Advice for new computer audiophile

[matthew]

Hello! I'm new to using my computer as a source for hi-fi audio in my also newly forming dedicated listening room. My interest is only in 2 channel, reference quality sound. My set up consists of a Jolida 302B integrated amp, an old 200 disc JVC cd player (my obvious need to upgrade to something else!) and Klipsch RF-82 speakers. The computer I would like to make my music server, is a Mac PowerBook G3 with 400 mHz processor, 2 usb and 2 Firewire ports. The sound is good with this limited setup, but I could definitely improve my sound by listening to HDCD's, etc., and technologies that would make better use of my CD's (which brings me here, and an excellent site btw). Being brand new I have some questions. Here goes:

 

1. From what I've read, USB is not favored over Firewire for streaming data to the dac. Am I reading this right? My USB's on the Mac are ver. 1.1. If I use the firewire option (or USB? based on your recommendations) would I be better off going to an external sound card that could "clean" the sound and output it into a more common dac input such as SPDIF or Toslink, etc? (there don't seem to be as many dacs that accepts firewire).

2. If I go the more common route with USB, would the fact I only have USB 1.1 prevent me from ever playing anything above 24/96? If that's the case, what is the sonic difference between a HiRez song at 24/172 played without downsampling (like it would through a USB connection) and one that downsampled from the USB 1.1 and then sent to an upsampling dac that then takes it back up to 24/172? Does that make any sense? hehe, I hope so.

3. Finally, if anyone has a common setup like mine (old PowerBook G3 mac, etc.) and has had the chance to try going out directly to a dac versus going to an external sound card + dac (external I'm assuming is my only option on a laptop like this), and the difference in sound quality and reliability...please, PLEASE let me know what you think!

 

Thank you kindly for the help, and look forward to contributing more in the future.

 

cheers,

Matt

 

[tfarney]

Hi Matt.

 

I can't go all the way back to a G3, but my system is based on an iBook G4. USB is fine. Firewire will give you the option of leaving your USB ports open for other things. You may need to plug some USB audio devices directly into the Mac, or use a really good, powered hub for best results, but USB is fine.

 

External soundcards, or DACs (digital/analog converter) don't "clean up" the sound per se, they do what they say they do. They convert zeros and ones to voltage that can be turned into music in an analog audio system.

 

There is a DAC already built in to your Macbook, but there may be two problems: 1) The inside of a computer can be a very noisy environment full of interference that will add noise and subtract resolution from your music. 2) Dedicated, external DACs come in all price ranges and some sound noticeably better than what is built into the typical computer soundcard.

 

And here are two things to bear in mind: 1) It might not be noisy inside your Mac. They are much more audio-friendly than most PCs. My iBook is very quiet. 2) Those dedicated, external DACs that sound noticeably better than what is built into the typical soundcard? They aren't necessarily the expensive ones. Buyer beware and all that.

 

With all of that said, you're on the right path, brother. A Mac, a DAC and your reference system is the headwaters of the watercourse way to listening enlightenment.

 

Tim

 

[markk02474]

I'm pretty sure Audio USB is only 1.1 and there are not any USB 2.0 receiver chips - 24bits * 96 kHz = 2.3 Million bits/second/channel. USB1.1 = about 8-10 Mbits/second. Firewire at 400Mbits/sec can do higher rates. The real issue is that the external device is stupid. We'd like it to buffer up the right amount of data from the computer and ask for more when needed. A musician wants to buffer very little so there is less delay between hitting a key and getting the sound. Playback doesn't have this requirement. Buffering is needed because the computer and the link to the DAC or spdif out won't be able to perfectly time when data is sent - they could be busy with other things.

 

The problem with SPDIF is jitter. With minute variation in when the input data is converted to an output voltage, you end up with a different voltage than you would have gotten if conversion happened at the exact time it should have. For music, anyway - if the number was constant, the output voltage would be constant, subject to dac error. The DAC getting the jittery signal can compensate for some of the variation with PLL re-clocking and sample-rate converters. Still, most people report less spdif jitter sounds better. The hot PCI card to use is one from Lynx with under 20 pico-second jitter. I don't know about cardbus or other alternatives for laptop.

 

Noise inside your computer is only one possible problem. The certain problem is not having enough room for independant power supplies to the dac and opamp buffers, and not enough room for good low-pass filters after the dac chip.

 

If you look at data sheets for DAC chips, you'll see that there are at least three power supplies and grounds: Digital, Left Analog, and Right Analog. Frequently there are more, with voltage references for L and R channels, though these are most often implemented as being decoupled off the respective analog channels.

 

The filtering after the dac reduces digital high-frequency artifacts above musical frequencies. These are usually implemented with resistors and capacitors. Both types of components have a wide range of sound quality and cost. Physical constraints dictate use of surface-mount parts, and thus the capacitors will be ceramic types. Within those, NPO types are better for audio because capacitance varies less with voltage. NPO types cost more, are larger, and are hard to distinguish from cheaper ones visually. Guess which kind get used most?

 

Opamps used to drive headphone-levels of output are usually not as good as the best ones that are only driving line-level outputs, so an external box can use an opamp for each need. An excellent sounding stereo opamp costs about $2.50. Most gear use $0.20 opamps.

 

Just about any consumer and most pro audio cards and dacs will benefit from modification. Better parts, power supplies, and minor design changes can help a great deal. The next level would be better clock circuitry. After that comes premium parts and power supply mods, space permitting.

 

If you downsample data, you are throwing it away. Upsampling later does not get it back! You may not lose all that much, however. Most DAC chip specs. indicate they don't resolve much better at 172 than 96k. The difference is going to be the low past filter they can use for each, with the one for 96k imparting more phase distortion into audio frequencies than the 172k one. You should check the datasheet for the dac chip being used to make sure it switches the digital filter being used. If all is good, upsampling back to 172k will use the less distructive filter.