Sorting out the bits

One of the ambitions I have had for a long time is to get all the binary bits from a musical recording through my chain to the loudspeakers. No matter what they make of them when they get there, which is another matter.

What do I mean? Well these days you can start with a CD, or a downloaded file which can be CD 16 bit/44.1kHz or upwards - my limit today is 24bit/96kHz as this the highest supported by my iMac and iTunes. A 24bit/96kHz file will carry a 120dB dynamic range and DC to 48kHz bandwidth. Not so good as an orchestra but a lot better than CDs.

But to throw in another difficulty in the way I insist on sending my music over my WiFi system, to avoid all those ugly wires and allow me to have the computer in one room and the system in another. So what I have is an iMac running iTunes, an Apple TV, and a spare Macbook also running iTunes. The Apple TV or the Macbook can be connected by an optical link to my DAC and the line out audio to my amplifiers.

A word about the DAC. I hate, if that is not too strong a word, all these DACs advertised as the best in sliced bread, but costing $1000s. This is ridiculous. The basic guts of a DAC are a couple of semiconductor chips, one to receive the SPDIF optical input and pass it to a decoder as a serial I2S signal. The decoder is the guts of the system and many semiconductor makers offer products for this function. However sorting the men from the boys there is only one company, English, that leads the field, this is Wolfson. The make both chips required, the WM8805 SPDIF and the WM8740 DAC. Together they cost $10! So why $1000? The problem is to find someone who makes a DAC with these chips. Fortunately a company in Hong Kong, HA INFO Audio Electronics, has seen the light and makes a rather neat DAC with these chips. It has digital SPDIF and Coax input up to 24bit/192kHz, and line and headphone output for under £100. Which is a great improvement over $1000-2000!!!

Then the amplifiers. Here again prices of equipment are ridiculous at anything up to the $5000 range. I am a great fan of class D amplifiers as I believe they can give excellent quality and performance, including transient rise times, and have high efficiency so they run cool. However looking again round the industry there are few amplifiers that quote a DC to 50kHz bandwidth. I believe we have to maintain the bandwidth all the way from the computer file to the loudspeaker terminals. And this means DC to 50kHz for a 24bit/96kHz source file. One company that makes such amplifiers is Hypex with modules up to quite high powers - 400W or so. I use their 180W models in a small case with a conventional power supply, the only requirement being to use massive capacitors of 10,000uF for both positive and negative rails. The result is a low cost amplifier (total about £200) which has excellent performance and DC - 50kHz bandwidth.

Getting the audio from the computer to the DAC SPDIF optical input can be done in three ways:

1. iMac > Apple Airplay > Apple TV > DAC. But this compromises the bandwidth as Airplay down-samples files to 16bit/44.1kHz. And this is what comes out of the Apple TV. So that is no good.

2. iMac > file copy > MacBook > DAC. Copy the 24bit/96kHz files to the MacBook and output SPDIF optical from there to the DAC, in this case the system does support DC-50kHz bandwidth all the way though. But files have to be copied.

3. Register both iMac and Macbook with Apple for “Home Sharing”. This allows the MacBook to see and retrieve the music from the remote iMac. And, wonder of wonders, it can retrieve 24bit/96kHz files over WiFi with NO down-sampling. This means that in this way high quality music files can be played over my WiFi system. (I Wonder why Airplay and the Apple TV down-samples? WiFi can easily carry the bit rates of the 24bit/96kHz music (2Mbps or so). Its probably because the Apple TV has a rather slow processor built-in??? Anyway Apple’s iTunes preferred music delivery system is sadly only 16bit/44.1kHz AAC compressed music...).

So now the problem is solved. If you are a newbie then I strongly suggest to use a setup of:

- Music Centre, either remote iMac or local Mac Mini - or both

- Itunes with Home Sharing enabled, HD tracks downloaded from the many on-line stores (Linn, 2L etc)

- The DAC and Amplifiers I mentioned above (Google them).

- Finally an iPhone or iPad with Apple Remote to control iTunes running on one or both of the Macs.

Don’t forget to enable 24bit/96kHz sound output using the utility Audio MIDI Setup on both Macs. And restart iTunes after you have done it.

A headless system can be created by using a Mac Mini as your music centre and controlling it by Screen Sharing / VNC Viewer from a remote iMac or iPad. In this way you can configure iTunes, buy HD track music and have a totally integrated low cost HiFi system.

Now about those loudspeakers which I want to drive from DC to 50kHz, what about those? Suggestions welcome.