Building the ultimate NOS DAC using TDA1541A

[Uchi Deshi]

Direct Interpolation- it's very interesting. Could you give us more details or post schematic? Is virtual multiplying Fs caused only by doubling TDA1541 and differential mode?

[MGH]

Hi ecdesigns,

This is great! I recently posted a thread asking if anyone has tried paralleling TDA1541 in a balanced configuration. As far as I know, you are the only one who is actually implementing this. Please let me know if you are planning to build a kit for sale. I would jump at the chance of buying it. How many 1541s can be practically paralleled in your design? Could it be piggy backed instead like the well known DDDAC1543 (www.dddac.de)? Good work!

[analog_sa]

Originally posted by -ecdesigns-
Photo was still too large, I rescaled it again

Is that wonderful white connector at the end of the coax handling the 75ohm spdif?

[-ecdesigns-]

Hi Andy,

Here is a photo of the new Quad DAC on the breadboard, below it is the High-End set I designed. On top is the input switchbox, in the centre is the 6 channel control amp, at the bottom is the fully symetrical rock-solid MOSFET cascode amplifier. I tried to post the schematics, but the file is to big, if I scale it down it's unreadable. I could email it and attach the original file (about 1,5Mb).

[-ecdesigns-]

Hi, Uchi Deshi

Thank you for your reply

Direct interpolation uses at least 2 DAC's, this creates 88.2KHz virtual sample rate and doubles the resolution. The Quad Direct Interpolation DAC creates 176.4 KHz virtual sample rate and results in 18 bit resolution. And yes sure you can use more DAC's in parallel, 8 DAC's will create 352KHz virtual sample rate and so on. How does it work? It involves slight timeshifting (delay) of both Data and Wordselect and connecting 2 pair of DAC outputs in parallel, then these two are fed into a differential amplifier after I/V conversion. So this system only needs 4 I/V converters. The easiest way of visualizing this system is comparing a 1 cylinder engine with say a 4 cylinder engine. The 4 cylinder engine runs much smoother and achieves much higher revs than a 1 cylinder engine would. Now the DAC's are the pistons, the crankshaft is the output stage and the ignition is the timeshifting. So all 4 DAC's receive their DATA in sequence (not simultaneously). So they output their sample on slightly different moments in time (one clockpuls after wordselect changes) and thereby creating addittional interpolated steps i.e. increasing the resolution. So basically it's a quite simple technique. Since an added sample always stays in the timeframe (64 clockpulses) of the original (large) sample, there is no phase shift. Schematic diagrams will follow.

[rfbrw]

In short, linear interpolation.

[aparatusonitus]

This is really getting interesting, excellent work -ecdesigns-, especially as I saved 8 TDA1541A all from the same batch for something like this Would I get something like 19-20 bit resolution with 8 of TDA's?

[-ecdesigns-]

Hi analog sa,

The white connector you referred to is actually the analog output. I use a custom made fully differential SPDIF interface like in professional sound studio equipment. So no 75 OHm coax cable, certainly no TOSlink, actually I am using standard SVHS cable for this purpose. So I have a normal and an inverted SPDIF signal to cancel out interference that causes jitter. Obviously I had to modify my CD player (added schmitt trigger differential stage) so it would output this clean jitter-free differential signal to start with. Why a schmitt trigger? to get very short rise times as long risetimes can cause jitter. And jou must make sure the CS8412 get's a stable low jitter signal, otherwise its PLL keeps fluctuating and makes matters worse. So if you want to get rid of jitter, tackle the problem by the source and go differentialjavascript:smilie('').

[-ecdesigns-]

Hi rfbrw,

Thanks for your reply,

You are right, the prototype uses a form of linear interpolation, but without the traditional form of digital filtering, no decimating, no brickwall filtering, and no phase shifts. So I called it Direct interpolation as interpolation takes place instantly without computations. The beauty of it is that you have a filterless NON oversampling DAC outputting 88.2KHz, 176.4KHz or even more. A pure clean signal. Please read my other posts were I explained the system.

[ezkcdude]

It's not linear interpolation, more like nearest neighbor. You still have a stair step pattern. Interpolation means that a newly created sample is some combination of the surrounding samples. This method should be called "Direct Repetition".