No, they shouldn't. The shields and metal enclosures should form one big conducting box around the circuitry. When you connect the shielding of the input and output signal wires anywhere else than straight to the enclosure, the shield that sticks into the box is a coupling loop that couples outside RF interference into the box.
All those insulated connectors are the main reason why audio equipment is often as leaky as can be for RF. Grounding for RF via a capacitor is a good bandaid, though. Another issue is poor connection between the panels of the enclosure.
Three-pong mains cords are a pain in the rectal region because of the hum loops they cause when the audio connections are unbalanced, fortunately most audio equipment is double insulated with no earth nowadays.
All those insulated connectors are the main reason why audio equipment is often as leaky as can be for RF. Grounding for RF via a capacitor is a good bandaid, though. Another issue is poor connection between the panels of the enclosure.
Three-pong mains cords are a pain in the rectal region because of the hum loops they cause when the audio connections are unbalanced, fortunately most audio equipment is double insulated with no earth nowadays.
Ok I am showing my age here but back around 1980 the Advent receiver was prone to RF. Advent was aware of this problem and recommended adding a small 270 pf cap across the phono input. ( or I believe that was the recommended value ) I never added those caps even though my Advent would some nights pick up Radio Canadia International . Now the funny thing is I don't live in Canadia I live neat Atlantia Georgia USA so I assume that station was over 2,000 miles away.
It certainly was "International"! 🙂
I used to live with a big FM radio transmitter almost as close as your antenna. It got into all sorts of things, including the sound card of our Commodore PET computer (circa 1980) We were surprised that FM would demodulate to audible, but with enough power I suppose it can. Never did find a solution, alas.
I used to live with a big FM radio transmitter almost as close as your antenna. It got into all sorts of things, including the sound card of our Commodore PET computer (circa 1980) We were surprised that FM would demodulate to audible, but with enough power I suppose it can. Never did find a solution, alas.
Loading an input with a capacitor is a poor solution that may not work at all. The problem is that the input is a very low impedance at RF frequencies vs the ~47K audio impedance. The best solution is a series impedance like maybe 100 to 1000 Ohms. You just have to reduce the RF current below the rectification threshold. For meg Ohm tube inputs, I use 100K but that would be too much for a phono input. A ferrite bead might also work and have zero effect on the audio. But the description implies some kind of grounding problem, but RF problems cannot be solved with "electrical" logic since any wire is a choke and an antenna. If you want to use a capacitor, the first place to put a cap is between the phono ground and the chassis, ie directly and not separated by a lot of wire. The cap should not be too large, or it may cause ground noise at audio frequencies, say 0.01uF.
Yes, this is for RF. See the Grounding and Shielding books by Ralph Morrison.
Isolated RCA jacks are for 60Hz ground loops.
Some try to have it both ways by isolating the RCA jack, but connecting a capacitor
between the jack common and the chassis. That doesn't work so well for RF, though.
Try more and better ferrites, or try looping the cable through it multiple times.
Don't squash the cable, though.
This is a nasty problem, but you should be able to solve it.
Many phono amps bring the screen and signal hot into the amp and attempt to solve the problem with an series mode RFI filter (lossy ferrite + Cap 'L' type pad) in the signal board. Often that is too late to try to fix it.
Its almost certain the the RFI is coming through common mode and its being picked up in the cable shield and then demodulated by one or more semiconductor junctions in the preamp.
On all my phono amps (and many commercial products designed with RFI mitigation in mind) I bring the cable shield and signal hot into the board as described above, but right at the connector, tie a 2-5nF (I've seen up to 10nF) cap from the shield to the chassis. This effectively shorts the shield to the metal housing at RF and makes it one 'enclosure' that very effectively keeps the RF out. In stubborn cases - ie high field strength situation, you will still get RFI coming through - clearly in your case, you are near a Tx. The high gain and use of bipolar input devices mean you have a ready AM receiver.
If you add a Fairite toroid as I've done below you now create a very effective common mode filter that will almost certainly resolve your issue. Without the toroid and with the RFI cap removed on my MC preamp, I get a load of European and French AM stations. With the cap and toroid, nothing - absolutely quiet.
Many phono amps bring the screen and signal hot into the amp and attempt to solve the problem with an series mode RFI filter (lossy ferrite + Cap 'L' type pad) in the signal board. Often that is too late to try to fix it.
Its almost certain the the RFI is coming through common mode and its being picked up in the cable shield and then demodulated by one or more semiconductor junctions in the preamp.
On all my phono amps (and many commercial products designed with RFI mitigation in mind) I bring the cable shield and signal hot into the board as described above, but right at the connector, tie a 2-5nF (I've seen up to 10nF) cap from the shield to the chassis. This effectively shorts the shield to the metal housing at RF and makes it one 'enclosure' that very effectively keeps the RF out. In stubborn cases - ie high field strength situation, you will still get RFI coming through - clearly in your case, you are near a Tx. The high gain and use of bipolar input devices mean you have a ready AM receiver.
If you add a Fairite toroid as I've done below you now create a very effective common mode filter that will almost certainly resolve your issue. Without the toroid and with the RFI cap removed on my MC preamp, I get a load of European and French AM stations. With the cap and toroid, nothing - absolutely quiet.
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@MarcelvdG
A lot of double insulated equipment uses clip on RFI/EMI chokes to raise the loop impedance and quash interference. I've found the clip on type ok for suppressing SMPS and/or CPU noise (in bad cases this comes through as a 'SHHH' sound - like white noise) but I found when I was experimenting that they id not help with the AM radio pickup problem - the toroid fixed it. I guess with the SMPS you can be picking up some very high HF because of the fast edges - the AM RFI thing is a bit different.
(Mark Johnson linked to a Ham radio operator's paper on all this stuff - that's where I got the Fairite toroid info - I forgot the name of the guy)
A lot of double insulated equipment uses clip on RFI/EMI chokes to raise the loop impedance and quash interference. I've found the clip on type ok for suppressing SMPS and/or CPU noise (in bad cases this comes through as a 'SHHH' sound - like white noise) but I found when I was experimenting that they id not help with the AM radio pickup problem - the toroid fixed it. I guess with the SMPS you can be picking up some very high HF because of the fast edges - the AM RFI thing is a bit different.
(Mark Johnson linked to a Ham radio operator's paper on all this stuff - that's where I got the Fairite toroid info - I forgot the name of the guy)
The Mani is not grounded, it uses a two-prong AC transformer brick. I’ve tried running the ground wire to ground directly, but it did not improve things.
I’ll continue experimenting. I’ll have to order a ferrite ring big enough to go around both cables. I might have a capacitor that will work, I’ll have to check…
I’ll continue experimenting. I’ll have to order a ferrite ring big enough to go around both cables. I might have a capacitor that will work, I’ll have to check…
Naw, get the right ones. As long as you are ordering the 10nF parts,
also get a couple of 4.7nF and a couple of 1nF just in case.
also get a couple of 4.7nF and a couple of 1nF just in case.
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Can do. My next project was going to be a phono stage anyway, maybe I should just accelerate my plans...
You seem to live in a different universe from me, Marcel. 😀
Most of my gear has 3-prong mains cords ... and I do not have any hum loops.
Well, that doesn't happen, in my system! Maybe electricity/RFI works diffeently in the southern hemisphere! 😀
Andy
Ceramic caps prevented my friend's Amstrad amplifier from picking up Radio Moscow back in the 70s!
I remember, in those pre-internet days, reading an explanation of exactly where to place them.
Was it straight across the phono input socket, Bonsai, or was it close to the first transistor?
I remember, in those pre-internet days, reading an explanation of exactly where to place them.
Was it straight across the phono input socket, Bonsai, or was it close to the first transistor?