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Old 13th November 2007, 02:30 PM   #1
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Default COD - Current Output DAC

Here are some pics of our new toy in testing: a PCM1794A-based DAC, which Russ has named the COD.

Click the image to open in full size.

Click the image to open in full size.

The board has spaces on-board for a simple RC I/V stage, but we are using a TXD module for the I/V stage, DC coupled with no instrumentation stage and no feedback caps.

The benefit? No caps directly in the signal path and a symmetric I/V for super-low distortion.

It's still early testing, but it sounds very good so far. In these pics I am pulling a single-ended signal off the TXD's balanced outputs because I don't have a balanced amp on the test bench.

More news as it happens...
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Old 13th November 2007, 02:56 PM   #2
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Hi All,

A couple of notes. I designed this DAC so I could experiment with different I/V stages for current output DACs.

The one Brian is using now is actually the I/V stage I am most excited about so far, that is using the THS4131 on the TXD as a symmetric I/V converter. The THD4131 is fully symmetrical and THD+N out of the I/V stage should be ultra low.

I also have some discrete stuff in the works I am wanting to test out.

The DAC board also makes allowance for a simple resistor I/V which you can use easily in conjunction with the "ballsie" module.

There are jumpers on the DAC board so you can run DC coupled or A/C coupled.

Cheers!
Russ
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Old 13th November 2007, 03:12 PM   #3
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I knew it was only a matter of time before you guys adopted PCM1794.
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Old 13th November 2007, 03:14 PM   #4
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Quote:
Originally posted by ezkcdude
I knew it was only a matter of time before you guys adopted PCM1794.

Actually have not really "adopted" any single DAC, there are actually several DAC modules we will be trying.

But yes, we are glad to give it a go.

Cheers!
Russ
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Old 13th November 2007, 04:40 PM   #5
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Very interesting. I'll have to give it a try when you guys get them ready for public consumption. Before the Opus, the PCM1794 was the DAC that interested me most.
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Old 13th November 2007, 06:26 PM   #6
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Normally I prefer salmon, which of course has that wonderful warm coloration, but COD is a much more neutral fish.

Will be watching with interest!

Terry

P.S. Brian, how can you possibly not have a balanced amp on hand.
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Old 13th November 2007, 06:29 PM   #7
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Quote:
but COD is a much more neutral fish
It is the potato of the sea...
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Old 13th November 2007, 06:43 PM   #8
00940 is offline 00940  Belgium
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I'm curious....

Why do you call the THS4131 a current-feedback opamp in the txd manual ? TI never ever claims it is, quite the contrary.

Why do you oppose feedback caps on the opamp I/V ? They seem necessary for proper I/V conversion with opamps : http://www.diyhifi.org/forums/viewtopic.php?f=15&t=262 And some filtering of the analog output never hurts. Just get some decent silver mica or polystyrene. It's like nothing's there.

This being said, the THS4131 should perform nicely. It was used by the Bel Canto DAC2 for I/V conversion
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Old 13th November 2007, 07:00 PM   #9
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Quote:
Originally posted by 00940
I'm curious....

1) Why do you call the THS4131 a current-feedback opamp in the txd manual ? TI never ever claims it is, quite the contrary.

2) Why do you oppose feedback caps on the opamp I/V ? They seem necessary for proper I/V conversion with opamps : http://www.diyhifi.org/forums/viewtopic.php?f=15&t=262 And some filtering of the analog output never hurts. Just get some decent silver mica or polystyrene. It's like nothing's there.

3) This being said, the THS4131 should perform nicely. It was used by the Bel Canto DAC2 for I/V conversion
Hello,

1) Very probably an oversight on my part, I guess I equated it with current feedback because of the inverting nature of the feedback and the need for low value resistors. Terribly sorry if that was technically inaccurate. In the end you must treat it almost exactly as if it were a current feedback op amp. So practically it matters little.

2) When have I ever said I did? I don't appose them at all. In fact there are spot for them on the TXD board.

3) I am quite sure it will.

Cheers!
Russ
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Old 13th November 2007, 07:02 PM   #10
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Default Re: COD - Current Output DAC

I must have misunderstood this ?

Quote:
Originally posted by BrianDonegan
The board has spaces on-board for a simple RC I/V stage, but we are using a TXD module for the I/V stage, DC coupled with no instrumentation stage and no feedback caps.

The benefit? No caps directly in the signal path and a symmetric I/V for super-low distortion.
Oh, and for the current feedback stuff, I just noticed it passing by. No big deal
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