Pi + PCM5102 DAC + TDA7850 4x50W Amp - Page 1
Forum section: electronics / projects

author: essele
author_rank: Frequent Contributor
timestamp: June 10, 2018, 01:20:34 pm
content: Hi,I've just started getting back into electronics again after quite a few years, and this is my first project that's getting to a nearly usable state, but I've already made some major schoolboy errors, so I'm sure there are many more, and I'd love to learn from others experience on this!This is a project to drive my garden speakers from an AirPlay source ... so it houses a small PSU, a Raspberry Pi (running Shareport-sync), an I2S DAC for good audio quality, and then a 4 channel amp.Attached is the schematic and PCB from version 1, there are quite a few errors I've identified, but I'm also getting some noise and I think there may be a few causes that I'd love to understand better. The basic errors...1. My 7805 doesn't have enough cooling, it's ok, but it sits at about 110 degrees C when streaming ... for the next version I've switched it for a TO-220 version with a heatsink, although I'm also going to look at the TO-220 format DC-DC modules for more efficiency (will that be a noise problem?)2. I completely forgot to connect ST-BY and MUTE on the TDA7850 ... doh! On the new version I'm going to attache these to GPIO lines (with a suitable buffer as one of them needs more than 3.3v) and then I can turn off the amp when nothing is streaming.3. I misunderstood the PCM5102 datasheet comments about not needing DC blocking capacitors and removed them from the inputs of the TDA7850, which completely broke the biasing (and took quite a while to figure out!)4. I was a bit optimistic about how good I am at soldering SMD and put quite a few components a bit close to each other. I managed, but it was harder than it needed to be.5. I left the SCK pin on the DAC connected to a trace that doesn't really go anywhere ... I don't think this is an issue, but I've grounded it in v2.So I've worked around these for my testing, and the board works really surprisingly well, but I'd really like to get it even better for version 2.The audio sounds fine to my very untrained ear, but if I turn the volume right down (but leave it playing) then I can hear quite a lot of hiss. It's probably ok given my use case, but I'd really like to understand it. If I stop the audio then the DAC goes into mute mode automatically and the hiss entirely disappears. If I scope out the DAC output I can see a fair amount of noise, there is a peak at 16kHz so I am wondering if it's either something to do with the I2S speed, or whether this is noise from the Pi, or noise from the PSU or regulators.So I have some areas that I'd like to understand a bit better, but any other feedback would also be really helpful!1. I have quite long power traces, for 12v, 5v and 3.3v ... I've made them reasonably wide, but could do more. I know this is likely to be a source of noise, but from my limited understanding of my scope, I can't see anything really horrible on any of these. Should I be doing something different?2. I've put ground planes on both sides of the board ... it seemed sensible and makes it much easier to route. Is there anything wrong with this?3. I have no idea why I stuck my 5v to 3.3v regulator right under the PI, there is plenty of space to move it down and out of the way. Would that be sensible or just a waste of time?4. For v2 where I'm going to feed GPIO's to control lines (with a buffer) should I put some kind of series resistor in there, and/or pull-down?5. The PCM5102 eval board doesn't seem to populate any of the 10uF electrolytics, surely that would only increase the risk of noise?I know that's a lot of questions, but any help would be gratefully appreciated.(Oh, and I know that my TDA7850 heatsink is too small for full power ... I do have neighbours to worry about so this won't be a problem, and I can limit max volume on the shareport-sync config)Lee.

author: JS
author_rank: Frequent Contributor
timestamp: June 10, 2018, 05:14:33 pm
content: Having more power than needed is a good source of noise of any kind, hiss in particular is one of the common ones. If you are not using the full power you could use an analog divider and maybe a jumper or an analog switch to leave it out when the full power is required.Don't let the regulator heat that much, if you are using a bigger one and heatsink you will need to find a place so it doesn't become a problem. The if the 3.3V reg is just for the ADC you could either connect it directly from 12V to ease the 5V regulator task or use the 3.3V coming from the RPI. You can go for SMPS but is quite tricky to get them right, I'd try to avoid having it on the same PCB than the audio unless you've build it and test it first. The alternative is to start with a 12V and 5V power brick, or an external 5V step down regulator with decent filtering which can go on your PCB, and use one of the cheap step down from flybay.JS