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Another iteration of DACS for Crossover

Posted 26th May 2012 at 03:00 PM by googlyone

I have never been happy using DACs to implement volume control. I guess largely because of the obvious degradation in resolution at low volume levels.

On my first DSP crossover I used an AD1939 CODEC, which has 24 bit resolution - though obviously lesser precision - and my approach here was to use a mix of steeped attenuator and digitally implemented volume control.

It worked well - though pretty shortly after I built an 8 channel PGA2320 based volume control.

For my new DSP crossover I started with a simple CS4398 in the DAC output. even when I built the first boards I KNEW I would be going back to integrate a programmable attenuator.

Why didn't I just start with it? **** knows.

At lease with the modular approach all I had to do was respin my DAC board to include a PGA2320 along with the CS4398. I already had SPI to the board, sop I can sneakily use these lines for the PGA 2320, as the CS4398 is in hard wired mode.

The DAC boards are still only 3.2 by 1.9 inches each - a tiny bit bigger that the original spin - but still small enough to keep me happy. Am waiting on the PCBs to ceom from PCB Cart -though I am stuck overseas for a few weeks so will have to hang off on doing much for the next month or so.

I also got lazy this time and had them professionally made - as I want about 15 of these things the time involved in making them is just too much. I would notmally do a board like this in the shed. At least this way the come out looking all pretty and silkscreened - functionally identical to home made but a load more sexy looking.

The software mods will be trivial - I hope.

Again - should anybody want the CAD files etc for this and / or the DSP feel free to ask. I don't offer pre made boards etc, but if you want to take a swing at DSP based audio, I am more than happy to give you whatever help I can.
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  1. Old Comment
    As an addendum to this, I was measuring the distortion of an external sound-card that I recently bought (that uses CS ADC's) and comparing this to my DSP.

    I had jigged a couple of DSP's so that I was driving one with a TOSLING cable and using it as a DAC, and using a second DSP with a TOSLINK output back into the computer. Essentially an enormously complex and expensive sound card.

    Much to my disgust, my DAC was showing levels of distortion about 10dB higher than the soundcard. DAMN what the....

    Mind you, we are talking distortion waaaay down in the -100dBc region.

    After several tantrums and several cups of coffee, I worked through an analysis of the issue. Guess what?

    THE DISTORTION WAS ALMOST ALL COMING FROM THE PGA2320!

    A long hard look at the PGA datasheet shows that this is not inconsistent with it's specs.

    A logical think about it says that this level of distortion is so low that I should be happy. But in the end of the day, the very thing I added to make the performance "better" has turned out to be the limiting thing in the design!

    That is what you get for "doing too much before you think".
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    Posted 25th May 2013 at 10:45 AM by googlyone googlyone is offline
 
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