A NOS 192/24 DAC with the PCM1794 (and WaveIO USB input) - Page 7 - diyAudio
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Old 17th December 2012, 02:44 PM   #61
qusp is offline qusp  Australia
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dont apologize man =) no need for that.

its just a pet peeve of mine, it is fashionable for the press and marketers to use this misleading term, we should just try not to use it here.
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Old 17th December 2012, 06:53 PM   #62
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Originally Posted by clivem View Post
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 17th December 2012, 06:57 PM   #63
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So you kept the 133R for IV conversion and used the balanced outputs. If I see well the picture, you put capacitors only at at neg outs. Why?
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Old 18th December 2012, 01:17 PM   #64
clivem is offline clivem  United Kingdom
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Originally Posted by crazyfrog View Post
So you kept the 133R for IV conversion and used the balanced outputs.
Yes and yes.

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Originally Posted by crazyfrog View Post
If I see well the picture, you put capacitors only at at neg outs. Why?
No. 0.22uF caps from the + to the top triode section (cathode follower) and the 2.2uF caps Doede supplied with the kit from the - to the bottom triode section (common cathode).

Sorry for the brevity, I'm running late.
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Old 18th December 2012, 04:28 PM   #65
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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.
I find that interesting. Hope Doede will answer...

Last edited by crazyfrog; 18th December 2012 at 04:29 PM. Reason: forgot something
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Old 18th December 2012, 06:27 PM   #66
clivem is offline clivem  United Kingdom
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Originally Posted by crazyfrog View Post
I find that interesting. Hope Doede will answer...
Pull up/down resistor, current source or sink used to null offset and stay within DAC voltage compliance, depending on whether DAC sources or sinks, is nothing new. I use it on both my TDA1541 DAC's.

I doubt there is a free lunch to be had here. (Although I'm basing that on my experience with other DAC chips, not the 1794.) Although the output may not clip until 4.5V, I suspect that distortion will rise rather rapidly, were you to use 233R in place of the 133R to obtain 2V RMS out, even with the 20mA at zero nulled with a neg supply and resistor or CCS. My gut feeling is that distortion is going to suck, each DAC being loaded by 466R. Somewhere I should have a -12V supply and a bag of IXYS10M45S. So I'll find out.
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Old 18th December 2012, 08:07 PM   #67
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Originally Posted by clivem View Post
Pull up/down resistor, current source or sink used to null offset and stay within DAC voltage compliance, depending on whether DAC sources or sinks, is nothing new. I use it on both my TDA1541 DAC's.

I doubt there is a free lunch to be had here. (Although I'm basing that on my experience with other DAC chips, not the 1794.) Although the output may not clip until 4.5V, I suspect that distortion will rise rather rapidly, were you to use 233R in place of the 133R to obtain 2V RMS out, even with the 20mA at zero nulled with a neg supply and resistor or CCS. My gut feeling is that distortion is going to suck, each DAC being loaded by 466R. Somewhere I should have a -12V supply and a bag of IXYS10M45S. So I'll find out.
I still have in mind to use my discrete opamps but it's better to use balanced outputs in this case. Much better if the offset is very low, no need for big caps...Not looking for high output as you can see. Anyway, your experiments will be welcome of course .
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Old 19th December 2012, 11:10 AM   #68
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Hi,

I might have been a little confused about how the internal ESD diodes on Iout is connected, as these are not in any TI documentation. My observation is that there must be a diode with cathode on Iout and anode connected to ground. Which limits the negative voltage swing to about approx. -600mVp.

My passive I/V on the Asus Essence One, i get approx 260mVrms af 0dBFS, per balanced output.

ASUS use PCM1795 in dual mono setup. I outputs are not paralleled, but summed after IV conversion in an opamp as seen op page 37 in datasheet. PCM1795 only have +/- 2mA per output. I let the IoutN and IoutP share the same Negative bias resistor. See picture. I have decoupled the shared node with a 47uF. This might not be need. (maybe some distortion would be canceled out here if there is no cap.?)

The resistors shown in the picture does not have the correct values as I have implemented, mainly because I use the input impedance of the summing opamp as part of the total IV resistor.

To get a higher output, I have tweaked the gain a bit in the summing opamp, to compensate. On the RCA output I have approx 1.8Vrms and I measure 1kHz THD+N = 0.007% (measured with AD of ASUS STX soundcard)

I think that 0.4% THD seems a bit high from your implementation of PCM1794.
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Old 19th December 2012, 03:26 PM   #69
clivem is offline clivem  United Kingdom
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Originally Posted by BachAudioDK View Post
Which limits the negative voltage swing to about approx. -600mVp.
Well, if that is also the case for the 1794.... If we can swing up to 4.5V without clipping, but only down to -600mV without clipping, there is little point biasing the outputs for 0V, apart from to get rid of cap coupling, which I don't have an issue with.

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Originally Posted by BachAudioDK View Post
I think that 0.4% THD seems a bit high from your implementation of PCM1794.
Yes, you might think that. But that's to be expected delivering 1.2V RMS from passive I/V resistor. On paper it might be bad, but hey, how many people with an interest in NOS DAC's live and die by on-paper specs. Measurement is an indicator. Ears are the final arbiter. I have been enjoying Doede's DAC and while I could pick at it and say this could be done better, that could be done better, at the end of the day the only thing that matters is that I am enjoying it.
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Old 19th December 2012, 04:06 PM   #70
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Originally Posted by clivem View Post
Yes, you might think that. But that's to be expected delivering 1.2V RMS from passive I/V resistor. On paper it might be bad, but hey, how many people with an interest in NOS DAC's live and die by on-paper specs. Measurement is an indicator. Ears are the final arbiter. I have been enjoying Doede's DAC and while I could pick at it and say this could be done better, that could be done better, at the end of the day the only thing that matters is that I am enjoying it.
I agree on your views that THD numbers are not directly a measurement on perceived sound quality. I also have a NOS DAC (Muse 4xTDA1543) which I have found optimal Rdac and Rbias resistor, to achieve 0.685Vrms with THD+N = 0.029 % and It sounds great too.

My comment on the 0.4% THD was more minded on, that this Chip (PCM1794) should be able to perform better even in NOS mode and with passive I/V.

Do you really need so high output of 1.2Vrms to drive your amp/speakers/headphones ?

(my guess is that during normal listning levels, your volume control is properly turned well below 12 O'Clock)

Cheers!

Last edited by BachAudioDK; 19th December 2012 at 04:10 PM.
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