Mac volume and external DAC

[rocdoc]

Some questions spawned by my technical ineptitude.
I use my iMac as a source, into a Fubar III USB/DAC into my AKG K601's.
Here are the points I don't get:
1. If I use a USB DAC, shouldn't all the sound processing go through it (USB connected, the computer preferences identify it correctly as the output hardware)? Why does the main computer volume control stay in the circuit?
2. That being the case, is there a certain level I should keep the computer output volume at? I notice that keeping it at max and working from there with the Fubar's volume gives me more dynamics than keeping it halfway and turning the volume on the Fubar higher. That would be fine, except it leaves me only the first about 10% of the volume range to work with. That's because I use high gain on the Fubar, again I hear a difference in dynamics, and TOdd from Audiophile Products thinks it's because the low gain adds one more resistor and switch to the circuit. Any thoughts?
Thanks for your insights.

 

[rocdoc]

REALLY? No mac geeks out there? Nobody interested in discussing something so sweetly geeky?

 

[rocdoc]

OK, an update.
I just connected the Fubar to my other mac, an iBook pro. It does what I know it's supposed to do, takes the computer volume control out of the loop, and now the Fubar does everything, and guess what, it sounds a lot better.
What is different in the settings on my iMac? Why does the sound control stage stay in the loop? I am guessing that in this case the DAC is still the mac's, and I am only using the amp.
HELP!!
Thanks

 

[pdennis]

If the iMac is connected to the FUBAR by USB, the iMac's DAC is not being used. USB only transmits digital signal.

 

[spyderx]

That's strange. I assume you've played around with the Audio MIDI Setup. On the Mac just set all volumes to MAX if they are adjustable (in addition to turning off all other sound processing settings in iTunes, VLC, whatever). It's been proven this does bit-perfect playback (at least with TOSLINK). I would assume this should apply with other playback mechanisms also, as you're not using software to attenuate the signal/output.

 

[rocdoc]

Thanks guys. I have an update actually. I played around more and tried setting it up as a new MIDI device. That seems to have done the trick. It now shows up in sound options as "aggregate device", with all sound controls in the mac disabled. And it sounds good. Does that make any sense? It's very strange. On the iBook I had to go through none of this, it did what I read it was supposed to do, no controls of the sound from the computer. On this iMac I had to jump through hoops as described above. Can anyone tell me if I understood this correctly, and now that it's set up as the "aggregate device", whatever that means, I am finally bypassing the internal controls? It also shows up on the USB hub, as it did before, working as before, with the computer volume and balance etc. controls in place. Weird.
Thanks again

 

[pkshiu]

What version of OS X are you running? I have the same setup, iMac Tiger to Fubar III USB.

It just show up as "PCM 2702" as an additional output device.
You should set volume to Max.

BTW I highly recommend using "SoundSource", a free menubar utility to switch easily between output devices. It will switch the system output vs other output independently if you want to.

 

[rocdoc]

I'm using 10.4.11
So, it's normal for the computer volume control to remain in the path? It does not on my laptop.
What have I done with the MIDI device set-up, to remove it?
And unless it's pure placebo, why do I think it sounds better now?
Technical stuff is hard...

 

[littletree76]

I have used the same Firestone Fubar3 USB DAC with various generations of iMac and Mac OS X (Tiger 10.4.9 / Leopard 10.5.2) without such problem. USB is a digital bus system and can only transmit audio data in digital format, thus system volume setting should not affect output volume at the speaker/headphone. Unfortunately I have replaced the Fubar3 DAC (USB) with SpitFire DAC (coaxial/optical) and sold the Fubar3 DAC some time ago.

Recall from my memory on setting up Fubar3 DAC with latest model of 20" aluminum iMac:

- Connect USB port of Fubar3 DAC to any USB port on the iMac (all 2.0 USB) but not through USB hub.
- In System Preferences -> Sound -> Output window, select PCM2702 as output source. All other audio outputs should be mute by the selection.
- Fire up Appplication/Utilities/Audio MIDI Setup utility. In Audio Devices window, select properties for the same device PCM2702. Set the sampling rate and bit width for audio output format. Note this step is optional if you satisfy with default settings. In the display you should notice that PCM2702 device is 16-bit instead of 24-bit which might limit dynamic range.
- Thats all and no aggregate device has to be created through Audio MIDI setup utility.

Note volume control on Fubar3 DAC not only control sound volume at headphone output but line output on back panel as well. Apparently there is only one built-in amplifier to feed both headphone and line outputs. I always use lower gain setting to minimize noise floor of any amplifier. In the worst case, contact helpful support staff of Firestone in Taiwan through support site: http://www.firestone.idv.tw

With Fubar3 DAC for external sound card and OSS-3D or iWow plugin for digital signal processing to enhance sound quality, you can have quite descent audio system to play music from iTunes. Another piece of software worth considering for normalization of sound level across all sound files is iVolume plugin.

 

[TSi]

For me, I use leopard, once i connect my iBasso D2, it turns off the mac DAC / volume and just goes straight to my dac.

 

[rocdoc]

littletree,
Thank you for the detailed reply. I figured out where the problem is: When I open the Audio Midi setup under devices, I select the PCM 2702 and instead of a properties dialogue the window is grey and says "output not supported". Now I'm really in trouble, aren't I? Again, on my other mac it worked right away, so it's not the unit. What do I need to do on the mac?
Thanks again

 

[rocdoc]

Also, are you saying that the DAC in the Fubar will only work at 16 bit instead of the original 24 bit? Maybe that's the difference I am hearing? It really sounds like a lot of the depth of information has been taken away from the music when I run it through the Fubar. If that's the case, why on Earth would someone even get an external DAC??

 

[PhaedrusX]

the USB protocol only supports the transfer of a 16 bit digital signal.

 

[rocdoc]

Thanks Phaedrus.
Hmm. What am I missing here? What is the point of a USB DAC then? Why on Earth would you want to hear compressed sound? That was what it sounded like to me from the beginning, without knowing this, and now that you confirmed it, I think I solved the problem. So, can someone tell me the secret here? Or are the USB DAC's to be used purely for mp3's etc., and not for listening to CD's? Because in that case it all makes sense.
Please share your knowledge.

 

[PhaedrusX]

redbook CDs (the most common kind) are packaged in 44.1khz 16 bit format.

you will not lose any information using a USB DAC, as it is taking in this entire signal (assuming you get bit-perfect signal out from your computer, which Macs do).

some DACs will then upsample the signal after it is received, others will not. that depends entirely on the type of DAC you are using (oversampling vs. non-oversampling). they just use different methods to filter out the noise.

if your computer volume is adjustable with your DAC connected, just max out the volume. likewise the iTunes volume. turn off the equalizer, and you should be getting the full signal to your DAC, no compression, no loss.

hope that helps.