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Old 11th November 2005, 11:14 AM   #1
dhayes5 is offline dhayes5  United States
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Default Need a DIY DAC Desgin for PCM1792

I have recently acquired two PCM1792 DAC’s and two OPA4134 4-channel OP Amps and am wanting to use them to build a DAC (am also trying to get an eval board but may be pushing my luck here).

Quite frankly I am not sure where to start. The PCM1792 will over sample but I keep reading about the virtues of non-over sampled DACS. I am also interested in ultimately having a tube output stage (but could start out with the op-amps).

Other thoughts I had were using these with a CS8412 or CS8414 audio interface receiver (or better if someone wiser suggests), possibly running each PCM1792 in monaural (something about separate signal paths appeals to me), perhaps building the output section with the op amps and such that it can be replaced later by a triode output stage.

Perhaps this system could be rigged with some toggle switches to switch the over sampling feature in and out – to allow me to hear the difference. However, the data sheet looks daunting and it seems you need some way to program the mode control registers – not sure how to do this.

I would also like to keep this design fairly simple (maybe could add the program features later) – though nothing about a DAC seems simple so this is probably a tall order.

I am hoping to get a dialog started with other DIY’er DAC builders to work out a good design. I am sure I will build more than one of these.

Do any of you DIY’ers have a design for a DAC that will utilize these chips?
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Old 11th November 2005, 12:36 PM   #2
Nisbeth is offline Nisbeth  Denmark
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AFAIK the PCM1792 is software-controlled, so you need a microcontroller to control the setup and configuration of the chip.


/U.
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Old 24th December 2005, 06:51 AM   #3
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I've recently built a PCM1792 DAC so I think I can answer a few of your questions.

First off you cannot run it in "non-oversampled" mode. It has a mode that turns off it own internal filter, but in that mode the data input has to be either 4X or 8X the sample rate. (it was designed for using an external digital filter).

I DID run this without a digital filter by using an FPGA to do a 4X data rate (each input sample was replicated so each sample became 4, no "interpolation" was done). This worked but sounded terrible, it seems the digital filter really is necassary with this DAC.

When using its own internal filter you have two modes, sharp and slow, they sound very different. In shap mode the sound is very clean and smooth, but also the soundstage collapses and its very "boring" sounding, the music looses its "life". The slow mode brings back the soundtsage and "life" but looses the high end air. I had one listening session where two people had opposite reactions, one loved the slow mode, one hated it for what it did to the high frequencies.

What I liked the best was a compromise, I did a 2X reclocking (again just duplicating each sample, not changing them in any way) and using slow mode, this gave the best of both worlds. This mode sounded WAY better than no digital filter mode and much better than the default sharp mode with 1X data going in (the 8X oversampling happens in the internal digital filter) The 2X data in and sharp mode also sounds better than the default 1X sharp, but not nearly as good as 2x slow.

The 1792 gets programmed from an external source everytime it powers up. It DOES have a default mode (I2S, 1X sharp mode), but if you don't want that mode you have to program it. I wound up implementing the programming in the same FPGA I was using for the data munging, but it would probably be easier with a PIC proccessor or something like that.

Its not that the default mode sounds BAD, its just that you can do way better by playing with it. In the 2X slow mode it sounds very similar to very well done NONOS DACs. (better than many)

This DAC has a high current differential output, the current is MUCH higher than most other current DACs, which makes certain things much easier. I wound up implemeting a differenital discrete stage, that is sort of similar to a D1 stage but with cascoded MOSFETS and higher currents. (take two D1 stages, put a common CCS on the bottom of each to get a differential amp and run at 30mA). This output stage sounds very good and works very well with the balanced output. It almost completely got rid of all the high frequency noise coming out of the DAC even wihout a filter. When run single ended there is quite a bit of HF grundge on the output, with the differential stage its gone.

Probably the simplest way to use one of these is to run in default mode (no programming) and buy the RAKDAC passive transformer output stage. It was designed for the 1794 which has the same output as the 1792. This will give you a good but simple DAC. You can then later on play with programming and trying different modes.

John S.
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Old 7th September 2010, 08:58 AM   #4
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John I like to know more about 2X reclocking, Can you explain some more details?
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