smps protection question

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RHC

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Sep 15, 2015, 8:33:30 AM9/15/15
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I have been playing with a couple of HV smps's I have purchased from alibaba with an audio circuit and have found a problem I was wondering if anyone had any suggestions for fixing.  Essentially they get very unhappy if one plugs in an rca line level input while the amp is turned on.  Here is an example of one of them. 


The unit doesn't die outright and still puts out the HV it was set to but it gets very noisy. You can see the power led flicker consistently, as soon as you put a load on it. 

Any thoughts on this are appreciated. 

Bob

JohnK

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Sep 15, 2015, 9:37:01 AM9/15/15
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I have a comment that is worthy of mention but may have nothing to do with your problem.
 
The use of RCA connectors can be tricky. The earth/ground makes after the signal connection. That means there is a possibility of very large common-mode voltages.
 
For instance, consider the usual mains CD player and mains amplifier [or DVD, VHS tape etc etc]. Most of the consumer hifi gear is double-insulated [in name only most of the time ! ]. Much of this equipment has noise suppression capacitors from the mains wires to the chassis resulting in the chassis being at a voltage-divided potential reference real earth/ground. Say that the two equipments are powered up but not interconnected. And, in the worst case, consider that the amplifier already has something else connected as an input that is providing a ground eg a turntable [or that the amplifier actually has a ground attached]. The powered up item you are about to connect as an input to the amp provides a voltage at highish impedance and at about half the mains voltage to the audio input (centre connection). This can damage circuitry.
 
Many manufacturer's user manuals say Do Not connect with power applied. Most of the time there is no damage. BUT we have all heard the intense hum when doing so - it is NOT all due to earth/ground loops.
 
 
Maybe you have a multi-potential problem. OR maybe it is due to the overdrive of the audio circuit during the connection being made.
 
More details of actual circuit needed. Look for things that you can rationalise eg earthing/grounding and connector styles. [By this I mean - can you add earthing/grounding so that all commons that can be at earth potential ARE at that potential; AND does it happen if you make the RCA connection and then apply the audio by raising the level suddenly?
 
If you know what I am getting at you will be able to dismiss the suggestion outright or try an idea based on it. IF you do not know what I mean by the double-insulated problem or by common-mode voltages then please ask more or read more before tinkering - you can inadvertantly make it worse if you don't correctly analyse the situation.
 
John K
Australia
[PS  re the doubleinsulation specs; have a close look next time you are inside gear. I had to add a remote control function to a modern turntable - it was the lightweight version and the chassis was plastic and not metal. The inetrnal wiring loom was the same production item and had a screw connecting an earth lug to the plastic in the same place that the cast metal chassis had. Before you ask - It was not a conductive nor dissipative plastic nor had buried mesh.
The tone arm and other metal things still protruded externally and were not separately earthed. The 230V mains still arrived on a PCB that held fuses etc - all open to the touch internally (not even cardboard covers). There was no double barrier - for instance the shield wire for the tone arm if not soldered or a piece of solder-wire dropped during manufacture could bridge the mains to the exposed metal.  ]
 
 
 
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gregebert

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Sep 15, 2015, 10:34:23 AM9/15/15
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My first suspicion is a ground loop, and they can be difficult to locate and fix. I'm assuming the audio and HV are on separate PC boards, so the solution is with cabling and perhaps a filter in the right location. It's possible an oscilloscope can help, but be aware that the ground-lead to your scope can add to the noise you see; there are several good techniques posted on the web.

When I built my first computer, the ground loops were SO bad that I could hear interference on an AM radio several houses away. It was amazing how easily I could trace the wiring inside the walls, because of the noise. After I removed the power-supply ground loops, there was no detectable noise outside the computer cabinet.

JohnK

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Sep 15, 2015, 11:54:17 AM9/15/15
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I agree with Greg that loops are troublesome; I would like a bit more info from Bob actually.  [Drawings, photos, (off-list if you like) ]. 
What power [watts] is expected of the supply? What amount of capacitive smoothing/filtering is on the high voltage?
This LED - it is across the HV?
 
Star earth/ground is definitely required inside any audio gear.  The power supply currents shouldn't flow in any of the signal wires.
 
"Essentially they get very unhappy if one plugs in an rca line level input while the amp is turned on" -
Did you mean there is audio signal on the RCA cable?
Does the problem occur when the RCA outer only is connected? ie connecting an earth/ground to the RCA outer perhaps?
I am not sure what you mean by 'noisy', but noise shouldn't be seen on the LED - it is 'too fast' to show; especially as I think you mean the LED flickers  ie very low/high volts for an appreciable time?.
 
I am very interested in whether your audio source is floating [and for that matter, if your amplifier is floating].
 
John K.
 
 
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Mich...@aol.com

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Sep 15, 2015, 12:48:00 PM9/15/15
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Don't mean to jump on your thread; however, I have tried buying from alibaba many times; however, I am in Washington.
 
Washington state (from contacting them) is the only State they are NOT allowed to sell/ship to.   I have had to pass up on a lot of kewl deals.
 
Car Key programmer (to avoide dealer fees) - found on another site for $40 more.
2 wheel balance board smart 10.   - found on another site for $75 more.
 
Michail
 
 
In a message dated 9/15/2015 5:33:34 A.M. Pacific Daylight Time, rhcc...@gmail.com writes:

I have been playing with a couple of HV smps's I have purchased from alibaba with an audio circuit and have found a problem I was wondering if anyone had any suggestions for fixing.  Essentially they get very unhappy if one plugs in an rca line level input while the amp is turned on.  Here is an example of one of them. 


The unit doesn't die outright and still puts out the HV it was set to but it gets very noisy. You can see the power led flicker consistently, as soon as you put a load on it. 

Any thoughts on this are appreciated. 

Bob

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