A few Raspberry Pi observations

GaryB

Senior Member
I've been following the discussions on the Raspberry Pi as an inexpensive endpoint but never had a chance to try one. I currently use a Sonore Microrendu feeding a Khadas dac and I wanted to see how close one can get to the Sonore Microrendu.
I had a Raspberry Pi 2B sitting around and I just picked up a Pi 4b this week. I booted both of them with Ropiee XL which gives equivalent functionality to the Sonore Microrendu. They can all act as Roon endpoints or DLNA renders or Airplay endpoints (via Shairport) or Squeezelite endpoints or HQ Player endpoints. For these tests I ran things as Squeezelite endpoints.

First up, I started with the Pi 2B powered with a Pwr+ switching supply. The Pi 2B doesn't have built in wifi, so I ran it off of a wifi bridge connected to the Pi's ethernet port. The Khadas was connected to the PI 2B's USB2.0 port and the Khadas got its power from the 2B.
Results were disappointing. Compared to the Microrendu, everything sounded flat and images were "stuck" to the speakers.

Next I tried the Pi 4B powered with a Cana 3.4 amp switching supply. For this test I used the built in wifi of the 4B. Again the Khadas was connected to the USB 2.0 port of the Pi 4B. This set up was considerably better than the 2B but not quite in the same class as the Microrendu.

Next I switched off the internal wifi of the Pi 4B and connected the 4B's ethernet to the external wifi bridge. I compared the Khadas connected to either the USB2.0 output or the USB3 output. The differences between connecting to USB2 vs. USB3 were small but perhaps slightly in favor of the USB3. This set up feels like it comes reasonably close to the Sonore Microrendu. I still need to do more listening comparisons between this and the Microrendu to decide upon how close they are but this is already quite good.

In the future I want to experiment with providing a separate linear power supply for the Khadas and see if that makes the Pi2B more acceptable. My suspicion is that many of the differences that I'm hearing are coming from the way they affect the power being fed to the Khadas DAC. I also want to experiment with linear supplies for the Pi's. So there's more to come.
 
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GaryB

Senior Member
The power supply is a diy unit based on a design over at diyaudio called the L-adapter. It's easy to adjust the output voltage anywhere from 1.5v to 20v, depending upon the transformer that one uses. I use it with a 6.3v r-core transformer, which allows me to set the output to 6.1v for the Microrendu. I can also adjust the output to 5.1v, so I'll experiment with this feeding the Raspberry Pi as well.

The rest of the system has the Khadas DAC going to a Redboy / Slagle AVC that I bought from @StevenZ last year. As purchased it was single input and single output, so I modified it to make it three input / single output. It's feeding a diy gainclone amplifier which drives Reference 3A mm de capo i speakers. It's a reasonably resolving system which makes these sorts of comparisons pretty easy.
 

GaryB

Senior Member
And the experiments continue.

First I tried powering the Raspberry Pi 4 from the L-adapter linear supply, with the output voltage set to 5.1v. Interestingly, I found that this was a slight step down in quality from the Cana switching supply. I should point out that I use a good power line filter with the switching supply. In the past (12 years ago :eek:) when experimenting with power supplies for the squeezebox, I found that a switching supply with a good line filter was better than a linear supply. So history repeats itself.

Next I wired an L-adapter linear supply to the Khadas. One wires the 5v supply to pins 1 (5v) / 21 (gnd) of the 40 pin GPIO header. The Khadas seems to first power up from the USB-C and then check the external power connection and if the external power is connected it switches over. When I powered up the Khadas before the Raspberry, it showed some strange behavior. I'd preset the L-adapter to 5.0v, but once connected to the Khadas, power dropped to 4.7v from the 5v and I couldn't get it back to 5v when adjusting the pot on the L-adapter board. Once the USB-c power was connected, the L-adapter voltage jumped to 6.1v and was easily dialed back to 5v. Which is why I think that the Khadas needs to see 5v power on the USB-C port before it sees the external power. Long term, I think it might be better to make a Y-adapter so the 5v linear supply can be applied directly to the USB-C connector.

What's nice about the current scheme is that one can unplug the L-adapter and switch back to getting power from the Pi4, which makes A-B comparisons easy. In terms of sound quality, the external supply was yet another step up in quality. It's easy to hear when powering on the external supply. The bass solidifies and the soundstage expands a bit. It's very nice. The Microrendu / Khadas powered by an L-adapter is also very good and perhaps very slightly better but it's awfully close at this point.

I also retried the Raspberry Pi 2 with the L-adapter powered Khadas. That improved things considerably but it wasn't quite in the same league as the Pi4. So I think it's better to retire the Pi2's and focus on Pi4's.
 

mred

Senior Member
Interesting notes about the switching supply.
What is the filter before it ?
 

MikeyFresh

Moderator
Staff member
First I tried powering the Raspberry Pi 4 from the L-adapter linear supply, with the output voltage set to 5.1v. Interestingly, I found that this was a slight step down in quality from the Cana switching supply. I should point out that I use a good power line filter with the switching supply. In the past (12 years ago :eek:) when experimenting with power supplies for the squeezebox, I found that a switching supply with a good line filter was better than a linear supply. So history repeats itself.
I have something similar called an AC-7a, produced for a number of years by Array Solutions, a Texas maker of high-end, high performance RF products. I noticed recently the AC line filter section of their website was void, and in checking further I was saddened to hear that these custom passive units are now discontinued due to the recent death of the 93 year old engineer, Ed Wetherhold (W3NQN), who had both designed and hand built them since 1998. I am proud to own one and wish I had bought more before his passing.

Interesting stuff in that 12 year old Audio Circle thread, you were quite cutting edge with the Slim Devices stuff way back then, which was way ahead of it's time, and a well established category leader at the time they were acquired by Logitech.

This all makes me want to drill out my Khadas T-1000 enclosure so I can fit a 2.1mm DC jack.
 

GaryB

Senior Member
Interesting notes about the switching supply.
What is the filter before it ?
It was discussed in the thread linked above. It's a DIY line filter which was first proposed by the late Paul Kaplan of Kaplan cables. Paul did a lot of experiments with different power line conditioners and his two favorites were the Audience Adept series of line filters and the Torus power conditioners. The Torus conditioners use some special noise cancelling transformers from Plitron while the Audience Adept conditioners are based on passive filters using a combination of capacitors and common mode chokes.
The line filter that I'm using is one that Paul called the "Felix" and it's similar in concept to the Audience Adept power conditioners. I've attached a copy of the schematic.

Felix.gif
 

GaryB

Senior Member
I think I may have been hasty in writing off the Pi 2's. Over lunch I reflashed a microSD card with Moode and got it set up on the Pi 2 feeding the L-adapter powered Khadas. This is starting to sound a lot closer to the Pi 4. So Moode must do a better job of stripping down the SW to the bare minimum. Probably still worth moving forward with the Pi 4's but it does mean the Pi 2 is still usable. Now I'm wondering how Moode will compare with RoPieee on the Raspberry Pi 4 when running Squeezelite. The permutations are getting a bit out of control. I may need to leave things alone for a while and just listen.
 
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mred

Senior Member
It was discussed in the thread linked above. It's a DIY line filter which was first proposed by the late Paul Kaplan of Kaplan cables. Paul did a lot of experiments with different power line conditioners and his two favorites were the Audience Adept series of line filters and the Torus power conditioners. The Torus conditioners use some special noise cancelling transformers from Plitron while the Audience Adept conditioners are based on passive filters using a combination of capacitors and common mode chokes.
The line filter that I'm using is one that Paul called the "Felix" and it's similar in concept to the Audience Adept power conditioners. I've attached a copy of the schematic.

View attachment 26108
Interesting, I have something similar in my application between the mains and transformer.
20200803_210714.jpg
 
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MikeyFresh

Moderator
Staff member
I think I may have been hasty in writing off the Pi 2's.
There is a camp that thinks the 2B is the best sounding Pi, but I'm not sure I follow the logic of why. The 2B supporters seem to think it has to be better sounding due to the fact it has no on-board WiFi or Bluetooth, however using a distro like Moode, one can fully turn off/defeat both WiFi and Bluetooth (or for that matter HDMI, or even the LEDs) so that shouldn't necessarily be an inherent sonic advantage if the settings are right.

One argument to the contrary is that the 4B has a separate data bus for Ethernet and USB, so if the DAC connects via USB (rather than I²S over the GPIO) then there can be packet noise and bandwidth issues with the older Pis, due to USB and Ethernet having a shared data bus.

On the modification side of things, the Pi 2 and 3B (but not the 3B+ or later) can actually be modified to have their switching regulator removed/replaced, not small surgery but it can be done, however changes made on the successor 3B+ and 4B make that essentially impossible.

Interesting that you seem to be yet another end user reporting Moode actually sounds better, there would appear to be secret sauce in that software, and it works on both the DLNA/UPnP and LMS/Squeezelite Renderers.
 

GaryB

Senior Member
Interesting, I have something similar in my application between the mains and transformer.
View attachment 26119
Similar but not the same. My experience has been that the off the shelf line filters such as the TDK unit in your photograph don't work as well as a DIY unit that uses higher performance Common Mode Chokes (CMC). The Coilcraft part shown in the schematic that I posted is really pretty special. It would be worth your while to put one of those together to compare. You might be pleasantly surprised.
 

GaryB

Senior Member
There is a camp that thinks the 2B is the best sounding Pi . . .

Interesting that you seem to be yet another end user reporting Moode actually sounds better . . .
The Pi 2b and Moode combination was a big improvement over the Pi 2b and Ropieee SW but it still wasn't as good as the Pi 4b and Ropieee. After running it for the last 20+ hours, it just seemed to be digital and tiring to listen to after a while. Maybe I should try replacing the 2B switching regulator. Do you have a link that describes that?

When I switched back to the Pi4b + Ropieee this morning, it's like I could relax again. Some level of digital grunge that I could only sense subliminally was gone.
 

mred

Senior Member
Similar but not the same. My experience has been that the off the shelf line filters such as the TDK unit in your photograph don't work as well as a DIY unit that uses higher performance Common Mode Chokes (CMC). The Coilcraft part shown in the schematic that I posted is really pretty special. It would be worth your while to put one of those together to compare. You might be pleasantly surprised.
I like the design you posted.
I had the same thoughts last night about trying that filter. I used the TDK because I got a bunch of them for free with some other parts I bought.
 

MikeyFresh

Moderator
Staff member
Maybe I should try replacing the 2B switching regulator. Do you have a link that describes that?
Quick reply from my phone here as Hurricane Isaias has knocked out the power.

I can't immediately find the thread nor even remember if it was on diyAudio, or Computer Audiophile (now Audiophile Style), but what I do recall was one guy did it and showed a picture of his modification but not a step-by-step tutorial, and then I think Greg Stewart also modded one of his boards too.

I can't find any of that now, however maybe Greg will remember. This goes back a few years, and I also remember there was some caution needed due to there being revisions to the Pis over the years whereby the power was slightly different for a v1 vs. a v2 of the same model (i.e. model 2B v1 vs. v2 etc).

Beyond the actual removal of the switching regulator, there were others who thought simply supplying the power through the GPIO rather than micro USB was superior, as even though that still flows through the switching regulator, it does bypass some protection circuitry thought to be sonically invasive. Doing so requires tightly regulated power to ensure the Pi is not harmed, but through the GPIO you can also then split out 5v, 3.3v, and 1.8v supplies.

Hopefully my power comes back soon and I'll be able to do a better search than I can on my phone.
 

mred

Senior Member
I connect my power to the GIPO also.
It would not be too difficult to feed it via USB and compare. Maybe an experiment for the future.
 

GaryB

Senior Member
Still can't find the original discussion of this I know is somewhere on diyAudio or Audiophile Style, however in searching I do see this very interesting thread on the Rune Audio site, same topic, only 9 pages.
Mike,
That link is great. In particular this diagram which shows exactly what to do on my PI 2b.
Through the help of online translation software, I know that it's telling you to remove the switching regulator pointed to with box #1 and then provide linear power of 5v, 3.3v, and 1.8v to the testpoint in the bottom part of the figure.
Thanks! Yet another little project.
Pi2bPS.JPG
 

GaryB

Senior Member
The rest of the system . . . DAC going to a Redboy / Slagle AVC . . . modified . . . to make it three input / single output . . . feeding a diy gainclone amplifier . . .
Here's a picture of some of that equipment along with the DIY power filter housed in Home Depot boxes and a McIntosh MR78 tuner. I've also succumbed and got yet another DAC to play with - this time the Soncoz LA-QXD1. This is designed by the same person who designed the Khadas, so it felt like an interesting continuation of the Pi experiments. More to follow on the Soncoz, perhaps in a different thread.

Electronics.jpgPi and Soncoz.jpgAVC and Soncoz.jpg
 

mred

Senior Member
Excited to hear your impressions.
I have heard some rumblings that this may be better than the tone board.
 

MikeyFresh

Moderator
Staff member
Looks really nice Gary. Though double the price of the Tone Board, you certainly get what you pay for there starting with that nicely machined and anodized enclosure. Add the Toslink input, coax input and output, selectable filters, and a fully balanced circuit, and that looks well worth $199.
 
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