Freq Band

Selector switch "pops" (isolated grounds)
« on: January 23, 2009, 11:24:59 PM »
I wired a rotary selector switch for 6 unbalanced external incoming signals. (preamp inputs)
I thought it was a good idea to put all the "grounds" for each signal, on a separate deck of the switch...so that only the selected input's ground is connected to the preamp....the other 5 grounds are not, until they are selected.
This was to prevent noise or loops from entering the preamp.

But I get a loud "pop" when switching between inputs.

Also note...on my selector switch....Right and Left channels switch decks are MBB....but Ground deck is BBM.

Can I jumper all the grounds together with a resistor (to eliminate pops), and still get the  ground isolation I desire ?


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Re: Selector switch "pops" (isolated grounds)
« Reply #1 on: January 24, 2009, 09:42:03 AM »
Resistor between the grounds doesn't remove the voltage between the grounds (that causes the pop). So you have to connect the grounds together or to live with the pops when switching.

JohnRoberts

Re: Selector switch "pops" (isolated grounds)
« Reply #2 on: January 24, 2009, 11:25:46 AM »
The first thing to look for is DC voltage. If a source is cap coupled but without a load to drain off any leakage current that cap could be sitting up at some voltage, so perhaps add a large impedance drain resistor.

Normalling grounds with unbalanced terminations can lead to signal contamination if non signal currents flow in those common grounds.

JR
John Roberts
http://circularscience.com
Tune it, or don't play it...

Freq Band

Re: Selector switch "pops" (isolated grounds)
« Reply #3 on: January 24, 2009, 12:26:42 PM »
The first thing to look for is DC voltage. If a source is cap coupled but without a load to drain off any leakage current that cap could be sitting up at some voltage, so perhaps add a large impedance drain resistor.
.....

Put a 47k resistor across each input's jack "signal and ground" ?

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JohnRoberts

Re: Selector switch "pops" (isolated grounds)
« Reply #4 on: January 24, 2009, 12:31:34 PM »
Yes, 47k (I used 100k as a default value) should be fine... IF THAT IS THE PROBLEM.

There will always be HF clicks, if you switch switch while music is playing as the signal is suddenly interrupted, and small DC offsets in DC coupled outputs (and inputs) will click too.

Good Luck.

JR


John Roberts
http://circularscience.com
Tune it, or don't play it...

Re: Selector switch "pops" (isolated grounds)
« Reply #5 on: January 24, 2009, 03:32:07 PM »
You can do a test by connecting all grounds temporarily together. If that solves the popping then ground potentials are the problem. If not then you have to eliminate (also) the DC potentials of the inputs/outputs.

I think in your case the biggest problem is not DC offset of the inputs/outputs but the potential difference between equipment grounds. First thing is to ensure that all equipment are well grounded (that their ground potential is as same as possible).

sodderboy

Re: Selector switch "pops" (isolated grounds)
« Reply #6 on: January 24, 2009, 08:17:29 PM »
That MBB switch on the signals is not hot either.  You are shorting two outputs together at each transition.  If you look inside a passive unbalanced switching box, all grounds are bussed together, L and R, usually RCA's bussed in a cluster.  Only the signals themselves are switched, usually with interlocking DPDT pushbutton switches.  Rotary is better with BBM.
I second the "ground as pop" theory.  To start with, all equipment should have 3rd pin power cords.  A lot of unbal stuff does not.  A simple source switch should not require any additional components if it is wired correctly with the correct switch or relay.  When I install my relay based monitor switchers, the popper is usually an unbalanced amp with a 2 prong cord.  Connect the chassis to power 3 rd pin and no pop.  My switchers, balanced, buss all L shields together and all R shields together.
Mike

AMZ-FX

Re: Selector switch "pops" (isolated grounds)
« Reply #7 on: January 27, 2009, 06:22:21 PM »
If there is even a small amount of dc leakage in the input capacitors, the imbalance will cause a pop when you switch the circuit. 

Adding pulldown resistors will cure the popping many times but you cannot put them on the jacks...  they have to be on the side with the leaky capacitor so they can pull the voltage down to ground...  see this article which explains further:  http://www.muzique.com/news/pulldown-resistors/

1M will usually be adequate for the pulldown resistors.

regards, Jack
Free Schematics and Projects - http://www.muzique.com/
My Guitar Effects Blog - http://www.muzique.com/news/
Follow me on Twitter - http://twitter.com/#!/AmzFx

sodderboy

Re: Selector switch "pops" (isolated grounds)
« Reply #8 on: January 31, 2009, 10:10:19 AM »
If there are leaky caps in a power amp input stage or DA output stage, shouldn't they be replaced instead of compensating for them in a passive switcher?  That's like adding buckets as the roof deteriorates, and then installing a pump system to empty them before they get full.
I have never needed any extra components in passive 1:X or X:1 switchers, but there are often grounding issues as mentioned before.
Mike

JohnRoberts

Re: Selector switch "pops" (isolated grounds)
« Reply #9 on: January 31, 2009, 11:10:08 AM »
All capacitors exhibit some leakage. Typical electrolytic capacitors driving very high impedance loads will develop DC levels that are audible when switched. It is common practice to provide a DC path to drain off this nominal leakage current. Some (all?) capacitor data sheets provide an equation for expected leakage current based on the product of farads and DC working voltage. Indeed excessive leakage can indicate a failure mechanism, but some leakage is normal and expected.

JR
John Roberts
http://circularscience.com
Tune it, or don't play it...


 

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