In the 2023 ARRL Handbook they suggest a method for determining if RFI is being picked up from the speaker cables. Disconnect the loudspeakers from the amp, attach a set of headphones with a (hopefully) short cable. If the RFI is present, it's not the speaker cables. Vice versa...if the RFI is not present with the headphones consider wrapping a few turns of the speaker cable in a large toroid choke.
Have you run the amp with shorted inputs and speaker connected for a test? Just wondering if it could be picked from whatever is connected to the amp. Also does it get worse with your source device powered off and the amp on? I once had a condition where if you turned off the preamp and left the amp on, radio pickup was somewhat worse. Seems a powered off device can act as a better detector.
Many feedback amplifiers become unstable if the capacitance across the input becomes too high, as this moves the pole in the feedback to a lower frequency, creating a 180 degree phase shift in the overall loop response, or positive feedback, before the gain drops below 1. However in the case of the LM3886 it appears designed to accept this capacitance without causing instability hence appears a good idea to include near the input terminals, particularly if RF problems are encountered.
Indeed.
LM1875 especially does not like excessive capacitance as well.
Using typical filter on the + input will murder the phase margin too.
Probably good to check with the 3886. The models are more than accurate enough to check
LM1875 especially does not like excessive capacitance as well.
Using typical filter on the + input will murder the phase margin too.
Probably good to check with the 3886. The models are more than accurate enough to check
I live smack in the middle of Calgary. I have several commercial FM radio and TV broadcasters within 10-15 km of my house. I can 'see' 69 wifi devices (2.4 GHz, 5 GHz) from my listening chair. The FM broadcast is strong enough to be picked up by an oscilloscope probe ground lead. So I'd say there's plenty of RF to go around, but it's not like I have a broadcast tower right next door.
Both fascinating anecdotes. But did any of those systems use an LM3886?
I'm not denying that RF interference is a thing. After all, there's a reason I fit all my products with EMI/RFI filters on their inputs. I'm just saying that I don't find the LM3886 particularly susceptible to RF interference, even in a setup like this:
Or this:
Both were some experiments I played with in the very early stages of the Modulus-86 development.
Tom
... which is why there are ceramic capacitors in parallel with the electrolytic ones in the decoupling network.
Tom
Obviously not but I did encounter RF when I was developing this kit with the LM3886. It was downright annoying. It's only a few components yet I had this RF issue. I have more than 20yrs experience designing discrete amps and never had this RF once.
I did some research and chanced upon an interesting schematic of the Yamaha HS80M Active Speaker.
Apart the 220pF across the +- inputs, they inserted two inductors (L601/602).
My guess is to eliminate RF.
Don't get me wrong, the LM3886 is an excellent chip amp on it's own. You have brought it to a whole new level with your composite design. The DIY community should be grateful for sharing your work.
I'm just high lighting that one can encounter RF which is what the OP is having. Not everyone will encounter RF but some people do. I did.
Regards
Mike
AmpsLab
Interesting. Could it be because you're missing the bulk capacitance on the PCB and that the other decoupling capacitors are pretty far from the LM3886 on your PCB?
Also interesting with the inductors in the Yamaha design. I fail to see how they would keep RF out of the LM3886, though. On the contrary, they would open the feedback loop at RF, which would make any RF issues worse. I suppose the LC formed by those inductors and the capacitor across the inputs of the LM3886 could do something, but if they do, I bet they're intended to tame any buzz or rail sticking that occurs when the LM3886 exits clipping.
If the issue was RF getting into the LM3886, why wouldn't Yamaha have added an inductor in series with the non-inverting input? For example in series with R602?
Tom
Also interesting with the inductors in the Yamaha design. I fail to see how they would keep RF out of the LM3886, though. On the contrary, they would open the feedback loop at RF, which would make any RF issues worse. I suppose the LC formed by those inductors and the capacitor across the inputs of the LM3886 could do something, but if they do, I bet they're intended to tame any buzz or rail sticking that occurs when the LM3886 exits clipping.
If the issue was RF getting into the LM3886, why wouldn't Yamaha have added an inductor in series with the non-inverting input? For example in series with R602?
Tom