Benefits of an external DAC


I need a sounding board from you folks.  I have a slightly upgraded Oppo 103 CD player that was a big SQ improvement over my Consonance tube CD player.  I need a DAC to take the stream from my iMac, turn into an analogue signal to my preamp.  Lots of folks rave about the benefits of an external DAC so, in progression, I got a Jolida tube DAC, a Schiit Gungnir, and a Channel Island Audio DAC.  I connect the Oppo to the CIA DAC with a coaxial cable.  I input the analogue signal from the Oppo into my preamp with RCA IC's.    

It is easy to switch between DAC in the path vs. the Oppo analogue signal straight into the preamp.  I tell ya, I have done the comparison between all three DAC's vs. the analogue signal run from the Oppo to the preamp and have been unable to discern any difference.   Zero, zip, nada.  I have done the comparison with lots of audio pals and nobody hears a wits worth of difference.

There clearly are lots of devotees using external DAC, but in my system (with a Don Sachs tube preamp, Pass Labs First Watt F5, and Spatial Audio M4 Turbo S speakers, with good cabling), I am perplexed as to why the use of an external DAC makes no difference in the SQ in my system. It sounds exactly the same.  

I am awaiting a demo of a Denafrips Ares DAC to see if this well-regarded DAC does something to the SQ -- one way or another.  So far, the employment of a DAC to improve the SQ of the signal from my Oppo has been an utter waste of time.  What am I missing?
whitestix
Whitestix - Let me know if you really hear a difference.  That DAC is a chip R2R and imo you should hear a difference in the mids mostly.  Lots of caps in there for the PS but nothing really new or innovative. The CIA is a basic dac that seems to use filtering for noise, not much info on the unit.  I would imagine that it has a smooth sound for digital but nothing special for the price.  Let us know what you are hearing please.
If you after really care about how the DAC sounds and not math I would very much recommend finding an older used DAC. One that is very well built with a good analog output stage and an extremely robust power supply. At the end of the day those things make more of a difference than sampling rates. Not knowing your budget perusing the listings here a few things jump out.

https://www.audiogon.com/listings/da-converters-lector-digicode-2-24-multi-bit-r2r-dac-2017-10-10-di...

https://www.audiogon.com/listings/da-converters-free-shipping-230v-50-60hz-2017-09-30-digital

https://www.audiogon.com/listings/da-converters-audio-logic-model-34-tube-dac-d-a-converter-10040-20...

https://www.audiogon.com/listings/da-converters-museatex-bidat-dac-2017-10-11-digital

It really seems like the industry has turned into a bit of the "DAC of the month club" with everyone chasing whats new and "hot". There is a lot of value, and great sound, in older DACS. Good luck!
OP, I don't think you or your buddies for that matter, are missing anything. DACs don't seem to make much a difference.  I did a similar experiment comparing DACs from a cd player, DVD, BluRay, Digital Pre-amp and even my TV using the same cd's and they all sounded pretty much the same.  The cd player also had a tube output which I really expected to change things but sadly it didn't.  Had a few friends over and none of us could pinpoint the differences. There were slight variations, but not enough that you could reliably tell which was the source in blind testing. The good news is that you can get by with a lot less boxes.  Just route the iMac to your Oppo or the TV, either should work, then to the pre-amp. 
Jond, I agree. That Audio Logic 34 would be something I would be interested in if looking for a DAC. 

@audioengr

Interesting. You advocate feeding a DAC with low jitter. However we all know that this is next to impossible as jitter is inherent in cabling and the way the clock timing is detecting at the receiving end.

I would say it makes equal sense to focus on a DAC that has the technology to reject all and any incoming jitter thoroughly below -140 dB. Is there something you don't agree with in designing robust mathematical algorithms and electronics to reject all jitter?