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Valve DAC from Linear Audio volume 13
Valve DAC from Linear Audio volume 13
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Old 20th January 2021, 06:35 AM   #1351
MarcelvdG is offline MarcelvdG  Netherlands
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Good to know that it arrived safely at the other side of the world.
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Old 21st January 2021, 04:11 AM   #1352
SonnyMarrow is offline SonnyMarrow  United States
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Making some progress! Everything installed other than output caps, the voltage regulators, and the lights (since I need to decide how to do those).

I've got a Maida PSU and a Pete Millett supply for the 6.3V. Just need to get the transformers, cases, and FPGA.

Might have wasted money on a voltage regulator, because I forgot that the FPGA will increase the power draw on the 5V rail. Went with this regulator, but it's max current is only 500mA. Will probably just go with the L4940V5 and save the fancy once for something else.
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File Type: jpg 20210117_180837.jpg (835.5 KB, 108 views)
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Old 21st January 2021, 05:55 AM   #1353
nautibuoy is online now nautibuoy  United Kingdom
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Sonny, regarding the voltage regulator current rating, that vendor (there will be others I'm sure) has LT3045 regulators that uses them paralleled for higher current;

LT3045

This vendor is the source of the regulator I use in my DSD version build, albeit the 500mA version, and I have used his regulators in other projects - they're good.
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Old 21st January 2021, 06:31 AM   #1354
MarcelvdG is offline MarcelvdG  Netherlands
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Looks great! How did you do the fine-pitched SMD soldering?

I oversized the 5 V supply a bit, because I didn't know what would happen when I changed the configuration file or how much sample-to-sample spread there would be. The last time I measured it, the current drawn from the 5 V supply was three hundred something milliamps if I remember well, but that was several configuration file updates ago. So your LT3045's current handling may or may not be sufficient.
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Old 21st January 2021, 08:12 AM   #1355
acg is offline acg  Australia
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Looking good Sonny! This is the first I've seen the pcb up close with some bits on it. Interesting to see just where the vertical resistors and some of the other taller components are located on the board.

You've not soldered on the 7 pin valve socket yet...perhaps you purchased a rubbish one like me where the pin sockets are too tight to push in the valve pins.

Marcel, is there likely to be a performance improvement with use of these ultra-low noise 5V regulators considering their output ends up in places like the clock and the flip-flops in the dac cores? I've not made my mind up how to proceed to be honest and have a number of options available but it looks like a "definitely maybe" to me.
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Old 21st January 2021, 10:38 AM   #1356
MarcelvdG is offline MarcelvdG  Netherlands
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I don't think so. I'd expect the noise of the regulator to be negligible compared to the switching spikes of the FPGA anyway, despite the frequency planning: as much as possible of the FPGA runs on clock frequencies that cause no interference when they should end up on the DAC reference or clock, but even then there are data-related sidebands. The flip-flops that drive the upper E88CC's run on -132 V and -137 V, so not on the 5 V supply.
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Old 21st January 2021, 12:52 PM   #1357
SonnyMarrow is offline SonnyMarrow  United States
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Quote:
Originally Posted by nautibuoy View Post
Sonny, regarding the voltage regulator current rating, that vendor (there will be others I'm sure) has LT3045 regulators that uses them paralleled for higher current.
I already ordered the 500mA one a few weeks ago, I think I'll just go with the recommended part since it's only $1 and won't negatively impact performance.

Quote:
Originally Posted by acg View Post
You've not soldered on the 7 pin valve socket yet...perhaps you purchased a rubbish one like me where the pin sockets are too tight to push in the valve pins.
That was not by choice lol, I ordered it with the others and the seller must have forgotten it. I'll order one when I get my tubes. Four resistors are also missing because I messed up when ordering. I also did not install the 75 or 110 ohm resistors yet, because I'm not sure what my input configuration will be.

Quote:
Originally Posted by MarcelvdG View Post
Looks great! How did you do the fine-pitched SMD soldering?
I used a magnifying glass and a conical tip on my iron. Put some flux down, then put a tiny bit of solder as close to the tip as I can, and barely touch the pads one at a time. They are so small, they heat up quickly, and the solder fills up the pad easily. Just had to watch for bridging, which I did have one but resolved it.
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Old 21st January 2021, 10:25 PM   #1358
acg is offline acg  Australia
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Quote:
Originally Posted by MarcelvdG View Post
I don't think so. I'd expect the noise of the regulator to be negligible compared to the switching spikes of the FPGA anyway, despite the frequency planning: as much as possible of the FPGA runs on clock frequencies that cause no interference when they should end up on the DAC reference or clock, but even then there are data-related sidebands. The flip-flops that drive the upper E88CC's run on -132 V and -137 V, so not on the 5 V supply.

The benefits of your well planned design. It looks like the clock 5V could be completely separated from the remaining 5V with omission of the L6 inductor and I assume that inductor is there to hunt a little noise from getting through to the clock.

I remember reading about the PSAudio Directstream Dac years ago and that the FPGA was considered a major source of noise within the unit, a significant contributor to its relatively pedestrian noise measurements. I may be wrong, or mis-remembering, but perhaps some of that FPGA noise in the ValveDac could be pacified with a power supply capable of reacting well to rapid current fluctuations...something with a large ultracapacitor at output. I remember seeing a scope reading showing the effect of an ultracapacitor filter at the output of a LDO that was subjected to rapid current pulsing...it looked to have made a significant improvement.
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Old 21st January 2021, 10:28 PM   #1359
acg is offline acg  Australia
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I did some searching...here is that scope shot. Not sure if it is worthwhile in this instance.
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Old 21st January 2021, 10:58 PM   #1360
MarcelvdG is offline MarcelvdG  Netherlands
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You could indeed remove L6 and use a separate 5 V supply for U17. Any jitter caused by ripple on the 5 V for U17 is supposed to be suppressed by the current steering gate made of E88CCs, but that suppression is imperfect, so maybe a separate supply or separate regulator could improve performance to some extent. I don't know, as I haven't tried it, but it certainly makes more sense than using an ultra low noise regulator for supplying an FPGA board.

For suppressing really rapid current variations, physically small capacitors work best. Hence the use of 10 uF X5R SMD capacitors at various places.

Regarding noise, as reported in the Linear Audio article, I measured noise floors of -85.76 dB(A) left and -91.29 dB(A) right with respect to a full-scale sine wave in the PWM8 mode. That's low enough not to bother me at all, but not nearly as low as some solid-state DACs.
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