Got it Tom, thx,
Don't want to go in deep discussion, but from my experience quite bit of demodulation happens in cables and source equipment already, there CMR in amp does the job.
Also , even if it is falling rapidly with frequency, typical old receiver like SSM2141 (which can be bettered today) still has over 35db CMR at 1MHz, which is useful. Needles to say source needs to have matched impedance's to do that, but its easily done.
Truth, I did not mention mobile networks and WIFI, just as I never had trouble with those GHz sources, it was always issue with sources in kHz to low MHz region..
Higher frequencies are easier filtered out at input, but if noise is demodulated already to audio range before input, only CMR will help.
Cheers,
Drazen
Don't want to go in deep discussion, but from my experience quite bit of demodulation happens in cables and source equipment already, there CMR in amp does the job.
Also , even if it is falling rapidly with frequency, typical old receiver like SSM2141 (which can be bettered today) still has over 35db CMR at 1MHz, which is useful. Needles to say source needs to have matched impedance's to do that, but its easily done.
Truth, I did not mention mobile networks and WIFI, just as I never had trouble with those GHz sources, it was always issue with sources in kHz to low MHz region..
Higher frequencies are easier filtered out at input, but if noise is demodulated already to audio range before input, only CMR will help.
Cheers,
Drazen
Yeah. The 2.4-5 GHz doesn't go very far and is easily filtered out or attenuated with shielding. I haven't had issues with those either. I do occasionally see the effects of wifi in my lab, but I can also measure things at the gnat fart level, so my use case is far from typical. I usually make a "circuit burrito" where I wrap the circuit in antistatic bubble wrap and then in tinfoil. The foil is then grounded. That gets rid of 2.4-5 GHz pretty quickly.
I haven't had significant issues with RFI since the world moved to 4G for cell phones and away from the 4.77 MHz PC clones that would get into just about any radio receiver they could find.
I usually run my oscilloscopes with the bandwidth limiter turned on. That turns my 400 MHz Tek 2465 into a 20 MHz scope which seems like a waste. But I can pick up ~10 mVpp of FM radio on the ground clip alone, so I enable the BW limiter to get a clean scope trace.
Tom
I haven't had significant issues with RFI since the world moved to 4G for cell phones and away from the 4.77 MHz PC clones that would get into just about any radio receiver they could find.
I usually run my oscilloscopes with the bandwidth limiter turned on. That turns my 400 MHz Tek 2465 into a 20 MHz scope which seems like a waste. But I can pick up ~10 mVpp of FM radio on the ground clip alone, so I enable the BW limiter to get a clean scope trace.
Tom
Exactly, this is different set up. OP has issue with demodulation coming in audio range, not lab equipment. What Im saying is that demodulation often happen before amp input, there filters cant help, good CMR can.
And I agree, as transmissions went up with frequency, ghost voices are gone away. Even for my own diy I cant confirm as I went fully differential while having Nokia 5110 (i think that was a model, anyway, in 90's) in my pocket 🙂 But before that, with single ended gear, AM still strong and early inverters in A/C units, ghosts happen from time to time.
And I agree, as transmissions went up with frequency, ghost voices are gone away. Even for my own diy I cant confirm as I went fully differential while having Nokia 5110 (i think that was a model, anyway, in 90's) in my pocket 🙂 But before that, with single ended gear, AM still strong and early inverters in A/C units, ghosts happen from time to time.
I see that R3, from the negative input to (the capacitor to) ground, is 1.4k whereas in all the example schematics it's 1k. The 1.4k with the two 20k in the feedback makes the high-frequency gain at 7. The datasheet says "The LM3886 is designed to be stable when operated at a closed-loop gain of 10 or greater" which a 1k resistor for R3 would set it at. If the gain of 10 is too much for you, make a resistive divider at the input to lower the input level.
Having the gain too low could make it unstable and like a regenerative receiver, on the verge of oscillation and too-easily picking up RF.
Would you be able to explain the method of demodulation? I fail to see how an interconnect cable can demodulate anything.
Tom
Hi, not because I don't know where it was happening then, but it is not only cable , it is all equipment plugged in amp by that cable Just suspect it was preamp with all interconnects as antenna, but don't know now, that was long ago
Copper cabling that has oxidized can have pretty good semiconductive properties. Both cuprous oxide and cupric oxide are semiconductive. One on the reasons RF conductors and RF cabling are plated with metals that have a self protecting (non semiconductive) oxide layer.
I would argue that in that case you can't really blame the power amp. After all, if you feed a power amp demodulated RF you'd expect to just amplify that, right? 🙂 But that's not to say that a twisted pair or capacitor across the input connector to the power amp can't help, even though they likely won't address the root cause.
Yeah. Various oxides can be semiconductive. I'm reminded of the lead oxide diode in a crystal set.
Tom
Hi Tom. yes , what you said is right and I don't blame amp, particularly not LM3886, I have 16 pieces at home (and Im not dealing with them, I'm amateur), all National as then is when I was buying them, that says something. ,They are lovely little chip,, no doubt, so many people like it with good reason.
I think today with stricter regulations and carrier frequencies going up , the issue of radio emissions is less, but my main point is when we spent so much effort on god knows what detail to get better sound reproduction, including ultra low thd that human cant hear, why not investing slightly more in at least differential inputs and outputs, if fully balanced system is too much. Pro people got it so long time ago, and also with good reason. What I mentioned about my ancient issues with external noises, it disappeared with differential inputs, seems it was all about common mode signals.
Technology is there, it helps with RF , mains noise that's everywhere and probably the worst thing, also with all so many SMPS and inverters, also everywhere, also with ground loops., and I believe it sounds better (but that is subjective)...... IMHO i don't see why should anyone design new piece of equipment with single ended inputs and outputs. So many threads here about grounding and noises from whatever, following good professional audio practice would reduce this type of questions a lot. That practice also includes RF filters here and there, I don't disagree, but as we concluded already, this is not main issue any more as piece of decent cable will filter out Ghz signal on its own.
Cheers,
Drašen...
By the way, what I found quite enlightening is your old article about grounding, where signal ground is directly attached to speaker return wire, this totally makes sense!
https://neurochrome.com/pages/grounding
https://neurochrome.com/pages/grounding
can I use cut frequency for this LC Low-Pass filter below 1MHz, like from 500KHz onward,
and how to check Q ≤ 1 and what frequency?
Also, can I use LC Low-pass filter made of L=1500uH and
C=22pF with cutoff frequency of 0.876MHz or
C=10pF with cutoff frequency of 1.299mHz
l like to use what I have in my stocks , and I have inductors with L=1500uH Part No: B82442T1155K050
That's nonsense! The filter should be designed for a cutoff frequency of approximately 1-2 kHz, and the overall Q factor can easily exceed 1, as long as the filter is sufficiently separated from the noise band. It's always best to prefer smaller inductors and use larger X2 capacitors. The filter's internal resistance is more important than an overall Q factor slightly above 1; as mentioned, it's a prerequisite to have at least 1 octave of separation from the noise band. Filters in the megahertz range are a joke and won't eliminate your interference.
What burden does Tom assume?
Furthermore, there was talk of a refrigerator, not of high frequency
The original topic of this thread is as in post #1. Presumably ham or CB radio, definitely no refridgerator. Tom's LC filter is for the signal input if I understood it correctly.
So you want to cut off the top 3-4 octaves of the audio band in an audio amplifier? Then you might as well turn the amplifier off. That'd likely eliminate the interference too... 🙂
You're also assuming that an inductor that's inductive at audio frequencies is still inductive at RF. I bet many inductors used in audio circuits would be well past their self-resonant frequency at CB frequencies (27 MHz).
If the issue is that RF gets into the amp somehow and is demodulated and presented as audio, reducing the amount of RF that enters the amp would certainly cut down on the amount of interference. At the same time you'd want the filter to have minimal impact at audio frequencies, which pushes the minimum cutoff frequency to at least 200 kHz (10x the highest audio frequency of interest).
If you also want good audio performance (for example, low distortion) you might want to push the cutoff frequency up a bit higher, which is why I suggested 1-2 MHz. Such an LC filter should have no trouble getting you 50-60 dB attenuation at 27 MHz.
Tom
Yeah. Refrigerators, furnaces, and other heavy loads can be good, wide-band emitters of RF if they are turned on and off with a relay. The arcing of the relay contacts works great as an arc transmitter.
I've certainly had prototype amps where I could hear a click in the speakers when my fridge or furnace turned on, simply because the arc of the relay in the fridge or thermostat was picked up by the amp. But, of course, that was more the case when my amps involved tubes and were sitting on a scrap piece of plywood in my living room.
Tom
So 1-2MHz range is the ideal minimum point, otherwise 500KHz can be a starting point for me.
can you please give some insights on Inductor values , where I should start from if those 1.5mH aren't suitable for such LC filter and some on caps values and types