A NOS 192/24 DAC with the PCM1794 (and WaveIO USB input) - Page 5 - diyAudio
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Old 5th December 2012, 08:07 AM   #41
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Thank you clivem but I kindly ask everyone to address all questions about WaveIO in dedicated thread located here. Let's try not to pollute Doede's discussions about his fine work! Speaking of that, I found plenty of PCM chips for my future decks but still waiting for an answer about the numbers... I guess four are enough but in your opinion Doede, eight are redundant?
Thank you,
L
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Last edited by Lorien; 5th December 2012 at 08:07 AM. Reason: speeling
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Old 6th December 2012, 01:00 PM   #42
jrling is offline jrling  United Kingdom
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Quote:
Originally Posted by dddac View Post
thanks for feedback. I checked the article from John and it was interesting in terms of getting keen on doing some next experiments with software filtering and up sampling and use some other players. Actually I never tried to listen and measure what happens if you up sample 44.1 to 176.4 or 192 and how it sounds or measure.

this week I finally get my new toy, an AP system two, so I am hoping to get some more insight by doing signal tests as well.

On a sad notice, if you read the thread of John, there is a lot of talking and opinions, but no real DIY which provides an easy route in improving sound for most hobbyists.... Nice for reading time, but not for "listening time"
I only have 44.1/16 music and now upsample it all to 176.4 (rather than 192 as 176.4 is a whole number multiplier of 44.1) using Secret Rabbit Code Level 0 (the highest quality) all performed in software within mpdPup to a WaveIO. It is excellent and gives a major boost to SQ without IMHO any unpleasant artifacts.

I was also very interested in John Swenson's article - but actually more from the point of view of the Ethernet transfer isolating the DAC from any other equipment in the chain. However, as you observed, there has been no news for some months on any DIY progress. I can only assume that is because the concept is being taken forward exclusively as a commercial product?
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Old 10th December 2012, 01:03 PM   #43
staki is offline staki  Switzerland
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Hi Doede and all other diyers !

I am very interested by this DAC, so much interested that I will buy one of them !

But, as there is no oversampling nor digital filtering, what's about ultrasonic noise (quantification noise) at the output ?
Is it reasonable to think that ultrasonic noise could interfere with the audio band and generate some intermodulation distorsion ?
Or could this ultrasonic noise damage some amplifiers or some tweeters at high volume ?

And, if I understand correctly, without oversampling it is practically impossible to put an analog lowpass filter at the output.
But, if one would oversample by software in the computer (using SOX for example) so all the files are played at least at 176,4 or 192 KHz sample rate (just like jrling does), would it be possible (and useful) to put an analog lowpass filter wih reasonable slope ?
Or would this filter destroy all the benefits of the technical choices of this DAC ?

I don't have any experience with non oversampling DAC's, so please all the explanations are welcome !

Chers, staki

Last edited by staki; 10th December 2012 at 01:12 PM. Reason: correction
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Old 10th December 2012, 09:45 PM   #44
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Default Resistors at 1794 output and 100K

Hi Doede,

You put 133R at 1794 outputs. I guess this is optimal value for biasing? Do you know if 130R would be right also?

100K is matched with 2.2 F?

Thanks
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Old 11th December 2012, 04:33 AM   #45
clivem is offline clivem  United Kingdom
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Originally Posted by crazyfrog View Post
You put 133R at 1794 outputs. I guess this is optimal value for biasing? Do you know if 130R would be right also?
I think Doede might be away on business at the moment.

IMHO, 130R I/V resistor is fine. Doede chose the resistor value to obtain a reasonable voltage output while keep distortion low. Decreasing the resistor size will reduce output voltage by a proportional amount. I think Doede stated a peak output current of 6.3mA per DAC, so......

133R: 2 x 6.3mA * 133 / 1.41 = 1.1885V RMS out.
130R: 2 x 6.3mA * 130 / 1.41 = 1.1617V RMS out.

Quote:
Originally Posted by crazyfrog View Post
100K is matched with 2.2 F?
The value of the output cap? 2.2uF into 100k load will give you a -3db point @ <1Hz. Again, fine.
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Old 11th December 2012, 08:47 AM   #46
Shinja is offline Shinja  Japan
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I suspect whether the delta sigma modulator runs properly.

Last edited by Shinja; 11th December 2012 at 09:04 AM.
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Old 11th December 2012, 10:57 AM   #47
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Thank you Clivem. Have in mind to try discrete opamps (Borbely) at output also.
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Old 11th December 2012, 02:47 PM   #48
clivem is offline clivem  United Kingdom
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Originally Posted by crazyfrog View Post
Have in mind to try discrete opamps (Borbely) at output also.
I'll post more when I have time later in the week, but I've already had a go with the Broskie Unbalancer tube output stage and ended up dumping the differential gain input stage (that I was running with a much lower value I/V resistor) and keeping the Broskie Cathode Follower and the original 133R I/V resistors. Output is still 1.2V, so on paper the only theoretical advantage would be if there was anything to be gained by cancelling noise common to using both the +/- output rather than just using the + output direct from the board, assuming a SE amp input. But to get the thing as quiet as possible I ended up with both regulated B+ and heater supplies for the tubes. (Aside from noise on the HT, these ears only find anything vaguely cathode follower related to sound good when regulated, so it's not just for the sake of quiet.)

I nearly jumped in on the questions from staki above about ultrasonic noise, but IMD, aliasing, images and noise generated by the sigma-delta modulator, seemed like too many cans of worms to be opened at the same time in one post. Anyway, while I was working on the tube output stage, I did have the chance to look at FFT on the outputs. Now admittedly my decent test gear is in storage and this was with a ten year old digital "sound-card" scope, (complete with useless software user interface that I suspect was designed by someone who has never used a scope), it is pretty obvious that the DAC chip itself has some on-chip filtering built-in. From what I saw I wouldn't bother with any form of external LPF. But having said that, there is always a corner case. From my point of view, using NOS DAC's with tube amps and transformers, where bandwidth ends up being limited....... But a couple of years ago I lent a NOS DAC without LPF on the output to a friend who lent it to a friend. They couldn't figure out why the amp they connected it to kept tripping the protection circuitry and shutting down. Of course, it was solid-state, and turned out the HF -3db point was something ridiculous like 300kHz. It didn't like my DAC! LOL.

Tweeters..... Unless you are talking about super tweeters, most of them tail off pretty significantly after 20kHz. It's a bit of a old wives tail that your blow your tweeters by using a NOS DAC without a LPF.

The bottom line. My modus-operandi, (following the "incident" above with the solid state amp), if I'm going to do any filtering at all, has been to put a small cap in parallel with the I/V resistor, when running into a tube gain stage without feedback, and aim for -3db @ 60kHz or so. Of course, as the OP says, if you do wish to use a "proper" analogue LPF, then up/oversampling is the way to go, but then you bring digital filtering back into the equation, regardless of whether you do it in hardware or software. I was actually quite pleased with what I saw (or rather didn't see) on the output of this DAC, to the point where unless I knew the DAC was going to be used with a solid-state amp that was likely to be a problem, I wouldn't bother with any additional analogue filtering at all. Having said that, my ethos tends to be KISS and that curing one so called "problem" tends to cause another. (And don't forget, assuming a 44.1kHz input, that with a NOS DAC you'll be be 3db down at 20kHz before anyone starts to get all bent out of shape about a stupid amount of attenuation being required at at 24.1kHz and how to avoid that biting into <=20kHz.) YMMV.
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Old 12th December 2012, 02:20 AM   #49
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Originally Posted by clivem View Post
Of course, as the OP says, if you do wish to use a "proper" analogue LPF, then up/oversampling is the way to go
Well, if I understand, a IV conversion circuit like OPC did (for an ESS DAC)followed by an active stage (my Borbely opamps or a tubes stage) would not be the perfect choice. That NOS converter has been (probably) "thought" having passive output.
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Old 14th December 2012, 01:46 PM   #50
staki is offline staki  Switzerland
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Originally Posted by clivem View Post
And don't forget, assuming a 44.1kHz input, that with a NOS DAC you'll be be 3db down at 20kHz
When Doede made the listening tests of his DAC, he was very impressed by the sound of the 176,4 or 192 KHz files. The 44,1 KHz sounded less impressive.

Could the "3 dB roll-of at 20 KHz" for 44,1 KHz files explain the perceived difference in sq ?
Or could it be that the quantization noise at 44,1 KHz is much more in audio band than the one at 176,4 or 192 KHz ?
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