Elimination of rather nasty hum?

[Andy_Leviss]

First, I'm going to skip over the balanced/unbalanced and combining mono aspects of this, since that's been aptly addressed in previous replies. So let's deal with the buzz. As a quick fix that will both confirm that the buzz comes from the laptop and get you through a show, try unplugging the laptop and letting it run on battery power. Many laptop power supplies are notoriously noisy, and inject hum into every part of the laptop. Often letting it run on battery power will clean it up.

--A

 

[cutlunch]

The problem with this last approach is that on the on balanced pc side, the ground and shield are connected, but on the balanced side, the mixer thinks it has the differential signal on pins 2/3 and that the shield on pin 1, SO the actual signal processing is different, and if there is any noise on the ground side of the unbalanced side it is going to cause you a problem.
Sharyn

Sharyn as far I know the standard soundcard output is unbalanced not balanced. If it was we wouldn't have so many problems and could use that rane note 109 that has been posted earlier. In my last post I stated that the shield is not to be connected to the PC.

 

[Van]

I still say your nasty hum is Inductive dissonance from the dimmers. If you're too close to a SCR system, I'm assuming they are SCR's of some sort, there is no way to remove3 the noise unless you a move the box or B. completely sheild the box.

 

[cutlunch]

I'm also wondering if an addition of a diode to the wiring of the device might not eliminate so possible issues with feedback, < electronic not audio> back into the outputs of the sound card.
I agree that using some external, possibly USB, device might not also help, Getting a clean signal in the first place is an issue.
Quote "Eboy87I highly reccomend that you do NOT lift the AC ground, people are killed that way.
I've never heard of anyone being killed by lifting the ground on Audio equipment, but I guess it could happen.

Van although you haven't heard of it doesn't mean it hasn't happened. Unless you can get hold of a detailed coroners report that states the earth was lifted it would normaly be put down to faulty wiring.

Musicans have been killed on stage when their microphones have gone live. This may have been because the earth wire came loose or was deliberately disconnected. Most of the time the shield on connectors land up being connected to the metal chassis. The electrical earth is also connected to the chassis. If you disconnect the electrical earth and the live wire comes into contact with the chassis it is now live. Because you have disconnected the electrical earth the fault won't be detected by the blowing of a fuse or circuit breaker. So now someone touchs the chassis and or the shield on a connected lead and they can become the path to earth and death.

So as you can see lifting the AC earth ( ground) can be deadly and should not be done.

The safest way to isolate sound equipment from a mains earth hum is to use a safety isolating transformer. This is if using DI boxes haven't being able to remove it at the signal ground level. Theorecticaly you could still get a shock but your equipment would have to have two electrical faults on the mains wiring at the same time. In which case it is quite unlikely that the equipment will actually work.

Please don't confuse ground lift at the signal level with lifting the electrical AC earth. Ground lift generaly refers to disconnecting the signal ground between two pieces of equipment. When using balanced systems the ground is not required to provide a signal path for the audio. In fact when it is connected it may become path for hum. That is why a lot of DI boxes have ground lift switches.

Just a cautionary note to us older members, including myself. I believe when it comes to safety we should always err on the side of caution in any post we make on this board. With experience we may be able to judge the level of risk involved. But this doesn't mean we should. What is the old fighter pilot saying " there are old pilots and bold pilots but no old bold pilots". However less experienced members may not and take it that if he says it works then ok.

I can remeber hearing years ago about airplane crashes that it normally is not one thing but a chain that leads to disaster.

For a recent example the Rhode Island tragedy. For example if the proper flame proof insulation had been used then maybe the pyrotechnics might not have started such a big fire so quickly. What if lower level pyro's had been used.

There is a discussion about fire safety " fire marsall" going on at the moment. In it people have mentioned wedging doors open to move stuff or because they haven't proper cable runs installed. This maybe fine 99% of the time but it just needs something to go wrong that this compounds the problem. The same with empty fire extinguishers, hose reels blocked. It all adds up.

Sorry for the rant but there have been a few safety issues raised on this board in the last few days.

 

[PhantomD]

So to reiterate again, the sleeve (ground) of the jackplug is to be connected to pin 3 of the XLR.. The resistor output is to be connected to pin 2 of the XLR. At the convertor end there is nothing to be connected to pin 1 of the xlr.

This is definite what I should try, then.

@ Van: I have iPRO dimmers (very HQ): http://www.lsclighting.co.uk/iseries.html

I have some kind of transformer here that came out of a Dick Smith kit. It has three legs on one side and two legs on the other.

I will leave this open for discussion before making a final decision - because every time I do stuff I get less and less lead to play with.

One of my friends said to make sure the Rane thing was using the standard XLR system - apparently there are two others, one of which is Japanese. He said he'd been running a system where some video camera guy had plugged his audio into the mixer and it killed the whole thing - the ground was swapped.

 

[PhantomD]

As a quick fix that will both confirm that the buzz comes from the laptop and get you through a show, try unplugging the laptop and letting it run on battery power.

Already tried with a batt-op CD player, still hums. Thanks anyway.

 

[cutlunch]

PhantomD

I think is unlikely that your mixer would have pin 1 on the XLR inputs being anything other than Cable shield (chassis ground). But if you are unsure then check in the manual for the mixer, they normaly show a diagram of the input wiring. If there is any difference then it will be that pin 2 and pin 3 sisgnals are swapped over. In your application this won't really be a problem.

If was doing this project in your situation and couldn't afford the DI box then yes that is the wiring I would try. I am not gauranteeing it will lower the hum but it should have some effect. If it doesn't it will really need a DI box.

The dick smith transformer would probably be unsuitable. If you can find the information on it check the impedances. It will probably be something like 8 ohm : 1k ohm or 3kohm : 3kohm. For this project we really need something like the latronics one which is 10Kohm ( for the laptop side) to 600 ohm for the mixer side.

Have you talked to anyone at school like the music teacher to see if they have DI box they can lone you.

The silicon chip DI box would not be that expensive to make and could be made in a couple of hours dependinmg on your skills. Even if you didn't use the full circuit you could make it with seven joints and no pcb required.

 

[SHARYNF]

Sharyn as far I know the standard soundcard output is unbalanced not balanced. If it was we wouldn't have so many problems and could use that rane note 109 that has been posted earlier. In my last post I stated that the shield is not to be connected to the PC.

TYPO meant to be UNBALANCED on SOund card, and Balanced on the mixer, to any late nights

Sharyn

 

[Chris15]

@ Van: I have iPRO dimmers (very HQ): http://www.lsclighting.co.uk/iseries.html

No offence, but just because they are high quality dimmers (and I agree with you there), does not mean they aren't the source of the problem. Certainly low quality dimmers will be worse, but all dimmers output EMF. The triacs or SCRs cut the waveform to do the dimming. This creates electrical noise - causing EMF. Then to reduce the effect of this on the lamp, the output goes through an inductr. This will also create some more rubbish EMF. Add these together, mulitply by 12 or 24 or however many dimmer channels are in proximity and you start to see how much rubbish EMF is around that will play havoc on the audio.

 

[Van]

No offence, but just because they are high quality dimmers (and I agree with you there), does not mean they aren't the source of the problem. Certainly low quality dimmers will be worse, but all dimmers output EMF. The triacs or SCRs cut the waveform to do the dimming. This creates electrical noise - causing EMF. Then to reduce the effect of this on the lamp, the output goes through an inductr. This will also create some more rubbish EMF. Add these together, mulitply by 12 or 24 or however many dimmer channels are in proximity and you start to see how much rubbish EMF is around that will play havoc on the audio.

I'm glad you brought that Chris up because of my earlier posts I didn't want to seem argumentative.

No matter the quality of dimmers, the fact remains that you are creating EMF pulses which create inductive vltage in other wires nearby. Even IPS strips which do not use SCRs and which do not use inductence coils can induce humming in a sound system. I looked at the specs for the dims in question, and although I could not find any internals it appears to me there is more than enough space for them to have hidden coils inside. In your original post you stated that moving the box isnot an option can I suggest powering down the dimmer pack then checking for hum ? It could be that your only going to gt the hum when the dims are moving up and down ie flucuating the power in the cables., However it is also quite possible that you biggest issue is just standard 60 cycle hum comming off the inductance coils .

 

[PhantomD]

I shall swap the ground/shield around to pin three instead of pin one and un-bridge the resistor output to be to pin 2 only.

I shall tell you how this turns out.

Will obviously try without dimmers on - I think the old circuit worked fine without dimmers on.

 

[SHARYNF]

while I am a fan of using a transformer, I think you have to make some mods to your diagram to make it work in your application

The 10k resistors are IMO too large, and will reduce the signal too much and cause you to need to amplify at the preamp increasing your noise, I would suggest 470 ohm or some where around there, not overly critical but this is OHMS not Thousands of ohms

To create a floating ground you then need to add a 20K (twenty thousand) ohm resistor across pin 2 and 3, leave pin 1 unconnected.

What you are likely to get with your present design is something that may or may not work and will vary based on a variety of conditions that all change. again A 50 dollar transformer isolation box will save you endless grief and give you a reliable connection

Sharyn

 

[cutlunch]

I shall swap the ground/shield around to pin three instead of pin one and un-bridge the resistor output to be to pin 2 only.
I shall tell you how this turns out.
Will obviously try without dimmers on - I think the old circuit worked fine without dimmers on.

Ok Phantom there is stills seems to be some misunderstanding judging by the diagram. I appologise for any confusion I caused.

You have it basicaly right except you don't use the braid/ shield to carry the ground to pin 3. The ground should be carried by one of the insulated conductors inside your cable. It does still connect where you have it shown.

One question are you making this up in box that has a jackplug in one end and xlr plug the other? Or are you wiring the jackplug and resistors straight onto one of the cable permantley?

If it is the latter then I would solder the shield/braid at the Xlr end by the mixer to pin 1. But make sure the other end of the braid by the jackplug is not connected to anything. This way hopefully interfence generated near the cable will drained away by the shield instead of getting into the two audio conductors. You can try this without the shield/braid unconnected if you want but if you connect the braid you should have less interference.

Sharon is right you could lower the values of the resistors but I would not go any lower then 2k2 ohms. This is because the resistors are there to prevent the signal from one channel of the soundcard damaging the other channel . The lower the value of resistor the more current flow back into the soundcard. For now I would not change them unless you find the voulme is to low. At the moment the hum is the main problem. Good luck. PM me if this is to confusing.

 

[PhantomD]

You have it basicaly right except you don't use the braid/ shield to carry the ground to pin 3. The ground should be carried by one of the insulated conductors inside your cable. It does still connect where you have it shown.
One question are you making this up in box that has a jackplug in one end and xlr plug the other? Or are you wiring the jackplug and resistors straight onto one of the cable permantley?

Volume is fine, hum isn't.

Currently there is a 4-conductor shielded cable going into a little plastic box on the wall near our dimmers.

Inside the box are the resistors/etc and the XLR jack penetrates through a hole (secured).

So I should use two of the conductors for audio, when they get into the box do the resistor thing and have an output to pin 2.

The shield is not connected on either end.

Use one of the other two conductors as a ground and connect a) where the shield currently is on the minijack and b) on pin 3 XLR.

Thankyou all for your help.

 

[SHARYNF]

HI what was the cable used for before? was it an AMX dimmer control feed that you are using the cable from. If you are trying to run a long distance via the various diagrams you have been shown then you are going to get hum unless you use the tranformer. The converter deals that you have been given are only good for short distances say under 10 feet at most.

Running an unbalanced feed over a long distance is almost a guarantee for hum.
Sharyn

 

[SHARYNF]

Ok
Sharon is right you could lower the values of the resistors but I would not go any lower then 2k2 ohms. This is because the resistors are there to prevent the signal from one channel of the soundcard damaging the other channel . The lower the value of resistor the more current flow back into the soundcard. For now I would not change them unless you find the voulme is to low. At the moment the hum is the main problem. Good luck. PM me if this is to confusing.

I don't think the flow back is an issue, what you are doing is creating a pseudo high impedance outinput with the resistors you have chosen it is hard to tell how much it might affect the frequency performance of the system with the additional loading.

From the other message I wonder just how long the cable is from the input to the mixer? long runs unbalanced is just asking for trouble and the diagram you have given really does not make it a balanced circuit with the noise canceling characteristics but really just a way to connect over a short distance an unbalanced signal to a balanced mixer

Still say getting a proper converter will save all sorts of grief work well, and not be subject to continual problems. It is one thing to make up a cheater cord for a quick fix it is another to make this part of an install which is what MIGHT be the case
Sharyn

 

[PhantomD]

How can I wire it so that it DOES have the noise cancelling characteristics (what I want).

I thought that with XLR the pins 2 and 3 had different polarity outputs and any interference will be the same on both pins therefore anything the same is cut out while anything different is output.

Am I correct?!

Therefore I was at a loss as to why bridging pins 2+3 (same signal) worked, albeit with a hum when the dimmers were on.

And now this circuit has confuddled me even more.

 

[Chris15]

How can I wire it so that it DOES have the noise cancelling characteristics (what I want).
I thought that with XLR the pins 2 and 3 had different polarity outputs and any interference will be the same on both pins therefore anything the same is cut out while anything different is output.
Am I correct?!
Therefore I was at a loss as to why bridging pins 2+3 (same signal) worked, albeit with a hum when the dimmers were on.
And now this circuit has confuddled me even more.

My guess is that the hum might be being induced into the piece of unbalanced cable coming into this box.

A proper transformer will give the desired noise cancelling ability as far as I know...

 

[SHARYNF]

This is what I have been trying to get across, probably because I have not been presenting it correctly or clearly

In a balanced line the two lines (pin 2 and 3) each carry the same signal it is inverted 180 degrees out of phase so that the signal that is picked up in the cable is canceled out and the sound you want is passed thru. In a balanced feed the two signals are referenced to them selves and the shield is only serving as a shield and does not carry any audio signal. When you make up these cheater cables you don't have that ability, The signal is referenced to ground but you are cheating and making this a pseudo inverted signal feed, so when you try to use the noise canceling aspects of balanced it does not really work.

The transformer isolates the two systems but also takes the signal and splits it across pin 2/3 referencing one to the other, and allows you to do the phase inversion. In a very short run under ideal circumstances you can get away with it and converting from Balanced to Unbalanced works pretty well. The reverse when the lead to the mixer from the source is long looses this noise rejection capability. The primary rejection comes from the referenced signals that are inverted and cancel out the noise, the secondary is that the shield operates as a Faraday cage, (where a metal cage with any voltage serves as a barrier for radiated energy) and takes any of that signal and simply returns it to the ground with no connection to the actual signal.

A long cable is in essence a receiving antenna. There have been lots of discussions about all this in regard to interfacing consumer (unbalanced gear) with professional balanced gear. It is a nightmare, since some consumer gear has double insulated 2 wire power systems some have three wire is chassis ground used for the signal or is it isolated, it goes on and on. The general consensus in the pro audio world is to take every unbalanced device and use a simple isolation transformer for all the interconnections, saving endless time trying to find out where the noise is originating.

Sharyn

 

[harry1989]

If your sound system and lighting system are coming from the same power source, you'll get a hum.

Would it matter that the lighting board isn't on half the time that we use the laptop?