Line and power filter question

[mike_mccue]

A while ago I asked about a box of Cor Com power filters I found in storage and learned a little about them... mainly that they existed :-).

I'm beginning to build a power supply for a tube preamp. It will use a traditional transformer > rectifier > filter design.

I just went and read a few things about line and power filters here:

Article - EMI Power Line Filters

EMI Power Filters Crush the Noise

I'm not so sure, but after reading the articles it seems to me that these filters:

1) do not apply to transformer based system but rather are helpful with switching power supplies.

2) are primarily meant to keep noise from leaving a device with a switching power supply and fouling some other device.

3) are meant to minimize noise to specific standards as required by law and often times need to be designed for specific charateristics.

I had previously thought the CorCom K filter filtered incoming noise... but now I think I may have had that wrong.

Can someone correct, elaborate, or confirm any of the notions I just listed?

As it stands I get the impression there is no benefit to using one of these CorCom K filter (that I have a box of) on a D.I.Y. tube based microphone preamp.


I appreciate any help or comments you can offer.

thanks,
mike

[mike_mccue]

Can someone correct, elaborate, or confirm any of the notions I just listed?


thanks,
mike

[Enzo]

It's a filter. It is caps and coils. It may be used to keep generated noise IN, as in most any digital circuit. It may be used to keep external noise OUT, as in sensitive equipment. For example in the arcade industry, and before everything went to switching power supplies, we had to have those on our video games and microprocessor based pinballs to prevent transient spikes on the mains getting through and causing glitches inthe digital system. I have a box of Corcom filters myself.

Using one on a guitar amp or mic pre or something can;t hurt, but I don;t know if the sort of high freq noises it stops are much of a problem. Especially on a tube circuit.

Grab one, either wire it up to a socket or use clip wires and add it in front of some amplifier. Make any difference?

Open up a filtered outlet strip, and you will find basically one of those, either in a single unit like that or made up of discrete parts, and a couple varistors as surge limiters.

[mike_mccue]

Thanks very much!!!

[Robert M. Martinelli]

EMI interferences can be divided in two main categories, "radiated" and "conducted".

The first ones, as their name suggests, are "transmitted" in air and can ( and sometimes do ) spoil other devices in the radiating device neighborhood, the second ones are those going out of the switching device through the mains line.

I' ve been working as an application engineer for a world-class AC drive manufacturer, so I have had to deal with EMI issues quite a few times in the past.

The problem with switching devices is that you always have some electronic device inside going on and off thousands of times a second ( e.g. think about the "carrier" frequency inside a PWM drive ) so these line filters are there to keep these devices' ( BJTs in the past, now IGBTs and MOSFETs ) switching noise from "electromagnetically polluting" the mains line. As a welcome side effect, being constituted by inductors and capacitors ( Pi filters used as resonant traps ) they also protect the device from transients coming the other way ( FROM the line ).

The worst EMI case I had to face back then was related to an AC drive installed inside a "tapis-roulant"... since the apparatus was classified as a "medical" device, the requirements were particularly stringent ( imagine what could happen to the heart of a guy carrying a pace-maker was the drive to spoil the pace-maker operation somehow.... ). We had to perform a very deep and thorough spectrum analysis on both radiated and conducted EMI, and ended up designing and installing supplementary filters and shielding the drive very carefully.

Our friend Steve ( Conner ) is very keen on switching circuits, so I hope he chimes in to explain the above concepts way better than I ever could.

HTH

Best regards

Bob

[mike_mccue]

Thanks very much for the explanation and the anecdotal example.

best regards,
mike

[Merlinb]

Such mains filters are often used in low-power devices such as preamps and CD players, whether they use switching PSUs or not, and there is no particular reason to use or not to use one. But they are not usually used in power amps due to the much larger peak currents required.

Of all audio devices, valve guitar amps are probably the least likely to give any noticeable benifit when using one. But if you've got a box full you may as well use 'em!

[mike_mccue]

Thanks for the additional info!

best regards,
mike