@BrianG- thanks for the suggestion. I tried it today and it didn’t change anything. The DAC in the C48 preamp I was testing is at least 10years old. I tried a pretty new SMSL SU-1 DAC (usb input) to compare it with and under the same settings for the QA403, below is what I got (SMSL on Left). The SMSL has 6dB of gain which is not adjustable, so I adjusted the C48’s gain to match that:
I rarely plan on testing DACs so this was more of a “QA40x” learning experience. With a 0dBV input the SMSL did not perform as well overall to the C48 with 6dB of gain. And I should mention that both measurements were made with the RCA (unbalanced) outputs on both devices…