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Old 14th May 2006, 07:27 PM   #41
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Default Direct Interpolation DAC schematics

Hi, all

This is what you were all waiting for, schematics! As promised I will now (try) to post the first schematic diagram of the Direct Interpolation DAC. This is basically all you need, it's the digital interface (delay lines) between the I2S interface and a bunch of DAC's. The SPDIF to I2S interface (CS8412, CS8414) is published so often that I will skip that part. This interface will drive 2, 4 or 8 DAC's. Exact connections to the DAC's is in the schematics. If you only want to use a twin or quad DAC unused shift-registers can be left out. I will post some differential output-stages next (as soon as possible). Basically there are 2 approaches:

1) Differential version: Odd DAC numbers 1,3,5,7 are parallelled and are the +
output that feeds into the + input of a diff amp using an OP-amp or a
differential triode stage. The Even DAC numbers 2,4,6,8 are parallelled and
are the - output that feeds into the - input of a diff amp.

2) All parallell version: Connect all DAC outputs together and feed a passive
(resistor) I/V converter. IMPORTANT: do NOT use the 2 or 4 inverted
data outputs labelled ND32,ND48,ND56,ND40 but use D32,D48,D56,D40
as you don't want to invert any DAC outputs.

Attention! when parallelling DAC outputs with a passive I/V converter beware of the significant voltage increase over the I/V resistor causing (massive) distortion. A common value for a single DAC is 33 OHm, full scale current is 4mA so the voltage across the resistor is 132mV, this is already way to high according to the TDA1541A datasheet (+-25mV max. to ensure specified distortion figures). In order to not make things worse use the following indication:

TWIN DAC: 15 OHms (120mV)
QUAD DAC: 8.2 OHms (131.2mV)
OCTAL DAC: 3.9 OHms (124.8mV)

But I have a better solution for significantly reducing distortion in a tube output stage: Use a differential triode stage (ECC83 or so). Now use a separate resistor I/V stage for EACH DAC output (you can use low values here to keep distortion low). Then use a summing node to add all separate voltages together (one + node and 1 - node), this way you get higher output voltages AND lower resistor values. A 12 OHm resistor results in approx. 50 mV voltage drop, not bad. In an octal DAC you will still get 400mV to drive your tubes! of course you can also try an I/V transformer with multiple primary windings, or use single I/V transformers and put the outputs in series.
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Old 14th May 2006, 08:01 PM   #42
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Default quad dac

Hi tubee,

I had a look at the circuit, what happens with WS? as this signal determines when the sample occurs on the DAC output, this is important for interpolation. I am curious what you did with the oscillator PIN's 16 and 17, parallelling these pin's somtimes causes massive distortion. I use a separate 470pF 1% polystyrene on each DAC. Then you can assign 1 DAC as "master" then connect 100pF capacitors from pin16 of the master to each of the other 3 DAC's, check with an oscilloscope if all oscillators are in sync now. And the outputs? did you put them all in parallel? By the way I just posted my schematics, I think you could easily modify your quad DAC javascript:smilie , you could use the 74HC164 instead of the 74HC166 I used, but you need 12 of them for a quad DAC. You are always welcome to listen to the new DAC and the sonic resonators.
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Old 15th May 2006, 02:32 AM   #43
MGH is offline MGH  United States
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Hi ecdesigns,

Thanks for replying on my thread regarding balanced, paralleled DAC. The question I had for you was if you were ever planning to produce a DIY kit of your balanced, paralled 1541 DAC for the rest of us who don't know how to design a PCB, but are OK at soldering. Even if you don't, your design is so intriguing I'll probably get the bread board out and try to put it together myself!
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Old 15th May 2006, 03:55 AM   #44
agent.5 is offline agent.5  United States
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Quote:
Even if you don't, your design is so intriguing I'll probably get the bread board out and try to put it together myself! [/B]

I agreed. I think quite a few of us will like to try it if there is a PCB. I think the PCB can also skip all the USB/SPDIF stuff in front too, as such functionalities are widely available, for example from dddac.
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Old 15th May 2006, 04:11 AM   #45
MGH is offline MGH  United States
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Correct me if I'm wrong. If you can get 400mV output with 8 chips, then with 20 chips, you can a full 1V output. You probably wouldn't need an active output stage (tube or opamp), if you have an active preamp. Right? I'm thinking simpler is better in this application. Doed's DDDAC doesn't require an active output stage because he gets 2V from his stacked 1543 chips, which may be related to the claimed purity/clarity in that DAC's sound.
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Old 15th May 2006, 07:10 AM   #46
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Compliance!
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Old 15th May 2006, 08:04 AM   #47
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Default Direct Interpolation DAC circuit boards

Hi, MGH

Thanks for your reply. Yes, I am designing circuit boards for this project. So other members can built this DAC or parts of it too because, it's fully modular, so you can experiment with different set-ups or modify your existing DAC. The photo of the quad DAC circuit board I posted earlier (the small module with 4 * TDA1541A on them) is in fact one of the modules of this new DAC (you need 2 of them for a octal DAC) . I am planning to make the following modules (circuit boards):

1) Differential schmitt trigger SPDIF driver (for low jitter)
2) Audio interface module 1: SPDIF to I2S
3) Audio interface module 2: USB to I2S
4) Direct Interpolation module (timing chain) for 2,4 and 8 DAC's that use I2S
5) DAC module with 4 * TDA1541A DAC's (this circuit board is ready)
6) DAC module with 4 * TDA1543
7) Differential output stage with OPA627 with/without Bessel filter
8) Differential triode / cascode output stage with/without Bessel filter
9) Main board for interconnecting the separate modules
10) Power supply with 3rd order line filter (note that every module already has voltage regulators built in for minimal interference, see photograph of the quad DAC module I posted, it has 12 regulators and 36 SMD bypass capacitors directly soldered to the chip's power supply PIN's).

Parallelling even more DAC's like the TDA1543 or TDA1541A is no problem, with the octal DAC version you can put 2 ore more DAC chips in parallel, for each DAC indicatad in the timing chain diagram I posted.

So if you have a lot of TDA1543's laying around you could use 8 of them in parallel for each of the mentioned DAC's, so the octal DAC get's 8 * 8 = 64 DAC's. Personally I think using 8 TDA1541A DAC's is more than sufficient, unless you use TDA1543 as this one has more distortion (see philips datasheets). As I mentioned before, getting a High output voltage is no proplem, with 33R resistors in an octal DAC you easily get 1V, but the problem is: the voltage at the DAC output must stay below 50mV (+-25) in order to keep distortion low. That's why I used an OP-amp I/V converter as it's input stays at virtual ground (0v) even at full scale current of 4mA or even higher.

But there is another solution: Use a very high quality OP-amp (OPA627 or AD equivalent) or a OP-amp using discrete components (perhaps a OP-amp using tubes) as I/V converter and a tube output stage, this way you get the best of both worlds: very low distortion and the tube sound you like so much.
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Old 15th May 2006, 10:02 AM   #48
guido is offline guido  Netherlands
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Default Re: Graphical example Direct Interpolation

Quote:
Originally posted by -ecdesigns-
Hi all,

Perhaps a graphical representation of Direct Interpolation says more than a thousand words The example shows 2 DAC's. The blue block (and blue lines) indicate one sample (64 * BCK).
Ok,

So you delay half the sampletime, not a full like the pic on the prev page.

Think the old cambridge (think it was them? UK stuff anyway) cd players with 4 tda1541's worked like that. '16x oversampling' with a 7220 to get to 4 and 4 dacs with delays to get to '16'.

Think there a few posts on that too. Just search
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Old 15th May 2006, 10:29 AM   #49
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Default Prototype Direct Interpolation DAC

Hi all,

For those who can't wait to hear this DAC, I will post a better photograph of the breadboard prototype so you have an example of how it can be done. I will post the differential output stage schematic diagrams part 1, very soon (my ancient DOS CAD system cannot export files). Then you got all necessary information to get soldering....Tip: by piggy-backing the 74HC166 (see photograph), the timing chain can be easilly build.
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Old 15th May 2006, 10:59 AM   #50
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Default Schematic diagram I/V diff amp

Hi, all

Here are the schematic diagrams of the I/V and differential amplifier stage part 1 that I promised.

Guido had a question about delaying samples:

A dual DAC: DAC 1 sample appears immideate, DAC2 sample 32BCK delay.

A quad DAC: DAC 1 sample appears immideate, DAC2 sample 32BCK delay,
DAC3 sample 16BCK delay. DAC4 sample 48BCK delay.

An octal DAC: DAC 1 sample appears immideate, DAC2 sample 32BCK delay,
DAC3 sample 16BCK delay. DAC4 sample 48BCK delay, DAC5 sample 8BCK delay, DAC6 sample 56 BCK delay, DAC7 sample 24 BCK delay and DAC8 sample 40BCK delay

So minumum sample delay is zero (DAC1), maximum delay is 56BCK pulses(DAC6) still less than 1 sample. All samples are delayed in the same manner (synchronous shiftregisters)

Hopefully this will clarify the delay "issue" ''






max. 48BCK delay for all samples and the octal DAC has max. 56 BCK pulses delay for all samples
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