Mains filter/conditioner schematic.

[Mark56]

Hi, I play in a rock band in Spain. The power supply in some venues has often caused an issue with bad hums and noises coming through our PA system. The voltage is usually around 230V (but can vary) and we manage to run all our present equipment from a 16A supply which is normal here for socket outlets.

I would be most grateful if anyone could post a schematic for a mains filter that will guarantee the removal of any hum or unwanted noise from the supply.

 

[Alec_t]

If the hum is more noticeable in some venues than others then it could be due to inadequate earthing of the mains installations.
A filter to suppress 50/60Hz would need to be very narrow band or else wanted bass frequencies would be affected too.
It would help to locate the source of the hum pickup if you could check whether moving your PA kit from one part of the venue to another has any effect on the hum.

 

[Mark56]

Hi Alec_t, thanks for the quick reply. Unfortunately moving a fully set up band and the PA system around a venue would not be practical to do and we have played all summer out in the open air at many different bars. There are just a couple of places where we got a hum through the power supply. The worst place we had it, it was intermittent and very loud, never did find the cause, can only add they had a lot of electrical equipment, fluorescent lights, fridges and freezers and air con. We were playing outside of the building with nothing other than some light bulbs within 30 feet of us.

We have also played in a place where there was no earth connection, this was clear to see in the large supply box hanging from a rope around a tree (the earth terminal was not connected to anything) In that venue we had no noise at all.

Not sure what you mean about affecting bass frequencies, I'm not looking to filter the audio to the PA system. I just wanted to build a unit we can filter the mains supply with to ensure we don't get any repeat of the problems we had at the venue I mentioned above which was clearly something in the bar causing a lot of noise to come through our PA. On that occasion we swapped out the PA amp for our spare, it got quieter, but didn't get rid of it completely. In 99% of places our PA is completely quiet apart from very low level white noise.

Reading between your lines are you suggesting this could be RF interference?

 

[Greg J.]

Newb reply (corrections desired). I believe he is referring to hum that might be coming from the main power supply. The normal Spanish 230V, 50 Hz power alternating current can be heard under some conditions. You can probably recognize whether your hum is at 50 Hz or not. A bass guitar at normal tuning will pass through 50 Hz exactly if you bend the E string 3rd fret, G to G#. So filtering out all 50 Hz "hum" the wrong way would also filter out some of the sound the band is playing. Identifying exactly what is causing the hum is the first step in eliminating it without using a filter that could affect your band's intentional 50 Hz notes. (There also may be overtones which you want to hear and some you don't.)

 

[Alec_t]

Reading between your lines are you suggesting this could be RF interference?

No. You can't hear RF frequencies. Hum is usually at mains frequency (or a harmonic).

 

[Mark56]

Thanks for the replies guys, I don't think we are dealing with a straightforward mains hum as that is filtered out by the smoothing capacitors in our PA and guitar amps and if there was a problem with any of them we would get the hum at every venue. And as you have stated it can't be RF interference.

So, as I stated earlier we had an intermittent and quite loud hum at one venue with only minor problems at a couple of others so it is more than likely due to a dirty mains supply. Would you agree with that? Could this be caused by an electrical appliance? a bad fridge or freezer thermostat that may be arcing badly while opening and closing perhaps. Obviously the band cannot fix the cause if it is a dodgy electrical appliance so what we need is a mains filter that will clean up any interference on the mains 230V supply so no matter where we perform we won't get this problem again.

 

[Bluejets]

You might try running your systems off a UPS that gives isolation.

Could be a rather large unit though. You will need to find what the power requirement of your system is under max conditions.

 

[Greg J.]

Is it possible that one of your pieces of equipment (or house equipment) is dirtying the line only under certain conditions? Btw, you could just try running 1-2 pieces of various equipment off a small UPS for test purposes.

 

[Alec_t]

it is more than likely due to a dirty mains supply. Would you agree with that?

It's a possibility. But 'dirty' mains implies frequencies higher than 50/60Hz. What hum frequencies did you experience?
Another possibility is a defective screen (e.g. an intermittent break or loose connection) on a mike/guitar cable, allowing mains hum pickup.

 

[Martaine2005]

Some good points mentioned above.
But it's only some venues!
More than likely that the Earth cable has been tapped into from another circuit or room/location causing a difference in the ground reference for the circuit you are using.
If this is the case, there really is nothing you can do. Maybe a generator, or run a long extension from a tested outlet that doesn't hum.
Also, if one of the venues has NO earth, refuse to play there. A guitar or mic can be lethal if a fault occurs.
Instrument amps must be earthed. Not sure about new ones, but always used to be.
Many artists have died from improperly grounded (earthed) equipment.

Martin

 

[Greg J.]

Some good points mentioned above.
...
Also, if one of the venues has NO earth, refuse to play there. A guitar or mic can be lethal if a fault occurs.
Instrument amps must be earthed. Not sure about new ones, but always used to be.
Many artists have died from improperly grounded (earthed) equipment.
...

Would a UPS be protection against that? All but the cheapest models claim decoupling, but I don't know how they do it. I have never felt like there is a transformer inside mine.

Mark56, you might get some more practical insight by hunting down an old sound reinforcement guru.

 

[Martaine2005]

Would a UPS be protection against that?

No, because it's still an AC source.
A good piece of additional safety would be to use a GFCI. A Ground Fault Circuit Interrupter. In the case of an earth fault occuring, it will shut off the supply in as little as 1/40 secs.
Have a read here.
You shouldn't leave home without one..

EDIT: It would help running off the batteries, but they wont last five minutes with all the equipment.

Martin

 

[Greg J.]

...
EDIT: It would help running off the batteries, but they wont last five minutes with all the equipment.

Martin

Why do you say that?

As an aside: Would a 1:1 transformer be any protection?

EDIT:

I would be most grateful if anyone could post a schematic for a mains filter that will guarantee the removal of any hum or unwanted noise from the supply.

Perhaps what you are looking for is a "power conditioner" product or schematic. If you'd have to wind your own transformer, perhaps the product is not so bad a choice. (I don't know to what degree this would address safety issues, though.)

 

[Alec_t]

Would a 1:1 transformer be any protection?

Yes, but it will weigh a ton! Tot up all the Watts demanded by your equipment at full blast to see what sort of power the tranny would have to handle.

 

[Mark56]

Thanks for all your recent posts and the advise given. As I am an electrician I am well aware of the dangers of using equipment with no earth connection, but thanks for the warning. I know we took a risk, but won't do it again if we go back to the same venue and they have not improved their earthing. All the bars we play in need to have there electrical equipment periodically checked for safety so, in general, we are quite safe. All electrical installations in Spain are protected with RCD's as standard.

The one really bad venue where we had an intermittent loud hum on the PA was almost definitely coming from something within the building and it is just that kind of situation I wish to build a unit for so it cannot happen again, something like this: http://www.chrisbrooksaudio.com/furman/furman-mains-conditioners.html
As you will read in the details, this unit protects against dirt in the mains supply produced by motors, fridges, freezers, etc and although intended mainly for computer equipment can be of great benefit to sound systems by cleaning up and regulating the mains supply.

I am quite sure the hum was not general 50Hz mains hum due to bad screening and from recollection it was above the mains frequency, but I can't possibly state what the frequency was. The noise was cycling on and off at random every minute or two, sometimes longer, and happened when no one was touching or moving any or our equipment, my best bet would be a fridge or freezer with a bad thermostat or faulty condenser on the compressor. If we performed there on a regular basis we could track it down and have it fixed but that isn't the situation.

Some UPS systems I have seen the specs on do include some form of voltage regulation, but not filtering to remove dirt, I am fairly certain the isolating transformer used would not filter out any dirt, it would simply pass it through. We also need about 3KW which is a tall order for a UPS, it would be huge and very costly.

So, going back to my opening post, it is a schematic I am after for a mains filter/conditioner. I have searched Google for quite some time and only found ready made units, but need to keep the cost down by building my own. Guess if I hit a dead end we will have to save for longer and buy a ready made unit.

I really do appreciate all the help you have offered so quickly, this is a great site.

 

[Greg J.]

Glad you took back control of your thread. Presumably you wouldn't want to get a single UPS for all your equipment. The lower priced ones are manufactured in larger quantities and each would weigh less and be easier to move. You can get a few thousand volt-amps (or watts) for $300 - $500 USD if spread across 3-4 UPSs for ones made for computers. Some might even be cheaper than building your own. I don't know anything about the ones made for audio systems. I'm sure they're more expensive if only because of their audiophile target market.

The right UPS shouldn't pass the dirt from their power sources, since they are using mains power to charge a battery and giving your equipment power from the battery. One of the purposes of all but the cheapest UPSs is to provide decoupling from the mains.

The link you provided for the Furman seems to say that it is not the same as what you would use for a computer. Attempting to read between the lines, it appears to be a conditioner that provides greater headroom than one made for a computer as well as a faster slew rate. I don't know if the ones made for computers would cause a noticeable degradation of sound. However, it should be relatively easy to test for yourself, since lots of people have computer UPSs that you might borrow. Check if the sudden loud notes are as crisp as you're used to. If I tried it, I would only try a computer UPS rated notably higher than what my equipment called for, however--or at least make a point of not stressing it.

Perhaps look for a UPS schematic? It very well might not state all the specifications you want, but I'm sure people would enjoy helping you modify it for rock band use. Coincidentally, I just ordered a replacement battery for my 850 VA UPS and it was only $20 USD.

 

[Mark56]

I do understand where you are coming from and thanks for posting again. But, we really don't need what a UPS will do even if it could successfully protect against mains noise as we would need several of them which will cost more than a single 20A mains conditioner which is all we need with our present set up. Having the back up power from a UPS in the event of the mains failing isn't a priority. Prices start at around $350 for a good quality 20A mains conditioner that has all the required noise and spike filtration without taken up the space and more financial outlay that would be required for a few UPS units. Everything I have read on the subject (and I have read a lot) recommends mains conditioners and not UPS units for high powered sound equipment. UPS units are great for home use to protect PC's, TV's and household electronic equipment and should be a 'must have' for sound recording so if the mains fails you have time to back up data before it is lost. The same goes for a desktop PC, but obviously laptops have their own battery to run off if the power fails.

I think you will find that a UPS provides power directly from its transformer to the connected equipment and only switches to the battery power via the inverter when the incoming mains supply fails or with other types it constantly provides the output power from the inverter which is permanently connected to the battery and the charging circuit, only switching to battery power alone when the supply fails. Some units also switch over to the battery when the supply voltage goes a long way off what it should be. I can't quite see how you could run the equipment from the battery while it is being charged, without there being a physical connection to the transformer. You would only have the output power coming from the battery alone when the input power is removed.

A mains conditioner provides an output that is clean of any dirt, a smooth sine wave that is regulated to stay at the required voltage irrespective of any dirt, spikes or voltage fluctuations that are present at its input. All the UPS units I have seen do not do the same job (or not as well) and it is only mains conditioners that are used/recommended for high powered sound systems.

As I have had no luck finding a schematic for a mains conditioner I think we will have to buy a ready made unit. Schematics are easily found on the web for UPS units, but not so for mains conditioners.

My next step will be a forum for musicians to get some recommendations from band members that use one. That seems like the best way to go from here.

Thanks again to everyone that has tried to help.