A NOS 192/24 DAC with the PCM1794 (and WaveIO USB input) - Page 6 - diyAudio
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Old 15th December 2012, 09:32 PM   #51
clivem is offline clivem  United Kingdom
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Originally Posted by crazyfrog View Post
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.
Sorry, not familiar with what OPC did or Borberly opamps.

It doesn't have to be passive I/V. It's up to you. Doede, stated his aims. (And also did his own custom transformer design with Sowter.) One eye on cost and another on simplicity. It's not like you have only +/- 1mA to play with, where you would have to add some kind of gain stage. And the 1794 seems friendly, (0.4% distortion, ISTR Deode said), to a value of resistor allowing a decent output. Transformers, opamps, tubes, jfets..... Or stick with a single I/V resistor. Pick your poison. I have tried a couple of options and I'm sticking with the BCF.
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Old 15th December 2012, 09:38 PM   #52
clivem is offline clivem  United Kingdom
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Originally Posted by staki View Post
Could the "3 dB roll-of at 20 KHz" for 44,1 KHz files explain the perceived difference in sq ?
Not IMHO.

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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 ?
No. That's not it either.

Don't think it is anything to do with not using a LPF and the images being shunted higher in frequency, either.

I have a theory that I've spoken to Doede about. I think he will be back this weekend and now that he has new test gear toys to play with, I imagine he will be busy testing and measuring.
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Old 15th December 2012, 10:23 PM   #53
clivem is offline clivem  United Kingdom
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Having had the chance to fiddle with various output stages for the DDAC1794, I've decided to stick with the Broskie Cathode Follower. (I make no apologies for being a tube guy.)

One thing I have found is that there is a benefit to be had using the +/- output from the DAC. Although I have not tried a transformer, (don't have anything suitable to hand), I expect that the benefit would be the same. You could even consider the BCF to be a sort of poor mans "active" tube transformer, converting a balanced output to single ended, but step-down, 2:1. (So if you had 1.2V single-ended from using one of the outputs, you'll still have 1.2V SE using both of the outs via the BCF.) Aside from any on-paper arguments about CMRR and noise cancellation, the bottom line.... more focus and less grain combining the outputs, as opposed to just using one single ended output. (Of course, if you prefer opamps...... )

Although I was using a differential gain stage in front of the BCF, I've gone back to the 133R I/V resistors and am just using the BCF to obtain a SE output from balanced.

200V HT from Tom Christiansen 21st Century Maida Regulator to the tubes. Heater supply is regulated too. ECC88's, 220R cathode resistors.
50k grid stopper and feedback resistors. That's all there is to it really.

I also swapped my original "junk" 7812 reg that was powering the main DAC board for a Salas shunt. To be honest, I couldn't hear any difference. Then I tried an eBay reg module based on the LT1764, claiming 40uV of noise. Couldn't hear any difference with that either. So it has stayed in by virtue of being last into the circuit. (I've ordered another to power the WaveIO board.)

I'll put it all into a case over the Christmas holidays.
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Old 15th December 2012, 11:21 PM   #54
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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

Last edited by BachAudioDK; 15th December 2012 at 11:50 PM.
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Old 16th December 2012, 04:10 PM   #55
clivem is offline clivem  United Kingdom
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Why don't you have a resistors on the Iout going to a negative Vcc voltage....
Doede must still be away on business. He can answer for himself. (I suspect that he will say the same as when 1704 was mentioned, needing additional -Vcc supply, that he wanted to avoid an additional power supply.)

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Originally Posted by BachAudioDK View Post
What is the DC voltage on your 133ohm I/V resistor at zero output?
Doede is using a Vcc of 8V and a 6.1k ref resistor, resulting in 10mA at zero. Mono mode, so x2, 20mA x 133 = 2.66V. Full scale current is 6.3mA peak per DAC, so swing up from ~2.6V to ~4.4V. (He determined that output clips at 4.5V.)
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Old 17th December 2012, 05:24 AM   #56
tuyen is online now tuyen  Australia
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Hey guys, I am wanting to go balanced out from this dac into balanced input into my preamp.

Does that mean I can bypass the coupling capacitor? How about the R load resistors?

cheers,
Tuyen

PS. your temp setup looks really interesting Clive. How does the tube rectification/output stage compare to the standard passive IV output? Do you use a valve preamp as well? IE. will I benefit from going an active tube output stage like yours if I already am using a decent valve preamp?
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Old 17th December 2012, 10:29 AM   #57
clivem is offline clivem  United Kingdom
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Does that mean I can bypass the coupling capacitor? How about the R load resistors?
My advice would be, unless you know that the inputs to your pre are cap coupled, stick with the coupling caps on the outputs of the DAC.

The I/V resistors? The DAC has current outputs. You need to keep the I/V resistors.

Quote:
Originally Posted by tuyen View Post
PS. your temp setup looks really interesting Clive. How does the tube rectification/output stage compare to the standard passive IV output? Do you use a valve preamp as well? IE. will I benefit from going an active tube output stage like yours if I already am using a decent valve preamp?
Well, the BCF I'm using although active, isn't really active in as much as it doesn't provide gain. It just combines the balanced outputs into a single output for SE use. In your case, where you'll be using both +/- outputs into your pre, there is no need to add anything if you are happy with the concept of passive output from the DAC board.

The problem with adding active tube stages to DAC's is that without feedback, they better be very low distortion, or you end up in the situation, where the passive out has reasonably low distortion to start with, that you increase it just by adding a tube stage. And without using very high transconductance tubes, you'll probably be adding more noise to the output than is there with the passive out.

IMHO, there is a benefit from using the balanced outputs from the DAC. Without getting into any audiofool clichés, eg. "it seemed like a veil was lifted", I didn't find it subtle. If you have a balanced setup to start with, you don't need to do anything. If you are single-ended, then it is worth thinking about transformers or the BCF.

Last edited by clivem; 17th December 2012 at 10:31 AM.
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Old 17th December 2012, 12:20 PM   #58
staki is offline staki  Switzerland
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Originally Posted by clivem View Post
If you are single-ended, then it is worth thinking about transformers or the BCF.
I have the opportunity to build a "no feedback" solid state output stage just like the one used in the Yamamoto YDA-1 DAC (PCM1794A dac chip in dual mono, with oversampling and digital filtering, but without analog LPF).

I will be able to compare with the passive output.
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Old 17th December 2012, 12:25 PM   #59
qusp is offline qusp  Australia
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instead of writing 'no feedback' perhaps we can convince people to write no global feedback? no need for the inverted commas then
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Old 17th December 2012, 12:39 PM   #60
staki is offline staki  Switzerland
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Yes qusp you're right ! I apologize.....
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