Looking for ideas (or an intervention) on how I could cobble together a basic balanced signal chain for as little money as possible since this is primarily an experiment to see if this can solve my RFI issues.. I'm thinking along the lines of hacking balanced output from one of my DACs and adding balanced input to an existing amp. The main hole would be what to use in place of the preamp that has remote volume control. I don't have any other sources than S/PDIF and I don't think I need the gain of a preamp.
Apple4K TV -> HDMI switch -> S/PDIF -> DAC -> Preamp -> Amp -> Asathor. My ICs are Belden 1694A from Blue Jeans Cable.
In my living room, I've been fighting RFI issues from 94.9 FM for several years since moving to the city where I can see three transmission towers from my couch about 1.5km away on the next hill. From my bedroom, I can see another three 5.5km away! I don't have these issues in my basement (audio only, no HDMI).
I jumped through a bunch of of hoops over the years with different equipment combinations (mostly DIY), ferrites, LC filters on inputs, cable management, etc with modest improvement. My commercial gear doesn't perform any better than my DIY gear.
Relatively recently I though I solved it when I found that RFI went through the roof when my amp's ICs and speaker cables where in contact with one another. But over time it seems come back and I start over and I seem to be getting inconsistent results. For exampl, today I did lots of cable tidying and had no RFI with the preamp inputs shorted and then it was back 15 minutes later.
Apple4K TV -> HDMI switch -> S/PDIF -> DAC -> Preamp -> Amp -> Asathor. My ICs are Belden 1694A from Blue Jeans Cable.
In my living room, I've been fighting RFI issues from 94.9 FM for several years since moving to the city where I can see three transmission towers from my couch about 1.5km away on the next hill. From my bedroom, I can see another three 5.5km away! I don't have these issues in my basement (audio only, no HDMI).
I jumped through a bunch of of hoops over the years with different equipment combinations (mostly DIY), ferrites, LC filters on inputs, cable management, etc with modest improvement. My commercial gear doesn't perform any better than my DIY gear.
Relatively recently I though I solved it when I found that RFI went through the roof when my amp's ICs and speaker cables where in contact with one another. But over time it seems come back and I start over and I seem to be getting inconsistent results. For exampl, today I did lots of cable tidying and had no RFI with the preamp inputs shorted and then it was back 15 minutes later.
Stations can change their RFI nature often. Use shorting plugs in the amplifier inputs, and see if
you still get any RFI over several hours. If not, it's probably not picked up by the speaker wires.
That is quite possible, especially in your situation.
I would tend to prefer audio cables having a twisted pair with a foil shield, instead of a coaxial cable
that is intended for RF. But your situation may be extremely difficult to resolve, except in the basement.
You can certainly replace the preamp with a passive 10k ohm volume control in a box.
you still get any RFI over several hours. If not, it's probably not picked up by the speaker wires.
That is quite possible, especially in your situation.
I would tend to prefer audio cables having a twisted pair with a foil shield, instead of a coaxial cable
that is intended for RF. But your situation may be extremely difficult to resolve, except in the basement.
You can certainly replace the preamp with a passive 10k ohm volume control in a box.
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I'll try that long-term amp test tomorrow. Thanks.
FWIW, I got some fun results with the shorting plugs connected to the amp via IC. Very good directional antenna.
IIRC, the only setup that was not susceptible was a Panasonic XR-45 digital receiver fed via S/PDIF but it died long ago.
With the single conductor coax cables that you have, the RFI could easily be picked up on the braided shield
and then be routed into the equipment. The cable shield should properly be an extension of the chassis,
but most RCA jacks on preamps and amps are isolated, and the shield enters the chassis to spray RFI all around.
and then be routed into the equipment. The cable shield should properly be an extension of the chassis,
but most RCA jacks on preamps and amps are isolated, and the shield enters the chassis to spray RFI all around.
I have 2 of these DC 12V-26V 100W Mono Channel TPA3116D2 Digital Power Audio Amplifier Board powered with 2 seperate old 19V laptop chargers, the sound signal is coming from sound card in PC then optical into a cheap DAC then into the boards via rca plugs I am running yamaha ns 890 speakers 2 completely seperate channels, it's the best quality sound I have ever had no hiss or hum what so ever in every way, more volume that I can ever use only using around 10% of power amazing, like to hear you thoughts. Will tidy it up.
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So, placing an interconnect with a shorting plug at the far end created an antenna with variable signal level as you moved it.
The usually way to RF proof audio gear is to add a bypass capacitor from the signal to ground, and a ferrite bead around the input signal conductor. The capacitor should have good RF performance, so high performance ceramics, and placed inside the amp. .001uf should do it.
The ferrite bead can be internal or external, and can be selected and sourced here. The bead material affects is frequency characteristics, so not just any old ferrite will work well, you want one that's good and lossy at 90MHz. A ferrite bead around the conductor carrying the RF acts as a passive filter, converting RF energy into heat. In a very high RF environment, like a high power AM broadcast station, they can get physically hot to the touch.
Perhaps its worth mentioning that a good RF ground helps too. The 3rd pin on an electrical outlet probably is not a good ground at 100MHz. It can be a challenge, especially in apartments. But thing low impedance, so heavy wire, wire braid, etc., to something really earth grounded.
RE: your thread title, balancing may not help because not many balanced input stages can deal with signals in the 100MHz range. Transformers may have enough inter-winding capacitance to let it right through. Active balanced circuits have limits to their CMRR that changes radically with frequency, and as frequiency moves well into RF, the aren't much good. It's better to kill the RF before, or right after it enters the device.
Balanced inputs are unlikely to solve your problem. At RF the CMRR of balance input isn't really useful. You need to do some basic trouble shooting to identify where RF is getting into your system.
First test is to have just the amplifier and speakers connected with shorting plugs in the input selected. Is there any RF breakthrough? If yes, change your loudspeaker cables to star-quad four conductor configuration which will give about 20dB reduction in RF pickup on the speaker cables. See https://www.canare.co.jp/en/products/cables/index.php?tid=4_008. Also fit a snap-on ferrite bead on the power lead at the amplifier chassis end of the power lead.
If you still have RFI with no input connected after adding the ferrite choke and changing the speaker cables your amplifier has a serious problem and there's not much point in trying anything else.
If you amplifier is clean without any external source connected then report back and I'll suggest the next test.
First test is to have just the amplifier and speakers connected with shorting plugs in the input selected. Is there any RF breakthrough? If yes, change your loudspeaker cables to star-quad four conductor configuration which will give about 20dB reduction in RF pickup on the speaker cables. See https://www.canare.co.jp/en/products/cables/index.php?tid=4_008. Also fit a snap-on ferrite bead on the power lead at the amplifier chassis end of the power lead.
If you still have RFI with no input connected after adding the ferrite choke and changing the speaker cables your amplifier has a serious problem and there's not much point in trying anything else.
If you amplifier is clean without any external source connected then report back and I'll suggest the next test.
Where your coax cables enter the equipment good practice is to tie the coax directly to the chassis via a 5-10 nF ceramic disc cap. Keep the leads as short as possible (1-2 cm max). Do the same on the source side and for all interconnects. What this does is makes the source, cable and receiving equipment a single Faraday enclosure to RF.
RF ingress like this is almost always common mode, so doing a few turns of the cables through a suitable Fairite toroid (after the caps above are installed) will also help attenuate RFI further.
You may also be getting some RFI through the power cables. The filter capacitors if fitted are usually film based and the wiring inductances high relative to RF, so RF just blasts straight through all of this - you have to take specific precautions for RF.
RF ingress like this is almost always common mode, so doing a few turns of the cables through a suitable Fairite toroid (after the caps above are installed) will also help attenuate RFI further.
You may also be getting some RFI through the power cables. The filter capacitors if fitted are usually film based and the wiring inductances high relative to RF, so RF just blasts straight through all of this - you have to take specific precautions for RF.
This morning I ran the amp with shorting plugs for a couple of hours with no hint of RFI.
I remembered I had an RC filter on the amp input so I opened it up to check the values: 681R in series followed by a 221pF ceramic cap tied to the RCA connector's shell. I also discovered this capacitor to rca jack solder joint was cold. This is the same channel I currently get RFI on when the preamp is connected. I repaired the connection but the RFI was still there with the preamp connected.
On a whim I tried the clamp-on ferrite cores that I don't remember making any difference in the past but lo and behold, the RFI went away when added to the amp side of the RCA.
Still curious why it was just the right channel.
Thanks for all the guidance.
I remembered I had an RC filter on the amp input so I opened it up to check the values: 681R in series followed by a 221pF ceramic cap tied to the RCA connector's shell. I also discovered this capacitor to rca jack solder joint was cold. This is the same channel I currently get RFI on when the preamp is connected. I repaired the connection but the RFI was still there with the preamp connected.
On a whim I tried the clamp-on ferrite cores that I don't remember making any difference in the past but lo and behold, the RFI went away when added to the amp side of the RCA.
Still curious why it was just the right channel.
Thanks for all the guidance.
What he asked....
I'll guess its demodulated audio since he mentioned an FM frequency specifically. So two things are going on, one is a parasitic ratio detector, the other is a nonlinear junction of some sort.