Chipamp EMI problem - Page 1
Forum section: electronics / beginners

author: MagicSm0ke
author_rank: Newbie
timestamp: September 19, 2013, 10:18:55 am
content: Hi everyone, I'm new here.I have recently built a small chip amp (with some National Semiconductor/TI power amplifier chips) and a basic power supply (toroidal transformer, bridge recitifier, filter caps). I've built the amp circuitry mostly based on the recommendations in the chip's datasheet.The amp works fine so far, except one thing. It is pretty sensitive to external interference, e.g. when I turn on/off a halogen lamp (low voltage halogen with a transformer) nearby, I can hear a (not very loud but audible) click sound in the speakers. Similarly I can hear a short click sound when my refrigerator turns on. This also happens if I connect no input source to the amp and turn its volume all the way down (which basically connects the amp's input directly to ground), so it looks like it is not caused by external audio-in wiring.Other than that, the AMP works nicely without any hum whatsoever, but I would like to eliminate this interference issue.Could you guys recommend what I might do to narrow down the problem? Could adding an X-capacitor to the amp's mains input help filter the noise? If so, how do I choose the right cap? Or is it more likely that the noise does not come through the mains connection? Maybe it is received through the amp's feedback loop or something?

author: amyk
author_rank: Super Contributor
timestamp: September 19, 2013, 10:20:53 am
content: Tried a shielding can?

author: MagicSm0ke
author_rank: Newbie
timestamp: September 19, 2013, 10:30:11 am
content: I've tried shielding the amp in a metal enclosure (which I have then connected to the circuit's ground), but unfortunately that does not make a difference.

author: ovnr
author_rank: Frequent Contributor
timestamp: September 19, 2013, 10:52:44 am
content: Sounds like your mains filtering is insufficient. If the power levels aren't excessive, try adding a LC filter to the main power supply rails.

author: G7PSK
author_rank: Super Contributor
timestamp: September 19, 2013, 10:54:09 am
content: You need more filtering on the mains input. Some common mode chokes and caps to ground perhaps.

author: MagicSm0ke
author_rank: Newbie
timestamp: September 19, 2013, 11:30:37 am
content: Thanks everyone for your replies.Power levels are fairly low, the transformer is a 120 VA model.Could you maybe recommend some website/resource where I can read about properly dimensioning such a filter? I'm a bit cautious when it comes to mains powered circuit parts. I've tried googling around, but did not find anything helpful.

author: ConKbot
author_rank: Super Contributor
timestamp: September 19, 2013, 12:13:23 pm
content: How much space are you working with? You could start off with a filtered IEC power entry module.http://www.jameco.com/webapp/wcs/stores/servlet/StoreCatalogDrillDownView?langId=-1&storeId=10001&catalogId=10001&categoryName=cat_3035&subCategoryName=Interconnects%20%2F%20Power%20Connectors%20%2F%20AC%20Power%20Filtered%20Receptacles&category=303510&refine=1&position=1&history=e15bndof%7CsubCategoryName~Interconnects%5Ecategory~30%5EcategoryName~category_root%5EprodPage~15%5Epage~SEARCH%252BNAV

author: MagicSm0ke
author_rank: Newbie
timestamp: September 19, 2013, 04:31:11 pm
content: Unfortunately, the available space in the case is rather limited. Looks like those filtered IEC power entry modules are too long to fit, but those modules are interesting nonetheless. The datasheet of that module has a circuit diagram and values for the components for the filter in there, which is a start.

author: 47MHz
author_rank: Newbie
timestamp: September 19, 2013, 05:39:56 pm
content: Is it LM3875, LM3886 or LM4780 amplifier? I bet the first oneCan you post some photos?Please check is it also happening when the speaker cable is maximally short. Chipamp has a very poor (minimalistic) input and output stage circuits that can cause problems.

author: fcb
author_rank: Super Contributor
timestamp: September 19, 2013, 05:47:47 pm
content: It sounds like the mains-bourne spike is getting through to your DC rails.Whilst the CMRR on these parts is *good*, it is actually not very good at HF (which is what these spikes are).Big inductors fitted pre-smoothing caps will help, but they'll have to be big - if the want you can fit a form of 'gyrator' to each rail (essentially a transistor with an RC filter on the base). You'll loose a couple of volts per rail, but it'll take out fast moving spikes.

author: MrAureliusR
author_rank: Unknown
timestamp: September 20, 2013, 01:01:45 am
content: A common mode choke with filter caps is a cheap and easy solution.

author: free_electron
author_rank: Super Contributor
timestamp: September 20, 2013, 02:07:09 am
content: is your input terminated ? it looks like your input is so high impedant itpicks up all kinds of crap. shunt a 1k resistor to ground at the input. that should sufficiently lower the input impedance to get rid of the noise

author: mmagin
author_rank: Frequent Contributor
timestamp: September 20, 2013, 04:33:54 am
content: Somewhat counter-intuitively, it is not that unusual for audio power amplifiers to pick up RF on the speaker (or headphone leads) and then act as a sort of wideband AM detector. This is particularly noticeable in the vicinity of a powerful AM transmitter, but it also fits this symptom (at much shorter distance.)It might be possible to determine if this is the problem by making the speaker leads as short as possible. If that has a noticeable effect on the behavior, then some approaches to limit how much RF can get into the circuit via the speaker leads (ferrite beads, etc.)

author: MagicSm0ke
author_rank: Newbie
timestamp: September 20, 2013, 06:35:55 am
content: Quote from: 47MHz on September 19, 2013, 05:39:56 pmActually, it is a pair of LM4700's (which can deliver 2 x 30 W into 8 ohms loads). I don't have a picture here at the moment, but maybe I can post one later.QuoteI'll check that.Quote from: fcb on September 19, 2013, 05:47:47 pmSo if the spikes are actually getting through to the DC rails, I guess I should be able to see them on a scope if I hook it up to the DC rails.Quote from: free_electron on September 20, 2013, 02:07:09 amThat's actually what I had in mind at first, too. So I connected the AMP's inputs directly to ground to make sure it is not that. I even temporarily connected the input very close to the IC's input pin to ground to make sure there is nothing wrong with the wiring between amp and volume pot and it has even less possibilities to pick up noise through the input inbetween. It did not make a difference, so I think I can safely assume this is not it.Quote from: mmagin on September 20, 2013, 04:33:54 amThis sounds similar to what 47MHz suggests. It is indeed somewhat counter-intuitive, at least to me. I would have thought the resistance of the speakers+wires would be too low to pick up such noise, but then again, my speaker wires a pretty long indeed, so I guess it is worth a try.

author: Marco
author_rank: Super Contributor
timestamp: September 20, 2013, 06:44:35 am
content: Quote from: MagicSm0ke on September 19, 2013, 10:30:11 amIs circuit ground earth grounded though?

author: MagicSm0ke
author_rank: Newbie
timestamp: September 20, 2013, 10:35:33 am
content: Quote from: Marco on September 20, 2013, 06:44:35 amNo, it isn't and I would like to keep it that way (to avoid ground-loops when connecting the amp to a PC or other equipment where circuit ground is connected to mains earth). Most hifi amplifiers seem not to be grounded at all and, even though some are sensitive to such noise as well, many of them aren't. So I assume it must be possible to fix this without earth-grounding the whole circuit.Edit: I've just reduced the speaker cable length to 1 meter each (this was the shortest I could easily do, I could try even shorter lengths if it is worth it, but this would require some more effort here). Unfortunately, the issue remains. I could not notice any difference compared to the longer cables (which were about 7 meters each).

author: SArepairman
author_rank: Frequent Contributor
timestamp: September 20, 2013, 04:38:35 pm
content: I wonder if a linear regulator would help

author: mmagin
author_rank: Frequent Contributor
timestamp: September 20, 2013, 05:26:40 pm
content: Quote from: MagicSm0ke on September 20, 2013, 06:35:55 amIf this is the cause, it's probably a common-mode signal, not across the pair. You could be picking up a signal on the speaker leads which act as an antenna, and at RF (even if it's DC-isolated), your power supply or audio input leads could provide the 'ground'. To really see what's going on in that case might require looking at the circuit in a non-conventional manner and some understanding of what's inside the IC.

author: MagicSm0ke
author_rank: Newbie
timestamp: September 27, 2013, 03:10:23 pm
content: In case anyone with a similar problem stumbles across this thread: I was able to solve the issue by putting a small capacitor (470 pF) across the positive and negative inputs of the amplifier chips. With the cap any high-frequency noise that makes it into either the feedback loop or the positive input is cancelled out by applying the same signal to the other input.