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Old 21st October 2002, 10:36 PM   #1
jwb is offline jwb  United States
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Default The third try, or Revenge of the PCM1704

The cheap DAC designs are done and passing from the stage of my thoughts. I now turn to another absurdly expensive DAC! This time I will design the whole thing in public, on this forum, with feedback from the prophets, elders, and acolytes thereof.

To make sure this DAC is actually different from the previous efforts, I will not use an asynchronous sample rate convereter. At least, I will not use the CS8420. I might think about the AD1896.

For the digital input I shall use the Burr-Brown/Texas Instruments DIR1703. The claimed output jitter is 75ps, and it can receive 32-96kHz SPDIF signals. Physical interfaces will be transformer-coupled coax and Sharp GP1FA551RZ fiber optic.

I think I will use the DF1704E filter and PCM1704U-K DAC again. It's a good combination, even if the pins are all in bad positions. Perhaps the oversampling could be switched on or off at will.

Let's see how the budget is working out so far:

DIR: $3
Filter: $16
DACs: $152
BNC connector: $9
Pulse trafo: $15
Fiber receiver: $2
PCB: $40
On-board regulators: $20
Reset pulse generator: $3
24.576MHz oscillator: $8
I/V BJTs: $13
Output capacitors: $160 or ??
Output XLR connectors: $12
Output RCA connectors: ??
Misc. passives: $20?
Power supply rectifier: $18
Power supply trafo: $18

So far: $509. Not too bad. More than the el-cheapo, less than buying it from Mark Levinson. This is still in the planning stages. Let me hear your best ideas!
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Old 21st October 2002, 10:51 PM   #2
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How many DACs? Maybe more should be spend on transformers? I used 8 trafos in my DAC and don't regret it.
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Old 21st October 2002, 10:58 PM   #3
jwb is offline jwb  United States
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Four DACs: two for left and right each. The PCM1704 makes this easy for us with the INVERT input.

How many windings do you propose? I think each pair of DACs should share the same power rail, to ensure that power noise is common mode. So, perhaps a winding for each channel of DACs, a winding for each channel of I/V conversion, and another one for the rest of the digital circuitry? That's four dual-secondary trafos. That wouldn't be too expensive but you now require a lot of extra rectifiers as well.

Nevertheless I like your idea.
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Old 21st October 2002, 11:14 PM   #4
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I went a bit overboard but I used separate windings for: input receiver, oscillator, filter, ea. DAC has 4 separate supplies (LT1085 and LT1036 , separate digital and analog) for a total of 8 supplies just for DACs with separate windings. Plus four windings for output stage. Total: 16 windings-8 trafos. Now ea. windings has 4 soft rec. rectifiers for a total of 64 diodes. I also used something like 24 caps for prereg. (off the PCB).

I didn't go balanced but I paralelled PCM 1704 (so I'm also using 4 of them). I also included output switched pot, so no preamp required. This was my first DAC and when I started building it I didn't have a clue about digital. Now I know a bit more and I'm ready for another, absurdly expensive DAC.

I will be participating in a project.
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Old 21st October 2002, 11:17 PM   #5
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Here is a pic. The trafos on left are stacked one on top the other, so there are 6 of them. I also use split bobbin type, because they are better. Right beside trafos there are 20 Sprague Extralytic caps under the PCB (so no see). Rectifier diodes are directly attached to the small trafos so no see either. On the right side under another PCB there are 8 more big filter caps.
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Old 21st October 2002, 11:25 PM   #6
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Originally the board didn't have a provision for separate reg for digital and analog part of the DACs. I attached extra regulators on a bottom and they improve sonics by a big margin.
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Old 21st October 2002, 11:28 PM   #7
alvaius is offline alvaius  Canada
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Nice work Peter!


jwb, if you are going for broke, why not use a DSP to do digital filtering and give yourself complete control over the process as opposed to relying on the filter you buy? Not much cost adder, but certainly some time adder.
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Old 21st October 2002, 11:32 PM   #8
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Quote:
Originally posted by alvaius
jwb, if you are going for broke, why not use a DSP to do digital filtering and give yourself complete control over the process as opposed to relying on the filter you buy? Not much cost adder, but certainly some time adder.
Who is going to write the software? As soon as I get a DSP, I suddenly need microcontrollers and EEPROMs. I write software for a living. I don't want to write software as a hobby, too.
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Old 22nd October 2002, 01:12 AM   #9
alvaius is offline alvaius  Canada
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EEProm yes, but why would you need a microcontroller?
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Old 22nd October 2002, 01:36 AM   #10
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Why not use the DF1706? It has 192kHz input. You can let the AD1896 do 4x OS and leave the other 4x to the filter.

Also, how do you get the DIF1703 to work properly with only one crystal? If you use a 24.576 mHz crystal, it's not a supported multiple of 44.1 kHz. Wouldn't you also need to employ a second crystal to handle 44.1 and 88.2 kHz sample rates? 11.2896 would work nicely at 256 and 128 fs respectively. How would one even implement a multclock system? Sorry, I'm kind of new to this.

Stu
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