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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 7th November 2018, 08:21 AM   #481
abraxalito is offline abraxalito  United Kingdom
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Yeah - you'll want to check that the AC is coming in fine to the rectifier, which is the middle one of the three on the reverse side of the PCB.

Then check the DC on the cap being fed by that rectifier. Then check the 47R resistor is in circuit - in normal operation it should have 5 or 6V drop across it. Seeing as you're getting 0V on the 5V test point it indicates either a short to GND or an open of the 47R or rectifier.

The shunt reg schematic can be found in the TI TL431 datasheet - the one I have is 'revised Jan 2015' its fig 31 on page 26 'High Current Shunt Regulator'. R1, R2 are 3K and the unnamed resistor at the transistor B-E is 300ohm.
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Old 7th November 2018, 08:45 AM   #482
nlgbo1 is offline nlgbo1  United Kingdom
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Ok, making some progress.
I resoldered the bridge rectifier. I now get 2.5v at the 5v test point to rca ground, and on pins 4 & 5 on the dac chips.
This is enough to make it work, but I think the output is a little low.
Just checking on the rectifier tests, is this the diagram you were referring to?

Thanks,
geoff
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Old 7th November 2018, 08:49 AM   #483
nlgbo1 is offline nlgbo1  United Kingdom
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Tests as follows:
47R voltage drop = 7.43v
A940 Rectifier AC = 6.5v
Capacitor DC = 2.5v

Thanks,
geoff.
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Old 7th November 2018, 12:52 PM   #484
abraxalito is offline abraxalito  United Kingdom
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The circuit you're showing is fig32 in my DS, fig31 is below.

2.5V on the DAC supply is just enough to get some current out but will give about half the output (so 1VRMS rather than 2VRMS). 2.5V might also lose you some dynamics as 3V is the minimum recommended working voltage for the DACs.

Given you're getting 2.5V check for a short across R1 (from the fig31 schematic) or perhaps the wrong value (should be 3k, its on the underside on my V1.1 board here). 7.43V across the resistor seems reasonable for only 2.5V output, the 2SA940 should just be getting warm.

Sorry can't understand 'A940 rectifier AC'. Ah on reflection maybe you mean the AC before the rectifier which feeds the 5V shunt
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Last edited by abraxalito; 7th November 2018 at 12:56 PM.
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Old 7th November 2018, 12:57 PM   #485
nlgbo1 is offline nlgbo1  United Kingdom
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Thanks abraxalito, you just beat me to it. Found the issue with the 3K resistor on the underside of the board, solder a new one in place and now getting 5V on the dac chips.
Sorry for the confusion, I meant 2sa940 = a940 (that is what is written on the chip!)

Thanks for all your help on this abraxalito, and on the 2 dac chips instead of 8. Sounds great so far! (although I don't have an 8 chip version to compare)

Thanks,
geoff.
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Old 7th November 2018, 01:06 PM   #486
abraxalito is offline abraxalito  United Kingdom
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Count yourself fortunate it was R1 which had the issue and not R2 If R2 was shorted the shunt wouldn't draw any current at all and would likely have taken out your two DAC chips through over-voltage. You do though have 6 spares so you'd have survived that scenario

Incidentally your first mod could be fitting a bigger value resistor in place of the 47R. Now you only have 2 chips to power there's no need for such a large current into the shunt, you could go up to 200R and get somewhat improved ripple rejection. A 0.5W resistor will do the job. The mod I'm doing replaces that resistor with a current source (LM317T) for even better noise rejection

Congrats on getting it playing music without any issues on the digital side. That would have been far more challenging to debug
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Last edited by abraxalito; 7th November 2018 at 01:12 PM.
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Old 7th November 2018, 01:29 PM   #487
nlgbo1 is offline nlgbo1  United Kingdom
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Ah, good to know. I've swapped the 47R for 100R for now, as that is what I had on hand.
Now to try out some different opamps, currently LME49710 installed, also have a couple of op134 in the parts box.

Thanks,
geoff.
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Old 9th November 2018, 01:57 PM   #488
abraxalito is offline abraxalito  United Kingdom
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Looking forward to hearing your remarks on how it sounds with the various tweaks.

I've done a few things to the v1.1 DAC I have here and its sounding very nice indeed. Here's a list - feel free to ask for further details on any of this stuff.

* reduced the number of DAC chips from 8 down to 2
* changed the 47R feeding the DAC shunt for an LM317T with 47R between its OUT and ADJ pins (giving a 26mA CCS). Kept the original 47R in series with the LM317T to reduce its dissipation and help stability (no heatsink required).
* installed LC (4.7uH and 3300uF) filter on the output of the DAC shunt
* changed the 150R resistors to discrete (2 transistor) CCS set to 50mA each. Kept the 150R in series with the CCS to reduce their dissipation.
* fitted LC (22uH, 1800uF Nichicon HZ) filters after the shunts
* reduced the shunt output voltage from 15V down to 10V
* fitted AD811 output opamps
* installed CLC NOS droop correcting filter between DACs and opamps
* loaded the 811s with 2.5mA 2 transistor CCSs

Having done all this it runs rather warm even with the lid off. To solve this I'll substitute a 9V transformer in place of the 15V one. The AD811s do run high quiescent (about 15mA) but they're worth it for the dynamic SQ.
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