp since the amplification as such is a purely analogue process.
If you're less strict then you can call power DACs (like this one) digital amps, but never ever is an ordinary class-d amp a digital amplifier.
 
Congratulations on winning the AES award!

I'd like to join the others and encourage you to share as much information as possible.

I'm particularly interested in your PRF reduction algorithm. I think this would be particularly beneficial for dealing with DSD bitstreams from SACD sources (completely bypassing the PCM format).

I'm curious if your signals to the output section drive the H-bridge upper sources and lower sinks in a direct complementary fashion (with dead-time between switching), or if you've used an extended technique that slightly varies the upper and lower switching moments.

The closest commercial products to your approach that I'm aware of are Sony S-Master Digital Amplifier processors with Sigma Delta modulation (i.e. Renesas M65817AFP). I don't think these have been discussed yet in this forum (I haven't played with them yet myself since I've been busy with the TI Equibit chips).

One thing that has become apparent to me is that for any of these modulation conversion technologies that use an H-Bridge power output section without feedback, it would be very easy to use an FPGA as a switch for easy measurement and listening comparisons between the various commercial modulators and self-developed algorithms.

From my own experiments, I found that I needed to begin with my focus on digital interfaces and power supplies for the amplifiers' output sections. I'm guessing that you haven't focused primarily on the power supplies. My initial conclusion has been that the power supplies are the biggest opportunity for improvement over existing commercial digital amplifier designs, more so than the modulators.

I can appreciate your wanting some interest before you spend the time and effort to document and present your work up to this point. I've achieved some excellent results with direct-digital interface and DSD-PCM conversion circuits for going between disk players and digital amps. I intend to post the details on them in the near future, but it's a lot of work to bring the information together in a presentable form. Also, I get hung up because I always know that there's some type of further improvement I could make.

As you mention, digital amps are finally at a point where we can consider making evolutionary changes and improvements. It's really good to start to be getting past the point of simply getting one to function.

Experimenting with this stuff does take a lot of time. The more people that are involved experimenting with this technology and sharing information, the better.

Regards,
Brian.:cubist:
 
I will be watching this thread closely to see how your project develops. This could get very interesting!

After reading your release paper I have more questions than answers, but I am sure you will get to them.

The first one I have is what kind of license will you be using? Public Domain? GPL? OpenCores?

This might become the worlds first Open Source Amplifier :)
 
JoeBob said:
Wow, really interesting work you've got there. The use of a 4-layer PCB as well as a FPGA will deter many DIYers unless a PCB is provided, if so it would be very interesting for anyone wanting to build a "digital amp"...

I'm also interested in seeing how this develops, and think that properly implemented digital amps may be the technology that usurps my interest in tube amps.

I also agree with JoeBob, that unless the PCB and FPGA were supplied (at reasonable cost) then interest from DIYers will be very limited. Most do not have the technical knowledge or equiptment to fault find or optimise a design this complex even if they do have the enthusiasm and interest (in spades). Previously, I looked at the Tripath gear but lost interest as the RF management was too difficult for me to deal with, not having my lab any more.

However, should you want a beta tester, I'll volunteer.
 
Hi guys,
This is a very cool project, and I am looking forward to more information on it.

I kind of quickly looked over your website, and had a few questions.

How do you do volume control? Maybe I missed it, but I did not see if it was in the FPGA, or in the analog amp section.

I also wondered how much the FPGA costs? I know the big Xilinx FPGA's can get expensive, but I am not sure which one you are using.

Also wondered how much free resources are left in the FPGA, for people to add things, if it gets to there. Maybe Xilinx FPGA's are better now, but I had issues with them a while ago, if you have critical timing in them, and you don't have enough spare resources, it can be tricky to compile one that will work write. We got around it by putting lots of constraints on routing.

Maybe these questions, especially the last one, is asking for too much detail at this point, but I was curious.

Anyway, good luck with this project.

Randy
 
The Xilinx Spartan FPGA's like these guys used are awfully cheap, randy. Nothing like the old big Virtex chips.

The two biggest issues for DIY'ers are the 4-layer board and soldering the FPGA. Xilnx FPGA's come in .65 and .5 mm pitch only.

A DIY'er would also have a HARD time checking his circuit out for RF nastiness. (Except by ear of course).


Personally I would love to try it :)

Assuming a PCB was sold with the SMD stuff pre-soldered, so we wouldn't have to worry about that or the PCB layout, I think something like this could do quite well.
 
Cause I can't imagine myself developing software for a company which earns money with selling the only hardware (as their property) that the software can work with...

We really don't consider ourselves a company who wants to make money by using other people's work. Most of us are still students whose goal is to design a very high quality amplifier, for you, and for us. This is only possible with a standard hardware. All the work has been done in that direction and that's why we are proposing our hardware. As many people already pointed out, the advanced technologies used in hardware tend to limit the DIY possibilities. This is only true if you want to produce and assemble everything by yourself. We knew from the very start it would not be possible for the usual DIYer to produce its own DAX (short for digital audio amplifier) since many aspects are critical. For us it would be relatively simple to have the printed circuited board produced since we already have the layout completed and a few units are already working. Moreover, since SMT is used for most components of the project (including the very fine pitch FPGA device), it would be wise to have the boards assembled too in order to ensure quality. Every aspect of the production could be discussed later as the interest grows. However, having the hardware in hand is only the beginning. First, as Brian said, because there’s a lot of work to be done on the power supply. This is a field of knowledge where a lot of people in the DIY community can bring something. Second, software coding just like the features to be added are endless and require a lot of efforts. In addition, we are sure there’re a great proportion of people who would simply enjoy to plug-and-play the DAX in their own project (power supply, case, etc.) and follow the development and upgrades.


The XC2S300E FPGA itself retails at US$50 in single quantity. Which is relatively cheap compared to other high-end devices. Furthermore, at 300,000 gates, this device is the biggest supported by the free Xilinx Webpack development tool.

We have no clue how much producing the printed circuit boards would cost. Obviously the price would decrease as quantity grows. By investing so much time and money in the project we lost track on how much “reasonable cost” is. At some point it will be required to settle on the price range for the project, but we are sure it will be a fraction of the TacT Millenium’s US$5,000!

Right now, the FPGA device (300,000 gates) is far from being fully used. We are talking about 20-25% utilisation. However, this is somehow arbitrary depending on the level of optimisation of the code (which is not on what we put the most of our time so far). Furthermore, we have to consider the thing we already plan to change and that would use more resources. Let’s just say this is not a big issue at this moment.

Concerning the license it is just too soon to figure what is best suited for the project, but the designation “world’s first open source amplifier” is exactly what we have in mind.

A DSP capable of executing a high-quality 2-way FIR crossover would be very useful addition to your project.

BruteFIR may be useful for code development.

Digital signal processing is done inside the FPGA eliminating the need for an additional DSP.


From my own experiments, I found that I needed to begin with my focus on digital interfaces and power supplies for the amplifiers' output sections. I'm guessing that you haven't focused primarily on the power supplies.

You are right! Our main focus was to develop a high quality digital system and output stage. However, as you stated, the power supply is probably the most important part of a digital amplifier. We chose not to focus on that as it can easily be changed. But now, it is exactly the kind of things where we want people to share.


Best regards,
Dax Group
 
what is all that about being a money issue?

How much are we talking about and what is it for. break it down will you?

The term "issue" is maybe a little bit too much. The only thing we were saying is that production of printed circuit boards will have to be considered. In addition, the FPGA device will probably also need to be included/soldered as the pitch is very fine. A group purchase for parts will also have to be considered since all the boards require the same parts and this will be definitively make it cheaper for everyone. Ulitmately, we could have everything assembled and tested, making the project more accessible.

Therefore, there's no way we can answer the question "how much?" at this moment.

Gabriel,
Dax Group
 
let me see if i'm right...

you want comments, feedback and advice from diy'ers to finalize a project you started?
good luck!

I have been browsing these forums for some 2 years now and I am still astonished everyday that when soemone comes up with a design, someone else will jack it up or crack it down.
It keeps me speechless!!!!!



I thought it was a case of needing funds to produce and sell.
Sorry for the confusion.

Jean-Pierre
 
Don't let other peoples skepticism or criticism get you down, what you're doing is pretty darn cool and alot of people would be interested when you get this out the door. But if you need feedback from us DIYers, I am sure I speek for everyone when I say, you're going to have to make it easy to reprogram, no one is going to want to keep the firmware it was shipped with for long (byt the sounds of it you already wanted to do this), not that your firmware will have anything wrong with it, but everyone's going to want to change things and add other features. Just wanted to give some ideas, in case you didn't already have them...

Oh, and you know, if you need a beta tester that's close *cough* *cough*, you know where to look (if I weren't obvious enough, well, me...)
 
what did I say.....

===you're going to have to make it easy to reprogram, no one is going to want to keep the firmware it was shipped with for long (byt the sounds of it you already wanted to do this), not that your firmware will have anything wrong with it, but everyone's going to want to change things and add other features.====



my ink wasn't even dry yet ......

Jean-Pierre
 
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