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MatM 30th November 2011 04:03 PM

Project FrankenAmp: 5.1 decoder/home cinema amplifier
1 Attachment(s)
First post on the subject, but this has been in the invention stage for a few weeks now...

It all started when my venerable Accoustic Energy 5.1 went on the fritz last month. Thanks to kids, etc, it's the only bit of hifi I've got left and is used for everything: TV, music, blueray, dvds, ATV streaming, etc, etc. My immediate response was to start trawling the usual suppliers trying to find a replacement home cinema amplifier on an extremely tight budget. Not only did I realise I'd be getting a load of rubbish for the cash (because I'm pretty skint - refer back to statement on kids), but I'd get a load of rubbish in a big metal box with far too many flashing lights, buttons, knobs, and everything else I'll never use and don't want sitting under the TV! Hence, only one solution, design my own...

This isn't going to be a spec-beating audiophile system with THD=0.0001%, its going to be a compromise solution, based on:
- cost (I'm skint)
- complexity (for diy build)
- power enough to fill a typically british living room (i.e. pokey)
- size and functionality (i.e. invisible, but very very audiable)
- 'good' audio quality (it must be as good as the digital sources driving it)
- energy efficiency (checked your electric bill lately?)

The main requirement is to beat the pants of any 200-300 home cinema amp you could buy. Hopefully we'll achieve something much better.

Ok, lets start with the immediate fall-over condition: proprietary licensing. Anything with a Dolby badge needs a dolby license. Hence, even though you see plenty of 1-chip dolby decoders, you can't buy them without waving the magic license. Not only is this licenses expensive, but it's also very restrictive in publication. I did a job a few years back that needed it and was told flatly "no" because my middle name isn't Pioneer or Kenwood.

So, a couple of beers later and the solution presents itself. Dolby Digital, aka AC-3 got standardised for ATSC audio transmission. As such, its not a free protocol (provided you don't call it Dolby): ATSC A/52. GNU already has an open source decoder library for doing this, as they have for DTS (but don't call it DTS, otherwise ker-ching).

This means we're going to need a reasonably decent micro with I2S ports galore. More later...

Next fall-over condition is complexity of design. HMDI inputs are right out, at the moment. We're talking gigabit LVDS SerDes signalling - this is definitely out the realm of my PCB skills and any micro I could buy. Perhaps the future evolution would be to put a cheap FPGA dev board in there, performing audio extraction. So, we'll stick with S/PDIF.

The overall system design works out to be:
- S/PDIF input processing with source selection
- central microprocessor
- 6 channel DAC output and electronic attenuators (need customisable attenuators for levelling 6 channels)
- user interface / control
- power supply
- amp x 6

I've attached an overview design showing very simplified makeup.

For the micro, I'm thinking of something very very new - actually came out in perfect time for this project: Beaglebone (google it)!! This is an embedded micro board running linux. The processor is an evolution of the TI OMAP processor which is essentially a DSP+ARM combo. Perfect for audio processing and comes with lots of I2S interfaces!!

I'll slowly upload the design over the next few days.
Got the SPDIF design: Wolfsson WM8805 - low cost, good performance and easily available
Got the micro hopefully sorted: (Beagle)
Got the DAC/attenuators: Wolfsson WM8766 - perfect fit!

Currently worrying about the amp. My immediate thought is to use Class D. Partially because they don't need massive heat sinks (breaking size requirement). Partially because they're very efficient (saving my electric), and mostly because my current amp is a class D variety (although proprietary solution) and I'm still in awe of the detail/clarify of reproduction.

I spent a long time worrying about power ratings. My current system has never seen the volume knob go higher than a quarter turn, so this is a good benchmark. Found it very difficult to get a proper power spec for it, so looked at the PSU rating instead, and boy was I suprised. The entire 5.1 system, including sub power, is running off a 12v/5A single rail supply!!! So much for "it needs to be 100w/channel"!! I'm hazarding a guess that because class D is ~85% efficient, you can effectively half the power requirements from a class A which is ~40% efficient...

Anyway, any comments/thoughts on the amp would be appreciated. Ideally needs to be single rail supply, to fit with Switch-mode requirement (got my eyes on an18V/8A from HP docking station or 15A/10A meanwell on RS). TK2050 chipset could be perfect, but thats 3x 16 just for the chipsets alone, unless I can find another supplier.

Right, I'll stop waffling. The point of posting is to get thoughts/feedback/suggestions/ideas/help/whatever??? If I'm going to the time/expense of producing my own designs, they may as well get used more than once!!


MatM 30th November 2011 07:46 PM

2 Attachment(s)
Draft schematic and PCB for S/PDIF input board attached.

Svendingo 16th December 2011 08:52 PM

Subscribed :-)

Sent from my DROID using Tapatalk

CECCO 17th December 2011 02:27 PM


I'm gonna put some infos toghether to build something like your amp.
Unlike yours, mine will be slightly simpler because (for me) TI processors are too complicated to program.
I'd go with a PIC18 Mcu controlling ST audio processors via I2C bus.
I just saw the S/PDIF I/F schematic and what I don't understand is why you used so many optical inputs.
Is just one not sufficient to get all the 4.1 data from eg.a DVD player?
Cheers and good work!


MatM 18th December 2011 06:50 PM

4 Attachment(s)
I'm basing most of the structural design on Eurocard-format 160x100mm sized PCBs. It makes life a lot simpler when mounting, as hopefully I can use eurocard enclosures with pcb guide rails already in place (look for verotec lbx series). This will work out as a single digital board, a single analogue board, then seperately mounted computer/control interface/etc.

The digital board schematic and PCB layout is now complete and attached. *NOTE: it's not tested yet*. This board contains a 6 channel S/PDIF to I2S converter (the input), then the 6 channel I2S to Audio (the output) Control of both interfaces is via common SPI serial bus, but each I2S line is kept seperate.

I've been testing a new (free) EDA tool for this application: Designspark. Alas most of my work stuff is done on CADStar, which I can't use for a home application. Designspark isn't bad for a free application!

So, with this post you get the digital board:
- schematic (pdf)
- pcb (pdf)
- gerber files
- entire designspark project (please point designspark to the included libraries before opening the project).



MatM 18th December 2011 07:02 PM


I'd be interested in knowing more at your mention of ST Audio Processor?

If you go with the full TI OMAP processor, then yes - it can get complex if you start delving into the DSP side. However, I'm probably going to be using "beaglebone" - when it finally comes in stock in the UK, which is a more standard ARM Cortex processor. It means essentially I'm programming Linux and only need to worry about the hardware for input/output. Remember: if you want to decode Dolby/DTS, then you need to do it in software, because you won't be allowed to by any hardware decoders without flashing your dolby license (especially not from ST - they're very pedantic and controlling of their markets).

For the S/PDIF inputs, I've settled on 4 optical and 2 co-ax. You only need a single input per device (dvd/etc), but I've personally got a few different devices I want to swap between (and it just so happens most of them are optical). If you only need a single input, then things could get much simpler, as you can either hardware-lock the WM8805, or even better; swap to a WM8804 which is a simpler device.

A total of x8 S/PDIF intputs can be supported on the chipset. I've just gone with x6 because it fits the PCB size. If a different combination of optical/co-ax is needed, just change the schematic - I've included the entire project here.


CECCO 19th December 2011 12:50 AM

Hi Mat,

The "Core" of my project will be a TDA7415 (497-11669-1-ND - 8,6€) controlled by a PIC18 Mcu (PIC18F47J53-I/PT-ND - 4,3€) programmed in C.
I will keep the things as simple as possible since this is the first digital amp I do, so no Dolby/DTS decoding.

Unfortunately, few days ago while searching the parts, I discovered that a lot of them were not accessible for DiYers.
Just to mention suppliers like Digikey,Mouser or Farnell, they have most of the intresting parts "not in stock" forcing you to buy a thousend or more pieces.

Also the Japanese parts are quite unavailable.
There are a lot good IC's with intresting functions made by Rhom, JRC, AsaiKasei etc....
Take a look here :
USB Audio Decoder ICs | Audio ICs | ICs | ROHM CO., LTD.
These chips are nowhere to be found plus, they don't give you the data sheet as well.

Said that, the part list will be determined by the "what's on the shelf" choiche.

I'm also getting infos on how to solder 0,5mm pitch LQFP64 without going crazy. That's gonna be funny. LOL.

CECCO 19th December 2011 12:53 AM

The power out section will hinge on a couple of STA516 class D amplifiers (497-11459-1-ND - 7,3€) driven by the STA321 (497-11050-ND - 9,5€).
Just to keep everything on the "digital side of the force".


i2k92 19th December 2011 01:38 AM

For amp combine this and this to get 6 channel.

This one is even simpler & cheaper : 7.1 TK2050

Have no idea about the quality, but I'm happy with other TK2050 board.

Fivetide 20th December 2011 08:39 PM


Originally Posted by i2k92 (
For amp combine this and this to get 6 channel.

This one is even simpler & cheaper : 7.1 TK2050

Have no idea about the quality, but I'm happy with other TK2050 board.

I'm looking at this exact board for 7.1 from Asus Xonar as the preamp , seems a very inexpensive way of making something that arcam sell for 1000's although I do have Arcam 250 AVR and thinking of stripping out video or cloning just the sound stage.

Also I came across this only today trawling at work , may be some tip and trick in the article you could use ?

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