A NOS 192/24 DAC with the PCM1794 (and WaveIO USB input) - Page 9 - diyAudio
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Old 27th December 2012, 06:57 AM   #81
dddac is offline dddac  Germany
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First of all, happy Christmas to all of you! Thanks for your patience on me being away and not answering questions ;-) BIG Thanks to Clive who has been so neat to cover for me during my absence and given all the right answers! Now I will read back and see what I can add to his comments....
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Old 27th December 2012, 07:01 AM   #82
dddac is offline dddac  Germany
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Quote:
Originally Posted by Panda2 View Post
Great project Doede!

Why 2 chips on 1 board? Is it dual mono?
Thanks for Feedback

Good question. The Point is that the PCM1794 in "digital filter bypass" mode only works in mono. Therefore you need one chip for Left and one for right. From a view of wanting to parallel Chips and have L & R separated as much as possible, this was very welcome anyway
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Old 27th December 2012, 07:11 AM   #83
dddac is offline dddac  Germany
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Quote:
Originally Posted by northernsky View Post
Hi,

thinking about getiing a kit, just a question.
I understood that no driver is required for Mac OSX 10.6 and later: am I right?

northernsky
Very clear answer... YES! Under OSX the WaveIO (actually every USB with the XMos chip as far as I know) works without any external drivers.

Some extra good news for the Windows community, I just received the new drivers for the WaveIO Board which are compatible with Windows 8. I already updated my download section. (and added Foobar 1.18...)
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Old 27th December 2012, 07:25 AM   #84
dddac is offline dddac  Germany
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Originally Posted by BachAudioDK View Post
Hi,

Why don't you have a resistors on the Iout going to a negative Vcc voltage to ensure the negative bias current of -6.2mA at bipolar zero ? Or am I missing some info in the diagram?

Page 25 of data sheet for pcm1794 show this. If you provide this resistor and negative bias current then you can trim the Vout to almost zero across your passive I/V resistor at Bipolar zero. And the you can use larger value of I/V resistor to obtain higher Vout, before THD rises, due to the conducting of the ESD diodes on Iout pins. The voltage on the I/V resistor will actually have a +/- swing with this negative bias resistor.

I use this principle with a negative bias resistor on a modded Asus Essence One, that uses PCM1795, that I have changed to passive I/V, I get very low THD.

What is the DC voltage on your 133ohm I/V resistor at zero output?

Cheers
Flemming
Denmark
This I like to answer, as it has to do with my basic design goals.

The whole trick of using DC bias (the 6k1 controls this at the ref pin) over the I/V load resistor, is to have a maximum voltage swing (I want maximum voltage output)

The good thing is, that the current output behaves like e very nice amplifier which is pretty linear. Measurements (I will post later) show that the distortion of the current source output is showing very nicely graduate reducing harmonic distortion from d2 downwards. This is what you see normally at good tube amplifiers

As the PCM1794 is not allowing you to create a negative Output voltage, there is no way you can have 1.2V rms swing around the Zero Position, hence you need DC bias. By the way this is 2,7Volt. The Output swings 3,4V p-p so the minimum voltage at the output is 0,7V and the max 4,4V. Both voltages have been found to be the max you can achieve without going into nonlinear distortion areas...

Even if it would be possible I would not do this, as you trade an Output capacitor (or Transformer) with an extra power supply being in the Signal way. I always like to avoid a situation like that....

As said on 133 Ohm one deck it is 2,7 Volt...
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Old 27th December 2012, 07:30 AM   #85
dddac is offline dddac  Germany
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Here are some preliminary test Images done by Triodedick on a one deck DAC connected with a spdif Receiver

Results speak for themself. Very linear, tube like distortion on the analog side and very low jitter
Attached Images
File Type: jpg FFT THD+n residu.jpg (135.9 KB, 671 views)
File Type: jpg Frequentierespons 48 kHz.jpg (100.2 KB, 651 views)
File Type: jpg J-test.jpg (158.8 KB, 652 views)
File Type: jpg Liniariteit.jpg (125.3 KB, 576 views)
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Old 27th December 2012, 10:27 AM   #86
jrling is online now jrling  United Kingdom
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Quote:
Originally Posted by clivem View Post
Is anyone (aside from Doede) running the WaveIO from a dedicated PS? I spent the day trying to hear any difference at all from it being powered from the USB bus or a dedicated power supply and I damned if I can hear any difference at all!
I am!

I have tried:

1. A Cincon SMPS open frame chassis type fed by a 240v dual bobbin transformer (7.5v)

2. A 12,000 mAh mobile phone charger (LiPo battery)

I noticed both having appreciable SQ benefit over USB power with my WaveIO. The SMPS was more dynamic, and the battery power (offline of course) was more fluid but still very detailed. I am not using the 1794 DAC attached as I have yet to buy one, but am poised to do so.

I can imagine many commenting adversely on both my choices of PSU from noise point of view. The SMPS though is used by a well known S/PDIF converter manufacturer and his view is that the switching frequency is so high as to be out of 'our' noise band. The battery will have some DC/DC conversion going on inside, but I thought I would give it a try and it works beautifully for me. If it didn't, I could always charge my phone with it and it was a v reasonable 32 from Amazon.

I watch your postings always with great interest as you are always trying new ideas which is great to see.
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Old 27th December 2012, 10:37 AM   #87
jrling is online now jrling  United Kingdom
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Quote:
Originally Posted by clivem View Post
Hair trigger volume controls are one of my pet hates. My downstream gear is designed on the premise that sources put out 2V RMS and that I want 3/4 of the volume control to be useable. I don't doubt that what I had with the differential tube input stage and the 10R I/V resistors would give a better THD measurement. Didn't sound as good as the 133R passive I/V! (And to be honest, my initial expectations before I ever heard the DAC, just knowing the value of the default I/V resistor, was a slew limited, muddy, crappy sound. That it doesn't sound that way and I prefer it to an active stage that on paper should result in better performance, go figure......)
Hair trigger volume controls are one of my pet hates.

Absolutely for me too.

I have Zu Druid Mk IV/Method Sub speakers which are 101 db efficient fed by a Bow Technologies ZZ-One 100w/channel integrated. So I have hair trigger issues! With 2v RMS output sources, I am on 7-8 o'clock max.

I would like to try passive output from the 1794 DAC and wondered if suitable I/V output component values could be suggested for low DAC output and maximising the lowest distortion hopefully able to be achieved by not pushing the output voltage swing so high (if you follow)?
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Old 27th December 2012, 11:18 AM   #88
dddac is offline dddac  Germany
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I know exactly what you mean. the unfortunate thing of passive I/ V is that normally, as in my case, the IV resistor is both DC bias determining as well AC current to voltage converter. that implies, if you lower the Rload, you also lower the DC bias and that lowers your voltage swing capabilities. trial and error or measuring is the best way.

another option is to split the 133 ohm load in let's say 100 and 33 ohm. if the 33 is at the gnd side and you tap signal at the middle, you reduce the output with (no calculator at hand) aprox 10-12 dB.... of course any mix is possible. positive extra, lower output impedance....
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Last edited by dddac; 27th December 2012 at 11:19 AM. Reason: typo...
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Old 27th December 2012, 11:22 AM   #89
dddac is offline dddac  Germany
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quick reaction to the SQ using USB power or external power...

Hard to understand, why Clive did not hear any difference. I know what I heard and several others at the explicit listening sessions we did. conclusion: totally worth it. I also know indirectly from Lucian, that other WaveIO users had same experiences. I have sent out the first power supplies to the early adopters now and of course very interesting for more feedback. I am sure we will have some more posts on this subject ;-)
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Old 27th December 2012, 11:25 AM   #90
dddac is offline dddac  Germany
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Originally Posted by crazyfrog View Post
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
130 would nr be a big issue.... but make sure to not go too far away from the optimum....

indeed the 100k is matched with the 2,2uF. but this is no tight thing. I used 2,2 so you can go as low as 10k input at the following stage ... there is some more on my web site about this....
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