When we connected just the DAC to power amplifiers , we found that in some cases due to mismatches in impedances’ ,transparency was affected ,some frequencies were attenuated , noise and distortion were also noticed . To alleviate this issue we decided to add a PreAmp between the DAC and the Power amp. We also noticed that when we use the digital volume control in the DAC using the ALSA mixer the control was as smooth and detailed as using an analog volume pot.
We are using two stage amplification using audiophile grade
Burr Brown OPA2134 opamp, resistors and capacitors . The first stage is a fixed gain stage which feeds to the pot this feed the second stage. Since it a dual mono stage we were able to achieve very low cross-talk.