[MrHeuristic]

Sorry, I guess I'm still a bit confused. Here's a screenshot of my sound preferences when I have a 3.5mm cable plugged into my iMac.
 
See how there's still a volume control on the output? To me, that means that the iMac is still amplifying the sound, rather than outputting a base line-out. And again, I will be going iMac —> Tube amp —> K702, not directly into the headphones. 

 

[bigshot]

It isn't amplified. Full blast is normal line level. The sound is just reduced when you use the volume control in iTunes. It's all software, not hardware.

 

[PurpleAngel]

Quote:

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.

Try the Hifimediy Sabre USB-DAC, $45 plus $5-$29 shipping, uses the same receiver chip and DAC chip as the ODAC ($150). I'm not saying the Hifimediy Sabre matches the ODAC,
but for only $52 (Hifimediy Sabre), it's something to try for a low cost.
 
Also here is a low cost "Premium' cable for connecting the USB-DAC to your tube amp.
http://www.monoprice.com/products/product.asp?c_id=102&cp_id=10218&cs_id=1021815&p_id=5596&seq=1&format=2

 

[MrHeuristic]

Quote:

It isn't amplified. Full blast is normal line level. The sound is just reduced when you use the volume control in iTunes. It's all software, not hardware.

 
Got it. Thanks for your help.
 
If the headphone jack on the iMac and Mac Mini serve dual purpose as level line outs, why doesn't the headphone jack on the iPhone/iPod do the same? I'm fairly certain that LOD connectors are required to get a true line level from those.

 

[PurpleAngel]

Quote:

 
Got it. Thanks for your help.
 
If the headphone jack on the iMac and Mac Mini serve dual purpose as level line outs, why doesn't the headphone jack on the iPhone/iPod do the same? I'm fairly certain that LOD connectors are required to get a true line level from those.

The line-out/headphone jacks on the iMac and Mac Mini are more like line-outs that pretend to be headphone jack.
also I think there is some reason that those line-outs need to have a high impedance
 
I believe the headphone jacks on the iPhone/iPod are really just expected to be plugged into low Ohm headphones, which means their headphone jack needs to be really low Ohm

 

[iEar]

Hi, I'm coming a few months late to this thread I know but hope someone will see the questions.
 
Did you find a way to get pure DAC signal out of the iMac to the amp?
 
I've been using an optical cable from the output jack on the back of the iMac
but find that the volume can still be adjusted with iTunes which to my limited understanding
would mean the iMac amp is still processing the signal. Would this be correct?
 
Have you had any other flashes of inspiration regarding getting sound out of the iMac?
 
Cheers,
Mark.

 

[bigshot]

Quote:

I've been using an optical cable from the output jack on the back of the iMac
but find that the volume can still be adjusted with iTunes which to my limited understanding
would mean the iMac amp is still processing the signal. Would this be correct?

 
No, that is incorrect. The signal is still digital. It's just being attenuated in the digital domain. (read volume level adjusted digitally). No sound quality loss.

 

[GSARider]

Interesting thread, is the conclusion that an amp plugged directly to a mac is just as good in terms of listening to a high priced Dac?

 

[bigshot]

Depending on the headphones you're using, you may not even need the amp.

 

[iEar]

Thanks bigshot! I've re-read the thread and now have a much better understanding of
the reasons for your answer! Do you have any info on which would be a better way of
connecting the iMac to the AV receiver? I use the optical cable now but wonder if
a the 3.5mm jack to RCA would be a better set up? Or, no difference? I'd test it
but I don't have such a cable! Thanks for you info in the posts above! much appreciated
 

 

[bigshot]

Optical cable or HDMI are best.

 

[GSARider]

HD800's, Amperiors, Momentum's mainly.
 

 

[bigshot]

check the specs on your cans. if they're less than 100 ohms, I'm told you should be good to go. above that, you might want a cheap amp.

 

[GSARider]

Well up to 300 ohms, don't need to check, I already know...

 

[OldHarbourGreen]

Joining the thread late.  iMac specs from Apple below.  S/PDIF has higher SNR and lower distortion.  

 

I/O ports

1. Headphone port

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 -43.0 dB.

During playback of a 1 kHz sine wave at -3 dBFS voltage level, 24-bit sample depth, 44.1 kHz output sample rate, 100 k load (unless otherwise specified), the audio output has the following nominal specifications:

1. Jack type: 3.5 mm (1/8-inch) stereo combo
2. Maximum output voltage: 1.4 VRMS (+5.15 dBu)
3. Output impedance: <24 ohms
4. Frequency response: 20 Hz to 20 kHz, +0.5 dB/-3 dB
5. Signal-to-noise ratio (SNR): >90 dB
6. Total harmonic distortion + noise (THD+N): <-80 dB (0.007%)
7. Channel separation: >85 dB

 

S/PDIF optical digital output 
The S/PDIF (Sony/Philips Digital Interface Format) optical digital output is automatically selected when an S/PDIF optical digital output device is detected on the external combination audio port. The S/PDIF optical digital output supports pulse-code modulation (PCM) and Arc Consistency Algorithm #3 (AC-3) audio formats with the following stereo data stream characteristics:

1. PCM: 16, 20, or 24 bits per sample at sample rates of 44.1 kHz, 48 kHz, or 96 kHz
2. AC-3: 16 bits per sample at sample rates of 44.1 kHz, 48 kHz, 88.2 kHz, 96 kHz

The S/PDIF optical output channel status conforms to International Electrotechnical Commission (IEC) 60958-3 consumer mode digital audio.

During playback of a 1 kHz sine wave (S/PDIF output format at 0 dBFS output level, 44.1 kHz sample rate, 24-bit sample depth, unless otherwise specified), the digital audio output has the following nominal specifications:

1. Jack type: 3.5 mm (1/8-inch) stereo combo
2. Digital audio signal-to-noise ratio (SNR): >130 dB
3. Digital audio total harmonic distortion + noise (THD+N): <-130 dB (0.00003%)