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TDA1387 x8 DAC: let's check its design, mod it -or not-, play music -or not! :(-
TDA1387 x8 DAC: let's check its design, mod it -or not-, play music -or not! :(-
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Old 11th February 2015, 03:05 PM   #21
infinia is offline infinia  United States
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suggest CFB op-amps as a min? large signal slew limits w/o prefilter
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Last edited by infinia; 11th February 2015 at 03:07 PM.
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Old 11th February 2015, 03:06 PM   #22
abraxalito is offline abraxalito  United Kingdom
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Yep - already tried AD844s, might have a go with AD846 but the feedback resistor's a bit low for that one....
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Old 11th February 2015, 03:11 PM   #23
infinia is offline infinia  United States
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OT sorry
what's a cheap newer DAC with diff outputs? I reckon they're all in one systems now. I^2C with microprocessors and clock recovery

is this unit worthwhile seems old-n - creaky
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Last edited by infinia; 11th February 2015 at 03:17 PM.
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Old 11th February 2015, 11:28 PM   #24
abraxalito is offline abraxalito  United Kingdom
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The only DAC chips I know with diff outs are S-D ones, kinda OT for this thread which is about getting the best out of the TDA1387.

Here's a filter I'm going to try after the opamp - its designed to do two things. First correct for the NOS droop and second attenuate OOB images produced by the DAC. The filter that's currently on the opamp output (220R, 220pF) has a -3dB over 3MHz so isn't doing much (if anything at all) to tame the DAC's images. My filter's 2 pole and it provides 24dB attenuation at 100kHz. The inductor's a Coilcraft MSS1210. The 50k resistor isn't part of the filter itself, its my volume pot
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File Type: jpg LeeNOSfilter.jpg (17.5 KB, 847 views)
File Type: jpg LeesNOSplot.jpg (81.7 KB, 831 views)
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Last edited by abraxalito; 11th February 2015 at 11:31 PM.
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Old 12th February 2015, 09:14 AM   #25
abraxalito is offline abraxalito  United Kingdom
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I've been listening now for an hour or so and this filter's definitely for keeps. The rise in HF response makes it sound just a tad more transparent and I'm getting more of an 'in focus' sound, closer to my reference DAC. Here's a hasty sketch of how to connect the filter (pink lines although only one RC network is shown in the sketch). I took out the square blue coupling caps as I already have input caps on my active speakers. If you need to preserve AC coupling move these caps to the output of the LC filter board - the Coilcraft inductors sit (face down) on my board where these caps used to be.

The resistors ringed in yellow are those to reduce to get a reduced supply voltage to the opamps. The green marks indicate the resistors feeding the shunts which are to be increased - add a 1W 100R in series with those already there by standing the existing Rs up vertically and placing the new Rs alongside.
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File Type: jpg LeeNOSOPfilter.jpg (154.2 KB, 836 views)
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Last edited by abraxalito; 12th February 2015 at 09:16 AM.
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Old 12th February 2015, 07:02 PM   #26
Malefoda is offline Malefoda  France
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Lower C42 sounds good to flatten the peak. Any counter-effect?
As for my tweeters being set to -1.5dB I will not allow that peak in HF (even If I have a 10K preamp).
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Old 12th February 2015, 07:25 PM   #27
matt_garman is offline matt_garman  United States
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TDA1387 x8 DAC: let's check its design, mod it -or not-, play music -or not! :(-
OK, you guys (well, mostly abraxalito) have convinced me I should try one of these. The only sellers I can find are based in China (Taobao/Cart100), which is basically shut down until Feb 25. Anyone know of any non-Chinese sellers?

I think I follow the mods being discussed here (and also on abraxalito's blog), but I might need some hand-holding. Hopefully you're all still interested in the board when I finally obtain one!

Also, is the PCB of decent-enough quality to withstand mediocre soldering skills?
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Old 12th February 2015, 09:27 PM   #28
Malefoda is offline Malefoda  France
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Hi Matt,
I'll still be here, right now I've done nothing ... but surviving a flu.
You'll try, that's nice as the meaning of the word still means the possibility of discard the item, in case it's not up to your expectations. I can't compare it to anything now so won't tell you it's fantastic or compare to billion dollar stuff or things like that.
That being said I'm a little bit worried about soldering stress of the PCB, Richard when you removed through-hole parts did you notice any weakness like vias going loose or tracks broken?
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Old 12th February 2015, 10:59 PM   #29
matt_garman is offline matt_garman  United States
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TDA1387 x8 DAC: let's check its design, mod it -or not-, play music -or not! :(-
Malefoda: sorry to hear you have the flu, hope you get over it quickly! Knock on wood, I've escaped the flu for many years now...

A random question: from looking only at the pics, combined with my just-enough-to-be-dangerous level of knowledge, it seems the PCB is fairly simple, especially if you ignore the power supply and USB/spdif input sub-circuits. Assuming I'm not too far off the mark, how hard would it be to design a "proper" PCB for the tda1387 DAC ICs (like they are doing with the tpa311x chip over in the class d amp forum)? Something like the Subbu DAC, a modular component that you can build as simple or complex system around?
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Old 12th February 2015, 11:09 PM   #30
abraxalito is offline abraxalito  United Kingdom
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Quote:
Originally Posted by Malefoda View Post
Lower C42 sounds good to flatten the peak. Any counter-effect?
Why do you want to flatten the peak? Are you running at 192kHz?

The peak is there intentionally because when running at 44k1 the DAC chips don't give a flat frequency response, they 'droop' about 3dB by 20kHz. There's an old post on my blog about this. So this filter is specifically designed to undo that drooping effect and give a flat response overall. If you really want a flat filter then I'll design one for you - just increasing C42 will certainly flatten it but might lower the corner frequency too, so you lose the highest freqs.

I omitted to mention that this filter's only suitable for use with RBCD source material - i.e. stuff sampled at 44k1. If there's interest I'll design one for 88k2 too.

About the PCB - so far no problems. Its of average quality, not as good as that in the DAC-AH but that one's exceptional. I've pulled out a few capacitors (lazily too, without using a solder sucker, just heat and jiggle) so far without any issues whatsoever so I think you'll be fine.

@matt - yes you're right, the PCB's quite simple, except for having island groundfills where most have an overall groundfill with the odd slot in. If anyone wants to go ahead and design a 'proper' PCB then start a thread and I'll certainly be a crowd member there As for your sourcing question - nope, no idea about any sellers beyond China. Guess patience is the word
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Last edited by abraxalito; 12th February 2015 at 11:22 PM.
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