[RESOLVED] Removing high frequency noise from an audio circuit - Page 1
Forum section: electronics / projects

author: microbug
author_rank: Frequent Contributor
timestamp: May 11, 2014, 10:23:08 am
content: I am making a bluetooth boombox. I am using a bluetooth module which runs off 5V and has an analogue output which feeds into my amp. During prototyping, I have been powering this from a separate power source (a backup USB charger running on an 18650 Li-Ion battery) for convenience, however I would now like to have it all running from the same battery.It appears that the outputs of the bluetooth module are not isolated, so when I am powering it from my 5V regulator (with a ground lead separate to that of the output) some current flows down the ground lead to the amp and is amplified to create high frequency audible noise. There is a high-pass filter on the input of the amp; it is identical to the example schematic in the datasheet (attached). I could add a low-pass filter to the input but as the noise is audible it would cut out some music as well. I tried adding a 100nF cap across the bluetooth module's power inputs, but didn't make a difference.Advice?

author: microbug
author_rank: Frequent Contributor
timestamp: May 11, 2014, 12:25:55 pm
content: Bump. I have been looking around for similar problems. I moved the bluetooth module's ground lead to the same point as the ground lead for the amp; that reduced the noise a bit. I removed the ground from the audio (mono) input to the amp but it didn't make any difference. I also tried some makeshift 'shielding' around the audio wire, tin foil (connected to ground with an alligator clip).I probed the audio input with my 'scope: it appears to be some sort of switching noise. I will try using a different 5V source, but other suggestions are welcome.

author: tautech
author_rank: Super Contributor
timestamp: May 11, 2014, 12:31:44 pm
content: QuoteNot

author: microbug
author_rank: Frequent Contributor
timestamp: May 11, 2014, 12:39:34 pm
content: Oops, it is now. (EDIT: attached to the first post)I tried using my bench PSU (noisy switch mode) and the module went nuts, just playing random sounds. It survived, but I think it is something to do with the 5V supply. I am using a Traco Power TSR 1-2450 step down converter at the moment...

author: microbug
author_rank: Frequent Contributor
timestamp: May 11, 2014, 03:33:23 pm
content: I tried adding a first-order low-pass filter, using R=3.3? and C=100uF for a -3dB cutoff of 482Hz. The noise, according to my scope, comes in bursts which occur every 1ms (=1kHz) so this should reduce it considerably, however the output of the filter looks much the same as the input. Any ideas?EDIT: Noise is around 40mV pk-pk.

author: bobwidlar
author_rank: Contributor
timestamp: May 11, 2014, 05:24:11 pm
content: First of all, have you used separate ground for analog and digitals and made a star connection at only one point? If your bluetooth module introduces high freq noise on power lines, then you might consider using some high frequency filtering ( series ferrite bead + cap kind of things) plus star connected power connections. I would rather use a separate regulator for analog + line filtering.You can get some idea of line filtering with ferrite beads here:http://www.altera.com/literature/an/AN583.pdfAnd for any kind of mixed signal PCB, this is a MUST to read:http://www.hottconsultants.com/pdf_files/june2001pcd_mixedsignal.pdf

author: microbug
author_rank: Frequent Contributor
timestamp: May 11, 2014, 05:40:18 pm
content: Do you count buck / boost converters to be 'digital'? If not then the only digital part of the circuit is the bluetooth module, which only has one ground connection. EDIT: If buck / boost converters are 'digital' I'll go and change the ground connections.Thanks for the links; I'll check them out.

author: bobwidlar
author_rank: Contributor
timestamp: May 11, 2014, 07:13:27 pm
content: Buck/boost converter is not digital, and since it's your voltage source you can't star connect your ground. But, your amp chip has different analog and digital supplies. Those should be separated (you can use same supply with your bluetooth module here, both are digital let's say), and you have separated digital and analog grounds for the amp IC. That grounds should be star connected at one point as close as to the board's external ground connection point.

author: microbug
author_rank: Frequent Contributor
timestamp: May 11, 2014, 07:24:04 pm
content: I didn't design the amp board but I do know that it has separate analogue and digital grounds. However, having just looked at the datasheet, they are probably connected very close to the chip as opposed to near the external ground connection (the pin description for DGND says 'Digital Ground. Connect to AGND locally (near the TA2024)'. Given this, I don't think there would be much point in having the bluetooth ground connected to the amplifier's digital ground, as the bluetooth ground is already connected to the external ground.I am starting to think about designing my own amplifier board using a better chip (the TA2024 isn't ideal for my purposes and is at least 8 years out of date).

author: David Hess
author_rank: Super Contributor
timestamp: May 11, 2014, 07:26:46 pm
content: This is a standard ground loop problem. It would be nice if the source included a separate connection for sensing the signal ground but I have never seen that feature unless I designed it myself.One of my solutions in the past has been to use some variation of instrumentation amplifier to subtract the ground noise between the output and input.

author: microbug
author_rank: Frequent Contributor
timestamp: May 11, 2014, 07:28:34 pm
content: QuoteAlso, there's no way I'm up to designing a bluetooth receiver.EDIT: If I could find one, I would use a bluetooth module with and I2S output and design my own amp...EDIT 2: The really weird thing about this problem is that it didn't exist until I switched from using external USB power to 5V regulated from the battery.

author: bobwidlar
author_rank: Contributor
timestamp: May 11, 2014, 08:18:59 pm
content: Aha, if that's ok with the battery than your error source is the USB, huh?Well I can't imagine your circuit, so I had no idea how you connected grounds etc.. Considering if USB is connected to a PC, than it's ground and power should be really noisy. For ground, I can't imagine what you should do but for power you should check out that ferrite bead + cap line filtering I mentioned before. Ferrite beads are to clean high frequency line noise.

author: microbug
author_rank: Frequent Contributor
timestamp: May 11, 2014, 09:00:46 pm
content: OK, thanks. I need to have another look tomorrow (it's 10PM here!) to figure out what the source of the problem is; it seems there are multiple components to the noise. That RC low-pass filter I mentioned did help a bit according to my oscilloscope, but the difference is not audible.

author: Niklas
author_rank: Frequent Contributor
timestamp: May 11, 2014, 09:26:12 pm
content: I would skip that R in the RC filter and replace it with a ferrite bead. Even low frequency variations in the current consumption can cause voltage fluctuations due to Ohm's law. Noise is high frequency, so that 100 uF electrolytic capacitor is pretty worthless at that range. Ceramic capacitors, 10 nF up to 1 uF, paralleled with 10 to 47 uF tantalum or electrolytic, could be a first try together with some BLMxx from Murata. The tantalums or electrolytics will serve as a local power buffer and the ceramics as a bypass for noise.Take that 5 V, filter it with LC or LCLC and then feed it to the audio circuit.

author: microbug
author_rank: Frequent Contributor
timestamp: May 12, 2014, 04:13:06 pm
content: Thank you so much for your help! Using Niklas' filter suggestion cut down on the 5V noise a lot, but I found most of it was generated by the boost converter on the amp's input. I feel kind of stupid for not realising sooner... I was drawing a wiring diagram when I figured it out.Thanks again!EDIT: For anyone who has a similar problem and finds this thread: I also noticed that there was just as much noise when I touched the 3.5mm jack on my amp input than there was when I had it plugged into the bluetooth module.