Elimination of rather nasty hum?

[Van]

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

Just because the light board is off doesn't mean power is not running through the system. You might not get the Transient hums associated with kicking on the system, or the build up hum you get when fading up or down a cue, but you'll still be getting the basic background 60/50 hz hum because of proximity. I'm hard headed and will stick by my guns, move the cables away from the dimmers, your hum will be reduced.

 

[SHARYNF]

I'm with Van remember that you are still running a lot of power lines to the dimmers, there is just a lot of stray ac interference.

Sharyn

 

[PhantomD]

Sharon I definitely see what you're saying about the transformer BUT the hum is penetrating the line BEFORE where a transformer would be put - in the short unbalanced run between the laptop and the box where all these resistors etc are.

The transformer won't do anything for the hum that's already there from up the line.

*sighs* I have had enough of this. Thankyou all for your help.

Can I ask a few favours of my ControlBooth saviours:

1) Can somebody provide me with a circuit diagram, taking all the above into account, that will do the best job in the situation and explain why it will work.

2)This has worked fine before in other setups with the same amount of unbalanced run, even beside the dimmers and all - except normally just taking the left channel cutting out the right and outputting it straight out into the system. The old cable, as mentioned in my first post, put L and R onto pins 2 and 3 respectively without any sort of resistor setup, making horrible sound and probably damaging the laptop unless I just inserted the TRS minijack only halfway into the laptop (obtaining L channel only).

I am starting to think Sharyn is right about there just being far too much resistance to the original signal.

The same amount of hum was probably always there (as with all unbalanced lines) - just there was such a huge amount of signal that the hum was miniscule.

If we could mix the best of both worlds (SharonF and Cutlunch), come to an agreement about the best resistor size and obtain a final circuit, I shall do it up and report back.

And finally, you guys are the BEST.

 

[SHARYNF]

If you look at the spec;s at rane's site, I'd say about 500 ohms would do it, the large resistors are also dramatically reducing the level of the signal. Do you have the cable to not try to convert stereo to mono and run the two signals independant back to the mixer and use two inputs? If you look at the comments I made about, re using three resistors, two about 500 ohms and then one about 20K it might improve things on your combiner

Sharyn

 

[PhantomD]

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.

Could you please explain this a bit more?

 

[tech_man]

Just a note I use my laptop for running sound all the time and if I have it plugged in to the AC power in my building I get a horrid buzz from the charger (on both normal AC and the sound booths isolated power). The way I fixed it is swapping around the plug so the ground is not connected. Now my theory is that since most laptop power supplies I have seen have only two wires going from the transformer the laptop. Now my thought is that since the transformer supply’s DC Power to the laptop that is no ground path in the circuit. So if I understand this right there is no ground path from the laptop to ground gust the power supply to ground.
Sorry if some one said this earlier

 

[PhantomD]

Same at home with my mobile phone charger and the radio in the lounge.

 

[SHARYNF]

Items with only two prongs on the plug are supposed to be polarized, and also double insulated. So if you have the plug in reversed, you are likely to get all sorts of ac noise

If you go back and look at the Rane http://www.rane.com/note109.html

you will see that in the combiners they use a 10 or 20 K hom resistor across pin 2 and 3, this separates the two and greats a grounding for the signal

Sharyn

 

[PhantomD]

Use of lower value resistors has helped the problem to an extent. I used 2200 ohm variety.

There was still however hum, much reduced.

I am happy to say however I have persuaded the Boss to wire the projector to the booth and have a direct audio input into our mixer (in stereo!).

Thanks guys.

 

[SHARYNF]

that is why I suggested the 400- 500 ohm, what you have is quite a bit of signal reduction with that resistor that you then need to amplify.

Sharyn

 

[cutlunch]

PhantomD on the chanel on the mixing board you connected the convertor to what was roughly your gain setting? Was it below 0 Db no amplification or above 0Db amplification?


I am glad you talked the boss into moving the source to the control booth as this should hope mean no hum induced from the lighting.

 

[SHARYNF]

I know where Cutlunch is going with this, and here is a caution.

Most of the connection on the mixer are probably mic level assuming you are coming in on a XLR connection. Your pc output is consumer line level, so you are about 40db above where a mic pre amp typically is. So in essence what you want to do is have as hot a signal as you can get padded down at the mixer but still not distorting, and them you are looking at minimal gain in the mixer.

What you are sort of doing is getting the noise level in this feed to be down at the overall level of the board, so it becomes less obvious.

Remember you most mixers you have input trim and then the gain on the pre amp, the input trim is mainly to prevent overloading on the input stage of the preamp, and the input fader is to bring the level up to 0 db

Sharyn

 

[Chris15]

Items with only two prongs on the plug are supposed to be polarized, and also double insulated. So if you have the plug in reversed, you are likely to get all sorts of ac noise

It may be worth pointing out at this point that all Australian plugs are inherently polarised by the angled pins, so there is never a danger of connecting them the wrong way round.

 

[SHARYNF]

That the plug is polarized is fine, but really does not eliminate the problem, you would be amazed at how much gear can be wired internally incorrectly, Or any hand/home made cable connection. US has on new gear that is two prong polarization, reduced the problem but certainly did not eliminate it
Sharyn

 

[Chris15]

My apologies. I stupidly forgot that all this is based on an assumption that the supply has been wired correctly... And I agree, any sort of other place where wiring occurs is room for error.

 

[SHARYNF]

The whole neutral and ground can get to be problematic. For sensitive audio type work, the best system is where the ground is isolated and continuous all the way back to the main ground point in the system. In your typical commercial install this is rarely done, since they are looking at grounds for safety and don't care about small currents that cause hum, they just don't want you to get killed.

So I have seen where the neutral is tied to the ground in sub panels, where the only ground connection is via the conduit, which is dependant on how good the screw connectors and physical connections and corrosion holds up.

I have also seen cases where the ground and the neutral are reversed. Many of the cheap testers don't detect this, since back at the main panel feed they are tied together.

This is why on a lot of concert deals, you bring your own distro, and have it wired in to the main panel, that way at least in theory you know everything in the path connection from that panel to your end device.
Sharyn

 

[Andy_Leviss]

If you go back and look at the Rane http://www.rane.com/note109.html
you will see that in the combiners they use a 10 or 20 K hom resistor across pin 2 and 3, this separates the two and greats a grounding for the signal
Sharyn

Actually, that resistor is a shunt, to keep the output impedance in the range the input expects to be seeing. It doesn't have anything to do with grounding.

 

[SHARYNF]

I think if you look at it, technically you have a floating output, and that that resistor helps create a floating ground. In a transformer setup you have a balanced output, in this system you have what would be called a floating output, since it is not referenced to ground but at the same time it is not truly referenced to itself in a balanced design

Here is a link not to a direct design, but gives more information about what I am talking about

http://sound.westhost.com/balance.htm#bal-vs-float

WOOPS

Sharyn

 

[cutlunch]

SharynF that link seems to be missing from your last post.

 

[Andy_Leviss]

I'm still not following you in reference to the Rane circuits, Sharyn. In all the ones I'm seeing, sleeve/pin 1 is connected straight through to sleeve/pin 1, so there is no floating ground. The shunt resistor is only between pins 2 and 3, the signal pins, so they have nothing to do with grounding.

There's a technique called forward referencing that involves a different wiring scheme to adapt from unbalanced stereo out to a pair of balanced inputs to cancel out hum, but that doesn't involve resistors at all (and again pertains to going to two outputs, not summing them down to mono). If you're a Syn-Aud-Con member, there's an illustrated article on the website by Rick Chinn explaining this, otherwise, I haven't yet found a good explanation in the public domain, although I'll keep looking.

--A