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Opamps for PCM1794 I/V conversion?
Opamps for PCM1794 I/V conversion?
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Old 12th December 2018, 01:03 AM   #21
abraxalito is offline abraxalito  United Kingdom
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If you're open to unconventional approaches then I can't recommend a CLC filter between DAC and I/V stage highly enough. Normally the I/V opamp is critical to the subjective SQ but with the filter installed in my own (multibit) DAC good SQ can be had from lower-spec'd I/V opamps. Which makes sense to me technically as with the filter, the I/V stage is no longer slewing at sample transitions.
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Old 11th February 2019, 11:33 AM   #22
samoloko is online now samoloko  Bulgaria
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how about opa861 for I/V
like http://designideas.cocolog-nifty.com...0828_00615.jpg
or discrete
Качественный дискретный выхлоп для ЦАП
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Old 6th March 2019, 12:57 PM   #23
samoloko is online now samoloko  Bulgaria
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Default OPA1612 as I/V

Hi Mikett,

reading your thread about ESS9028PRO Moving from 9038Q2M to a 9028Pro board I thought that you are charmed from using OPA1612 as I/V http://www.ti.com/lit/an/sboa237/sboa237.pdf
I don't know why they Inject bias voltage and In pcm1794 specification http://www.ti.com/lit/ds/symlink/pcm1794.pdf there Is no biasing of NE5534 used as I/V
did you change that opinion
and how about using OPA1622 or ADA4898 at output as differential summing and low pass filtering

Quote:
Originally Posted by Mikett View Post
I have the single PCM1794 version of this board.
The AD797s with the dual adapters work well on this board.
Next thing is that these op amps need good decoupling so you might want to ensure that the power legs are decoupled very closely to ground using the appropriate caps.
On this board the AD797s are very sensitive to the IV stage power supply quality.
LM317/337 were lackluster with lack of real bottom end control and power with an emphasized lumpy upper midbass and no high frequency detail and definition.
Substituting a Jung Sulzer power supply driven by NE5534 did improve things nicely but not enough.
Adding a Jung Didden Super Regulator did open things up markedly and it started to sound in the correct direction. Still a bit of lumpiness in the low end but missing the detail and transparency found in a decently modded ESS9028 and likely 9038Q2m Dac. ( not op amp rolling....but power supplies)

Finally I added a matching power supply to this board driven by 3x5534 op amp rails. A power supply being sold for $20+ USD. It did not match the Jung Didden Super Regulators but I would put it a step above the Jung Sulzer Regulator with one caveat.

The caveat is that the PCM1794 chip is driven by an LT1761 that taps off the IV stage power supply. What I did was to remove this tap by removing the SMD jumpers and completely removing the LT1761-5 on the board. Then I directly fed an op amp driven power supply to the 1794 chip. This improved things a lot better BUT, again the speed of the 5534 could be discerned as lacking definition and transparency.

Given the low current requirements of this 1794, I suspect my next step is to create an op amp buffer power supply (AD797) to drive the 5V current requirements of this 1794. This is the next stage. Using something like an AD797 configured in a non inverting buffer configuration with a low noise 5 V on the +input will likely do the trick.

So without the last suggested mod, and even with the prior mods to the 5V, the 1794 lacks behind a current ESS chip dac well modded. With a good 5V supply to the 1794. It could possibly get close.

OP amp rolling compared to the suggested mods and not supplying top end power supplies is completely underwhelming. The 5v current supply to the 1794, needs to be stable and fast reacting and stable as heck....high speed op amps are needed. Spend the efforts there....not in the choice of op amps.

Finally, I will suggest a current boosted OPA1611 over the AD797 in the differential amp leg. It appears to sound better than the AD797.

Focus on the power supplies.....fast and stable with reasonable low noise.

Visit the huge thread on modifying the 9038q2m and learn about the importance of the current supply to the DAC chip. That is the weakest leg and then onto the IV stage and power supplies. I've given up on LT304x supplies. Indeed they are low noise but they are too slow and unstable dynamically...they give a very smooth sound but they lack detail and microdynamics and imaging in the sound. Been down that road.

Last edited by samoloko; 6th March 2019 at 01:09 PM.
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Old 6th March 2019, 01:26 PM   #24
Markw4 is offline Markw4  United States
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OPA1622 is not DIY friendly, and the 8-pin DIP versions of OPA1622 from China, assuming they are not fake, do at a least fake using the ground pin on OPA1622 properly. I kind of assume they use some resistors to make a voltage divider to bias the ground pin half way betwen the + and - power pin voltages. But, that's not really ground, and the power pins may have noise, ripple, etc., on them that a true ground plane ground would not have.

Otherwise, OPA1612 is a very good choice when very low distortion is wanted, such as with high quality 24-bit dacs. If OPA1612 doesn't sound good, then there is a problem somewhere, maybe with power supply impedance not being low enough across the entire audio band up to around 100kHz, and or too much RF/HF leakage from the dac chip getting into the dac analog ouptuts and no corrective action taken in the dad board design.
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Old 7th March 2019, 06:05 AM   #25
samoloko is online now samoloko  Bulgaria
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Mark,

thank you for reply

good to know that you like OPA1612 as I/V for quality 24 bit dacs
do you recomend to try It with not just only pcm1794 but also pcm63, pcm1704
what Is your choice for power supply to pcm1794, analogue and digital : Salas shunt, LT3042, ADM7150, cap multiplier

what Is your choice for OPA used as diferential summator / low pass buffer - what do you think about OPA2132 or ADA4898
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Old 7th March 2019, 06:35 AM   #26
Markw4 is offline Markw4  United States
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samoloko,
While OPA1612 is good for low distortion 24-bit dacs, it may be important what clock noise coming out of the dac looks like. For Sabre dacs it is very low level, but there is some HF/RF junk. It can be wise to filter out a good amount of any HF/RF junk before an I/V amplifier. Sometimes a 2,000pf cap can be enough to do the trick, connected from the dac output to ground (I/V input to ground), or maybe connected differentially for differential output dacs. Whichever way works best for a particular dac is best, so it may require trying both ways, and maybe trying a bit bigger caps. For older dacs with even bigger clock pulses, more pre-I/V stage filtering may be appropriate and further help sound quality. Also, for 16-bit dac chips, there may not be a lot of benefit from using an ultra-low distortion opamp. A faster opamp may be more tolerant when using dac chips with more substantial, higher amplitude clock or other HF/RF noise.

Last edited by Markw4; 7th March 2019 at 06:39 AM.
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Old 7th March 2019, 11:14 AM   #27
samoloko is online now samoloko  Bulgaria
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Mark,
please advise about propper power supply for pcm1794 digital and analogue part, and your choice for OPA used as differential summator / buffer
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Old 7th March 2019, 11:49 AM   #28
Markw4 is offline Markw4  United States
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samoloko,
I am using OPA1612 for IV and differential summing/filtering. The +-15v opamp power needs to have low impedance across the audio band up to 100kHz or so, IMHO. That can be done passively with enough fairly large and expensive film caps, or it could be done with an active voltage regulator. Active regulators work by having a lot of gain in a feedback loop, so depending on how much phase shift is caused by components in the load, such regulators may risk instability and oscillation if sufficient care is not taken. They also need to sense the load voltage where the voltage is used or very close to that, not long wires away on another power supply board. Our fellow diyaudio member eziitis has reported what he finds to be good results using the three transistor version of Nazar regulators, which are pretty cheap and easy to build. eziitis also says he hasn't had any stability problems if he uses only X7R ceramic bypass caps at opamp power pins, and skips any tantalum or electrolytics on the load side of the regulator. Probably, a set of dedicated + and - Nazar regulators located on right next to a 3-opamp output stage would be appropriate and likely to work well. However, I must say for the record that I have not directly compared a set of the Nazar regulators eziitis likes with what amounts to about $100 worth of Wima mks4 film caps. Of course, there are other regulator options too, such as perhaps classic Jung regulators (diyadudio store sells bare PCBs to make a set of them). I will attach a schematic below of +- Nazar regulators recommended by eziitis and add notes offered by him as follows:

"The shunt transistor type depends on how much power it should deal with, so current set by R15/39 and output voltage. their value can be calculated R=(1.25-Ueb)/(0.3/Uout)+Iload
Ueb typically 0.65V, 0.3 stands for the max wattage of the transistor in this case. so if one wants to pump more current then something like BD139/140 should be used. LMs need some 4V overhead, and the whole thing radiates a little heat. anyway, the noise is in uV range, but the impedance is what rules. elcaps better not very low ESR, the reg may start to oscillate. ceramics perhaps better np0, but i am fine with x7r so far."

Regarding my own use of passive film caps, more info at: ES9038Q2M Board ... and a few posts following that one. My most successful experiment with film caps was using Wima mks4, with 2 identical sets of five film caps soldered in parallel (2 independent sets of 5-soldered-together-parallel-caps), and with one of the sets connected from each of the +-15v rails to ground. Values used for each of the sets of parallel caps are: 10+10+22+33+33 ( all values in uf). As I said, it costs about $50 per rail, so not cheap.
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File Type: png ! Screen Shot 2019-02-14 at 23.23.02 (eziitis).png (71.2 KB, 40 views)

Last edited by Markw4; 7th March 2019 at 12:06 PM.
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