iMac Sound Quality

"I'm not positive about this but, in general, I think you are better off when you use the USB out than a other out's."

Pardales- the USB output is powered by the PC's PSU and will transmit it's noise to the USB converter/DAC. The iMac's digital output is fiber, so the PC's PSU noise is isolated from the DAC.

Of course there's the issue of which output has lower jitter.

Better off with USB output for sound. First off, just because something is fiber does NOT mean it is the better choice. In this case if your goal is to isolate then yes you can use the optical connection. But...USB is a bi-directional connection, so timing errors are dealt with on the way in and on the way out. Not the case with your optical. Doesn't mean jitter is eliminated, don't know there is a connection that does that yet, but it is certainly better for sound quality.

If you want the best of both worlds, isolation and sound quality, you can simply use an Optical USB cable. One called Opticis is one that has been successfully used but people who want to isolate and use USB. Altough the PS on the Opticis has been called into question in the past, most people who have used it speak hightly of it.

I can say, from experience that I use a Kimber USB on my system from a MAC, and it is as quiet as a church mouse, and dynamic peaks come from BLACK backgrounds. So for me personally, isolation is not worth it.

At the high end, the best solutions have been USB converters that reclock the signal at a high level of accuracy to eliminate jitter, such as the Empirical Audio Freeway or Off-Ramp, or the Wavelength Brick or Cosecant. Also at the high end are networked devices, such as the Slim Devices Transporter, which uses the ethernet to pass the signal from the computer to the DAC. If you are at the high end, it is easy to find a good solution.

Since my budget does not currently allow me to be at the high end when it comes to getting the signal from my computer, I have the same question that you have. I am running audio from USB to an M-Audio Audiophile USB and then running S/PDIF to a fully tweaked-out MF X-DACv3. I get a large sound stage, great dynamics and an overall musical presentation, but insruments could be better defined and the sound could be smoother, as is evident in comparison to my ModWright Sony 999ES. The problem is prior to the DAC, because it goes away when I connect the Sony to the DAC with a glass Toslink (i.e., using the Sony as a transport).

I have been reading a lot of posts and I can share what I have learned so far:

1. There is a huge difference between TosLink cables. The good ones use a large number (say, 128) of glass (not plastic) fibers and highly polished glass lenses. There is an Audiogon member who sells goods ones at a very reasonable price. Just search for "glass Toslink". The bad ones have helped to give optical cables a bad reputation, although according to Gordon Rankin at Wavelength, there were also problems with the early optical transmitters and receivers.

2. The optical connection on the iMac (which I have) requires a mini-Toslink adapter and I have not been able to find any that say that they are made of glass fibers, with highly polished lenses, as opposed to cheap plastic. This leads me to suspect that they are cheap plastic. I haven't been able to find any information on how much of a difference this makes, given the short length of the adapters.

3. If you need to do a long run of the Toslink cable (say, over 3 feet), it is recommended that you use an extender to boost the signal. Specifics on the quality of these extenders is also hard to find (glass or plastic innards?).

4. Posts that compare direct Toslink optical to the use of a USB/digital converter are hard to interpret if they don't indicate whether or not they used a quality glass cable for the comparison.

5. There are many posts that give extremely high marks to the Trends Audio UD-10, which can be had for about $130. It can run off of USB bus power, but will sound better with the optional battery-powered power supply. But many say that it works best with an external 5 volt, 1.5 amp linear, non-switching power supply, which is the same power supply used on the Squeezebox 3, which power supply you would have to buy separately from a company such as Digikey. For even better results, Bouldercables.com will modify the power supplies. There are many threads on the Trends unit on Audiocircle.com.

6. The HAGSUSB as also received some favorable reviews. It can be purchased already assembled or as a half-kit.

7. For use as a USB to S/PDIF converter, some have said that comparison tests show that the Trends is better than an unmodified Squeezebox 3. Others, however, have said that the Squeezebox 3, modified by Bouldercables.com, is better by far.

7. At least on the Mac, the built-in USB audio drivers are high quality and are said to be better than the proprietary drivers of some USB/SPDIF converters. One post indicated that, on a Mac, both M-Audio's Audiophile USB and Edirol's UA-5, using their special drivers, were inferior to the Edirol unit running off of Apple's built-in USB audio drivers. Unfortuately, the Audiophile USB cannot be used without its special driver.

8. If you are using a converter that runs without the installation of special drivers, it is reported that you can use the Audio MIDI Setup.app (in the Utilities folder of your Mac) to turn on software-based upsampling -- useful if your DAC does not upsample and your converter permits an upsampled signal to be passed to the DAC. Since my converter requires a special driver, I have not been able to test this.

I may have run on a bit long, but perhaps you can benefit from my research over the past week or so, with a bit of a Mac persective. I am also waiting for a definitive answer to the questions, "What is the best moderately-priced USB/S/PDIF converter for use with the Mac?" and "How does that compare with a direct optical connection from the iMac to the DAC?"

Cheers,
Bill

Bill-

Thanks for your reply.

My son just a MacBook, so I hope to test it's optical output
using a Monster Cable ICable.

If you look at my system, you'll see I'm using a modified SB3 with a linear psu.

Aloha,

Dan

PS- On the subject of glass verses plastic Toslinks, see:

http://www.lifatec.com/toslink4.html

Dan,

Thanks for the link on the plastic cables. It is good to hear that they now come with higher purity/transparency materials.

You may find this link interesting: http://www.impactacoustics.com/links/optical_interconnect_basics.html

I would like to hear about your experience with the MacBook's optical out.

Cheers,
Bill