[MrHeuristic]

First, I'd like to say hi — this is my first post. I'm looking to make my first foray into headphones. As a college student with just a pair of Logitech Z2300 computer speakers, I'm excited for the upgrade to a headphone amp and headphones. 
 
I've already purchased a TU-882 amp (I know, I know, tubes, I could probably have gotten a SS with more accurate amplification for cheaper. I thought a kit would be fun though, and it's not OTL) and I'm looking to pair it with the AKG K702.
 
Now, pretty much everywhere else on Head-Fi, I see recommendations for expensive DACs. Some members even suggest spending the majority of one's budget on a fancy DAC, and claim a massive difference in sound quality. 
However, from my short reading time in Sound Science, I've noticed that there's a massive amount of confirmation bias, wish thinking, and unevidenced poetic ramblings in the audiophile community (lots of parallels to the religious community, actually).
 
So, I'd like to ask here: is there an objective, audible difference between an external DAC and the internal DAC in, say, my iMac?
More specifically, has anyone else here with both an iMac (or Mac Mini, MBP, or Airport Express) and a DAC done any blind ABX testing between the onboard DAC in the Apple computer and the external DAC? 
 
I'm already out $600 for the headphone and amplifier, and if I can avoid buying another component on my college budget, I will.

 

[stv014]

Quote:

Originally Posted by MrHeuristic /img/forum/go_quote.gif
 
So, I'd like to ask here: is there an objective, audible difference between an external DAC and the internal DAC in, say, my iMac?

 
It is hard to tell without having reliable and detailed measurements of the iMac DAC available. The quality of onboard audio is quite variable.

 

[bigshot]

The DACs in Apple products have always been consistently top quality. If there would be any measurable improvement in going to an external DAC it would certainly be beyond the threshold of human perception.

 

[nick_charles]

Quote:

 
It is hard to tell without having reliable and detailed measurements of the iMac DAC available. The quality of onboard audio is quite variable.

 
Indeed ! - my old Lenovo laptop had something like an -8db cliff at 10k 
 
Quote:

The DACs in Apple products have always been consistently top quality. If there would be any measurable improvement in going to an external DAC it would certainly be beyond the threshold of human perception.

 
Generally yes but it does not hurt to do some measurements. All one needs is a decent ADC connected to a 2nd computer run the analog outputs from source A and source B into it, align, trim  and compare the results. I've done this with my CD players and found surprising differences sometimes (admittedly normally output levels) 

 

[bigshot]

The Mac Mini was designed to be an AV machine out of the box. Apple has always been focused on sound quality. I mastered CDs on an ancient 8500 AV and I run 5:1 sound out of my Mac Mini. It's as good as it gets.

 

[nick_charles]

Quote:

The Mac Mini was designed to be an AV machine out of the box. Apple has always been focused on sound quality. I mastered CDs on an ancient 8500 AV and I run 5:1 sound out of my Mac Mini. It's as good as it gets.

 
Sorry to seem contrary but the OP was after empirical tests or objective measurements, remember where we are !
 
I love the sound out of my $90 DAC/amp but that is not data either just another anecdote.

 

[TMRaven]

Recent apple computers have great dacs.  I'd be hard-pressed to tell the difference between the onboard audio of my 09 iMac and HRT Music StreamerII/ ODAC.  The most benefit you'll get from an external dac at this point would be a more efficient external amp and elimination of any computer noise.

 

[MrHeuristic]

Quote:

...and elimination of any computer noise.

I have noticed that on my iMac, when plugging in cheap earbuds (some skullcandy in-ears, they're all I have to test it until my K702 gets in) to the headphone jack, I can hear an audible white noise or hiss when the iMac is amplifying and nothing is playing. After about a minute or two with no sound output from the OS, I can hear the iMac stop amplifying sound, and the hiss goes away. If I play anything in the OS, the hiss again comes back moments before the audio starts playing. I believe this is called a high noise floor? 
 
I don't notice it on my Logitech speakers, though, so I'm wondering if it will be noticeable if I directly amplify the iMac's output and use some better headphones. If, when my amp and K702's arrive, and the high noise floor is still a problem, an external DAC will prevent this?

 

[TMRaven]

That is a high noise floor, yes.  It has more to do with the amp of the imac and not its dac most likely, and is also a result of the earbuds' very high efficiency.  Lots of very expensive 'audiophile' amps can exhibit the same results with very efficient in-ears as well.  I was moreso talking about any kind of electrical interference that you might hear when say, scrolling on a safari window or opening applications etc.

 

[MrHeuristic]

Quote:

That is a high noise floor, yes.  It has more to do with the amp of the imac and not its dac most likely

In that case, if I were to skip buying a DAC for my setup, how would I isolate the iMac's DAC output from its internal amplifier?
 
My amp has RCA inputs and a 1/8" line in. If I were to connect the iMac's headphone jack directly to the line in, would the amplifier be amplifying the already-amplified signal from within the iMac, or just the bare DAC output? If not the latter, how would I achieve that?

 

[bigshot]

The Mac Mini has analogue line out. Just connect that to your amp and you'll have no problem. You don't need to use the headphone jack at all.

 

[MrHeuristic]

Ah, so it appears the headphone jack on the iMac is also the line out, even when using regular 3.5mm cable (not optical mini Toslink)? From the spec page at Apple, I get this: 

Line/Headphone output

The headphone output is automatically selected for audio output if no external S/PDIF optical digital output device is detected. The headphone output supports a stereo data stream at bit depths of 16, 20, or 24 bits per sample and at sample rates of 44.1 kHz, 48 kHz, or 96 kHz. The headphone output volume can be adjusted from 0.0 dB to -64 dB.

 
So, I guess my next question is, if I'm connecting to my amplifier with a regular old 3.5mm cable, do I set my iMac to full volume? How do you do this with your Mini?

 

[bigshot]

The Mini has a true analogue line out too.

 

[MrHeuristic]

Quote:

The Mini has a true analogue line out too.

 
Yes, but when you amplify that, do you set the volume output on the Mac Mini — in the operating system itself — to full volume? minimum volume? Or does it not matter?

 

[bigshot]

You can adjust it in iTunes. It doesn't matter where you set it. I keep it about 75% so I have room to turn it up from my iPhone if I want.