Computing stuff tied to the physical world

The world of audio

In Hardware on Dec 13, 2012 at 00:01

There’s a huge world out there which I’ve never looked into: audio. And it has changed.

It used to be analog (and before my time: vacuum tubes, or “valves” as the British say).

Nowadays, it’s all digital and integrated. The common Class-D amplifier is made of digitally switching MOSFETs with some cutoff filters to get rid of the residual high-frequency this generates. Leaving just the “pure” audible portion to drive the speakers.

With the recent switch to a new small TV, away from the Mac Mini, for our TV & music system, I lost the original hook-up we had, which was a (far too cheap) little analog amplifier driving (far too expensive) speakers we’ve had here for a long time.

So now we have this TV with built-in tiny 2.5W speakers blasting to the rear – a far cry from the sound we had before. And no music playback capability at all in the living room right now. Not good!

Our needs are simple: CD-quality music (we’re no audiophiles) and decent TV sound. I am going to need a setup soon, as the Christmas vacation time nears.

Trouble is: the sound source for our music is on the Mac Mini server, which is in an impossible place w.r.t. the TV and the speakers. So my first thought was: an Airport Express. It can play over WiFi, and has optical audio output. But… the AE draws 4W in standby. And turning it on for each use is awkward: waiting a minute or more to get sound from the TV is not so great.

The other options for music are an Apple TV or a specially-configured Raspberry Pi.

The only remaining issue is how to get sound from line-level analog audio or (preferably) digital audio to the speakers. I ended up choosing something fairly simple and low-end, a component from miniDSP called “miniAMP”:

DSC 4302

This takes all-digital I²S signals and produces 4x 10W audio. It needs a 12..24V @ 4A supply, i.e. a simple “brick” should do. But that’s just half a solution: it needs I²S…

This is where the “miniDSP” component comes in (the SOIC chip at the top is a PIC µC):

DSC 4301

So the whole setup becomes as follows – and I’ll double up the miniAMP (one for each channel) if the output is not powerful enough:

Screen Shot 2012 12 12 at 23 11 57

The miniDSP takes 2x analog in, and produces up to 4x digital I²S out. The nice part is that it’s fully configurable, i.e. it can do all sorts of fancy sound processing:

Screen Shot 2012 12 12 at 23 21 17

This is perfect for our setup, which includes old-but-incredibly-good separate speakers for the highs and the lows. So a fully configurable cross-over setup is just what we need:

Screen Shot 2012 12 12 at 23 23 11

The way this works is that you set it up, burn the settings into the DSP front-end via USB, and then insert it into the audio chain.

It’s tempting to start tinkering with this stuff at an even lower level, but nah… enough other things to do. Although I do want to look into auto shut-off at some point, to further lower power consumption when no audio is being played. But for now this will have to do.

  1. This timely – I recently tried to use the Raspberry Pi in my living room for Christmas — I was disappointed in the built-in audio output from the Pi — some sort of PWM gizmo — so I added a cheapo USB audio dongle, but it only accepts 48 kHz samples, and so the PI’s cpu was used up resampling the 44.1 kHz music into 48 kHz in software…

    My project is on hold untilI I find a better solution. I will stay tuned to see what you decide…

    • Well, this is probably not going to help you, but I’ve decided to use an AppleTV box for now. Not very DIY’ish, but it was plug and play for me. Recent RPi’s have some changes to better support I2S, so there will probably be a lot more developments in that area. Will definitely track it.

      IOW, my setup is: iTunes -> WiFi -> AppleTV -> HDMI -> TV -> SPDIF coax -> miniDSP -> miniAMP -> speakers. Not 100% there yet, but the analog setup works.

  2. hi jcw, I m impressed by your contribution in the arduino area, and I m just starting myself with rfm12. when it comes to audio, I see where you come from and the direction you take, as I probably went trough this journey myself last year! it costed me a lot of money and time to reach a point where I m proud of my audio system and just amazed by its quality.

    minidsp is one of these boards I bought which will end up one day on ebay. that is a very very great product, sure. but once you put the fingers (and both ears) in audio, its a continuous moving forward feeling where the expectation is 2 time higher the day after. some sort of exponential attraction to heaven (or hell).

    Some site like diyaudio.com are amazing and plenty of good ideas and good facts.

    If I have a single suggestion I d say, if you want to try the digital audio and the dsp world like for doing cross over, pay a very special attention on digital to analog (from dsp to dac and preamp): – first make sure you put a fifo between the DSP and the DAC , yes this is the most amazing component here. – second choose the best precise clock of the world (cannot be below 100 bucks). yes this is the other most amazing component :) – chose the best DAC chip like WM8742 with minimal passive output like 600/600 transformer. – fourth make sure you provide your dac chip with a decent power supply.

    then you ll start feeling better;

    about class A, well, amazing too, but only listening can help you to decide. I cannot move mself out of traditional current feedback amplifiers like accuphase ….

    good luck for your journey and keep us posted ! fyi, here is my personal experience in building up my own power bi-amp. in french, but the picture might be self explaining :) http://www.homecinema-fr.com/forum/travail-de-groupe-diy/diy-quad-406-sur-base-accuphase-t29956757.html

  3. Thanks for the info (French is no problem for me). You’re aiming for considerably higher-quality audio than me, I suspect. I am very pleased with the miDSP/miniAMP setup. So far, anyway.

    I still do have a 2x60W Kenwood amp – looked up the specs and the schematic the other day, and it’s DC to 400 KHz @ 3 dB. But it’s unwieldy, and not bi-amp, so not perfect for separate high/low speakers.

    Right now I’m still using the various built-in low-end DACs, but once I get SPDIF hooked up, the chain will become all-digital. It’ll be very interesting to find out whether I can actually hear a difference…

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