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lingDAC - cost effective RBCD multibit DAC design
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Old 17th July 2018, 07:14 AM   #31
abraxalito is offline abraxalito  United Kingdom
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Yeah I originally preferred the 1545 to 1387 because it consumed about half the power and also was able to scale its output current up to 2mA. I built up a few DAC-AHs where I replaced the 1543s with 1545s and even increased the number of chips up to 32. However on careful listening I couldn't get rid of some lowish-level noise modulation on piano (a large proportion of the music I listen to has piano in it) which never has appeared on 1387.

TDA1311 has voltage output so does not lend itself to passive output filtering. LC passive filters have lowish impedance and at line levels draw too much current for comfortable drive.
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Old 17th July 2018, 07:51 AM   #32
kazap is offline kazap  Australia
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Originally Posted by abraxalito View Post
I sourced all my TDA1387 on Taobao,...... they're so cheap .......

Thanks for sharing all your experience and expertise with the TDA1387. Given TDA1387 are so affordable is there any merit using multiple chip designs for a high performing DAC?


Are there three topologies for implementing multiple chips? Stacking them, analogue bit cascading them in time and segmenting MSB and LSB for eg 24bit processing? Do you think any of these approaches is useful for the TDA1387? Can any of them bump its performance up towards say a PCM1704? Thanks.
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Old 17th July 2018, 08:12 AM   #33
abraxalito is offline abraxalito  United Kingdom
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Quote:
Originally Posted by kazap View Post
Thanks for sharing all your experience and expertise with the TDA1387.
You're welcome - I'm glad that there are people finding it useful.

Quote:
Given TDA1387 are so affordable is there any merit using multiple chip designs for a high performing DAC?
I've been asking myself that exact same question for quite a few years now. I guess it all depends what's meant by 'high performing'. Certainly one of the TDA1387's weaknesses in today's digital market is that its only 16bits hence the noise floor is determined by that. I would guess that a lower noise floor can be achieved by paralleling outputs and driving each DAC independently from a DSP. Its not something I've tried so far, given there aren't so many DSPs to choose from which have at least 4 I2S ports on them (I'd say 4 would be the absolute minimum worthwhile to parallel).

Quote:
Are there three topologies for implementing multiple chips? Stacking them, analogue bit cascading them in time and segmenting MSB and LSB for eg 24bit processing? Do you think any of these approaches is useful for the TDA1387?
I've done stacking and while its useful for getting more output current, it doesn't in my estimation result in better SQ. Its not like the 1543 where there are considerable linearity errors which, when averaged out result in a straighter line for INL.

I do have plans to build a spaced-in-time DAC for implementing NOS droop - effectively an analog FIR DAC-filter. I did build one with about 19 1387s a few years back, however that was in the days prior to my getting involved in LC filters and fancier power supplies.

Quote:
Can any of them bump its performance up towards say a PCM1704? Thanks.
If by performance you mean noise, it would need a fair amount of complexity to get down to the 1704's noise floor (-120dB) - probably at least 16 chips. If you're interested in bettering the SQ of the 1704 then I'd say lingDAC already does that (on 16bit material) with only two chips - to my ears 1704 is a little bit 'soft' in comparison. About half-way to a 1543.
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Old 17th July 2018, 04:03 PM   #34
matt_garman is offline matt_garman  United States
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lingDAC - cost effective RBCD multibit DAC design
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Originally Posted by abraxalito View Post
The aim there is to give the 'passive' DAC as close as possible the same voltage on its outputs as the active one. In the basic configuration only one DAC's outputs are taken to the I/V stage, the other DAC is there solely as a 'dummy' to balance out the current drawn by the active DAC (the current draw being signal dependent). I figured to get the best cancellation it might be best to give both DACs the same output voltage, but I've not actually gone and verified this.
Hi Richard! I only just now saw this thread, very much subscribed.

For those of us interested in balanced output: is it possible, instead of a dummy load for DAC #2, you could instead run it into a whole second filter+buffer, right?

I too have a v1 lingDAC that Abraxalito was kind enough to send me, but time constraints have prevented me from playing with it.
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Old 18th July 2018, 12:23 AM   #35
abraxalito is offline abraxalito  United Kingdom
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Hi Matt - welcome to the thread.

Yes, for balanced output its necessary to connect four twisted pairs to the DAC board - one pair for 'in phase' and the other for '180o phase'. These pairs then go to two filter-I/V boards and then on to two buffer boards, for a total of 5 boards to create the balanced lingDAC. If unbalanced outputs are required I recommend a trafo after the buffers.
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Old 19th July 2018, 05:28 AM   #36
abraxalito is offline abraxalito  United Kingdom
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Default Filter I/V board schematic

Here's the schematic for the 2nd board in the three-board set - it accepts the 0.6mA peak current output from the DAC board, filters it and then converts the current to a standard 2VRMS level.

Filtering is done by a 5th order LC which incorporates the NOS boost. Inductors (TDK SLF7045T range) do need pre-selecting to get an accurate frequency response - off the reel they're generally +/-20% tolerance.

The discrete transistor I/V stage is DC coupled and so there needs to be some tweaking to set the output DC offset to mid-way (3.8 - 3.9V). This is accomplished with some of the 'NF' resistors which adjust the bias currents coming from two-transistor current sources.
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File Type: pdf Schematic_0DAC 2V filter.pdf (55.8 KB, 69 views)
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Old 19th July 2018, 05:43 AM   #37
jambul is offline jambul  Belgium
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Richards,

Thanks for sharing, getting more complicated but must be worthed on desktop rig, but since i just using a portable i ended up with this kind of iv
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Old 19th July 2018, 07:31 AM   #38
Bartb is offline Bartb  Slovenia
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Quote:
Originally Posted by abraxalito View Post
Here's the schematic for the 2nd board in the three-board set - it accepts the 0.6mA peak current output from the DAC board, filters it and then converts the current to a standard 2VRMS level.

Filtering is done by a 5th order LC which incorporates the NOS boost. Inductors (TDK SLF7045T range) do need pre-selecting to get an accurate frequency response - off the reel they're generally +/-20% tolerance.

The discrete transistor I/V stage is DC coupled and so there needs to be some tweaking to set the output DC offset to mid-way (3.8 - 3.9V). This is accomplished with some of the 'NF' resistors which adjust the bias currents coming from two-transistor current sources.
Very nice.
I also see some Non Fitted capacitors (C14, C29) in the filter, what is their purpose?
Thanks B.
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Old 19th July 2018, 07:32 AM   #39
kokoriantz is offline kokoriantz  Lebanon
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Max supply voltage for tda1387 is given 5.5v . Is 6v you use, alright?
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Old 19th July 2018, 08:35 AM   #40
abraxalito is offline abraxalito  United Kingdom
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Originally Posted by Bartb View Post
I also see some Non Fitted capacitors (C14, C29) in the filter, what is their purpose?
Good question - they're for tweaking the frequency response when the inductors aren't equal to their nominal values. As I mentioned above, the tolerances on inductors are pretty wide and at the extremes can make significant changes to the frequency response of the filter. Some variation in inductor values can be compensated for by adjustment of capacitor values, hence the NFs.

I do plan to do a more thorough analysis of the sensitivity to inductor values of my filter and I do have (in theory) a modification of this filter to accommodate a wider range of values. Still to be tested in practice though....

@kokoriantz - the absolute max supply is 6V but the recommended max is 5.5V. I've not had any problems so far running at 6V but if you'd like to run at 5.5V - the only penalty is a slightly lower maximum output - then you'll need to make changes to the LED string to reduce it by 0.5V. Currently there are two yellow and one red LED, I'd guess two green LEDs would have about the right voltage. The other LED locations will need bridging with 0R resistors.
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Last edited by abraxalito; 19th July 2018 at 08:38 AM.
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