Our new mixer prototype described at https://ez.analog.com/message/165262#165262 suffers from noticeable output noise with its inputs turned down. To trace its source I modified the SigmaStudio project to switch a full-scale 1 KHz tone directly into the relevant output DAC. By measuring the level at the mixer's balanced output with this tone switched ON and OFF, we originally found only a 65 dB S/N. Then we discovered that our millivolt meter's 300 KHz bandwidth was allowing out-of-band noise into the measurement. Attempts to limit this bandwidth by RC filter at the meter input proved unsuccessful, so my friend hit on the idea of using a handy ADAU1452MINIZ board loaded with a modified version of the project at https://ez.analog.com/message/147007#147007 -- we have to be creative where I work, as there won't be a AP on the bench anytime soon. Reading the -1452's on-screen level meter yielded a more respectable S/N of about 80 dB, yet this remains below our expectation.
Running the same test on a ADAU1701MINIZ board shows a much better S/N of 96 dB at the board's headphone jack. Thus the question, what is making this mixer prototype so much noisier? Perhaps those with experience with noise could help us find out why. Here's what I know so far:
- The noise is pink / white in character, not a buzz.
- The relevant schematic portion (DAC to balanced output) is shown below.
Audio stages run on 24 V with a 12 V pseudo-ground provided by a "Rail Splitter" IC. 3.3 V supply for the SigmaDSPs is a buck converter from the 24 V. The board is four-layer with generous ground planes and bypass caps. A passive output reconstruction filter is shown above (we're switching to active in the next proto). I don't remember the IC numbers although the op-amp is a low-noise type we've been happy with in other products, and the balanced driver is a ADI part.
- A buffered, packaged 12.288 MHz oscillator provides MCLK for all six -1701s.
- TDM audio runs through all the DSPs, although the above test connects only the switched oscillator to the DAC.
Any suggestions on how to quiet this thing down, or how to track the cause of the excess noise, would be appreciated.