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