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The Blog

New PCB's are in and the quality is excellent !

It was decided to evaluate our new PCB manufacturer with one of our board designs to see what the quality was like before we commited to the other 8 boards with this same supplier. Due to reader feedback we modified the original Digital Audio IO board to incorporate more digital input options among a few other mods. Originally this board was layed out using 2-layers but due to the added complexity we have gone the full monty and used a 4 layer board design with dedicated ground and power planes. Like the main DSP processor board, using 4 layers should improve noise and EMI and reduce jitter which after all is in keeping with this being a hi-end audio project.

A new class of Digital Servo Amplifier drives woofer distortion down to vanishing low levels !

A new class of digital servo amplifier has been developed which improves on our previous design by at least an order of magnitude. One can expect a 20-30 dB improvement in loop gain over our previous design which was already high by industry standards. A record breaking maximum loop gain of 68dB (2500x gain) is now being achieved whilst maintaining unconditional stability ! This is unprecedented in the design of servo controlled loudspeaker systems which were typically limited to a maximum loop gain of 20dB (10x gain) due to maintaining stability criterion. Loop gain is important in a servo controlled woofer system because it directly indicates how much distortion reduction one can achieve as well as how accurately the woofer can track the input signal. The more loop gain the better the control over the diaphragm motion and the lower the distortion.

For the first time a loudspeaker servo system is now approaching the loop performance of a moderate feedback amplifier. The further increase in loop gain over the previous design helps counteract the rise in distortion at low frequencies due to the combination of suspension and box compliance non-linearity’s along with the use of current drive. We can now apply a servo system to a woofer and improve its performance by orders of magnitude whilst not being limited to woofers with exotic and expensive motor designs.

We took a high powered car speaker and added an accelerometer to it and then applied our servo controller to it. This particular speaker had a very stiff suspension and was not really designed for reproduction below 30Hz. But still, the improvement in distortion at 15Hz whilst being driven to almost amplifier clipping levels from a 1000 watt amp is shown below.

Lanzar OPTI-1232 driver with no servo
Lanzar OPTI-1232 driver with servo

How it works

The digital servo processes the incoming signal and compares it with the motion of the speaker diaphragm via an accelerometer that is attached to the loudspeaker voice coil. When the diaphragm departs from its intended motion the servo corrects it by applying a corrective force to compensate for any deviation which corrects both speaker non-linearity’s and transient response.

With our servo controller these corrections are made at 50,000 times a second so when the woofer is not moving as fast as it should, the servo senses this and applies more current to make it move faster. Likewise when the signal stops, it detects that the woofer continues to move when it shouldn't be and the servo applies a braking force to stop the woofer much quicker and more effectively than an open loop woofer could possibly respond. The result is a text book transient response with no over or under shoot and no distortion. What remains is the original music !


DSP Project featured on diyaudio.com

  To get the ball rolling and invite participation and feedback from readers I decided that diyaudio.com was the best forum platform to promote this. Indeed this has proved to be the case with many encouraging posts and emails with questions and new features. Sometimes an alternative point of view can pinpoint potential problems or new ideas that have been completely overlooked and I am happy to say this has been the case. As you can tell this project is aimed at the diyer - although not neccesarily open source but that doesn't make it less attractive as a project. We encourage discussion of ideas on the forum.

DSP based integrated multi channel DAC/Preamp/Crossover


Some readers have asked if it is possible to build a SOTA no holds barred, price no-objective DSP based multi-channel Preamp and Crossover. I'm not sure about the price no-objective requirement but I've been thinking about a project like this for a while. Firstly I wanted to wait for the dust to settle down a bit in the low cost audio DSP arena before I attempted anything major - after all it would be pointless re-inventing the wheel and building yet another low cost DSP audio board.

Observers will note that the main DSP board concept shown below is similar to our DSP Servo board. In actual fact most of it is identical except for the codecs in the servo section which have been replaced by the ESS Tech ES9018 32 bit Sabre DAC and ES9112 32 bit Sabre ADC. Still the layout of the Sabre DAC, ADC, power supply, IV convertors and associated clock distribution network is no slouch to get right and has been meticulous and uncompromising. There are eight other support boards not shown here. Everything is modular so the design can be upgraded later on for special customization requirements.

People may ask why I chose the ES9018 Sabre DAC and not some of the other reputable DAC's. Well essentially the ES9018 has an exceptional reputation and performance to go with it, being used in both low-cost and hi-end DAC offerings. It is used in the newest Benchmark™ DAC as well as the hi-end Weiss™ and the recently released Mark Levinson™ integrated amplifier which speaks volumes for it. It also is a multi-channel and highly integrated device which is ideal for this application, and bang for the buck is unbeatable IMHO. Still, there is more to a design like this than the DAC and once the hardware has been settled on the rest is up to the firmware that runs on it. We will be able to re-use a lot of the code that was used on the DSP Servo board because the code is written using highly optimized object oriented C++ language and since the hardware is almost identical means that portability is assured.



The user interface is being finalized....


In our up and coming flagship subwoofer we have decided to go all out with a front panel user interface and full color QVGA LCD display. This enables us to customize the user interface and allows us to expand the functionality by upgrading the software. Front panel access also means that it is easy for the user to setup and adjust the subwoofer without having to access the back of the unit. The following are screen copies from the actual LCD display.


Panel designs ready for CNC milling....


Complex panel work requires precision CNC cutting equipment to get everything to line up perfectly. There are no less than 4 panels that need to be fabricated for our up and coming flagship subwoofer design. This is a departure from conventional subwoofer designs which use a single plate amp and controls mounted at the back of the subwoofer enclosure thus making it hard for any adjustments without having to move the subwoofer enclsoure. Most of these designs are set and forget but because we have incorporated DSP processing we can add a variety of software controls and switches which don't have to be physically located at the back of the subwoofer. Our comprehensive and flexible front panel user controls is just one of many features that we will be incorporating into our designs.

Shown below is the front panel which consists of a 5 button user interface, one rotary control, full color QVGA LCD and various status LEDS. Front panel controls are a rarity on most subwoofers so adjustments are awkward if not impractical. Even designs boasting DSP control provide minimal interaction for the user simply because the DSP code is not accessible to the designer so it's almost impossible to implement anything on this other than a few prescribed filter functions.

Shown below is a preview of the DSP panel plate which mounts on the back of the subwoofer. It has numerous audio input and output combinations as well as many others that you don't normally see on a subwoofer.



Audiophile grade class-D amp and matching switch mode power supply.....


We've spared no expense in choosing the right amplification in our up and coming flag-ship subwoofer product. It just so happens that Hypex have exactly what we need and it is well specced and documented. We didn't want to go down the route that so many other subwoofer manufacturers have gone through and chased down the price at the expense of performance, reliability, features and functionality !! We've heard so many disaster stories from owners of low cost subwoofers because the manufacturer skimped on the amplification and left the customer with an unworkable or unreliable product. This is because subwoofer vendors who don't offer performance or value can only compete on price and the only direction forward is to chase down the price until the product becomes questionable. Even vendors who offer higher power amps and multiple drivers are not really offering anymore in terms of value than their cheaper cousins. Most of these designs could be acomplished by any diyer with the ability to build a suitable enclosure and design simple EQ filters.

We decided from the outset that chasing down the price was not an option for us nor was building yet another run of the mill vented or sealed box subwoofer with the same generic plate amp and some EQ to flatten the response. This has already been done to death so we have taken a fresh approach and approached this from a completely different angle. Instead of chasing down the price of what is an otherwise a no-frills design we have added significant technological value and performance so much so that our subwoofer designs can no-longer be classed or compared to existing designs. In fact our subwoofers can't be classed as subwoofers in the traditional sense even though they can still be configured to operate as such. If that sounds confusing then at this point in time we can't go too much into details at the moment because a lot of it is propriertary but by all means keep visiting this website for updates.

We also didn't want to participate in the SPL wars where louder is better and a substitue for quality when it is not ! Our main focus has been quality of reproduction but this has not been achieved at the expense of quantity which is why we have specced in a 2 kilowatt audiophile power amp. With our subwoofer designs you can have your cake and eat it too !


Putting the finishing touches to the metalwork.....

Shown below is the outline of the plate amp and subwoofer plate controller that will be used across a broad range of subwoofer models. This is a 12 inch by 13 inch form factor. Not shown is the metalwork for the front panel controls. The plate controller also features the Hypex 2000 watt class-D power amplifier and Hypex 3000 watt SMPS.

Boards are in and assembled ready for testing.....

Prototype boards are in and assembled for testing. Our Interface adaptor board is up and running and we are able to achieve accurate voice coil current sensing using a bridged class-D amplifier even with outputs that float at half the rail voltage such as that used by the Power Physics A-1014 1000 watt class-D amplifiers. This means that high common mode rejection of high voltage and noisy signals is mandatory. Low noise current sensing of the voice coil is an integral feature of our servo design and is needed to provide current feedback for the current servo amplification. The Interface adaptor board can accomodate other amplifier topologies such as single half bridge or full bridge ground referenced amplifier topologies. This is no mean feat but with our propriertory symetrical balanced drive configuration we have now achieved accurate and low noise current sensing with a variety of amplifer topologies whether they be full-bridge or half-bridge arrangements.

Shown in the photo below is our new Interface adaptor board piggy-backed on to our DSP Servo board which is piggy-backed onto the back of a Power Physics C-1014 combo chassis. This board has a number of other functions besides the current sense interface such as the main supply DC regulators and preamplifier boosters for some amplifiers such as the Hypex which we bypass on their boards. To the bottom left of the photo you will note the User Interface board which consists of a variety of LED indicators, push buttons, rotary encoder and Infrared remote control receivers. Also to the bottom right we have our new USB Interface board. Not shown is the Audio Interface or IO Board which consists of a variety of XLR balanced and RCA un-balanced inputs and outputs including one XLR input for a microphone input used for measuring the speaker acoustic rersponse as well as user room correction feature.

Progress Report


Finalizing the next revision board updates. We have added two new boards to make it better for design for manufacturability. The Plate IO board will contain all of the audio connections on a single board and will mount directly onto the aluminium plate which typically mounts in the back of the subwoofer or active speaker.

Previously we had separate boards for different audio IO connections and the connectors were all right angle connectors. The main problem is preventing air leaks through the connectors so a new approach was sort without having to resort to using lots of silicone sealant to seal the connectors which can become very messy and also make servicing difficult. The Plate IO board and DSP Servo board will be contained within a sealed aluminium enclosure and the Interface board will seal off this enclosure as well as being the conduit between the DSP servo and the internal enclosure volume. The Interface board will feature additional functionality that was found to be lacking in the Power Physics A-1014 module and it will also allow us to interface to a number of other vendors class-D amplifier modules such as the Hypex UcD OEM700 and UcD OEM2000.

Implementing a current servo and current sensing with a bridge output stage can be a bit tricky but we have finally nutted it out and the Interface board caters for half bridge and full bridge output stages even if they float at half rail voltage such as the Power Physics designs do.

Although the DSP Servo board Rev:B is essentially finalized we won't be getting the boards made just yet as the currently working Rev:A boards will suffice and prototyping fine pitch SMD boards can be a royal PITA. The main issue was getting the Interface board designed to overcome the limitations in the Power Physics modules. The Plate IO and Interface boards all use through hole technology so it's not to bad for prototype assembly. I don't mind prototype SMD assembly and wave soldeing 176 pin QFP packages but 0402 chip resistors can drive me batty after a while.

A website is born !
The Australian AV and Hifi Show
  This time the show was located at the Intercontinental Hotel at the Rialto building in Melbourne which was a much better venue than the previous one in 2011 which as I recall was small, hot and stuffy with no air conditioning ! Lots of imported gear as usual mixed in with the usual local mobs. It's good to see the local mobs standing their ground in an ever competitive field.




Last Updated April 9, 2015