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.
Yes and yes.
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.
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
.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.
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.
Attachments
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.
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!
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Run it into a virtual ground and I'm sure it would be possible to get very near the datasheet spec.
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......)
Doede has added the DDDAC Power supply to his 1794 webshop - 5 and 12 volt / 1A in kits and assembled versions. Scroll down on this page
DDDAC 1794 NOS DAC - Non Oversampling DAC with PCM1794 - no digital filter - modular design DIY DAC for high resolution audio 192/24 192kHz 24bit
He has also updated the page about power supplies accordingly. Scroll down
DDDAC 1794 NOS DAC - Non Oversampling DAC with PCM1794 - no digital filter - modular design DIY DAC for high resolution audio 192/24 192kHz 24bit
Looks neat
Per
DDDAC 1794 NOS DAC - Non Oversampling DAC with PCM1794 - no digital filter - modular design DIY DAC for high resolution audio 192/24 192kHz 24bit
He has also updated the page about power supplies accordingly. Scroll down
DDDAC 1794 NOS DAC - Non Oversampling DAC with PCM1794 - no digital filter - modular design DIY DAC for high resolution audio 192/24 192kHz 24bit
Looks neat
Per
Lucien is the person to give you a definitive answer to that question. That's what it says on his website. Your question might be better asked in the WaveIO thread.
Yes, I have been. When I first thought about it, higher sample rates, higher current draw, so any differences are likely to be more obvious at higher sample rates. I've listened to a bunch of native high-res material along with other stuff that I have software upsampled 44.1k -> 176.4k. Even had the DAC plugged into my headphone as well as speaker rig. Maybe what I used to consider to be golden ears have turned into cloth ears. LOL. I really can't hear any differences. Nothing subtle. Nothing at all.
I've also tried the laptop as well. So I've been listening bus powered driven from my Logitech Touch and bus powered from my old Samsung NC10 laptop running Linux.
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I'm not Doede, but I'll bite....
Yes, configured for dual mono, so one DAC chip for left channel, the other for right channel.
Either from the pre-amp that you plug the DAC into, or in software on the computer or whatever else you are using to feed it via USB.