Allen and Heath GS3000 ground hum

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James Porchik

Active member
Joined
Oct 26, 2007
Messages
35
I've been recording in a friend's studio.  He has a GS3000--the individual channels are clean when recording, but when assigned to the monitors there is a hum.  The more channels assigned, the greater the volume of the hum.  The mix busses hum as well, which means all of the mixes sent to the artists are really painful.  This is a 60 cycle hum, not a hiss.  Can anyone offer any suggestions for where I should start troubleshooting?  I have experience making diy projects and doing electronics labs at the community college, just not fixing consoles.
thanks,
James
 
James Porchik said:
I've been recording in a friend's studio.  He has a GS3000--the individual channels are clean when recording, but when assigned to the monitors there is a hum.  The more channels assigned, the greater the volume of the hum.  The mix busses hum as well, which means all of the mixes sent to the artists are really painful.  This is a 60 cycle hum, not a hiss.  Can anyone offer any suggestions for where I should start troubleshooting?  I have experience making diy projects and doing electronics labs at the community college, just not fixing consoles.
thanks,
James
Typical longitudinal noise in a mixer. Can be many things, starting with noisy PSU. Can you check with a Variac? Sometimes increasing the mains voltage gives better regulation, hence less noise. Also check any source of magnetic field under the mixer. Many times, I've seen the PSU and the monitor amp tucked under the mixer.
 
I'll check out the installation for sources of magnetism, but I know that the monitors are self powered and the psu about 2 feet from the board.  However, who knows what else could be nearby.  I don't own a variac, but I'll see if I can borrow one.  How much are we talking about increasing the voltage?  The next question is, could the line voltage be low?  I'll check that out with a meter.
 
I was thinking this morning that if I put the power supply on a scope, I should see a 60 hz sine wave riding the dc, correct?  I would also guess that it will be very low voltage.  Am I thinking about this correctly?
 
First unplug everything and just listen through the headphones.  That way you can rule out interface issues.

I have the service manual if it gets really hairy. 
 
James Porchik said:
I'll check out the installation for sources of magnetism, but I know that the monitors are self powered and the psu about 2 feet from the board.  However, who knows what else could be nearby.  I don't own a variac, but I'll see if I can borrow one.  How much are we talking about increasing the voltage? 
You have to stay within safety limits; don't increase voltage more than 10% above nominal.
The next question is, could the line voltage be low?  I'll check that out with a meter.
It's a possibility
 
Problem partially solved: The neutrals and grounds were tied together at the service box.  Now, the neutral goes back to the utility and ground goes to the ground rod outside.  I haven't seen this yet, but the studio owner told me he did it a couple of days ago.  I did listen to the board and most of the ground noise subsided EXCEPT in the aux's.  The aux's are pure misery to listen to.  There is a minor amount of 60 cycle hum elsewhere, but now is the time (I think) for us to isolate the board as Brad suggested listen to each section for noise.  There are no sources of magnetism within 3 feet of the board.
 
That's interesting information on grounding.  I had no part in changing the electrical service, neither was I consulted.  The studio owner had an electrician do it.  It did seem to clean up some of the noise, but the 6 aux outputs seriously nasty amounts noise that sounds like ground hum.  I looked at my initial post and I wrote that it was the monitors, but that's not true: it's the aux's and the aux's supply the "artist" headphone mixes.  Everything else is pretty quiet.  I have the board out of the studio right now and did a visual inspection.  The only internal problem I could find was a broken solder tab on an input xlr.  I've looked at the schematics in the troubleshooting manual, but nothing leaps out at me on the aux schematics.  I'd like to put this problem to bed and move on, but I'm not sure what to do next.  Any thoughts?
 
Just a thought - looking through the service manual I noticed that the Aux Outputs are balanced. If your headphone amps also have balanced inputs then maybe you can disconnect the ground connection at one end of the cable in case there's a ground loop created between the two chassis.

If the headphone amp doesn't have balanced inputs, then check your cables as there may be a grounding issue depending on how the balanced jack at the Aux output is wired to the unbalanced jack at the headphone amp input. If you got a line isolation transformer/box handy, try putting this inbetween and see if the hum disappears.
 
that's a good idea.  When we took the board out of the studio, we put in a different boards so we could still have the routing/monitoring.  The new board is a allen and heath live board.  It cured all the noise problems in the monitoring.  Headphones are clean and quiet.  I assumed that was because the new board had cleaner aux outputs, but maybe not.
I have a more general question about reading the schematics for this board.  Until now, the schematics I've dealt with have been for smaller devices that are originals.  This made them very easy to read. I'm having a problem associating all parts of the circuits together.  It seems like the circuits stop dead with an arrow and cryptic writing that I have a hard time reading due to the copy/scan generations.  They pickup elsewhere on the page.  I suppose the do this to save space.  Any advice on how to organize these for myself and better trace the circuit would be much appreciated.
 
I just went through the outputs on the board.  Aux 1 and 2 have noise, but it is mostly hiss.  Aux 3-6, Studio 2 and Control Rm Alt all have hum.  Studio 1 is pretty clean and control rom main has no noise.  I did the checking with all the channels muted and the aux channel sends all the way down.  The aux outputs, studio outputs and main faders were all the way up.  I used a presonus headphone amp http://www.presonus.com/products/Detail.aspx?ProductId=20 that claims to be balanced.  I listened with a balanced trs cable and I listened with a trs cable with the shield lifted on one end.  I also tried a guitar cable.  The results were approximately the same.  As a comparison, I listened to the output of the gl2400 live board that we're using in the control room as a temporary measure.  The GL2400 has a little his when turned up, roughly similar to Aux 1 and 2 of the GS3000, maybe a little less noise.  Bottom line, it has a mean ground hum.  The chassis ground stars were cleaned and tightened with no audible change. 
I have a scope, but I'm waiting for the probes to come in from an ebay seller.  Once I get them I want to put it to work.  Please correct me if I'm wrong: I should be able to put the scope ground on chassis ground and probe signal ground and signal hot separately correct?  If there is a ground problem, signal ground will show the noise or am I off in left field here?  Also, if the board is not connected to anything, there should be no ground noise at the output, right?
 
Two more things.
1) The Gl2400 live board has unbalanced outputs.
2)The manual mentions this:  "To connect an unbalanced source to a balanced console input, link the  - input (...ring terminal of a TRS jack) to 0v earth (...or the jack sleeve terminal) at the console.  To connect a balanced console output to an unbalanced destination, link the - output to 0v earth at the console."  The normal monitor setup that this board drives is the 6 aux's go to a Hearnet hub.  The specs for that say it's unbalanced,  I'm wondering if the cabling suggestion above will fix the problem and if it will have any effect on the presonus headphone amp. 
 
This is probably the obvious, but I'll report my observations (to myself):
The ground noise rides the dc +/- with the same magnitude waveform as I observed on the outputs of the auxs.
The channel outputs are near silence.
The aux's have very low gain in general.  The have to be run near full volume.
I ran the output of the aux to a radial d.i. box and that removed nearly all noise, but there was further attenuation of the signal.
Is it normal for this board to have low gain on the aux's?  It seems abnormal to have so little headroom and this is a major contributing factor in the noise.  
 
Hi,

I too am having a wierd issue with an Allen and Heath GS3000,which is a great desk by the way! 8)

The Power Supply transformer buzzes  rather loudly when the desk is connected to it and thus under load.The strange thing being that when the PSU is turned on, but not connected to the desk,the PSU doesn't hum at all and is dead silent. :eek:

What the hell's going on?I have disconnected everything in the studio from the utility sockets so only the desk is connected,eliminating the possibility of any ground loops and it still happens.

Also is there any chance of hooking me up with that Service Manual?It would be very helpful indeed.

Thanks


 
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