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Stack-a-DAC filter module taking shape

Posted 28th April 2014 at 01:27 PM by abraxalito
Updated 29th April 2014 at 12:36 AM by abraxalito

I'm prototyping this baby which will probably be the first Ozone board to go to PCB layout. It implements the 10th order Chebyshev filter shown earlier and is intended to be mounted atop a 'hexacap' passive shunt. The DAC stack in the centre is made up of six TDA1387s. Input will be I2S and output unbalanced audio 40mV RMS.

Update - over on a thread about AD815 Eldam asked about the inductors - how many I need to buy to get the values in the schematic - so I'll talk about that a little here.

I have bought a big reel of 1k pieces of these 680uH inductors. The price was around $100 but I think I probably should have paid a little more as although the reel is marked up as TDK, they might well be fakes. I suspect that because their appearance isn't as professional as some 1mH parts I bought earlier from another seller. Added to that is the spread of values - having measured at least 100, only one has shown up as 680uH (within 10uH). The rest have varied from 800uH...
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Old

First transportable Ozone incarnation

Posted 27th April 2014 at 02:48 PM by abraxalito
Updated 28th April 2014 at 05:33 AM by abraxalito

I took this to the park this morning, it survived the journey in my backpack but it exposed a dodgy RCA connector. The case I used (temporarily) for it is a cylindrical tea carton - the red 'tower' behind. Battery power comes from 3 NiMH AA cells giving around 10hrs of listening.

I've just done a mental estimate of the BOM cost of this DAC (labour cost is way dominant, but this is DIY right?) and it turns out to be dominated by the cost of the caps in the top three tiers (the small black ones). These are real Japanese Rubycons, chosen for their diminutive height (16mm) vs the Chinese ones at 25mm height. So if you didn't mind adding 3cm to the overall neight you could build this with all Chinese caps and save a few dollars. As it is the total parts cost is under $30 including batteries. If you wanted S/PDIF input (currently its I2S) you'd add a couple more $.
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Old

Ozone Chebyshev filter

Posted 18th April 2014 at 02:39 AM by abraxalito

I've munged the earlier 8 inductor filters down to only 5 - this one is using off-the-shelf TDK inductors, SLF7045-681, albeit they need to be hand-selected to get the correct values as the production tolerance is 20%. X7R caps sound just fine, surprisingly

FR is -3dB at 17,8kHz with around 0.25dB of passband ripple.
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Old

One Farad capacitor array

Posted 3rd April 2014 at 03:08 AM by abraxalito
Updated 4th May 2014 at 04:53 AM by abraxalito

This array of 127 10,000uF caps measures -j2.9mohms at 50Hz which I reckon makes it comfortably over 1F in capacitance. ESR is 1mohm @ 50Hz.

The individual caps were measuring on average 8,500uF. Although they're marked 10V (the brand is ChengX a slightly shady Chinese one) they're not good for anywhere near this voltage. I reckon they're in reality 6.3V and they seem to have decently low leakage at this working voltage. I shall be using them at 5V or below.

Update1 - seems I wasn't using enough copper in the wiring. I've now built a second one with 2.5mm2 wiring around the loops and 1.5mm diameter cross links and ESR is down to 0.5mohm @ 50Hz now, decreasing at higher freqs. I have enough caps left for a third one...

Update2 - now the first array has a basement level giving 2.1F total with ESR below 0.4mohms @ 50Hz. There are six paralleled TDA1387s atop the combined hexagons giving unprecedented LF dynamics. Amp long, long overdue for an upgrade.......
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