New CS4398 plus PGA2320 DAC board set to work

Just finished set to work of the new DAC for the digitial crossover. This includes a PGA2320 programmable attenuator on the output of the CS4398 DAC.

This replaces the home made PCB thai I was using on the CS4398 output. I went the professionally made board mainly because PCBCART was able to deliver 16 of these to my house for $140, that includes tooling!!! The actual boards were like $3 or 4 each!

The quality of these baords is fine - though I must admit that I am not breaking any new ground in PCB technology here.

The board is only double sided - I thought about adding a seperate ground and power plance - but the routing density is so lot it would be crazy. The whole back side only has a handful of tracks on it - and is a ground plane in itself. The top side even has room for significant ground fill.

All the digital stuff is in between the connector and the DAC, with a few SPI lines up the left hand side of the board to the PGA2320, though these are only ever active when the volume control is being used - otherwise they are totally static.

I kind of angsted over the ground planes - and am still notn absolutely convinced that it is "just right" - thre is a single plane for ground here that combines digital and analogue. The reaon I would up with this was that there will be multiple input and output grounds - which all need to be locally referred to the ADC or DAC. I just can't see how tieing seperate ground returns to the "Motherboard" will result in anything but ground loops / sources of error.

I will be testing this both on single channel and multiple channels during following tests - I think I have it right, but might be embarrassed yet

Set to work was both heartening and disappointingly straight forward. Heartening in that I was combining two mature designs together, to which end problems would have been a bit embarrassing. Disappointing in that my planned weekend of playing in the workshop was cut unfortunately short by the task being complete.

So the system is all up and running. The Crossover now implements:
- Channel offsets for each independent channel, which the ADAU1442 implements in the DSP.
- Overall volume control implemented by the PGA2320

The reason I have done this is to allow the user to program in an offset in the DSP gain profile allowing high positive gain in parametric and CD horn equalisation, with a matching attenuation for the channel. The result being guarantted no clipping on the channel.

I considered implementing this in a manner transparent to the user, whereby setting a parametric EQ with say 10dB peaking gain would:
- Set an -10dB offset on the input gain for all channels, and
- Set a +10dB gain on the output volume of the system.

This all seemed to be a bit too much like "polishing the cannonball".

I now need to work out if it is practical to test the perfromance of this DAC / volume control combination. I don't have access to real high end audio test gear - the stuff I have is a mix of analogue (notch filter etc) and sound card based test gear. I am pretty sure that testing this DAC will simply be an exercise in baselining the test equipment I am using...

I now need to package the DSP crossover into a nice box and do some listening and m aybe have a few glasses of red...