Grounding Issue with Monitor Switcher on Neotek

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EWalter

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Sep 25, 2018
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Hey guys,

I'm installing an Elma 4 pole, 3 position rotary (non-shorting) switch on a Neotek Series 1 and I'm having some grounding issues that I can't make sense of.

The switch is wired at the control room output of the master section and is unbalanced signal. Switch position 1 goes to a pair of balancing cards on the output of the console, connected to a pair of powered monitors. Switch position 2 goes to a pair of TS jacks on the console, connected to an unbalanced power amp.

I'm getting a buzz from whatever pair of speakers is not getting signal. i.e. When on switch position 1, the speakers connected to switch position 2 will buzz. Both output jacks are connected to chassis ground. On the switch side all grounds (input & both outputs) are tied together. If I jumper (with alligator clips) the input poles of the switch to whichever switch position is not in use,  the buzz will go away, but of course, then I have signal at both pairs of speakers.  :-\

The amp is on the same circuit as the console, but I've tried ground-lifting it anyway, without any luck. I've also tried disconnecting all grounds at the switch, as well as lifting both outputs and sending the input to chassis.


One strange anomaly: The passive monitors have a low freq. ground hum/buzz whereas the active monitors (which receive their input from balancing cards in the console) have more of a white noise sound and it's only in the left speaker. I've had this same white noise sound happen with these when ground was not connected properly at the output cards, and solved it by tying them to chassis, but that's not working here.


I'm not sure if there is some other type of component I need to add to make this work, or if there is some special grounding trick that I'm missing.

One thing I just thought of while typing this up is that the switch is balanced and my signal is unbalanced, so I'm only using half of the switch. Could Poles A&B be isolated from C&D? Currently I'm using A for Left and B for Right; should it be A-Left & C-Right? I'm going to try this out next unless someone tells me otherwise!

Thanks for your help!
 

Bo Deadly

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Dec 22, 2015
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Draw a sketch of the switch wiring and take a picture of it with your phone (under bright lights at angle to minimize glare) and post it on imgur with a link here.

You could use the commons of each pole for output to monitors / speakers and then ground unused signals. Then you will really use 4 poles.
 

Monte McGuire

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Jul 26, 2013
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346
It'd be nice to see the actual circuit, but it sounds like the sends to the unused amps are floating when not selected by the selector switch. Normally this should not make buzz, but what could be happening is that when the amp send is driven, the impedance of the signal lines are very low, and the line thus does not pick up hum. When a destination is not selected, the cabling to the unselected amp is floating, and picks up interference because its impedance is very high and the wire picks up electrostatic interference.

One simple way to solve this could be to add some "tack down" resistors to the amp sends, connected between the switch signal output to ground, one for each signal line. A value of 10kΩ or20kΩ might be enough - maybe even higher like 47kΩ or 100kΩ might work, but try 10kΩ first. By doing this, all of the output lines will be "tacked down" to ground when they are not selected. The 10kΩ value won't be a difficult load, and even though it's in parallel with the amp input, a competent line stage should be able to drive the tackdown resistor and the amp input impedance in parallel easily.

I could be wrong, but it's a simple trick, worth a try. If that doesn't work, you need to document what you have and show us what the overall circuit of your connection looks like.
 

EWalter

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Nashville
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Thanks, Monte! That's basically what I'm thinking is happening, but I didn't know any such tricks. I'll definitely try that Tack down method and let you know how it goes!

I drew a block diagram of the part of the circuit in question. I tried to draw it very simple and straight forward - hope it clears things up for y'all:
ph5pOSW.jpg



Let me know if you have any questions!
 

Monte McGuire

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Jul 26, 2013
Messages
346
Thanks for the drawing - definitely helpful. Since you only have two destinations and a pair of unused switch wafers, you could instead do what squarewave suggested above, which is to use the unused part of the switch to ground the un-selected amplifier inputs.

To do that, you would ground points C and D and then connect C1 to A2, D1 to B2, C2 to A1 and D2 to B1. In this way, an amp is either connected to A/B (your signal source) or connected to C/D (ground).  This would be much more of a "guaranteed fix".

You still might get some clicks as you operate the switch, since the amp inputs will all float briefly as the switch is changed. Also, the new destination might have charged to some odd DC voltage from leakage from the floating amp input circuitry, which would get discharged when you switch it to "on", making a little click. The tackdown resistors will prevent that by making sure that all inputs have a defined DC value at all times. For this use, you could use a 47kΩ resistor - there's no need for something as small as 10kΩ, and it will reduce the loading to negligible values.

Best of luck, and enjoy your Neotek!
 

EWalter

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Awesome! I’ll give that a try and let you guys know how it goes.

Thanks again for all the help!
 

Bo Deadly

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Dec 22, 2015
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You can't leave inputs "floating" like that. Flip the switch around so that the "common" terminals are outputs. Then you can ground unused inputs.
 

EWalter

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Sep 25, 2018
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Nashville
Hey Guys,

I just did the ground jumper swap trick and it worked! Thanks so much for your help!

Monte - I am getting the electrical pops and had read somewhere about doing something similar to your tack-down method, but with capacitors. I'm going to use your method with resistors, but It's not entirely clear to me where the resistor will go. Do I put them in place of the jumpers I just installed? i.e. between C1 & A2, etc...?

Thanks!
 

MrG

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Jul 22, 2018
Messages
82
Location
Nashville, TN
Hey all,

EWalter and I make up Reel Recording, which is where my Neotek is. Erik has been working on getting this solved but is still experiencing a pretty significant pop (moves speaker drivers) switching between A and B _only on the right speaker_. His next move is to try capacitor tacking down.

I try to think of things in a low level logical way to avoid overcomplicating, so I find myself wondering how this could possibly be like this, and where our theory knowledge may be lacking.

Questions:
-we are going in and out unbalanced - if we went in and out balanced, would this theoretically solve the problem?

-we are balancing the signal on the one set of outputs and not on the other. Could the balancing procedure be bleeding back, and is there a way to control that?

-the sound of popping - is it hot touching another hot, or hot touching ground? Is there a switch layout to avoid this, like using A and C outputs instead of A and B?

-why would this problem plague only the right speaker?

Thanks for any help.

Mark
 

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