aclac

M.Nats 1176 serious hum problem (SOLVED)
« on: November 23, 2009, 04:25:54 AM »
HI, I am new here... finished REV.J 1176 project couple days ago. Two comp. unit inside 2U metalbox with M.Nats PSU. It seem to work well, but... hum is totally LOUD expecially, when I unplug input XLR connectors from mixer (even -20 dB). When I have XLR conneced input, this hum is not so bad, but I can't live with it. I had 2 VU lamps (8V, 50mA) connected with 270 ohm serial resistor in the 30VDC rail, but it gave more hum to circuit and I loose them of... SAD, because I'd like them back. I think, that this hum is caming through PSU, but I don't know how I test it? I tried to take power transformer (toroidal) 24-0-24 VAC (90VA) out from box, but it was not the main hum reason. The XLR- input pin 1 is grounded and so is power transformers center tip... and main supply's ground wire. The wires inside the box was made like instrustions said, and... rotary switch cables are not twisted as the instructions, is this OK? As I though, that hum sounds like 50 Hz transformer... and I can't measure and eliminate it. Don't KnowHow? This project was very interesting and I'd like thank everybody here, because I was get lot of information here and without you I maybe could't start this project. Please, help me with this problem, I tried get solution here and everywhere, but not got yet. I can give sound sample or foto of my project, if this help you? Thanks...
« Last Edit: December 25, 2014, 12:11:00 PM by aclac »
Things happens...


Re: M.Nats 1176 serious hum problem
« Reply #1 on: November 23, 2009, 09:35:06 AM »
Well lit pictures always help.  Perhaps someone will see something you missed.

Biasrocks

Re: M.Nats 1176 serious hum problem
« Reply #2 on: November 23, 2009, 01:36:58 PM »
The wires inside the box was made like instrustions said, and... rotary switch cables are not twisted as the instructions, is this OK?

Why, yes it does matter. 

Go back and redo the wiring per the instructions, twist the wires where required and use shielded cable where needed.

Pictures would help.

Mark
http://SharktankPro.com

"I'd rather use an SPX90 than a UA plugin....." Joe Barresi

aclac

Re: M.Nats 1176 serious hum problem
« Reply #3 on: November 23, 2009, 03:17:21 PM »
OKAY...
Here is the picture;    http://muusikoiden.net/dyn/users/66548.jpg
And here is the guide, what I followed;    http://mnats.net/1176-wiring-rotary.html
Hmm. there is no twisted pair, but pusbutton version has?

Key things are, that if lamps are ON, brrrr- sound is louder! And when XLR- input is without connector the hum is huge. It is true, that left side get little bit different hum (because main transformer is closer), but I think, that it is not main reason. PSU DC- voltages are OK. As you have told, maybe I most twist all DC- cables and long VU- cable and DC- cable, whitc carries GR signal to VU- meter? As you see, there is little diodebridge to VU- meter, but they works guite good... I think so?

- aclac -
Things happens...

Biasrocks

Re: M.Nats 1176 serious hum problem
« Reply #4 on: November 23, 2009, 06:29:36 PM »
OKAY...
Here is the picture;    http://muusikoiden.net/dyn/users/66548.jpg
And here is the guide, what I followed;    http://mnats.net/1176-wiring-rotary.html
Hmm. there is no twisted pair, but pusbutton version has?


That black cable that you're using doesn't look shielded to me.

This is an example of shielded cable.



http://www.mogamicable.com/Bulk/micr_cables/console_cables/console.html

You must use shielded cable where it's specified.

Mark
http://SharktankPro.com

"I'd rather use an SPX90 than a UA plugin....." Joe Barresi

thedude

Re: M.Nats 1176 serious hum problem
« Reply #5 on: November 23, 2009, 07:03:26 PM »
Hi aclac! I've also had some hum probs with this build. Solved it with some ground lift switches and better wiring(twisting some wires etc.) First of all im not the best when it comes to grounding, so feel free to correct me if im wrong. I've took a look at your pic and is see that your output pin1 is grounded at the same point as your input pin 1. In the wiring instruction it says to only connect the input pin1 to the chassi. Maybe this could cause a ground loop. But if your shield on the xlr-output cable is not connected to pcb-ground i guess it could work anyway.

When it comes to the lamps, i have a theory but i can be completly wrong about this. It could be a good idea if someone ho knows better could fill in here ;D.

When you connect the lamps to your regulated 30 volt rail, you get a higher load on the power supply. Maybe the PSU caps aren't big enough for the current you need and it could cause some ripple on the PSU output. Try to connect lamps on the directly from the transformer secondary, that is how i did it on my RevD and it works fine. On the G1176 board the "VU" is also taken before the regulated voltage.

Cheers

thedude

Re: M.Nats 1176 serious hum problem
« Reply #6 on: November 23, 2009, 07:08:26 PM »
Oh now i see, Mark you got better eyes than me ;D no shield there.

aclac

Re: M.Nats 1176 serious hum problem
« Reply #7 on: November 24, 2009, 06:28:08 PM »
Yes... this black cable is shielded (http://muusikoiden.net/dyn/users/66611.jpg), but there is no pair inside one shield. It was too hard to get thin sielded flexible pair and I took this cable from old hifi headphones :o There is also blue and red shielded cable. Forgive me, because my picture is too bad! But it is not so bad, because thedude found one funny thing. Only XLR input pin1 is grounded, but I took shielded wire to an extra connector... and the same shield continues to output side... but there is no groundpoint anyway. I lose extra connectors shield and hum was little bit stronger? I'll put it back.

Now I have twisted DC- cables and... hum is still there, maybe weaker? I'm not sure is it too loud or is it not, but lamps takes 54 mA and gives more hum. As thedude told, can I have some ripple there? Is there some information, how I measure it, and what is tolerance in this case and how can I eliminate it? I have M.Nats PSU (http://mnats.net/psu.html) and it gives exactly +30/-10 VDC. I'm very grateful for your answers everybody. Aclac
« Last Edit: November 24, 2009, 06:39:39 PM by aclac »
Things happens...

Biasrocks

Re: M.Nats 1176 serious hum problem
« Reply #8 on: November 24, 2009, 07:56:24 PM »
Yes... this black cable is shielded (http://muusikoiden.net/dyn/users/66611.jpg), but there is no pair inside one shield. It was too hard to get thin sielded flexible pair and I took this cable from old hifi headphones :o There is also blue and red shielded cable.

Well, I suspect that not having a properly shielded cable (one with the conductors fully contained in the sheild) is a large part of your problem.

Mark

http://SharktankPro.com

"I'd rather use an SPX90 than a UA plugin....." Joe Barresi

regularjohn

Re: M.Nats 1176 serious hum problem
« Reply #9 on: November 24, 2009, 09:54:34 PM »
Power for incandescent lamps should be drawn from the transformer secondary, where it's still AC.  That's likely why the lamps are causing some of the hum.

It sounds like you've got a ground loop going on.  What are those connectors between your xlr's for, and why are they connected to the xlr's?

Are both channels sharing the psu ground?  If so, only ONE of the channels should be connected to chassis ground.  Otherwise you'll have a ground loop.
« Last Edit: November 24, 2009, 10:00:13 PM by regularjohn »
SATANOSCILLATEMYMETALLICSONATAS


aclac

Re: M.Nats 1176 serious hum problem
« Reply #10 on: November 25, 2009, 02:32:15 AM »
Yes... this black cable is shielded (http://muusikoiden.net/dyn/users/66611.jpg), but there is no pair inside one shield. It was too hard to get thin sielded flexible pair and I took this cable from old hifi headphones :o There is also blue and red shielded cable.

Well, I suspect that not having a properly shielded cable (one with the conductors fully contained in the sheild) is a large part of your problem.

Mark
Both conductors has own shield, watch carefully my picture...
Things happens...

aclac

Re: M.Nats 1176 serious hum problem
« Reply #11 on: November 25, 2009, 03:15:41 AM »
Power for incandescent lamps should be drawn from the transformer secondary, where it's still AC.  That's likely why the lamps are causing some of the hum.

It sounds like you've got a ground loop going on.  What are those connectors between your xlr's for, and why are they connected to the xlr's?

Are both channels sharing the psu ground?  If so, only ONE of the channels should be connected to chassis ground.  Otherwise you'll have a ground loop.
OK, I can move lamps to AC later. They are not connected now. This extra connector is made for my stereo pre amps insert path (not balanced) and I haven't even tested it yet. But, both channels is sharing the psu's internal ground and none has connected to chassis ground? There is powercord's shield, transformers secondary center tip and both input XLR:s pin1. Do you mean that i loose other XLR pin 1 from chassis?
« Last Edit: November 25, 2009, 04:08:18 AM by aclac »
Things happens...

Kamel

Re: M.Nats 1176 serious hum problem
« Reply #12 on: November 25, 2009, 03:59:33 AM »
yes, the loop seems to be here, tie the ground of each xlr directly to ground chassis, near the ac input socket

aclac

Re: M.Nats 1176 serious hum problem, my missunderstanding?
« Reply #13 on: November 25, 2009, 08:20:24 AM »
HEY

My hum problem might be only missundestanding? When I has G1176LN input and output pot in full gas I got some hum, but I never can't use this kind of volume (over +8dB) in recording situation. All little noises in room, even breath are 10 times stronger than this hum. I didn't understood that this little thing can do so much amplifiering into signal path? I tryed record some song and bass- guitar and I couldn't notice any hum and even compression is working... but ratio board seems to behavious like inverted? Anyway those lamps I'll put transformers secondary side (AC) and twisted DC- cables might be better now as they are now. Can this be like that... solved so simple and my mistakes? Aclac
Things happens...

regularjohn

Re: M.Nats 1176 serious hum problem
« Reply #14 on: November 25, 2009, 09:29:37 AM »
Yes, I've found that usually any problems I have with builds are simple mistakes.
SATANOSCILLATEMYMETALLICSONATAS

Biasrocks

Re: M.Nats 1176 serious hum problem, my missunderstanding?
« Reply #15 on: November 25, 2009, 10:01:16 AM »
HEY

My hum problem might be only missundestanding? When I has G1176LN input and output pot in full gas I got some hum, but I never can't use this kind of volume (over +8dB) in recording situation. All little noises in room, even breath are 10 times stronger than this hum. I didn't understood that this little thing can do so much amplifiering into signal path?

As a general rule for gain staging, most 1176's spend their time with their input at the 10 o'clock and the output at the 2 o'clock position give or take.

Mark
http://SharktankPro.com

"I'd rather use an SPX90 than a UA plugin....." Joe Barresi

Sleeper

Re: M.Nats 1176 serious hum problem
« Reply #16 on: November 25, 2009, 12:35:53 PM »
Quote
As a general rule for gain staging, most 1176's spend their time with their input at the 10 o'clock and the output at the 2 o'clock position give or take.

Definately something to be aware of and a bit of a shock the first time you use an 1176. 
mine live closer to 9 and noon on almost everything.

aclac

Re: M.Nats 1176 serious hum problem
« Reply #17 on: November 23, 2014, 10:00:33 AM »
OKAY, back to beginning. I was forced to build new M. Nats revJ G1176 compressor, and...

I have one case, 1 toroid transformer and 1 PSU- board to supply 2 compressor unit. First, I was make same kind of mistakes than 5 years ago and solved those rather easily, but... I noticed some interesting things. I made some silly tests to eliminate this last minor hum problem and lastly decided to cut off ground wire from PSU to both PCB:s...?     Here is my PSU:s schematic: http://mnats.net/files/MNATS_PSU.pdf

Now, I have toroid transformers CT and PSUs GROUND connected to ground near AC input jack and compressor PCBs are without 0- wire.  Why... because the wiring instructions says, that only one point can been connect to ground from compressors PCB- board. This is made with input cables shield wire through input XLR pin1. Wiring instructions says, that input cables both ends shields to be connected, in my case (Input Wiring Option B (transformer balanced input)). It work OK now, and I think, that there might appear ground loop, if I connect back my PSUs 0 to compressor PCB. Otherwise I need to take toroid CT straight to PSUs CT- position and 0 from PSU straight to comp PCB, but... maybe then my PSUs 0 is really 0, because it floating?


Can anybody help me to get some focus, how I really should connect my transformers CT, PSU- output and input XLR without ground loop? I think anyway, that CT or PSU 0 rail should be connected to my ground point. Thanks for everybody - Aclac -
Things happens...

aclac

Re: M.Nats 1176 serious hum problem (SOLVED)
« Reply #18 on: December 25, 2014, 12:29:53 PM »
I need to tell you what solution I found...

I got back my old diy 1176 stereo comp to do some improvements inside it and... I noticed that hum is totally louder than my new 1176. First I tried my new psu to old comp and hum was gone, hmm... interesting. Okay, it seems that old psu has something wrong it. It has 2x24VAC (90VA) toroid transformer, which is 3 times stronger than new one (2x24VAC, 30VA). Is that problem..? Nope, the broblem was toroid transformers secondary coil wiring direction, what? Yes, I changed other secondary coil cables the other way and voilâ, problem has gone! It took 5 years to solve this problem, I'm a happy man  ;D
Things happens...


 

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