Return-to-zero shift register FIRDAC

Member
Joined 2007
Paid Member
But it you know sometimes it is in front of your eyes but you can't see it.

Well, yes, I do indeed know about this as well :magnify: ... Currently unable to fathom why a Kulish cell transistor amplifier performs very much differently when given one of Viktor's oscillator signals (1 kHz, - 155 dB 2H) and a DSD NoDAC signal. When fed by Viktor's oscillator the Kulish reaches - 108 dB THD for a 4 transistor circuitry whereas when fed the DSD NoDAC signals it is down to - 80 dB THD. Basically no idea why this happens ... :umbrella:

Anyway, returning to your "challenge" & just thinking aloud here: Have you tried to measure the signal levels on the outputs of the 74LV574As? I reckon the cut-off frequency here would be ~50 kHz so if you have a good multimeter you may be able to measure and compare the output levels already here (e.g. a 1 kHz signal). Are they identical?

Cheers, Jesper
 
I am struggling with another issue which I cannot solve, so asking again for some help.
I have 2.5dbV difference between the channels. The channel with the lower output has also a small amount of 2nd harmonic, the other is clean down to -100db.
Input signal: 1000Hz, -3dbFs
Output 1: -5,75dbV
Output 2: -3.24dbV
I have traced the signal back to U12,U5 with similar results

Both levels are lower than I would expect. Is the level difference the same for the positive and the negative sides of each balanced output?
 
Well, yes, I do indeed know about this as well :magnify: ... Currently unable to fathom why a Kulish cell transistor amplifier performs very much differently when given one of Viktor's oscillator signals (1 kHz, - 155 dB 2H) and a DSD NoDAC signal. When fed by Viktor's oscillator the Kulish reaches - 108 dB THD for a 4 transistor circuitry whereas when fed the DSD NoDAC signals it is down to - 80 dB THD. Basically no idea why this happens ... :umbrella:
Are you measuring THD or THD+N including the ultrasonic quantization noise of the DSD signal?
 
The common-mode loops built around U8 and U15 on the filter board do their very best to always make the signals at the outputs of U4A, U4B or U11A, U11B balanced. Hence, if anything goes wrong before the sense points of the common-mode loops (R36-R48 or R79-R91), it is likely to affect the positive and negative outputs equally.

Could you measure at the DAC outputs with the filter disconnected? When the DAC board outputs are not loaded by the filter and the DAC is playing DSD, I would expect DC levels close to 1.24 V at pins 2, 3, 4, 5, 7, 8, 9 and 10 of connector P3 of the DAC board.
 
Member
Joined 2007
Paid Member
Hi Marcel .. & thanks for your feedback on my comment about the THD variations.

Are you measuring THD or THD+N including the ultrasonic quantization noise of the DSD signal?

I actually didn't mean to divert the thread with this - just was a little frustrated by FFT results that appeared to be "all over the place" without any - apparent - reason. Anyway, I think I will wind up this topic here and instead send you a brief PM later in the day.

Cheers, Jesper
 
I have 1.245 VDC on every pin except pins 2 and 3 which are at 0.622 VDC
I have checked U7, U9 for connectivity, all pins are soldered, no short circuits and the resistors are also ok.
Also resistance between U7 and U9 pins is 6k02.
Also checked for connectivity and short circuits, back to U20
 
  • Like
Reactions: 1 user
It seems that precisely half the FIRDAC for the positive side of the left channel doesn't work. Do you measure connectivity between U7 pin 2 and U9 pin 3?

If you can measure this without causing short circuits, what DC level or what signals do you see at U7 pins 19, 17, 15 and 13 and U9 pins 18, 16, 14 and 12? The best places to probe those signals are probably the left sides of R47, R43, R40, R37, R52, R54, R58 and R60. They should all be switching between 0 and 5 V with an average value of 1.25 V, but I suspect four of them are stuck at 0 V.
 
I tried to re-flow U9 in order to remove it for replacement, when new chip arrives.
Instead of removing it I decided to re-solder it and now it works.
Oomph! I am very lucky on this.
Now both channels are the same, no visible distortions down to -100db.
What a great project !!!!!
Thank you so much.
Next step after some listening sessions is to replace NE5532 which I really do not like (others do), with OPA1678.
 
Last edited:
  • Like
Reactions: 1 users
Just about to put an order in to complete my ADC so rather than wait until November to complete the backordered bits from Mouser USA for the RTZ, I'll be ordering from Farnell UK for speedy delivery with little delivery cost. One thing I forgot previously are the standoffs and connectors - what did people use for their standoff lengths?

Slightly annoyingly I'm still missing a C16 33nF 2% NP0 but the closest I can get from Farnell is 5% - I'm tempted to order the two caps and replace the soldered and missing with the new caps, depending on what capacitance reads. When the new cap arrives - I'll updated it.
 
Go for it Nick.
As for standoffs it depends on how you will connect the 2 PCBs.
If you put DAC3-filter above DAC3, as intended, you need at least 13mm, lets say 15mm spacers, because the highest parts are C4, C6 and C9 with 13mm height.
The signal and PSU inter-connectors you will use has also some impact on the height, though what Marcel suggested in the BOM seems ok.
Another matter affecting the total height, length and width is the digital signal feeder you will choose to use,
Amanero or other with compatible pinout like DSDit or BBBreclk from Pavel designs can be pluged directly below DAC3 pcb.
The excellent I2SoverUSB PCB must be put on the side.
So choose your preferred I2S source before ordering standoffs.
I went with Pavel's designs (very elegant, handy and well documented), preferring ethernet wiring for many reasons.
See post 419,
https://www.diyaudio.com/community/threads/return-to-zero-shift-register-firdac.379406/post-7268682
Periklis
 
  • Like
Reactions: 1 user
Thank you for the info about the i2s boards - I'm considering a thin flat DAC case so this will work nicely.

I'll probably go with the i2soverusb - it has some flexibility and it's isolated. I've designed and built a 4 layer clock pcb for the ADC (24.576MHz) so I will have the option at a later date to add an external CCHD clock and the option to use the existing ADC clock for tests (has a 1:3 fan out buffer on board). I found isolating the STM32 from the ADC on the i2s stream dropped the noise floor by 27dBV so I'm all for isolation!
 
Next step after some listening sessions is to replace NE5532 which I really do not like (others do), with OPA1678.
I am currently using NE5532 and have OPA1678 on order since Dec last year, estimated delivery is Nov. :eek:

Have you actually got some 1678 and have you done real listening tests between them? I am really interested to know your observations and conclusions!

Well done finding and fixing the issue with your DAC!
 
Actually I have tested OPA1678 on this precise DAC.
Since I do not like the sound of NE5532, I find them very nervous, though it is difficult to describe sound with words,
I ordered some OPA1678s from Aliexpress, which I installed initially.
Having all these issues with the output and seen no obvious reason for this, I started suspecting that the opamps I used was not genuine.
So I replaced them with NE5532s which was bought from Mouser for this particular reason, from the beginning.
Well it was not their fault, though the Ti Logo is not exactly crisp and clearly printed on them, but the sound of them, if I can say, despite the fact that the DAC had some flaws was much better, smooth and spacious.
Now i am expecting some OPA1678 from LCSC.com as atomdiy suggested (see post 474).
LCSC is the parts supplier in the same group with JLCPCB, from whom these project's PCBs are built and I decided to give them a try.
When they arrive and are installed I'll report back.
Meanwhile I am auditioning the RTZ DAC with NE5532s and despite my bias with them, it sounds really nice because it is a brilliant design.
Everything else is of less importance and should not become a reason to postpone the built.
-Periklis
 
I continue to be impressed and delighted with the RTZ DAC. I tend to listen to streaming stations online, via Squeezebox, Soma FM, etc. I listen all day long as I work from home. My speakers are behind my work position (due to the room layout and location of desk) but I find myself amazed at the sheer depth of the image behind me, very much further out than the walls. Plus the "sharpness" of the sound, something just right about it. My ears and brain love it.
 
  • Like
Reactions: 1 users