AUX hum on Soundcraft 200b

[spitfire1956]

The church I attend has been donated a Soundcraft 200B which we wish to use for various wired and wireless microphones and also with musicians. When we recently set up two active foldback speakers for the musicians and singers we found a very high level of hum coming from the foldback speakers when they are driven from Aux outputs. However, there was no hum at all when we switch to using Group outputs. All cables in use are balanced.

Does this indicate a fault on the Aux outputs? Is the Aux output earthed differently to the Group outputs? and can they be reconfigured internally?

We also tried using an Aux output to feed our hearing aid loop and also find it is picking up some hum and there is cross-talk to the Aux bus from a channel we are using for playing CD's.

[Matt Syson]

Hi
Is the level of the hum alterable by using the aux master level pots?
If so then it may be that there is a channel or frame fault which has the aux busses shorted to ground or possibly elsewhere.
If the aux master pots DON'T alter the hum then it is more likely a grounding issue.
Read all the other 'ground hum' threads and try the various suggestions.
Matt S

[spitfire1956]

Hum levels dont change with the Aux pots, so we're assuming grounding problems are the likely root cause. WIll the Aux outputs have a different grounding arrangement to the Group outputs ? (possibly different output stages too?).

[Matt Syson]

Hi
I have not looked at the schematics for the 200 outputs but it is possible that the auxes may not be balanced. Rather than getting too deep into this you could make up a 'special' cable where you cut the ground (pin 1 of XLR) at one end. You could make a 'better' version by cutting the ground and inserting a capacitor say 0.1 uF (anything small will do) which will help keep out mobile phone and radio but prevent significant 'mains hum' loops.
A more expensive way would be to use transformers with again 'ground lift'.
Having active foldback monitors is one way to encourage loop problems as there is a possibility that there is some difference in ground from the desk to where the monitors are plugged in. If the stage area is any distance from the desk it is even possible for them to be on different mains phases.
You COULD try running the mains for the monitors back along with the audio cable, so they are 'powered' from the same socket as the desk but this is getting a bit messy.
DO NOT cut mains earths on ANY equipment.
Matt S

[spitfire1956]

Thanks for the suggestions Matt (much appreciated) - we'll try them out and see how we get on.

[sameal]

i think the 200B may be eligible for the Eddie cilettie mod.

Console Modification: Soundcraft 200b grounding

[Matt Syson]

Hi
Possibly but it is not the basic issue here.
The 'Eddie' mod is not necessarily the best thing to do anyway as although chunky (American) cabling may look sexy, it is not necessarily the best way to achieve this 'improvement' and potentially creates a series of ground / bus loops.
Matt S

[spitfire1956]

We've had all the channel modules and master boards out, cleaned up contacts and found a possible loose earth and seem to have eliminated a large amount of the hum. Might consider the documented earthing mods as a final resort. Thanks for your help and suggestions gentlemen.

[Barish]

FWIW, I had applied Eddie's mod on a 200B in the past and it had really improved the signal to noise ratio.

B.

[sameal]

Originally Posted by Barish

FWIW, I had applied Eddie's mod on a 200B in the past and it had really improved the signal to noise ratio.

B.

i did it to a desk of mine and had no problems to speak of while i had it.

[polymono]

Well, i know its an old thread but i'm having exactly the same problem on the aux's with exactly the same mixer. I've removed everything and by building it back up I'm pretty sure its not the power supply. My group outputs are also clean.

Matt if your still around you mention:

Originally Posted by Matt Syson

Hi
Is the level of the hum alterable by using the aux master level pots?
If so then it may be that there is a channel or frame fault which has the aux busses shorted to ground or possibly elsewhere.
Matt S

This seems to apply to me as all is clean until i bring up the master aux pot. Any tips on rectifying this? Would be most appreciated. I'm a bit of a noob to analog desks.

Many Thanks,

[Matt Syson]

Hi
Try the obvious first. big transformers (amplifiers, power supply etc) near the desk and so on. If you were brave you could go for the Eddie mod suggested above. I would exhaust other possibilities first however.
If yopu unplug all except the monitoring does it hum then?
There are loads of 'hum' threads, please look at a few as the problems are often similar, just change the name of the gear!
Matt S

[polymono]

thanks for getting back to me matt. Really appreciate it. Have checked a few other hum threads and I'm currently overhauling my entire setup as you suggested. I had a look at the grounding and made sure everything tight etc. i think i might have a couple of dogdgy channel modules that are contributing too. Thanks again for the advice, much appreciated. Wish me luck

[polymono]

ok, i've been troubleshooting off and on for 3 days. Main issue is the 50hz hum.

At the moment its looking like an internal grounding issue. Everything is disconnect from the mixer and the mixer is the only thing connected to power in the room. using headpones to monitor hum. Does this sound familiar to anyone: Raising master faders and monitor output pot to maximum makes the hum go away. Bring either fader pot down at all slightly and hum begins to return...??

[polymono]

Have had a play around with the grounding and tried a few different configs which changed the hum slightly, my raising/lowering the fader no longer affects the hum.

However the hum is still there. Am i being unreasonable expecting there to not be a small low frequency hum with nothing plugged in? It doesnt seem that audible right now with nothing connected, prob -80db overall noise floor with faders down.

Could the psu be at fault? The original psu has been replaced with a studiomaster EP2 psu. How can i tell if it is failing?

thanks in advance anyone

[Radardoug]

Is that noise level with group/aux masters at their nominal zero's? If so, you are doing about as good as you would get out of a Soundcraft.
If that is with group and aux masters down it is meaningless, you don't operate the console with the masters down!
What you want is the noise level with a typical mix set up ready to go. By all means start with nothing plugged into the console, but you need to find out your noise level when you start plugging in mikes and especially di's. DI's will cause ground loops through the inputs which will really show up any issues.

[polymono]

hey, doug thanks for the reply - well yeah at the moment i'm still working on it with nothing plugged in, but last time i checked, with everything plugged in and 3 aux sends with returns my noise floor was about -55db and you could clearly hear low frequency hum in that! I definitely have a few ground loops from various bits of equpiment which i've been reading like crazy trying to figure out the issues, and i will eventually put it all back together as I'm giving it all a complete overhall again. For now though i'm concentrating on the board alone.

I'm just curious as to how much hum to expect, i see the eddie mod claims to quiten the board hum down by 12db, so i'm presuming there must be some hum in the first place to quiten it down by!! Maybe i've got it as quite as it can be now?

As i say, i'm still learning about ground loops but as they seem to multiply as i plug things in my aim was to get things as quiete as possible to begin to lessen the multiplication possibility (?!!)

[Matt Syson]

Hi
It is good you can put figures to this but WHAT are you reading? You need an osciloscope ideally so you can 'see' what it is, either hiss, hum or oscillation, or a combination.
With master controls set 'flat' (unity) you would hope for HISS at around the early '-70dB' sort of level with hopefully a trace (or less) of powerline hum. Is it powerline frequency, or 'double', which could well be problems with the power supply unit.
Upending or turning the desk through 90 degrees may reveal changes in HUM level. which would point to something being picked up which although it shouldn't, is more info to go on.
You may be able to 'null' it out by rotation.
Stereo busses are usually a bit lower in hum on 'cheap' desks as the measures to make it quiet cost money and take up resources.
Matt S

[Radardoug]

I agree with Matt as to noise floor. If you can get it to that with nothing plugged in, then at least it's then ready for the next step, which is plugging in external devices.
This is where the pin 1 mod really comes in. I would recomend that at the very least you should make sure that all your XLR pin 1's wire direct to the ground at the power supply input, and that this connection does not go onto the channel strips. Alternatively connect pin 1 to chassis on each one locally, but make sure that the connection is very good.

[polymono]

thanks again for all of the advice guys..

I know its getting a bit removed from the original topic, but unfortunately i'm going a bit doolally with this right now. Here's where i am:

Nothing is plugged into the mixer, monitoring on headphones. There is a small amount of internal hum to begin with. I've moved the psu of the mixer and plugged it in 6 feet away via an 8 way. Plug in pc, firewire audio interface to the 8-way and i get a load of hum in my cans before i even connect any audio leads between the two. Have tried moving things around, everything is out racks, room is upside down!!

Feeling a bit lost :(

[Ike Zimbel]

What frequency is the hum? If it's 120hz, it points to the filter caps in the power supply.
+1 to what Radar Doug and Matt Syson have already said.
best,
Ike

[polymono]

ok, thanks for all of the input everyone, things are begining to move along now..

I experimented with lifting pin 1 on a cable and connected my interface to the mixer and ground loop dissapears! Thanks Doug for that.

I have been reading through the manual for the board, and it also suggests that there should be only one central earth.. star point. Now, i'm trying to learn, i really am, but how does this work? Doug, i appreciate your guidance but i'm so new to this! How do i make a star point?

To get started i'm planning on buying a bunch of jack plugs and wiring up some cables with the pin lifted on each. Is this recommended? Safe to do?

thanks again for everything all,

[Radardoug]

A starpoint ground is a common point where all earth connections are connected.
A wire runs from each desired earth point back to the star point. The Soundcraft way of wiring, with a long ribbon plugged in to all the modules is about as far from the star point idea as you can get.
BUT!
Ground is a nominal concept of a surface at zero potential, and is not actually possible, because as soon as you introduce conductors with resistance, then the surface is no longer at zero potential. So all we can do is try to get as close as possible.
The pin 1 on a cannon is for a sheild connection. This sheild should not carry any desired signal currents, but should drain off all undesired currents. Soundcraft violate this principle by connecting pin1 to the signal grounds in each module, thus introducing undesired currents into circuits which can amplify them.
So, to cure this, we try the star ground. Select the nearest point to the mains ground that you can access in the console. This is where the ground enters the console on the power supply connector.
OR
Run a completely separate ground from the mains ground to a point in the console.
One problem is the way the power supply grounds are run in the Soundcraft power supply.
Ideally there should be a wire from the chassis of the power supply to the chassis of the desk. This cable should only connect to the mains ground and the power supply ground in the power supply, and the chassis of the mixer.
Then connect this ground with individual wires to every pin1. OR use the chassis as a mass ground, and connect all the pin 1s at each socket to the chassis. DISCONNECT any other connection to the pin 1s.
You can extend your star point with a heavy copper busbar inside the console
and then make shorter connections from the pin 1's to this. You can also use another busbar to connect all your signal grounds from each module. Then connect these busbars at your starpoint. You must disconnect all other grounds, as they will intoduce earth loops.
Or throw the whole thing away and buy a mixer from a manufacturer who knows how to wire grounds.
In the era of the 200, Soundcraft had absolutely no idea of how to do grounding, and sold probably thousands of dodgy mixers. And they are still out there haunting us!

[Matt Syson]

Hi
Part of what Radardoug has said is true, however some is not. Star grounding is an 'idealised' form which works in some places and not in others. It does not embrace the concept that the SIGNAL MUST follow the ground otherwise itself will produce a 'loop' thereby getting back to the original problem.
The ribboning in these particular desks has a 'defect' but not necessarily the one for which it is being blamed. The main issue is that pin 1 of the XLRs MUST go to chassis only (as has been noted), and the phantom supply for the mics should also be referenced to this.
The 'Eddie' mod strives to overcome the issue by massive brute force, whereas a much reduced copper option is possible.
Many fail to understand and implement some of the laws Mr Faraday established many years ago.
Getting circuit principles from a conceptually single point system into a large 3 dimensional object is quite tricky. All conductors of the circuit having resistance, inductance and capacitance to others and all have the capacity to act as aerials both receiving and transmitting.
Matt S

[Radardoug]

Which bits aren't true Matt?

[Matt Syson]

Hi Radardoug
It is elements of some of the different points you make rather than whole statements being incorrect. It is quite difficult to explain briefly.
There should be 2 'groups' of power circuitry. One being the 18 Volt supplies and their 'ground' which runs up the ribbon, thus preventing hum from being induced into the power and ground and mix busses.
A separate system should be involved with the phantom power which correctly appears referenced to pin 1 but should not be linked to the audio ground but to the metalwork at the XLR for the reasons you state.
There is then a problem with unbalanced signals leaving and entering the desk which should be converted to a 'pseudo balanced' system by providing 3 pin connectors with sleeve to chassis and ring connections going to analogue ground at the module near where the 'send' amplifier is situated. This would reduce any 'ground currents' from being forced into the desk (ribbon) ground.
Of course this is all quite tricky on an already built desk.
The phantom supply should have resistors added in series (in each module) to limit induced 'ground loop' currents and a decent size capacitor coupling the 48 V supply to PIN 1 at the XLR.
Yes a 'star' point is a useful concept but it can easily fall down if the audio signals don't follow it's physical routing because unfortunately signal requires a 'loop' to actually work.
Inside a desk is a whole nest of loops, usually followed by an amplifier to make things worse.
Had Soundcraft used all balanced connections to the various back panel sockets and got the phantom referenced only to the chassis there would not be an issue with hum pickup.
The One of the Neve desks uses stacks of ribbon but because the phantom is applied at the patch, and all signals follow ground there is no hum problem.

This was not a personal attack, just making a point that 'star grounding' is not necessarily what you want. It is certainly useful in a wider studio environment but this is mainly because it enforces a 'system' rather than a mess of plugboards and goodness knows what.
Matt S

[Radardoug]

Not taken as a personal attack Matt, and your clarification adds greatly to the discussion. Part of the problem is trying to explain fairly compex situations to others who may not have the understanding.

[polymono]

Guys thank you so much for all of the input.

Doug i read your post, re-read it and re-read it again and bugger me its actually starting to sink in a bit! Searched around alot and read up about telescoping etc. Thanks for all the effort to you and Matt for what you added.

I've taken all of my gear out of racks, seperated it all from each other, and have prepared a buch of pin 1 lifted cables which will be shield connected at the mixer end only.Think i can hopefully get the ground loops under control now with a bit of work this weekend and then the internal grounding stuff will come later. Think i will try the eddie mod a bit further down the line once i've ironed things out everywhere else.

On a side note I'm a bit worried that my neighbours electronics are causing problems on the internal stuff, as he is directly above me as i am on basement level. Even though my equipment is all connected to one path now, i'm guessing he might have gear connected to seperate outlets that are conflicting and causing me internal hum. I've found that the internal hum is is variable at different points of the day that seem to correlate to his usage, so seeing as we share the power source this would make alot sense. I think this has been the source of alot of my doolallyness - when i have only a mixer plugged i would get different responses each time i approached it! Might have to visit him at some point this weekend...

Thank you so much again for all of the info. Its been invaluable to me.

I'm working through it all this weekend so will keep you informed...

[Matt Syson]

Hi
Having a variable factor that is 'uncontrolable' is a pain. It means that you will have to be more carerul than ever to get your installation right.
One 'problem' with star earthing that stems from poor signal routing (loop between ground and signal) is that it is moderately easy to make it appear 'excellent' due to cancellation.
If this is going on and all your cabling and the environment are 'static' all well and good. In your situation you may get it to be 'good' by cancellation, then your neighbour comes in and does whatever he does and it will change the situation.
I would use a bolt on the rear of the mixer as a 'star' point and ensure that the supplies going into the desk are floating. This bolt should be as near as possible to the origin of the power input wiring in the desk. The metalwork of the PSU box MUST stay grounded by it's mains lead for safety.
Mod the desk wiring of the XLRs as Doug suggests taking pin 1 to chassis only.
Wherever you have balanced connections available, use them and 'adjust' your wiring so that the best use of the balancing capability can be made. Roughly speaking only unbalance at the point where it has to be unbalanced. Be prepared to drop the shield at that point.
It may be you will need a couple of transformers for isolation on the odd piece of gear.
Build up the wiring getting the desk happy into headphones, then add monitors and 'multichannel' devices (convertors etc).
You are looking for loops. Many are unavoidable but knowing where they are is part of the battle.
Matt S

[polymono]

well its been a long while but i've narrowed things down a bit!

I had a lot of ground loops that were fixed by lifting the shield on cables at one end. I was able to reduce alot of hum by moving the psu of the desk far away from the power packs of my fx boxes etc and i was able to reduce the master hum a bit further by attaching a ground wire from the master module to the bus bar internally, something i noticed possible after seeing an option for "P1 Chassis GND " on the master. I also looked re-routed alot of the internal grounding for the psu and cleaned things up, cut new ends etc which helped considerably and cleared pretty much anything left of the hum.

Master is now definitely quite enough for working!

Now, with everything quite on the master and all ground loops, interference out of the way i can hear the original problem in all of its clarity - The AUX hum - loudly!

So everything is clean except for the AUX outputs which hum. I have had the ability over the last two weeks to use a 2nd mixer as parts and have tried swapping the entire aux board which makes little difference. I have also swapped out the main output module wich also made no difference. I have also noted the "P1 Chassis GND " on the aux board and have attached some ground wire to this and fixed it the internal bar, but it has made no difference.

The hum is clearly audible when plugging headphones straight into the aux outputs and is too loud to work with.

Has anyone come across such noise on there AUX channels before? Again, the problem is only on the aux, nowhere else!

thanks guys,