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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 6th February 2015, 07:03 PM   #11
Malefoda is offline Malefoda  France
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OK so let's state you confirmed that it is a good modding base for a cheap and yet great sounding DAC (anyway it's a bit stupid from me as I don't know compared to wich other DACs... and I hope I don't expect too much in hoping beating the VRDS 10's OEM soundcard...).

As for saftey, there is a long nude track wich is meant to touch the case, very poor contact I see but it's here, just need to wire the earth from ICE socket to that track left alone. But I'll cut plastic sheet wich where used as PSU's internal isolation.

As for USB daughter board, the seller pointed these out:
L108USB daughter card, L4399, L1852 series USB extended sub card
L2706USB combined with L9018, L4399 series of decoder using PCM2706 daughter card
CM6631 24bit 192K USB L4399DAC decoding card used for different steps
I know nothing about data through USB, don't they send, correct error and get bit-perfect? Any card better sounding? Or anywhere else to buy cheap?

I find USB to S/PDIF here and there wich chips references, is teh CS8416 fed with S/PDIF? Sending what to the TDA1387s?


For my own PCB, I have old spare lytics caps, I guess they are dead as shelf life if very short. And still a bunch of fresh SPEC, FM and ZLG. SPEC at pure digital supply (5V CS8416?) ZLG at close decoupling (330F/16V under the TDAs?)... OPA2132 ready to go. I may wait you report as a crash test dummy before fry mine

Thanks Richard and fellows for having a look.
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Last edited by Malefoda; 6th February 2015 at 07:09 PM.
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Old 7th February 2015, 01:36 AM   #12
abraxalito is offline abraxalito  United Kingdom
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I took a look at your linked USB modules. If you don't need to send 192kHz to the DAC over USB, I'd go for the first one as its also the cheapest The normal way of implementing USB on Chinese DACs is for the USB receiver to convert to S/PDIF, this output is then routed into the CS8416.

Isn't OPA2132 a dual opamp? No duals here, only singles - you'd need a pair of OPA132s.
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Old 7th February 2015, 04:39 PM   #13
Malefoda is offline Malefoda  France
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Damn I have no Fet single opamps...
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Old 9th February 2015, 06:20 AM   #14
abraxalito is offline abraxalito  United Kingdom
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I've buzzed out the connectivity of the opamp-DAC interface now. Its using active I/V with the opamp as a transimpedance amp. The feedback network is 620R in parallel with 2n2 and there's an output filter of 220R and 220pF. The resistors aren't the same value on mine as the silkscreen markings - my SS has 750R and 75R.

Given this new info, its clear why NE5534 is a poor choice - a transimpedance amp needs unity gain stability and 5534 doesn't have that without the Ccomp =22pF (not fitted here).

Back in the days when I was playing with opamp I/V I found LM6172 gave the best result, but I don't have any LM6171s (the singles) so I'll play with some other 'real' CFB amps and report back. At present I'm listening to the AD845s but I've taken out of circuit the 2n2s as I found lower feedback caps sounded better when driven from my AD1955 DAC.
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Old 10th February 2015, 12:00 AM   #15
abraxalito is offline abraxalito  United Kingdom
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A day's listening and I'm really, really impressed with this DAC. Taking out the 2n2s has considerably reduced the harshness which was apparent on more complex material (orchestral strings, choral) so much that I'm not sure its still there. I'll continue listening.

My favourite opamps for the output stage are still AD845s, I've tried AD844 and AD847 too. All sound very good, just my impression is that AD845 is very slightly sweeter and AD844 makes lower level details slightly more apparent. AD847 sits between the two. Note that to try AD844 you definitely need to remove the 2n2s.
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Old 11th February 2015, 03:10 PM   #16
abraxalito is offline abraxalito  United Kingdom
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Played some more with the power supply this afternoon - found on closer inspection that the TL431s are configured - together with the heatsunk transistors - as 'amplified' shunts (fig31 in TI's DS). The transistor is lowering the shunt's output impedance and boosting the dissipation. These shunts are fed from the res caps via 150R either side. Seeing I've now lowered the supply voltage to around 8V per rail, I've also increased the 150R to 244R (added a couple of 0.5W 47Rs in series with each). Current through this chain of resistors is about 50mA now.
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Old 11th February 2015, 03:28 PM   #17
Malefoda is offline Malefoda  France
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OPA602 and LM6171 on their way home now. What was the purposeof the 2.2nf caps? In fact I don't even see wich one they are on pcb o_O'
Also some opamps show better specs with higher rail voltage, why lower in your case?
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Old 11th February 2015, 03:50 PM   #18
abraxalito is offline abraxalito  United Kingdom
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I've marked the caps to remove with a red X - the 2n2s are the blue ones. To the right are the supply decouplers (grey coloured).

I've lowered the supply voltage primarily to be able to get more capacitance out of the X7R ceramics - they lose a substantial proportion of their capacitance under higher bias. It also allows use of physically smaller electrolytics (10V rather than 16V) so its possible to get more capacitance in closer to the ICs. I also would like to experiment with biassing the opamps into classA - this will raise the dissipation so I prefer to minimize that by reducing the rails to what's really necessary, no more.

I think the purpose of having the 2n2s across the feedback resistors is two-fold. First to limit the slewrate and secondly to provide a bit of low-pass filtering. There's a considerable downside in SQ to having them though, which isn't expected under the normal ways of understanding opamp behaviour. I suspect it has something to do with putting glitches on the supply rails when they're present.
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Last edited by abraxalito; 11th February 2015 at 03:54 PM.
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Old 11th February 2015, 04:00 PM   #19
infinia is offline infinia  United States
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I reckon more attention to SE current DAC to op-amp inputs would yield the greatest improvements. pre-filter, grounds, etc. too bad it's not a true differential IV interface.
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Old 11th February 2015, 04:02 PM   #20
abraxalito is offline abraxalito  United Kingdom
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You're suggesting passive I/V then filter prior to the signal seeing any opamp? That's what I do in my own designs but I'm curious to see what gets lost (or gained) by doing it another way. I've just cobbled together a 2-pole (LC) output filter with NOS correction that I'll try out over the next day or so. Schematic to follow....
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