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Old 19th February 2013, 01:44 AM   #21
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Is it possible that your laptop smps is sub par and is putting a lot of noise back onto the mains, and that in turn this may be causing the Power Supply in the amp to oscillate?

Certainly just having the two devices plugged into the same power board without any other interconnection implies that the interaction is happening via the mains leads, unless it is air borne noise that is being picked up by the amp. In any case proximity is probably a factor. If you have a scope you might want to check what sort of noise there is on your mains with and without the laptop SMPS plugged in.

If it is a noise problem then a solution may be to get a good quality IEC filter and an IEC female socket mount them in a metal box, get a suitable power cord (they type they use on UPS's) and run your laptop power supply through that.

edit: does the amp have a filter on it's mains input?

Tony.
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Last edited by wintermute; 19th February 2013 at 01:46 AM. Reason: ask if amp has filter.
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Old 19th February 2013, 02:08 AM   #22
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it's possible but AFAIK it is the original SMPS.

tomorrow I'll be permanently removing the front end and its associated supplies. anyway, I've always wondered if there is some sonic degradation because of the DS1666 chip. with the bonus that I'll be switching to balanced inputs.

my best guess: the front-end transformer is breaking down under certain conditions. off it goes.
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Old 19th February 2013, 12:48 PM   #23
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Hi I hope I can be of help here.

We have all sorts of strange power connection devices these days.
Usually we can get a reasonable idea what we are dealing with by inspecting the label for input power. The oldest and best insulating device is the so-called wall-wart isolation transformer. It is normally indicated as allowing a single input voltage of either 120VAC or 230VAC 50 or 60Hz. The output can be any voltage usually less than 24V AC or DC. A DC one of this type has diodes inside, but no capacitors to smooth out the AC ripple. This type of power-supply has an isolation transformer in it. This transformer has windings that are wound in two separate compartments on the transformer bobbin. The isolation of this sort of power supply is often very good and rarely causes any problems other than some either induced or audible hum. When this type of supply is overloaded it overheats and may melt in severe cases. The final result is a mains fuse blowing because the mains connected primary overheated and shorted out. Normally the primary to secondary isolation remains intact.

The second type of power line-connected supply is a offline-switching supply. This is normally identified by being very light and small and often, not always, have a range of line inputs from 100VAC to 240VAC this last indication is a true give-away that this is an off-line switcher. What is done here is that when the line-voltage enters the box, it finds a simple high frequency filter, an inductor or two plus a capacitor. Thereafter we will find a few rectifying diodes, and a high voltage electrolytic capacitor. This electrolytic capacitor represents a lot of problems. To be physically small and inexpensive it has poor electrical characteristics. It will age and eventually burn up and short out. Hopefully this happens after the warranty time has expired. After this capacitor there is a switching circuit that chops the rectified DC across the electrolytic capacitor into a high voltage AC waveform with a frequency from 30kHz to several 100kHz. This high frequency waveform is transformer coupled to the lower secondary voltage winding. This lower voltage is rectified and some messaging circuit, usually an optocoupler sends a signal to the offline primary switcher.

We now have many things that can go bad. The insulation of the high frequency transformer can break down or less frequently the optocoupler could break down. A laptop for instance is usually powered by such an offline-switcher. If the switcher develops an insulation breakdown, a laptop would work fine without external devices connected, but as soon as something else is connected especially if it is ground or earth connected then we have a disaster. The line voltage will be connected directly to ground through the wall connected supply.

Modern electronics is full of these Offline-Switchers. Tvs, desktop computers, CFL lights, Battery-chargers, inexpensive audio gear Laptop power-supplies etc.

They all use essentially the same circuits cheapened to take the last penny out. They all produce intolerable RFI and EMI (Radio Frequency Interference and ElectroMagnetic Interference) rendering shortwave and radio reception useless. The RFI may also produce audible sounds in Audio-electronic equipment because so much Audio is now done digitally.

This became a long dissertation, but I did it for completeness.

Hans J Weedon.

Last edited by HJWeedon; 19th February 2013 at 12:54 PM.
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