Analog vs. Digital

[cutlunch]

The major problem is that there is an annoying persistant hum that comes through the speakers. We have narrowed it down to a wiring problem because when we take the same equipment and use it off site with new wiring it works perfectly. Also, all the power for our sound and lighting equipment is earthed at the same point - from what I've been reading that causes ground loop problems...I may be wrong but that's my understanding. The hum is annoying but not so bad that we can't use the sound system.

To help locate the problem a little bit more info is needed.

Are the amplifiers in the control booth with the mixer or are they backstage?
Also does the hum change as you change the level of lights especialy when the levels are below 40%. If the hum doesn't change then it's not the dimmers dirtying up the power supply.

Another thing you can try is bring in an electrical isolation transformer. Ie the type to stop you getting electrocuted when working outside. Make sure it has enough current rating and plug the sound equipment into it. This will isolate it from the lighting gear. See if this helps.

You say you take the equipment and it works with new wiring but you don't say what new wiring. Is it just the leads from the speakers or do you change leads from the mixer to the mixing desk.

You don't say where you think the faulty wiring is from mixing desk to the amplifiers ( via any processing). I would start by disconnecting the amplifier input leads then turn the amp up and see if the hum is still there. Then step back to the mixer. If you have any processing gear in between take it one piece at a time. Is the hum on only one channel or both? If it is only one swap leads to see if you can isolate wiring that is faulty.

Does the hum only happen when you have a certain channel on the mixer up. If so what is connected to this channel. Swap the input to a different chanell and re-check.

If you find a lead at fault either replace it or if it is fixed wiring check the soldering on each end. Also use a meter to check for open and / or circuits.

While I was doing this I would keep a log of each step. This way you can go back to the powers to be and say you have found where the problem is. If you can specify where it is they are more likely to pay to repair it. If you just go to them and say it's in the wiring but not where they have no idea of how much it is likely to cost.

Also they will listen to you more if they can see you have thought out this problem in a logical manner.

Also if you can isolate the faulty wire you maybe able to convince the Music Teacher to hold off on a new desk until the wiring is fixed. You maybe able to say tactfully to them that if we get this new desk and it doesn't fix the problem we will look like fools.

Also think of this as a good learning oppurtunity. If you continue in theatre this is the sort of problem you will have to solve half an hour before a show opens.

 

[mcgart]

I'll have a stab at replying to those things you've mentioned above. Just before I start, I'll just say that I'm relatively new to this sort of stuff so I may not give the proper run down of the problem.
We don't use any amplifiers as all our speakers are powered. Our setup is as follows: Sound from the board is sent first through a reverb processor and routed back in through an FX return. That mixed sound is then sent through a graphic equaliser where it is finally sent from the control booth at the back, about 20 metres to the FOH speakers. Similarly, we have monitor and bass speakers running out of monitor and aux sends, but they do not run through the equaliser. Instead they run straight from the booth to the speakers, also about 20 metres away. We receive hum out of all the speakers.

When I move the sound board and speakers (minus the equaliser) to another location, and rewire both the power in and lead connections between board and speakers, the hum is not present. And I am pretty certain it is not the equaliser, as on one occasion I took that offsite too and it didn't add any hum.

Just thought I would provide a general overview of our setup. At the next chance I get, I will run through some of those troubleshooting options you described.

As for the dimmers affecting the sound, there is no noticeable difference when they are over 40%, so I am beginning to think they may have nothing to do with it.

I'll keep you posted with my results, but any other suggestions would be welcomed....thanks for the start Cutlunch

 

[Eboy87]

See if you can find a circuit(s) that isn't feeding the dimmers, and plug the gear into it. See if that gets rid of your buzz. I agree with whoever said there's a ground loop problem: dimmers + audio + same circuit = a world of bad (ok, not a world, but it ain't good).

 

[cutlunch]

Mcgart you did a great job of describing the setup. It has given me some more specific clues. If you get time, posting the make & model of your equipment may be helpful but we can work without it for now.

Where do the speakers get their power from? Is it from power points on stage or do you run extension leads all the way from the sound desk. If the speakers are powered from the stage I would try plugging them into to the same power point as the mixing desk. This is just a temporary idea so you can use an extension cord and only connect one or two speakers so you don't overload the circuit.

If this makes a difference you now know the problem is in the power supply. Now for some solutions:

1) I have already mentioned using an electrical isolation transformer. Borrow one to try this out. Put the transformer at the mixing desk and feed the desk, reverb, eq from it. Does this make a difference? If not try powering the speakers through the transformer.

I would probably try the transformer first. I would would be fairly confident this should fix the problem.

2) If it doesn't or you can't get a transformer then I would try modifying the audio cables that feed the speakers. You will either have to be able to solder yourself or have someone who can help you.

I would start with only one speaker and lead. I am assuming that you are using XLR leads. Go to the XLR plug that plugs into the back of the speaker. Take the cover off. You want to disconect the wire ( shield) connected to pin 1. The lead could be just a collection of bare copper wire or it may have some insulation around it. Don't cut it, unsolder it. If this same wire also appears to go to a little metal tag that is connected to the metal case of the plug disconnect it . You don't have to put the cover back on the plug yet if you are careful. Plug it back into the speaker and see if the hum is still there. If it has gone then you can cut back the shield and tidy up the plug. You will need to do this on the lead that plugs into each speaker. But first check that the speaker doesn't have a ground lift switch. That is the eqivalent of what we did by unsoldering the lead.

In fact you probably should check the speakers for one of these before you disassemble the plug. If the speaker has a ground lift switch then put it to the ground lift position and see what happens to the hum. If the hum goes away this is the answer. Some of your speakers may not have a ground lift switch so you will have to modify their input leads as above.

If neither the transformer or modifying the cable (ground lift) reduce the hum then you must have a very unusual problem. Then you will have to get outside help in.

For the isolation transformer either the Shop Class or the janitors may have one you can borrow. If the leads and plugs look to damaged on it it might pay to get it checked by an electrician before using it.

 

[SHARYNF]

Just some clarification
In general having both the mixer and processing equipment and the amps all connected to the same power source and ground will eliminate the hum.

In your setup I don't think the problem is going to be solved with a transformer on the ac supply.

In general it is a combination of the equipment in the booth and the powered speakers not being on the same power source. I would definitely try the same power to both, so make sure you plug in the powered speakers, and the mixer and all the procession all to ONE circuit. This then has everything on the same power source and ground.

The transformer that you could look at would be more likely to be an isolation transformer on the line feeds to the speakers, again not likely to be on the ac side of things, especially since the dimmers don't seem to affect it.

It is also possible that you have a cable where one of the connections in the cables has gone bad, so I agree open up the connectors and check them, you might want to get a cable tester.

All this assumes that the cable being used is balanced two wire with shield and usually xlr connectors. If for some reason it is only two wire including shield than that is probably your problem. I have found that connecting the mixer/processor rack and the speakers usually fixed the problems, typically there is a ground connection problem in the power system that is weak or has a grounding point somewhere else along the line between the outlets and the main panel breakers

Sharyn

 

[Schniapereli]

So, is it better to have a balanced TRS cable on just a regular wireless lav. What's better with the signal (since it is just MONO) as opposed to an unbalanced TS cable? Is the hum the only benefit? When would an unbalanced cable be used. (do they just make them because they are cheaper, or are they preferable in some cases?)

When are Digital Mixers used? I know the Cadac J-type is an analog mixer, and is the standard for broadway theater. Do they use digital ones for rock groups, or stuff like that?

 

[Chris15]

So, is it better to have a balanced TRS cable on just a regular wireless lav. What's better with the signal (since it is just MONO) as opposed to an unbalanced TS cable? Is the hum the only benefit? When would an unbalanced cable be used. (do they just make them because they are cheaper, or are they preferable in some cases?)
When are Digital Mixers used? I know the Cadac J-type is an analog mixer, and is the standard for broadway theater. Do they use digital ones for rock groups, or stuff like that?

A balanced TRS cable is a seperate matter to stereo or mono. Could you please clarify what you mean when you say "just a regular wireless lav"? Do you mean the input cable to the transmitter or the output cable from the output cable from the receiver? Balanced cables are not used for inputs in my experience. The transmitters very rarely have balanced inputs. The output from the receiver, supposing that we are talking about radio mics other than the absolute bottom of the line, I would be expecting a balanced output on an XLR. Some will have a 1/4" TS output as well. Given the option, why wouldn't you use the XLR output? Unbalanced cables are NORMALLY fine for short distances, but for long distances or in environments with high electrical noise levels, balanced cables are better.

 

[mcgart]

Thanks for the detailed reply!

The speakers all get power from different points on stage - totally seperate from where the sound console is drawing power from. Your solution of using them off the one power source sounds promising as I remember when I took the equipment offsite and plugged it off the one source the hum was gone.

I have read on several websites that removing the connection of Pin 1 can help with the hum. I haven't tried this yet but next chance I get will give it a go. I am not entirely sure whether our speakers have a ground lift switch as I couldn't find one when I looked. We currently use Meyer UltraSeries speakers in our theatre.

Thanks for the advice...will implement it and get back to you.

 

[cutlunch]

Mcgart in my earlier post to you I mentioned trying out powering the speakers via an extension lead from the sound desk power supply. I probably should have expanded on the point more.

As SharynF says running everything from the same power source is the ideal. But it poses some questions for you to get answers to. If you are going to do this then first you need to know the total power load for the equipment. This should be just a matter of adding up the loads marked on the equipment. Then the harder part is finding out what load the circuit you want to connect to is rated for. You might need to talk to the schools electrician (electrical contractor) . The electrician may also be able to show you if any of the stage points are on the same circuit as the sound desk. If you are lucky they maybe able to re-wire a stage point to this circuit so you don't have to run extension leads.

 

[Schniapereli]

I'll rephrase the question.

We don't have enough XLR cables to connect all of our Shure wireless receivers to the Mixer. We use as much as we can though. We have a few TRS cables, (but not much), but we have a lot of TS cables (which are also conveniently short). What sound quality is sacrificed from using a TS cable connecting the receiver to the mixer as apposed to using a TRS cable? (Since The signal is MONO, and the cord goes about 3 feet...)


(...and, if anybody knows, if they don't use digital mixers on Broadway, do they just use them for music/rock groups?...)

 

[Eboy87]

(...and, if anybody knows, if they don't use digital mixers on Broadway, do they just use them for music/rock groups?...)

Who says they don't use digital on Broadway? I know Spamalot is using DiGiCo's D5T, Bombay Dreams had a PM1D. I've seen other tours come through where they've mixed the pit on a DM2000.

I still prefer analog boards at this point, it just seems counter intuitive to me to only adjust one channel at a time like you do on most digital boards, but they make up for that in recall ability.

As far as the 1/4" TS vs. XLR, I'm sure that if it's a short run, it won't matter too much. But, and someone correct me if I'm wrong, won't you then be connecting a mic level source into the console's line level input? Unless of course you plan on using a 1/4" to XLR adapter. Then again, I don't know what the recievers' output is. Wait, disregard that. I'm pretty sure they're line level. Oh well, it's finals week and my brain is overloaded. I'm gonna quit before I ramble on further.

 

[mcgart]

I don't think the load on the circuit will be a problem - currently all the speakers run off the same circuit but on a different source to the sound board. Judging by this I think we should be able to safely run them off the same circuit as the board. In any case, I shall check with the relevant person. Hopefully the will then be able to rewire and provide a more permanent solution.

 

[SHARYNF]

Here are some thoughts

Audio amps or the amps in speakers rarely ever take the full rating of the amplifier. Amps feeding subs might come close. but audio is not a constant signal so the loading is not very high. SO I agree you can probably run it all off the same power source. It is important to realize that the potential problem is the path from the power point back to the breaker panel. I have seen electricians argue it is all on the same breaker it does not matter. This is correct ONLY if there is dedicated isolated ground feed from the outlet back to the panel. Many electrical contractors skimp and use grounded outlets that use the electrical conduit for the ground. Usually this is safe, and if they use a little tester it shows no problem but, from tons of experience, with audio with high gain the small ground currents and loops cause a different potential and cause hum. Again Electricians understand safety but not necessarily the best setup for sensitive equipment. In addition if anything else is plugged into this circuit and its grounding is not totally clean, this will also introduce hum into the system.

Re balanced vs unbalanced

If you want to have a whole lot less problems, make sure anything connected to your mixer is either balanced itself of via an isolation transformer.

Basically the bottom line is this
In an unbalanced circuit ts for instance, the shield/ground of the cable is part of the signal. In a Balanced Circuit trs xlr, the Shield?ground in NOT part of the circuit and ONLY serves to basically feed the small signals in the shield to ground and eliminate them from the audio circuit.

So when you use a ts to connect your wireless receiver to your mixer, you are feeding the ground noise from the receiver to the input of your sustem.

XLR or TRS cables are not expensive, and you will simplify and clean up all sort of additive problems if you eliminate them.

I have seem people bring in a piece of consumer gear, with only a two wire mains connection, take cheap rca outputs, put rca-ts jack adaptors on them and then wonder why they may have hum and noise. Will it always cause a problem NO, but that is what drives you crazy, in that it is not a black and white situation rather a detective exercise to fine what is causing the problem.

SO IMO get the proper xlr or trs cables to connect your wireless mics to your mixer

Sharyn

 

[TechiGoz]

I find it amusing how topics just flow on in conversation and are now talking about detailed problem solving! Wow.. its just great! But that it just a side thought

Anyway, back to the issues at hand. It seems, mcgart, that you are suffering a definate case of the ground loop syndrome known as the mixedlightandsound effect. Not really, I just made that up. Everybody's suggestions sound great, and I'm sure that at least ONE of them will work, if not more.

When you say you have taken the equipment off site and it has worked without hum, that is a clear indication to me that it is more of a power problem than a wiring issue, however it could be both. Certainly having your speakers come from the same circuit(s) as the lighting dimmers and desk will have an effect on the hum/buzz. So try what everyone else has suggested, i shant rewrite it here, and see how things go.

Good Luck

 

[mcgart]

Thanks TechiGoz,

I believe you to have more knowledge of this problem as you have actually experienced the change when we take it off site. Considering you have worked in the theatre where the problems are originating, and you have a knowledge of the equipment, what solution would you suggest I should try. Judging from your post your recommending I try to have the speakers running off the same source as the board. Do you believe this will overload the circuit, considering we also have the equaliser, mixer, mics and other audio equipment running off it. Just interested to know what you think. Thanks!

 

[TechiGoz]

No problem Mcgart,

You are correct! From my experience working in the theatre and moving the equipment off site, and from the observations that I have seen, it comes down to two possibilities in my opinion. Possibilities that have already been mentioned by a myriad of other replies to this post.

a) Power. I do think that part of the reason we have and still are experiencing the hum is because of the mix in grounding by lighting and sound. If we were to test (which we will) the power, there is some possibility that it may over load the circuit. As a result, we may change the circuit that the lighting comes off and link two for sound.

b) Wiring, as you have said, is most likely another cause due to the state of the theatre which you know so well. This is another thing we will fix/test to see if it help. We shall talk more at length via email.

Thanks to everyone who has posted their opinions! They are really helping our quest!

Cheers!

 

[highschooltech]

I know several bands that use digital. Cassy and the Sunshine Band use a Yamaha PM5D. I know this because i was just trained on one today by their monitor mixer

 

[saxman0317]

I'll rephrase the question.
We don't have enough XLR cables to connect all of our Shure wireless receivers to the Mixer. We use as much as we can though. We have a few TRS cables, (but not much), but we have a lot of TS cables (which are also conveniently short). What sound quality is sacrificed from using a TS cable connecting the receiver to the mixer as apposed to using a TRS cable? (Since The signal is MONO, and the cord goes about 3 feet...)
(...and, if anybody knows, if they don't use digital mixers on Broadway, do they just use them for music/rock groups?...)

It depends on the show, venue, and tech. Ive seen broadway shows that do have digital, some with analog, and some with both actually. Blue Man is notroius...i would love to work wih their tech crew one day. I got myself into one of their mix decks at a show i went to, and there were 4 seperate boards for sound, 3 analog, 1 digtital. I was talking to the guy who ran it all, and he just simple said that with all the stuff that they had going on, they found it a whole lot easier with some things to be on analog for the easy of live edit, and some on digital because of the amount of aux. effects they were feeding through the channals. As far as running shows with the two, i refuse to work a live band or the equvilent with anything but analog...i dont like not being able to see everything thats going on with my EQ's and such when doing something so fluid and busy. Plays and such though arent bad..their alittle slower moving.
As to the first issue of thispost, we had the same problem a few times inn one of the theaters i work at. Our electrician rigged up a homemade grounding box with a bunch of transistors and circuits...maybe you can find some stuff online on how to do it? We had half a dozen of those and just ran everything off it isolating our gear and it worked great.

 

[Stu]

Our theatre is in kind of the same boat -- except that they have decided to go ahead with the digital console. Trying to decide between the Mackie TT24 and the Yamaha PS9. Any recommendations?

 

[audioslavematt]

Our theatre is in kind of the same boat -- except that they have decided to go ahead with the digital console. Trying to decide between the Mackie TT24 and the Yamaha PS9. Any recommendations?

Go with the LS9. A lot of people think the Mackie is a lot clunkier to use and the onboard effects aren't as good as the LS9. Not only that, but the Yamaha will most likely get more respect if you have outside groups come in and use the facility.