TDA1387 x8 DAC: let's check its design, mod it -or not-, play music -or not! :(- - diyAudio
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Old 4th February 2015, 08:38 PM   #1
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Default TDA1387 x8 DAC: let's check its design, mod it -or not-, play music -or not! :(-

I've lost the first post I've done... damn!
It's about a tiny 8x TDA1387 DAC, how we can get the best out if it, if it's even worth it...

In short and less organized (than my lost text):
Under 100USD the cased device, Euro/China mains voltage, caps are from old stocks or worse, not the same in each device... (got one for a friend).
CS8416, S/PDIF to DACs converter?
7 pins header "USBinput", any daughter-board needed as my Mac does not see any device when plugged?
STC seems to be a controller of sources selection, or USB input related?
LT1028 in audio path even if I've only seen some in super-regulators, 5 supplies, +/15 opamps, 5/3.3v CS8416, 5V for the TDA1387s?
Cheap PCB but high potential?

All I've written lost for ever... ok guys, look at that and comment. Don't know how's the sound yet as USB failed on my computer and my VRDS is not ready.
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File Type: jpg PCB.jpg (451.8 KB, 907 views)
File Type: jpg BackPCB.jpg (352.3 KB, 865 views)
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Old 4th February 2015, 11:15 PM   #2
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Great pics, very clear.... Soldering all looks done by hand. I agree there's a missing USB module to fit into the SIL connector. I would guess the STC uC is in charge of the front panel display, probably decodes the status lines out of the DIR9001 and turns those into the sample rate LEDs?

What I like is they've adopted ground 'islands' and aren't using one single ground fill over the whole board. I spent many hours hacking the groundfill on the DAC-AH into manageable pieces so having it partitioned already is a great start Ooh, just noticed there are TL431s being used for regulation - does that mean the heatsinked devices aren't the usual 3-terminal regs, rather discrete transistors? I'm coming to like this more and more
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Old 4th February 2015, 11:25 PM   #3
infinia is online now infinia  United States
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ooh how I hate global ground fills for non RF , it's a lazy mans solution for sure > each sub-circuit needs its domain.
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Old 5th February 2015, 05:03 AM   #4
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I've got mine slowly warming up now. Most noticeable on orchestral material is that the HF has the 'false detail' effect which makes it rather fatiguing to listen to over longer periods. Violin strings are overly strident. In my experience this is down to the glitchiness of CMOS DACs generating IMD somewhere in the following opamp. When I originally played with TDA1387s/1545As I tamed this effect with a long string of ferrite beads between the two. Perhaps this time I'll see if the effect can be reduced by attention to the opamp's HF power supply impedance, just for fun...
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Old 5th February 2015, 05:09 AM   #5
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Good thinkin' there, Richard ...
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Old 5th February 2015, 05:20 AM   #6
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Taking a look at the layout there could be a common-mode noise problem in that there are a couple of 220pF caps feeding directly into the opamps' +ve inputs from a dirty ground (the output ground). So first things first, I shall cut the thick track along the red line and route the output ground back to the trafo (indicated by the green arrow).

Over on the reverse side, the 10k loads are unnecessarily low (I take it they're to charge the output caps up) so I'll increase them to 1meg.
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File Type: jpg TDA1387mod2.jpg (15.1 KB, 761 views)
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Old 5th February 2015, 06:24 AM   #7
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On opening it up, I discover mine has NE5534s, nothing fancy like LT1028s. I couldn't resist doing a couple of other mods on the first round - those are reducing the supply voltage to the opamps by paralleling a 13k resistor on top of the 10k. I only have 25V ceramics to do the decoupling and 30V of opamp rail isn't required to drive 2VRMS CD levels. Hence now the TL431s are programmed with 2k and 5k6 giving 19V total. Out came the grey rectangular polyesters and in went some 10uF X7Rs (4 per chip), but directly across the power pins (4 and 7), not to AGND. There are already some lytics handling the to-ground decoupling.

Even before I turned up the volume on this it was apparent the depth has improved. Strings are still rather strident but less fatiguingly so. So looks like the ferrites between the DACs and the opamps might be next. Though another possibility is to try some JFET opamps in place of the bipolar 5534s......

Incidentally a warning is in order regards electrical safety. This DAC wouldn't pass CE because its not earthed yet doesn't have classII quality double insulation. In particular I'd recommend some insulating tape on the inside bottom of the case as nothing but air remains between live tracks and the ally chassis.

Tried some AD845s they indeed sweeten the strings somewhat, and now at last the piccolo is sounding more like it should....
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Old 5th February 2015, 07:27 AM   #8
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Next up is replacing all the pin7 decoupling caps (pin7 is the TDA1387's reference voltage) with 1000uF 'lytics. At present those caps are 10uF ceramics - when I've tried ceramics on my designs I've not much cared for how they sound in this position. Not easy to describe why. Anyway with these 8 caps (on the underside) replaced by 'lytics (on the top as not enough clearance below) I notice an improvement in the LF - more low-level details become apparent, like page turns. Also there was a slight 'nasal' quality to the strings which has diminished.
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Old 5th February 2015, 08:34 PM   #9
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Oh man you already done that much!
Beside having mailed seller for USB card info and taken OPA2132s out of drawer I'm enjoying reading your mods and maybe try some soon
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Old 5th February 2015, 10:56 PM   #10
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I need to figure out the schematic for the output stage before I go any further. Haven't worked out if there's passive I/V or active opamp I/V. But its jolly listenable now, nice sense of depth down to lowish frequencies. Not as clean as my Ozone Pagoda (which is my reference) so I wonder how many mods I have left before I have to bite the bullet and install a passive AIF.....
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