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tda1387 dac pcb "front end"
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Old 20th May 2018, 04:13 PM   #201
matt_garman is offline matt_garman  United States
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tda1387 dac pcb "front end"
Update on the balanced RPI HAT design: it appears it generates some low-level hiss. With lower-sensitivity amps and/or low-sensitivity speakers, you may not notice. But I recently paired the DAC with a new amp. Compared to what I was using, this amp appears to be a bit more sensitive to source quality. The result was that during quiet passages of less-than-perfect recordings, the hiss was sometimes noticeable enough to be distracting. See this post for more info.

So looks like my next project is to try to clean up some base noise on this DAC. (Hmmm, what about pre-I/V filtering? I wonder if anyone's tried that? )
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Old 20th May 2018, 09:20 PM   #202
GORDMUS68 is offline GORDMUS68  South Africa
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Finally had some time to do a quick setup of the SE HAT today.


Rpi3b + kali - piCorePlayer squeezelight and LMS - through mediocre workshop amp and speakers.



Very listenable and no grounding problems etc.



Next a more permanent case and upgraded power supply, as have JRiver will set that up on tinkerboard and move to our everyday system, for some proper listening comparisons.
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Old 21st May 2018, 05:27 PM   #203
stellarelephant is offline stellarelephant  United States
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I have been toying around with the Garman 1387 hat for the last several weeks. Greg Stewart kindly loaned me a treasure trove of 5V PSU goodies to try in my Pi stack. He encouraged me to try powering each board separately, and I must admit, each time I added a PSU, clarity went up another notch. Here's my current stack using 4 PSUs. (Temporarily the DAC is jumpered to the Kali to avoid sharing power between them--until I remove that 0 ohm resistor on the Kali to defeat this feature)

Top to bottom:
Gstew modded 5V Jameco linear wall wart -> Garman 1387 hat
Gstew modded 5V Jameco linear wall wart -> Allo Kali
Gstew modded 5V Jameco linear wall wart -> IanCanada IsolatorPi
IFi iPower -> Raspberry Pi
All PSUs are going through a shared CLC via Greg's power conditioner plug.

So far I prefer the sound of Greg's 5V wall warts to everything else I have tried on each level of the stack. This includes: an additional iPower, a 12V linear wall wart into an eBay LT3042 LDO, and some of Greg's modded 12V warts into Acko AK375 LDOs with supercaps added. (Today Greg suggested that I may not have warmed these supplies up enough go to realize full performance.) And also the iPower into a Jan Didden Silent Switcher--probably my second favorite 5V supply so far. Grounding for any of these supplies is essential to reducing noise/hum. I have wires from the ground rails on both top and bottom of the isolator board going direct to earth.

Anyhow, I am very pleased with the beautiful transparency of the rat's nest you see here. I feel I may be getting somewhat close to the upper limits of Pi-Fi. My next step will be trying to replace the iPower on the Pi with a linear 5V supply capable of 2A output. Even with the isolator in place, the Pi's PSU plays a very audible role IME. I am also doing some comparisons between this and other DACs, including DIAL, LingDAC, ApplePi and hopefully Boss.
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Old 21st May 2018, 07:13 PM   #204
matt_garman is offline matt_garman  United States
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tda1387 dac pcb "front end"
Quote:
Originally Posted by GORDMUS68 View Post
Finally had some time to do a quick setup of the SE HAT today....
Awesome! Glad you got it up and running. Always encouraging to see my "babies" all grown up and starting their own lives.


Quote:
Originally Posted by stellarelephant View Post
My next step will be trying to replace the iPower on the Pi with a linear 5V supply capable of 2A output. Even with the isolator in place, the Pi's PSU plays a very audible role IME. I am also doing some comparisons between this and other DACs, including DIAL, LingDAC, ApplePi and hopefully Boss.
Although the RPi can technically draw up to 2A, if you are using it purely to shuffle PCM bits onto the I2S bus, power utilization should be well under 2A. I don't know what power-saving tweaks Volumio does by default, but in general, with Linux on the RPI, it's not terribly hard to disable hardware you're not using, e.g. HDMI, onboard sound chip, wifi (if using RPI3), etc. The CPU frequency can also be changed, and I have mine fixed to the lowest possible speed.

The Raspberry Pi Zero is somewhat appealing in this regard, except that it lacks a builtin Ethernet controller.
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Old 22nd May 2018, 06:52 AM   #205
GORDMUS68 is offline GORDMUS68  South Africa
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Hi SE,


Could you elaborate on the mods done by Greg to those Jameco adapters, any particular reason why are they superior to the others you have auditioned.


I see your LingDac in the pictures, time permitting I too am muddling through a 'Pi-Fi' iteration, will be interesting to read your comparison once you have viewed all your Pi-Fi options.


Best of luck and looking forward to your observations.


Matt - haven't had much time with your 'baby' hence my reserved comments. At the very least I now have a small taste of the potential of the 1387 you all talk about, indeed this is going to be very enjoyable -


Thanks again all pioneers I too hope to become a 'pundit' !!
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Old 23rd May 2018, 06:12 PM   #206
Alexandre is online now Alexandre
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When I experimented with the 1387, I found there is a "correct" voltage for the output node. Where the noise will be minimised.

This happens because of the continous calibration. With each wordclock transition it will calibrate one of various current sources. This current source will be disconnected from the output node (the "spare current source" will be used in its place), calibrated, and reconnected. If the voltage at the output is the same of the internal calibration circuits then there will be no voltage jumps during this process. Thus noise and glitches are minimised. You can figure this from the various philips papers.

Actually, to make this simpler, think of it as finding the correct voltage between output and V+. You can vary the supply voltage to the dac chips instead of changing I/V resistors. Find the point for lowest noise. Important, must be done with all input signals present, of course. With the dac playing silence.

Thanks,
Alex




Quote:
Originally Posted by matt_garman View Post
Update on the balanced RPI HAT design: it appears it generates some low-level hiss. With lower-sensitivity amps and/or low-sensitivity speakers, you may not notice. But I recently paired the DAC with a new amp. Compared to what I was using, this amp appears to be a bit more sensitive to source quality. The result was that during quiet passages of less-than-perfect recordings, the hiss was sometimes noticeable enough to be distracting. See this post for more info.

So looks like my next project is to try to clean up some base noise on this DAC. (Hmmm, what about pre-I/V filtering? I wonder if anyone's tried that? )
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Old 24th May 2018, 04:49 PM   #207
matt_garman is offline matt_garman  United States
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tda1387 dac pcb "front end"
Hi Alex, thanks for the input!

Quote:
Originally Posted by Alexandre View Post
When I experimented with the 1387, I found there is a "correct" voltage for the output node. Where the noise will be minimised.
Just out of curiosity, what kind of I/V were you using when you experimented with the tda1387?


Quote:
Originally Posted by Alexandre View Post
This happens because of the continous calibration. With each wordclock transition it will calibrate one of various current sources. This current source will be disconnected from the output node (the "spare current source" will be used in its place), calibrated, and reconnected. If the voltage at the output is the same of the internal calibration circuits then there will be no voltage jumps during this process. Thus noise and glitches are minimised. You can figure this from the various philips papers.
Makes sense. I'm curious, then, if you did any experiments with changing the sample rate? If the calibration happens on every wordclock, then changing the sample rate of the source material out to result in a corresponding change in re-calibration rate, which should in turn affect the noise. Presumably measurable, I'm curious if it's audible? I might give this a try just because it's easy.


Quote:
Originally Posted by Alexandre View Post
Actually, to make this simpler, think of it as finding the correct voltage between output and V+. You can vary the supply voltage to the dac chips instead of changing I/V resistors. Find the point for lowest noise. Important, must be done with all input signals present, of course. With the dac playing silence.
In my current RPI HAT designs, it's hard to change supply voltage because I'm getting it from the RPI (5v). I'm also using this very simple common-base active I/V from Abraxalito. See attached schematic image.

BREF is the reference voltage for the transistor base, and is generated from a TL431 used as a shunt regulator. With a BC807, BREF=2.8v. I've also used the SSM3J327R MOSFET here, with BREF=2.5v.

Do you have any suggestions on what could be tweaked here to possibly reduce/remove the noise?

Thanks!
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Old 24th May 2018, 05:44 PM   #208
Alexandre is online now Alexandre
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Matt,

I was using a simple passive I/V. I found that a carbon resistor gave me a more pleasant sound. Removed some edginess and "metallic" characteristics. It was around 2K.

I had a strong preference for higher sample rates. The residual noise was indeed afected, but that was not the only reason I prefered it. It sounded better.

About that I/V, Iīm not too fond of it, the base current is one concern. Maybe it isnīt a problem if the hfe is very linear. Will it be linear at such a low current (say 100uA to 1mA)? Iīm not sure.
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Old 24th May 2018, 05:46 PM   #209
Alexandre is online now Alexandre
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Are you using digital volume?

Because if you arenīt, then youīll be reaching zero current through the BJT (isnīt the tda1387 current output going between zero and 1mA?)
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