RF Interference in Speaker Cables??? (video)

[DonH56]

But this is wrong. "There is always feedback around that output stage, so RF present at the output will follow the feedback network to the input of a gain stage, where it will be detected and amplified."

Experts that don't know how negative feedback works.
The RF at the output goes to the negative input and is reduced.

That is exactly the mechanism I have seen many times over the years for RFI. The feedback control itself is non-existent at RF since the frequency is well beyond the unity-gain point of the loop, but the RF signal is coupled through the feedback network to hit the input or driver stage where it can be rectified (mixed,) to audio and amplified. Too many amplifier designs do (did anyway) not consider this and left off a simple RC snubber (or Zoebel) circuit to route RF on the speaker terminals to ground before it could get further inside the signal path.

FWIWFM - Don

 

[fpitas]

That is exactly the mechanism I have seen many times over the years for RFI. The feedback control itself is non-existent at RF since the frequency is well beyond the unity-gain point of the loop, but the RF signal is coupled through the feedback network to hit the input or driver stage where it can be rectified (mixed,) to audio and amplified. Too many amplifier designs do (did anyway) not consider this and left off a simple RC snubber (Zoebel) circuit to route RF on the speaker terminals to ground before it could get further inside the signal path.

FWIWFM - Don

The output Zobel does that, as well as offering a real impedance to the followers so they don't oscillate from cable reactance.. A Zobel at the speaker end is the cherry on the cake: https://sound-au.com/cable-z.htm

 

[Cbdb2]

That is exactly the mechanism I have seen many times over the years for RFI. The feedback control itself is non-existent at RF since the frequency is well beyond the unity-gain point of the loop, but the RF signal is coupled through the feedback network to hit the input or driver stage where it can be rectified (mixed,) to audio and amplified. Too many amplifier designs do (did anyway) not consider this and left off a simple RC snubber (or Zoebel) circuit to route RF on the speaker terminals to ground before it could get further inside the signal path.

FWIWFM - Don

Not doubting this happens just how and how often (Ive never seen it). Audio amps can and do have unity gain past 1mhz and this does reduce the RFI.

Generic spice audio amp, nothing special. The max V at the amp out is about 1mv at 100Mhz but its still reduced 20db. After 100mhz the E-C capacitance of the output transistors shunts the RF to the rails.

 

[DonH56]

Not doubting this happens just how and how often. Audio amps can and do have unity gain past 1mhz and this does reduce the RFI.

Generic spice audio amp, nothing special.

I am pretty sure Jim Brown knows how feedback works. I think I do, but who knows, tough to identify keyboard warriors these days...

I was a tech during the CB craze of the 70's and 80's and it was a fairly common problem at the time for me. The CB band is around 30 MHz. There were also AM (~1 MHz) and FM (~100 MHz) as well as TV (wideband) towers, and the occasional HAM, that caused interference. I rarely saw issues from HAM stations as they tended to be cleaner than CB (illegal) amplifiers and HAMs tended to be savvy enough to help their neighbors themselves. Don't really know how often it happens now; anecdotally much less frequently judging by how rarely I run into it these days (only a few times in the past few years). But, I am no longer "in the biz", so don't have a good baseline reference.

For the record (no pun intended), phono stages were by far the worst offenders I dealt with, and adding an RFI filter/snubber got to be common. Tricky to do without messing up the cartridge matching, natch. IIRC I tended to use an LCR network instead of a simple RC snubber to isolate the cap. Been a while. I still have an old LM381 phono amp I built (one of my first home-brew PCB designs, ink and acid) that has an LCR input filter on it with little hand-wound coils.

The culprits I have dealt with more recently are dimmers and LED replacement lights with their noisy little SMPS supplies. But by and large the culprit has not been via the speaker wires, except in one or two cases, over the last say ten years or so.

Whatever - Don

 

[Cars-N-Cans]

Not doubting this happens just how and how often (Ive never seen it). Audio amps can and do have unity gain past 1mhz and this does reduce the RFI.

Generic spice audio amp, nothing special. The max V at the amp out is about 1mv at 100Mhz but its still reduced 20db. After 100mhz the E-C capacitance of the output transistors shunts the RF to the rails.

It’s kind of useless at RF these days since things go so high up. Example would be cellphones, while being in the GHz range still transmit at an audible rate and can introduce interference.

 

[restorer-john]

Don't really know how often it happens now; anecdotally much less frequently judging by how rarely I run into it these days (only a few times in the past few years). But, I am no longer "in the biz", so don't have a good baseline reference.

As you mentioned, it was people running dirty linears for their 27MHz and local AM stations.

When we first moved to this state in the late 1970s, there was a local AM station on 1197kHz (4GG), which in those days, had its transmitter about 7km away. I could look up its historic ERP, but it was well into the hundreds of kW IIRC.

My father was putrid that half his HiFi picked up 4GG if he used long speaker cables in the downstairs listening room. I was about 9yo and that local station meant even the simplest three component 'crystal' radio (slug tuned coil, germanium diode and ceramic cap) would bring in that wretched (pop music he hated) station on a salt crystal earpiece. I used to build little custom crystal radios to sell to kids at school in grade 6.

 

[Astoneroad]

I used to build little custom crystal radios to sell to kids at school in grade 6.

I'm willing to wager that the fact that you did this is of no surprise to anyone here. I wouldn't be surprised if one or two of your grade 6 rigs aren't still in some of your classmates memory boxes or on a shelf in their office. At what age did you take on the name of restorer john?

 

[mansr]

At what age did you take on the name of restorer john?

And what was it before, Destroyer John?

 

[Cars-N-Cans]

When we first moved to this state in the late 1970s, there was a local AM station on 1197kHz (4GG), which in those days, had its transmitter about 7km away. I could look up its historic ERP, but it was well into the hundreds of kW IIRC.

The AM station right behind my house is 50 kW. It’s amazing what it can do. Everything becomes an AM radio, and AM radios only get that one channel, no matter where the dial is.

 

[Music1969]

And what was it before, Destroyer John?

I noticed @mansr isn't around the forum much anymore

Enjoyed his technical contributions around here

 

[restorer-john]

I noticed @mansr isn't around the forum much anymore

Enjoyed his technical contributions around here

Shoot him a PM.

Maybe taking a break. Maybe just had enough of ASR. Who knows.

 

[Andysu]

I am pretty sure Jim Brown knows how feedback works. I think I do, but who knows, tough to identify keyboard warriors these days...

I was a tech during the CB craze of the 70's and 80's and it was a fairly common problem at the time for me. The CB band is around 30 MHz. There were also AM (~1 MHz) and FM (~100 MHz) as well as TV (wideband) towers, and the occasional HAM, that caused interference. I rarely saw issues from HAM stations as they tended to be cleaner than CB (illegal) amplifiers and HAMs tended to be savvy enough to help their neighbors themselves. Don't really know how often it happens now; anecdotally much less frequently judging by how rarely I run into it these days (only a few times in the past few years). But, I am no longer "in the biz", so don't have a good baseline reference.

For the record (no pun intended), phono stages were by far the worst offenders I dealt with, and adding an RFI filter/snubber got to be common. Tricky to do without messing up the cartridge matching, natch. IIRC I tended to use an LCR network instead of a simple RC snubber to isolate the cap. Been a while. I still have an old LM381 phono amp I built (one of my first home-brew PCB designs, ink and acid) that has an LCR input filter on it with little hand-wound coils.

The culprits I have dealt with more recently are dimmers and LED replacement lights with their noisy little SMPS supplies. But by and large the culprit has not been via the speaker wires, except in one or two cases, over the last say ten years or so.

Whatever - Don

27Mhz , i blew a tv up that was about around maybe 90 feet away ? one my Dad's friends and he had every right be red face when came around other , no day
19 no year 83 , as turned out had worlds first fastest VHS player x100 speed , when TX ing with 200w the VCR speed increased when using remote it increased in speed even faster ! then tv went out ! ? curious ? switched it off then on and came back on ,
i was puzzled why the SWR was reading in the red for over , no week ? thought it had rain water in the base - coil of GPA 1/2 wave 18 footer ,
it tuned out i had centre core on the PL 295 dry solder joint and re soldered and SWR was back in the green around 1.5 ratio ?

so i had maybe demon RF signal output and maybe why every other breaker was moaning about my signal phantom-ing in other channels

but i manged to blow a tv up , and felt bad about that , almost had Cobra 148 GTL-DX locked away , my Dad was not pleased
i guess the neighbor if when had tv repaired may be a protection surge circuit that needed replacing ( just guessing )

i have superstar 360FM now its same as Cobra 148 GTL-DX minus the roger bleep which is only a capacitor if i soldered one in , i more less have same power amp 200w , ant is silver rod 1/4 wave 22 footer and i maybe hundred or so below actual but i got to , usa few times and italy

and no interference when using 200w it doesn't break over the THX system which is 40kw , if only my base station was 40kw

filters on tv's today have better grade and digital and other higher frequency if only manufactures made better tv's as they would have known of the CB radio and uk gov ruined CB in uk with lousy FM uk 27 that legalized nov 5th 1981 and nobody was going to dance to the uk gov , myself included not giving up the Cobra , had small 40 ch FM 4w and it still interfered with some neighbors tv's expect for my Dad's and the base ant was near the tv ant , ? don't care to know why its 1980's maybe some gremlins was attached to ant ?