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Old 6th September 2014, 07:40 PM   #1
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Default TDA1387 continuous calibration dac

Hello all,

I want to start a thread about this unconventional dac chip from Philips. I see considerable amount of topics and implementations about other chips from same era (say TDA1545, TDA1543) but such interest didn't involve with TDA1387 somehow.

I wasn't aware of this dac chip until trying to find out which chip was the behind of old Creative AWE64's sound. I surprised with the existence of this eeprom look-alike small chip.

Since this dac accepts i2s signals directly, I decided to build an external dac feed by Amanero like usb/i2s interface. But I couldn't find an I/V stage (or a complete schematic of course) which specifically calculated for this dac's output capabilities and requirements. If you have any recommendations and design offers I appreciate any contributions for this thread.

Regards.
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Last edited by terranigma; 7th September 2014 at 03:45 PM.
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Old 6th September 2014, 08:36 PM   #2
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I remember Creative AWE64.

Was that a non-oversampling sound-card then?
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Old 6th September 2014, 09:23 PM   #3
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Quote:
Originally Posted by Kastor L View Post
I remember Creative AWE64.

Was that a non-oversampling sound-card then?
Since the chip supports up to 384khz sampling rate, we can't consider the every design based on tda1387 should be the non-oversampling. I don't know design details about AWE64 then have no idea about oversampling capabilities of it.
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Old 6th September 2014, 11:34 PM   #4
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Thanks for opening the thread terranigma, its a great DAC chip to talk about and I've plenty of experience to share from a couple of years of playing with it.

Its a very close relative of the (older) TDA1545 but has two clear advantages for DIYers over that chip. First its I2S and second its output can go down to 0V which makes designing the I/V stage easier. In fact just a resistor to 0V is all that's needed and this sounds better than a resistor to a supply voltage within the output compliance range (which is the solution for TDA1545).

Given the wide output compliance (0-3.5V with a 5V supply) there's a wide choice of resistor values that work well. I started out using small values (50 to 100R) because of my intentions to use a following passive filter. It turns out that for lower impedances the inductor values are smaller, hence easier to wind when custom values are needed (fewer turns, less chance of losing count, thicker wire is less likely to snap).

The disadvantage of using low value I/V resistors is lower output level. This can be overcome though by stacking chips - I've stacked up to 8 so far. More recently though I've become more adept and confident at inductor winding for passive filters so have been moving towards higher impedances. Being able to delete an active amplification stage and only use a buffer and transformer I feel will bring benefits in SQ as in my experience the amp stage's power supply has been the limiting factor. A pure classA buffer I have a hunch will be less limited by power supply quality.
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Old 6th September 2014, 11:41 PM   #5
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The arrangement I currently use is TDA1387 -> resistor to 0V -> LC filter -> AD815 post amplifier -> ferrite core trafo.

The transformer serves two useful purposes - first it converts the balanced output of the AD815 to unbalanced, second it provides noise isolation between DAC and amp. Even with a balanced input to the amp the isolation is worth winding a trafo for. Ferrite core trafos are considerably cheaper than any bought solutions, which generally use steel (cheaper) or nickel or mu-metal cores (expensive). Ferrite while having poorer flux capability than metallic cores does have the advantage of being an insulator which improves the isolation between primary and secondary.

The AD815 has a lot of attention paid to its power supply and its outputs are loaded down by current sources. I'll talk in more detail about this if there's interest.
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Old 7th September 2014, 12:42 AM   #6
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Thank you abraxalito.

Do you drive tda1387's in balanced mode, thus 8*4 stacks for stereo? Is stacking 8 of them still not providing enough voltage swing?
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Old 7th September 2014, 12:49 AM   #7
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Yes - the stack of eight I did (as a mod to a Muse 1543 DAC) was 2 * 4 in balanced mode. I then used an AD830 to create an SE output. Sounded pretty good for an unfiltered DAC.

Stacking them doesn't increase the possible voltage swing - that's limited by the output compliance. It increases the voltage swing for a particular I/V resistor though. A single DAC can give the full voltage swing if the I/V resistor's around 3.5kohms. For a standard level CD output (which is 2VRMS) then running balanced can achieve this - followed by a 1:1 transformer to create an SE output.
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Old 7th September 2014, 03:29 AM   #8
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Ok, I got it now. Maybe off topic but can you go into detail about your inversion process of i2s signal please?

I have Potato PO74G74 flip flops. I plan to use them for re-clocking i2s. I wonder if these chips also capable of generate inverted signal for a particular signal.

http://www.potatosemi.com/potatosemi...t/PO74G74A.pdf
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Old 7th September 2014, 06:24 AM   #9
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Thank you for sharing your practical experience.

I suppose 3.5V compliance does not mean it still sounds good at 3.5k R_iv ?
If you use 4x in parallel and 100R, it will only give you 0.28Vrms out.
Are you then using AD815 to make up for the gain ?

When using 5V supply, does the digital inputs have to be 5V as well, or 3.3V will also do ?


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Old 7th September 2014, 06:42 AM   #10
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For inversion of the I2S I have found a spare inverter inside my QA550 wav player amd route the data signal through it.

I haven't noticed any SQ differences correlated with the size of the I/V resistor, though its true I've never tried as high as 3.5kohm. My current implementation uses a much lower supply voltage (2.7V) which is compatible with supercapacitors - so the compliance then is about 1.2V, I'm using around 1.2kohms. Yes the AD815 is needed to make up some gain - 6dB is lost in the passive filter for a start. Its also used to correct for the NOS FR droop which is over 2dB by 18kHz (the cut-off frequency of my LC filter).

The digital inputs are nominally TTL compatible, irrespective of the supply rail - 0.8V(0) to 2V(1) does fine IME.
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