Here are some pics of our new toy in testing: a PCM1794A-based DAC, which Russ has named the COD.
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...
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
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...
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
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
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
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
00940 said:
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
Re: Re: COD - Current Output DAC
Sorry I understand your question a bit better now.
Ah, I think what Brian was referring to there was his particular test at the moment he had no caps, and more particularly the lack of output caps (AC coupling) was what he was trying to point out.
I don't think either of us (Brian or I) have any aversion to using filter caps in the TXD itself for I/V. Sorry if it sounded that way. I hope it is more clear now.
Perhaps the better to thing for us to say is that such things as filter caps are completely optional. And no DC blocking caps are required at all, because of the common mode rejection of THS4131.
Cheers!
Russ
No worries.00940 said:
Sorry I understand your question a bit better now.
Ah, I think what Brian was referring to there was his particular test at the moment he had no caps, and more particularly the lack of output caps (AC coupling) was what he was trying to point out.
I don't think either of us (Brian or I) have any aversion to using filter caps in the TXD itself for I/V. Sorry if it sounded that way. I hope it is more clear now.
Perhaps the better to thing for us to say is that such things as filter caps are completely optional. And no DC blocking caps are required at all, because of the common mode rejection of THS4131.
Cheers!
Russ
00940 said:Everything's clear now
Thanks, and actually your point on CFB is absolutely correct. It is something I should correct in the TXD documentation. I wrote that manual when the device was still pretty new to me, and the whole symmetric feedback was just soaking into my smallish brain.
I have actually designed a discrete amp very similar to the THS4131 and I know it is voltage feedback. So it stands to reason the THS is too.
Cheers!
Russ
Testing underway!
I have my COD (I have a couple of them) working.
I have tested the following scenarios:
1) Passive I/V, AC coupled (22uf Oscons) with 470,390,221, and 100R resistors for I/V into a Ballsie. The Ballsie gain is not very high so I would not go much lower than 100R with a stock Ballsie, but you could if you want. The input impedance of the Ballsie is high so 390 or 221 of output impedance (or close to that) is absolutely no problem at all.
2) Active I/V with TXD at 750R for the feedback resistor. Rf can be adjusted to get the output swing you desire.
Honestly both I/V options sound excellent
The noise floor is as low as the Opus, which is to say, quiet.
I will have to listen to each setup for a while longer to give an opinion on which I like best, right now I would put it at a dead tie.
Important note, filter caps are much more important on the COD than the opus. I used 1nf on the TXD and 1.2nf/22pf on the Ballsie.
Cheers!
Russ
I have my COD (I have a couple of them) working.
I have tested the following scenarios:
1) Passive I/V, AC coupled (22uf Oscons) with 470,390,221, and 100R resistors for I/V into a Ballsie. The Ballsie gain is not very high so I would not go much lower than 100R with a stock Ballsie, but you could if you want. The input impedance of the Ballsie is high so 390 or 221 of output impedance (or close to that) is absolutely no problem at all.
2) Active I/V with TXD at 750R for the feedback resistor. Rf can be adjusted to get the output swing you desire.
Honestly both I/V options sound excellent
The noise floor is as low as the Opus, which is to say, quiet.
I will have to listen to each setup for a while longer to give an opinion on which I like best, right now I would put it at a dead tie.
Important note, filter caps are much more important on the COD than the opus. I used 1nf on the TXD and 1.2nf/22pf on the Ballsie.
Cheers!
Russ
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