• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

SMPS experts come on in please!

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aardvarkash10

aardvarkash10

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Joined 2007
#1
gaaaaah!

Bought a new LED HDTV (Samsung) and sat it over the top of my pre- and power amps (both simple CRCRC power supplies). They all connect to a common wall outlet

woah! Wholesale high frequency interference... I'm guessing from an SMPS in the TV.

Suggestions?
 
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#3
aaah, forgot to mention - they are not connected yet except at the phase a neutral of the power outlet. This is RF stuff... or coming through the power supply.

And the TV is above ground ie no Earth terminal on the main power lead.
 
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#4
Schematics of the PSU sections ? If you're not using ceramic or foil filtering capacitors, add them. You might also want to consider a (common mode) RF choke. You can grab the entire section out of an old PC PSU, it's the small PCB attached directly to the mains socket.

Before this is in place I wouldn't even bother looking for problems elsewhere.
 
#9
I'd start with one of those input power filter assemblies found in computer switching supplies and similar. Corcom (Tyco) makes all sorts of line filters - something like this mounted in a box might help..

2VK1: CORCOM/TYCO ELECTRONICS: Interconnects

Tightly coil up the line cord from the TV so that it is not an effective radiator (antenna) and perhaps add a ferrite core as well..

None of this is absolutely guaranteed to work, and the set itself may be radiating RF into your components. (Not just conducted EMI through the line) It may be that moving the components would prove more effective..
 
#10
:-( Already have one inside the pre-amp which supplies power to both pre and power amps...

I'm using a bog stock three-core IEC cable to the pre-amp where it is switched and then fed to the filter, then on to the pre-amp circuit and out via a female coupling and a short three-core cable to the power amp.

Are you suggesting I put one into the TV power supply too?
 
#11
aardvarkash10 said:
Are you suggesting I put one into the TV power supply too?
Click to expand...

Try to power TV through the filter to find out if it is the source. Powering preamp through the filter may make things even worse, if you connect TV's ground to your audio ground, for example feeding video and audio from some DVD or blue ray or whatever color source. Filter are needed to protect mains from contamination.
 
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#12
I am interested in this too. I am in the process of troubleshooting my 6L6 PP amp. Perfectly quiet in the workshop, hook it up to main system, I get a 'whistling' through the speakers. I have tracked it down to the power supply for an external hard drive causing the problem. I have a Mac Mini as a media unit with an external 2GB drive. When the drive is 'working', I get the whistling. Have tried the suggested power supply filters on my amp, with no change. Will try filtering power supply of external drive, but this is not so easy as it will require building/buying a box for the filter and associated plugs/sockets.
Cheers,

Chris
 
#13
Hi Chris;

it is the case when balancing transformer helps. You can't remove ground loop adding a filter to your amp, it makes it even worse. A filter contains a couple of capacitors in series, connected between phase and null, and a middle point of such a voltage divider is grounded. That means, whatever comes from outlet between phase and null, causes spikes of half of it's voltage on the amp's ground.
 
#14
Wavebourn said:
Hi Chris;

it is the case when balancing transformer helps. You can't remove ground loop adding a filter to your amp, it makes it even worse. A filter contains a couple of capacitors in series, connected between phase and null, and a middle point of such a voltage divider is grounded. That means, whatever comes from outlet between phase and null, causes spikes of half of it's voltage on the amp's ground.
Click to expand...

Thanks Wavebourn,

I did not think of the problem as a ground loop initially. While trying to isolate the problem I realised that not only was the amp in question making the 'whistle', but also two other amps in the system. The whistling would happen on the other two amps even when the amp in question was turned off. I discovered that if I removed the input cable or the power cable from the amp in question (with power off), that the whistle stopped in the other amps. I concluded at that point I had a ground loop problem.

I have a grounding scheme like the one on Rod Elliot's site Earthing (Grounding) Your Hi-Fi - Tricks and Techniques When I lift the zero volt line from the chassis (leaving safety ground connected of course), I get dead quiet. I may try experimenting with removing cap in the back to back diode//resistor//capacitor setup or varying some values...

Cheers,

Chris
 
#15
aardvarkash10,

are you sure it's a "conducted" problem? can you try putting one layer of (iron) sheet metal inbetween, just to see if it is not radiated magnetic emission that is causing the problem? should be a fast and cheap experiment, e.g. a baking pan should do .....
 
#16
problem fixed! basic really😱

I replaced the 100 ohm resistor separating the pre-amp's signal ground from the chassis ground with anti-parallel diodes.

Hum gone. New SOP in place - antiparallel diodes on everything from here on!

Thanks to all for their input.
 
#17
Glad you fixed the problem. It took me a while, but I sorted mine out too. I had used the scheme used in my link above, so that is why I did not think I had a loop problem to start. It turns out that there was a short in my setup (one of the diodes I think). When I eventually worked this out and replaced it, problem solved! Amp has ZERO hum!

I have read that diodes most often fail short circuit, so worth testing them before wiring up.

Cheers,

Chris
 
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