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Are line harmonic suppression filters ever used for audio amplifiers?
Are line harmonic suppression filters ever used for audio amplifiers?
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Old 19th September 2019, 10:36 PM   #11
trobbins is offline trobbins  Australia
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Hari,
Such line harmonic filters can be used industrially for many reasons.

If you are hoping that the filter will improve the voltage waveform seen by the amplifier then have you confirmed an improvement (by measuring mains AC voltage harmonic levels before and after application of filter), or are you just using your ear?

If mains voltage distortion is your concern, perhaps because you have measured what is available to the amplifier, then you need to appreciate that the distortion may include a lot of distortion forced on the mains AC by both nearby equipment, as well as your amplifier itself !

Similarly, if you apply a harmonic filter to your AC mains then it will become a low impedance path for harmonic currents that originate from anywhere nearby , as well as from your amplifier. That could mean substantial current levels coming in to your house/building/room, from industrial equipment nearby, and those current levels could exceed the trip levels of protection devices.

I would strongly recommend you do not add circuitry to the AC mains supply unless you have appropriate measurement equipment to know what it is experiencing, and have in place appropriate protection devices and have a professional awareness of what is and could be going on.
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Old 20th September 2019, 07:18 AM   #12
Hari is offline Hari  India
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Thanks trobbins for your inputs. I have been using this for around one week and have two MCBs in the path which has not yet tripped. Also i have physically touched the inductor and capacitor and they run cool even after 2+ hours of usage which means there is no major power loss across them. But what you mentioned is also very logical. Perhaps connecting this to an 1:1 isolation transformer would be more safer. What do you think?
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Old 20th September 2019, 09:54 AM   #13
DF96 is offline DF96  England
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Quote:
Originally Posted by Hari
Actually i tried building one with CFL chokes and capacitors and found the tube amplifier power transformer pretty cold even after 2+ hours of use.
I can't see the connection between filtering harmonics and a cool transformer. Does the filter remove DC too?

Be aware that the main source of harmonics will be PSUs; yours and others on the same mains circuit.
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Old 20th September 2019, 10:04 AM   #14
mindutis is offline mindutis  Sweden
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I am building a mains filter now, and it is not only for EMI but also to remove any DC in mains to prevent power transformer from buzzing etc. I have already built this filter some years ago and now building another on. Just this time not on a DIY pcb, but traced with eaglePCB. Already got all the parts.

Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.

Not in english, but you can translate it i am sure Audio schemos leopard

Will be used with 1kW isolation toroid also.
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Old 20th September 2019, 11:01 AM   #15
zigzagflux is offline zigzagflux  United States
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There are active and passive harmonic filters, which operate very differently. You are asking about passive filters, so I will restrict this post to those for now.
Passive filters are not used to block (like say an EMI filter does). They are tuned in such a way to sink harmonic currents that are known to be problematic. These currents do NOT come from the supply, they are generated by the load. Filters are normally placed near the harmonic-generating load such that those known harmonic currents do not propagate up into the supply system; they are sunk by the filter. Technically, they provide a lower impedance than the supply system, so it is a current divider.

Without the filter, the harmonic current must pass through the entire supply system impedance, creating a harmonic voltage drop. This is what causes flat-topped sine waves on the voltage supply. The utility (near the point of generation) is a very clean harmonic-free sine wave. The further away from that generator you get the more harmonic voltage is created by the harmonic currents passing through it, and you see this at your electrical panel.

With regards to your power supply, it is a harmonic generator, it is part of the problem. If you were able to provide your amp with a perfect voltage free of harmonics, your amp will throw back at it a plethora of harmonic currents. This applies to capacitor input, choke input, and yes even switching supplies that are power factor corrected (though they are pretty darn good).
Your voltage supply, not having 0.0000000 ohms of Thevenin impedance, will therefore distort due to those harmonic currents the amp has generated.

Now, you come out and install a passive filter near the amp input. This might improve the voltage supply if you have carefully selected the resonant point of the filter to match the harmonic current(s) you know the amp is producing. By the nature of your question, you have not knowingly matched any filter performance, so you should not expect a voltage source improvement. In fact, what you could have done is caused a sink for harmonics generated else where in the home, which will drop voltage across your branch wiring and add additional harmonic voltage drop (distortion).

You should also be aware that a passive filter as a general rule will NOT improve the voltage waveform present at the panel; this is largely dictated by the utility and the bulk power grid. You are wanting to restore harmonic voltages that have dropped between the supply and point of use; you would not obtain this with passive filters. That would be the role of active filters. In short, you cannot expect a passive filter to clean up the voltage waveform; at best you can expect it to reduce the voltage distortion created by your amp, and that only. In relative terms, your amp adds very little distortion to what exists on the line.

There are, for some people that have the software tools and understanding of power system resonance, who can create voltage rise by overcorrecting at the point of use, utilizing passive filters to act as 'little generators', but this is not within scope of your application, not by a long shot. My suggestion is don't even try to pursue this. I do this on occasion, and it requires careful analysis of frequency response and damping to ensure the passive components are not overloaded. Metallized film is generally discouraged; oil filled film/foil preferred. UL/FM Global are starting to frown on oil filled capacitors, so those of us that have to try and design custom filters for industrials are running into compliance issues. Then there is the challenge of load variability - passive filters work well when the load is known and constant. Turn circuits on and off, and performance changes, sometimes unpredictably.

If you are willy-nilly installing LC filters hoping for improvement, I would suspect you are either accomplishing nothing or are resonating in bad ways with parallel loads, potentially overloading the filter components and adding distortion. Have you intentionally selected the R of your RLC filter (there is always R present) for proper Q? If not, I would say it is nonideal, and could cause problems.

Not to argue about it, but if you have no measurements (FFT) of with/without filter, then I would bet money you have effectively gotten lucky, and done nothing with the filter addition other than dropping some voltage. You have 117Vrms at the receptacle, feed into the filter, then have 115Vrms at the amplifier. That's all you have accomplished, which might cause your transformer to run a bit cooler. It isn't easy to develop strong resonance in real world residential power systems, so you have avoided the worst case scenario, but have not acquired the goal of 'cleaner' voltage.

If you have a sound card, a filament transformer and FFT software it would be easy to prove with/without performance.
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Old 20th September 2019, 11:18 AM   #16
mindutis is offline mindutis  Sweden
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This long text was dedicated to CLC filters that sits in almost every electronic equipment. The main reason they are there is to fullfill the requirements for some EMC Directives. And they are "standard values" filters. Not calculated for each task usually.
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Old 20th September 2019, 12:35 PM   #17
djmiddelkoop is offline djmiddelkoop  Netherlands
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Hari,

Maybe you are using a Ferroresonant power supply ( aka constant voltage transformer )

Click the image to open in full size.

What is a Ferroresonant Power Supply? - Sunpower UK
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Old 20th September 2019, 07:48 PM   #18
trobbins is offline trobbins  Australia
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Hari, I suggest you remove the line filter. Imho, get to know your audio system better by looking at actual performance, rather than apply a 'solution' when there is no problem.
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Old 25th September 2019, 03:20 PM   #19
Hari is offline Hari  India
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Thanks to everyone for their reply and letting me know about pitfalls with the implementation. What i have done now is added a series of common mode chokes on the line and neutral ( 3 of them in series) which is giving me an inductance of 40mH and a resistance of 0.1 ohms. i have added an interference supression Y capacitor 0.01uF at the input side and output side. No X capacitors between line and neutral. This will reduce any common mode noise and supress any RF and EMI i think.
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Old 26th September 2019, 03:24 AM   #20
wg_ski is offline wg_ski  United States
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Audio gear *generates* strong 3rd and 5th harmonic currents, due to the rectifier power supplies. If your incoming waveform were a little “flat topped” it won’t hurt anything - it will make life easier on your rectifiers by increasing the conduction angle and actually improving the power factor. If there is 3rd and 5the harmonic voltage present (which a flat topped sine will), there will be real power delivered in the 3rd and 5th harmonic currents drawn by the rectifier/capacitor load. The odds actually go up much higher, but most of the reactive power is accounted for in just the 3rd and 5th.
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