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Old 15th June 2019, 02:42 PM   #4541
Markw4 is offline Markw4  United States
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Quote:
Originally Posted by ClaudeG View Post
Any idea of the price range, Mark?
Not yet.
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Old 15th June 2019, 02:54 PM   #4542
Markw4 is offline Markw4  United States
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Originally Posted by damiangt3 View Post
I'm hoping to make a start on the I/V and I have a question on the selection of resistors/capacitors on the -ve feedback loop. I can see on Jens' design he's using 470pf and 1.27k, on ESS application note 100pf/272 and on Mr Slim's 330pf/820. How are these determined?
The resistor is chosen first to give a desired I/V stage output voltage swing. The value is chosen according to ohm's law, R = E/I, where E is the peak output voltage and I is the maximum dac output current. There is also a DC offset that has to be added which is Vref = AVCC/2.

The cap should be NPO or C0G, and is chosen to give a particular time constant in combination with the resistor. tau = RC. That sets the first filter corner frequency.

There are some tradeoffs involved with the resistor value. If we keep it small-ish (such as 820-ohms for ES9038Q2M), the fact that there is a Vref offset voltage can used to bias the I/V amp to always run class-A. Since Vref=AVCC/2=1.65v for Sabre dacs, if we keep the I/V output swing to less than +-1.5v, the opamp output will never go below ground and will be running class-A.

On the other hand we could use a bigger resistor to get more voltage output from the first stage, if we think that would be of more value for our needs.
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Old 15th June 2019, 03:02 PM   #4543
damiangt3 is offline damiangt3  United Kingdom
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Quote:
Originally Posted by Markw4 View Post
The resistor is chosen first to give a desired I/V stage output voltage swing. The value is chosen according to ohm's law, R = E/I, where E is the peak output voltage and I is the maximum dac output current. There is also a DC offset that has to be added which is Vref = AVCC/2.

The cap should be NPO or C0G, and is chosen to give a particular time constant in combination with the resistor. tau = RC. That sets the first filter corner frequency.

There are some tradeoffs involved with the resistor value. If we keep it small-ish (such as 820-ohms for ES9038Q2M), the fact that there is a Vref offset voltage can used to bias the I/V amp to always run class-A. Since Vref=AVCC/2=1.65v for Sabre dacs, if we keep the I/V output swing to less than +-1.5v, the opamp output will never go below ground and will be running class-A.

On the other hand we could use a bigger resistor to get more voltage output from the first stage, if we think that would be of more value for our needs.
Thanks Mark, I really appreciate your response.
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Old 19th June 2019, 06:51 PM   #4544
KoAP is offline KoAP  Russian Federation
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Markw4, means the chip ESS9038Q2m can work with dsd512, but why plt with AK4137 can not convert to dsd512, although it has a generator of 22.579MHz.
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Old 19th June 2019, 08:03 PM   #4545
Markw4 is offline Markw4  United States
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Originally Posted by KoAP View Post
..why plt with AK4137 can not convert to dsd512, although it has a generator of 22.579MHz.
It can output DSD and half its own clock rate, and maximum clock rate for AK4137 is around 25MHz. That means 12.5MHz DSD256 is its practical limit.
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Old 20th June 2019, 05:54 AM   #4546
Markw4 is offline Markw4  United States
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Thought I might mention that for the ES9028PRO dac I am preparing at this point, I decided what I thought was mostly holding it back as of a couple of days ago was the Trident-AVCC power supply (even running it with 7v input and with 33-ohm load resistors). To find out, I built an AVCC board much like the one we recommend in this thread, in fact I used the same schematic. To meet the AVCC current requirements of ES9028PRO, I used dual AD797 opamps running off their own dedicated +-15v power supply. I find that I definitely prefer the opamp regulator again in this situation. A good finding, I think, for those that went ahead and built the recommended AVCC board for dac modding from this thread. For those preferring the simplicity of a pre-built AVCC solution, I think I can say that Trident-AVCC can do pretty well, and much better than stock, running from 7v input power and with a 33-ohm, 1/2-watt load to ground for each channel. Please remember, if considering running Trident-AVCC from 7v input, to read the part numbers on the ADM7151 chips and make sure that is what you actually have, and that they are therefore safe to run at more than 5v input. Some ADM715x family regulators are limited to a 5v maximum input level.

Last edited by Markw4; 20th June 2019 at 06:01 AM.
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Old 20th June 2019, 07:10 AM   #4547
ClaudeG is online now ClaudeG
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Very interesting Mark, thanks again for this and sharing with so many details

I am pretty sure all this experience and options you have already handy will enable to gain a lot of development time around the AKM or whatever DAC chip

Have a nice day

Claude
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Old 20th June 2019, 08:09 AM   #4548
Terry Demol is offline Terry Demol  Australia
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Quote:
Originally Posted by Markw4 View Post
Thought I might mention that for the ES9028PRO dac I am preparing at this point, I decided what I thought was mostly holding it back as of a couple of days ago was the Trident-AVCC power supply (even running it with 7v input and with 33-ohm load resistors). To find out, I built an AVCC board much like the one we recommend in this thread, in fact I used the same schematic. To meet the AVCC current requirements of ES9028PRO, I used dual AD797 opamps running off their own dedicated +-15v power supply. I find that I definitely prefer the opamp regulator again in this situation. A good finding, I think, for those that went ahead and built the recommended AVCC board for dac modding from this thread. For those preferring the simplicity of a pre-built AVCC solution, I think I can say that Trident-AVCC can do pretty well, and much better than stock, running from 7v input power and with a 33-ohm, 1/2-watt load to ground for each channel. Please remember, if considering running Trident-AVCC from 7v input, to read the part numbers on the ADM7151 chips and make sure that is what you actually have, and that they are therefore safe to run at more than 5v input. Some ADM715x family regulators are limited to a 5v maximum input level.
Mark, good work.

WRT ES9028Pro:
- what is current requirement for AVCC and
- how did you combine the AD797's?

They can be a finicky opamp stability wise but do have very good OP drive.
Another option is ADA4898 which has more OP current, is faster and comes
in a dual. Subjectively it's anyone's guess which will sound better for that
application.

Do you know what the OP Z / current is of ES9028Pro?

Thanks,

T
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Old 20th June 2019, 02:37 PM   #4549
Markw4 is offline Markw4  United States
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Terry,

I measured input current to the Trident regulators at about 130ma each when they were driving the added 33-ohm loads. The loads draw 100ma, and the ADM7151 regulators probably draw a little, so I figured around 25-30ma range is what each of the AVCC opamps would have to supply.

ESS measured performance before for an earlier Sabre dac using AD797 and found lowest audio output THD with that opamp used for AVCC. I also used them before with good results, except they didn't like when I added film caps to the OPA1612 output stage +-15v power. Rather than fix the AD797 issue at that time I just replaced them with OPA1612 and left it at that since the dac at the time was an ES9038Q2M.

In this case the AVCC regulators have their own +-15v supply free of film caps. Eventually, I may see if AD797 can be made stable with film caps on the +-15v rails that I use for the output stage opamps.

The approximate schematic is attached below. 22uf Wima mks4 film caps were used for opamp output capacitors without problems. There was no need for Vref dividers on the AVCC board so I left those out. They are on the dac board and I used 1k resistors instead of 10k to help reduce I/V stage input bias imbalance and the HD that can cause (mostly 2nd harmonic).

In this case the AVCC regulator includes an on-board LT1086-5 as a pre-regulator for a plug-in 3.3v LTC6655 reference module. The AD797 are mounted on a single Brown Dog dual opamp adapter so the socket can be wired for standard dual opamps. The top opamp also has a stick-on heat sink.

The AVCC board and opamp outputs are physically located as close to the ES9028PRO AVCC inputs as I could manage. That is in accordance with advice from ESS.
Attached Images
File Type: jpg ESS9038Q2M_AVCC_V03.jpg (348.3 KB, 186 views)

Last edited by Markw4; 20th June 2019 at 02:44 PM.
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Old 20th June 2019, 03:01 PM   #4550
Markw4 is offline Markw4  United States
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Quote:
Originally Posted by ClaudeG View Post
I am pretty sure all this experience and options you have already handy will enable to gain a lot of development time around the AKM or whatever DAC chip
Claude,

Nice to see you are still here. Thanks again for your summary of the German audio magazine dac reviews.

To some extent your guess about experience so far helping with future projects is probably correct. However, in the case of AVCC, I think we have probably reached the limit for simple opamp AVCC regulators without pass or shunt transistors. For ES9038PRO and AK4499, more current will be needed and other types of regulators topologies will have to be considered. Perhaps some time will be needed to find a preferred solution going forward.
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