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Still plagued by noise from passive RCA switch

ToTo Man

the band not the dog
I posted about this last year so apologies for the deja vu but I'm still no closer to solving the problem.

I am using this passive RCA switch to route the line output from my amplifier to one of two tape decks (my amp doesn't have tape loops) but am finding it is adding 50Hz noise to the signal on the LEFT channel.

i have measured the internal noise of my ADC (Focusrite 4i4), the noise of my amplifier connected directly to the ADC, and the noise of my amplifier connected to the ADC via the 2-way switch.

Can someone please explain why a passive switch would cause in a huge increase in 50Hz and its harmonics, and why only in the left channel?

NOTE: I have amplified each recording by +75dB to make the noise easier to see. (This really equates to a boost of +65dB because the ADC records a 2Vrms input at -10dBFS instead of 0dBFS so you need need to boost it by +10dB for unity playback level).

ADC internal noise:
51070671438_185787a6cc_c.jpg


Amp (off) -> ADC:
51071360036_771d84cb50_c.jpg


Amp (on) -> ADC:
51071456627_19a0877421_c.jpg


Amp (off) -> switch -> ADC (switch set to ADC):
51071359856_e604f1e54e_c.jpg


Amp (off) –> switch -> ADC (switch set to other output):
51070664373_4db75ba715_c.jpg


Amp (on) -> switch -> ADC (switch set to ADC):
51071456582_c121ca2fb4_c.jpg


Amp (on) -> switch -> ADC (switch set to other output):
51071359811_270333cd1e_c.jpg
 
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Open it up and have a look? Buy a different one?

My guess would be that the switch has all the grounds tied together. Not that that's always a problem, but I've always heard you should cherchez la ground loop.
 
What should I be looking for when I open up the existing switch, or when looking for an alternative? I've never had any issues with ground loops before so don't know the troubleshooting process.
 
What should I be looking for when I open up the existing switch, or when looking for an alternative? I've never had any issues with ground loops before so don't know the troubleshooting process.

I would be looking for a poor ground connection somewhere, either loose or broken. Possibly a bad solder joint.
Presumably bought too long ago to simply contact Fantronix for a replacement (or advice even)
 
What should I be looking for when I open up the existing switch, or when looking for an alternative?

Open it up and post a pic' here.

I am not familiar with one of these switches, but reading between the lines, as @booja30 effectively suggests, if the earths are ganged together, desolder them and only reconnect the ones essential for what you are trying to do. If that cures the problem, you know where to start
 
Unfortunately the box looks to be inherently flawed as all the earths look tied together - no matter what you do it will result in hum loops simply by having things connected to it. Presumably the earth continuity between the inputs and outputs is going through the case.
Yes, grounding appears to be through the case. This is the reply I got from Fentronix today when I asked them about the 50Hz noise:

"I think its unlikely to be the switch itself. Its quite unusual. Usually with these they go off and we never here another thing. Even in noisey environments the metal box usually provides a lot of protection. The Ground conncetions are usually through the case, but you could try adding direct short connections inside between the connectors. I can't remember what you are connecting too be it could also potentially be a grounding issue between the connected devices especially if it decks. If for some reason they aren't connected well or at a slightly different potential that could cause issues. Either that or a ground loop introduced somehow."
 
Problem is very few switch box makers actually show you the internals so I'd be taking a punt buying another one, and the ones on eBay are either super-cheap looking or super-expensive with nothing in between. It would be better if a pfm member could make one for me. :)
 
Or take it to a good service engineer who knows what he is doing...

Easily said ... the last electronics repair shop in our town closed at least 8 years ago ... presumably if he was still open he would be closed anyway.

During the last lockdown I managed to track down a local bloke via extensive searching when I was needing a surface mounted chip replacing, his reply was that his 'pending' list was at least 3 months long... but he would get back to me when he had a space .... I'm still waiting.

Unless of course you were actually offering to help the OP out? :)
 
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Problem is very few switch box makers actually show you the internals so I'd be taking a punt buying another one, and the ones on eBay are either super-cheap looking or super-expensive with nothing in between. It would be better if a pfm member could make one for me. :)

I would offer to rewire the one you have - I can solder but unfortunately I am no EE so can give no guarantee you wouldn't be worse off ;)

However - as the box uses the case for ground it is very likely that the ground tags and the insulation washers will be missing on the sockets which would mean buying new sockets or bodging the wiring/insulation somehow.

I think you would actually be better off following up the QED suggestion above :)
 
This thread is looking very similar to the Chinese DAC thread :-s

Case needs to be isolated from every RCA and should either have its own safety earth connection or be linked to a single output RCA.

You'll need a new case or new RCA connectors. Soldering isn't very pro either and neither is using all the same colour wire.

Get rid and buy a Goldpoint one
 
I'm somewhat hesitant to try another vintage unit after my experience with the Sony SB-300. All of the push switches were heavily oxidised and in need of a thorough cleaning, which I was about to embark on, but then I read a couple of posts over on AudioKarma stating the SB-300 needs to be flushed out every few months to prevent intermittency. I don't have the patience for that kind of maintenance. I made some test recordings of the SB-300 when I got it and IIRC the noise level varied depending which set of inputs I plugged into, which I found rather odd at the time. I'll need to locate those recordings and do a deeper analysis of the noise to see if it's the same 50Hz problem I'm getting with the Fentronix.

I'd order a Goldpoint but I wouldn't be able to manage the rotary dial with my cerebral palsy. I need a box that either has rockers or push buttons.

I'm also constrained by the size of the switch box. Two Fentronix boxes, each measuring 89mm wide, are just small enough to sit side by side between my two decks. If the boxes were any bigger I'd need to order a new table:

51073442868_6f0c93feff_o.jpg
 
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