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
I'm sure Yamaha tried adding an inductor in series with R602 and found that it didn't help.
I think RF is picked up inside the chip amp, right at the input.
Because of that, no external filtering will help.
They solved it by tapping the RF from the output and injecting it into the -ve terminal.
That's where the inductors come in.
So basically, they are using the CMRR method to cancel out the RF.
Just an educated guess.
Regards
Mike
AmpsLab
I think RF is picked up inside the chip amp, right at the input.
Because of that, no external filtering will help.
They solved it by tapping the RF from the output and injecting it into the -ve terminal.
That's where the inductors come in.
So basically, they are using the CMRR method to cancel out the RF.
Just an educated guess.
Regards
Mike
AmpsLab
Interesting thought. I'm wondering what you're basing it on aside from that one bad experience that you haven't seem to have gotten quite to the bottom of.
If that was the case, they'd be marked as wire links in the schematic. No company of that size will pay for inductors that don't do anything.
And if you're now sure that Yamaha found the inductors ineffective, why did you mention them in support of your argument. I'm confused.
Look. I'm not denying that you've had an experience with RF pickup in an LM3886-based amp. But I am inviting you to look for alternate explanations instead of simply blaming the chip. If this was a widespread issue, we'd all know about it by now.
Tom
@ tomchr
Interesting thought. I'm wondering what you're basing it on aside from that one bad experience that you haven't seem to have gotten quite to the bottom of.
I did. I just solder a 220pF across the + and - inputs.
If I still get RF, I'll just abandon using the LM3886. It's not a big deal.
@ tomchr
If that was the case, they'd be marked as wire links in the schematic. No company of that size will pay for inductors that don't do anything.
Precisely the point. So, explain to me why the inductors are there?
@ tomchr
But I am inviting you to look for alternate explanations instead of simply blaming the chip. If this was a widespread issue, we'd all know about it by now.
This is not the first time I hear RF complains from the LM3886. Check it out yourself.
Tom
I understand you are making a living off this IC.
You give me the impression that I am trying to jeopardize your business by "blaming" the chip?
Rest assured, I have not intention of doing that.
The OP has a RF issue.
I offered a solution.
Try it. If it works, great.
If not, find another way.
End of story.
I really don't have time to argue with you.
I've got more important matters to attend to like settling my estate.
Mike
AmpsLab
Interesting thought. I'm wondering what you're basing it on aside from that one bad experience that you haven't seem to have gotten quite to the bottom of.
I did. I just solder a 220pF across the + and - inputs.
If I still get RF, I'll just abandon using the LM3886. It's not a big deal.
@ tomchr
If that was the case, they'd be marked as wire links in the schematic. No company of that size will pay for inductors that don't do anything.
Precisely the point. So, explain to me why the inductors are there?
@ tomchr
But I am inviting you to look for alternate explanations instead of simply blaming the chip. If this was a widespread issue, we'd all know about it by now.
This is not the first time I hear RF complains from the LM3886. Check it out yourself.
Tom
I understand you are making a living off this IC.
You give me the impression that I am trying to jeopardize your business by "blaming" the chip?
Rest assured, I have not intention of doing that.
The OP has a RF issue.
I offered a solution.
Try it. If it works, great.
If not, find another way.
End of story.
I really don't have time to argue with you.
I've got more important matters to attend to like settling my estate.
Mike
AmpsLab