Help with some hum
enginefire
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#1
5th April 2013
Old 5th April 2013
  #1
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Thread Starter
Help with some hum

Hey all
I am connecting a pair of western electric 227 d preamps into a metal Hammond enclosure. I have a regulated PSU in the box as well as a small transformer and a power switch (lighted) .

My problem is this :
When I have both units connected to the power source, I get hum. When I have either unit connected on its own, I get no hum at all! Just great sound. I would really like to use these as a pair

Any ideas as to how to eliminate this problem? I suspect it is a grounding issue as the hum is low pitched (not hiss) I am guessing 60hz but I have not measured. Btw the caps in the pres have been replaced.

I will post a picture tonight to visualize the wiring and layout, any other info im leaving out?
#2
5th April 2013
Old 5th April 2013
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Richard Crowley's Avatar
 

Two possibilities that immediately leap to mind:

1) Connecting TWO preamps into the metal enclosure is causing some sort of ground loop that isn't a problem with only ONE preamp in the circuit. Do you know about "star grounding"? There are way too many variables here to warrant further discussion without seeing details of exactly how you did this "transplant".

2) Are you sure the power supply is capable of powering TWO loads without "drooping" (which sometimes produces the symptom of hum.)
Do you have a way of metering the power supply voltages to see if there is any difference between one load and two loads?
#3
5th April 2013
Old 5th April 2013
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Ike Zimbel's Avatar
 

+100! And, once again, the actual frequency of the hum would be very helpful. 60hz supports option 1, above, and 120hz leans toward option 2.
Hopefully your pics will shed some light on the overall grounding scheme. I have seen other racking projects go terribly wrong due to the ground implementation being, let's say..."inventive".
Best,
Ike
enginefire
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#4
5th April 2013
Old 5th April 2013
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I have thought about that, some kind of ground loop between the chassis of the two pres.

Here is a photo and a description of the grounding.

Voltage with one pre -23.8
Voltage with two pres -22.4

-24v goes to -24v in
0v goes to grd
Connecting grd to the input transformer ground removed the hum when I was testing each unit one at a time.

Right now I have PSU ground connected to pre 1 circuit ground, then one branch to input trafo ground and another branch to pre 2 circuit ground and from there another branch to pre 2 input trafo ground. I have experimented with connecting pin 1 on in/out xlr to ground, no change in hum, but it did reduce some higher frequency noise.

The power line input ground is connected to chassis.

Testing the two pres powered but not physically touching each other does not change hum.

Originally I had each pre with its own ground line to PSU ground.


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enginefire
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#5
6th April 2013
Old 6th April 2013
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If it's any help, this is what I get when I took up a scope... Admittedly, im not too sure how to really use a scope in any real practical manner, just learning how to read volts and offset. I was hoping to figure out the hum frequency but I dont know how to do that.



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#6
6th April 2013
Old 6th April 2013
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Ike Zimbel's Avatar
 

That power transformer looks suspiciously small to me...it might be working too hard under load OR, the regulator might be working too hard to maintain 24v because the trafo is sagging.
Best,
Ike
enginefire
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#7
6th April 2013
Old 6th April 2013
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I tried with another trafo and regulator (a much bigger one) that I used to power a pair of scully 280's. The voltage stayed steady at -24 v with both pres attached... Unfortunately the hum also stated steady.

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#8
6th April 2013
Old 6th April 2013
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Hi

Might not the AC transformer be radiating a field your audio transformers are picking up?

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enginefire
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#9
6th April 2013
Old 6th April 2013
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Thread Starter
Quote:
Originally Posted by Geoff_T View Post
Hi

Might not the AC transformer be radiating a field your audio transformers are picking up?

Posted from my iPhone
Yes that was an issue when I was testing with only one pre, moving the pre around varied the hum. I adjusted the position of the ac trafo until the hum was only barely audible with all volumes at max.

I thought that was the end of that, but as soon as I wired up the second pre the hum return. It sounded the same as the AC trafo interference hum, except it did not change when I moved the pres around (the ac trafo was stationary, just to be clear) .

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#10
6th April 2013
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Ike Zimbel's Avatar
 

Quote:
Originally Posted by enginefire View Post
I have thought about that, some kind of ground loop between the chassis of the two pres.

Here is a photo and a description of the grounding.



Right now I have PSU ground connected to pre 1 circuit ground, then one branch to input trafo ground and another branch to pre 2 circuit ground and from there another branch to pre 2 input trafo ground. I have experimented with connecting pin 1 on in/out xlr to ground, no change in hum, but it did reduce some higher frequency noise.

The power line input ground is connected to chassis.

Testing the two pres powered but not physically touching each other does not change hum.

Originally I had each pre with its own ground line to PSU ground.


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Hi,
When you say "branch" do you mean that the wires daisy chain (ie they are in series)? I think you should go back to the separate line to PS ground for each.
Another question: Does PS ground (0v) connect to chassis ground? It should. This is often done through a 10 ohm resistor.
After that, I would have all of the XLR Pin-1's go directly to chassis and not run them to the modules.
Best,
Ike
#11
6th April 2013
Old 6th April 2013
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Geoff_T's Avatar
 

Quote:
Originally Posted by enginefire View Post
Yes that was an issue when I was testing with only one pre, moving the pre around varied the hum. I adjusted the position of the ac trafo until the hum was only barely audible with all volumes at max.

I thought that was the end of that, but as soon as I wired up the second pre the hum return. It sounded the same as the AC trafo interference hum, except it did not change when I moved the pres around (the ac trafo was stationary, just to be clear) .

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Hi

I don't recall whether you said the a.c. transformer was toroidal or EI core (square looking beast).

If the latter, the EMF radiation can probably be detected on Mars.... Either go for a toroid or don't mount the transformer inside with the other components.

Just needing clarification....

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#12
6th April 2013
Old 6th April 2013
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Richard Crowley's Avatar
 

Enginefire said that it was hum-free with EITHER of the preamp modules alone. To my way of thinking, that would preclude magnetic field coupling from the power transformer.
#13
6th April 2013
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Geoff_T's Avatar
 

Quote:
Originally Posted by Richard Crowley View Post
Enginefire said that it was hum-free with EITHER of the preamp modules alone. To my way of thinking, that would preclude magnetic field coupling from the power transformer.
Hi

Unless the core was saturating as it could not power both...

Obscure chance, I know....

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enginefire
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#14
6th April 2013
Old 6th April 2013
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Thread Starter
Thanks for all the advice...
Ike
Originally I had the grounds on individual wires connected to the PSU 0v, when that didn't work I tried the series connection with no change.
I will try connecting the chassis ground and 0v with a 10 ohm, I had tried a few alligator clips with no change already.

Geoff
I tried another external PSU with a trafo more than twice as big, no change. The original PSU is a square, not torroidal.

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#15
7th April 2013
Old 7th April 2013
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Hi

Have you considered shielded wiring instead of the loosely twisted connections to the XLRs?

Won't fix the hum necessarily but is good general practice....

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#16
7th April 2013
Old 7th April 2013
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Richard Crowley's Avatar
 

It seems like it would be easy enough to separate the power supply away from the preamps temporarily to confirm or eliminate the magnetic field possibility.

Twisted pair like that isn't a bad construction method in that kind of situation. A grounding problem sounds more likely to me. Exactly where is everything connected. Particularly all the grounds. For example how are the XLR inputs handled? Does pin 1 go directly to the ground pin on the preamps? Is it also connected to the chassis ground at the connector?
enginefire
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#17
7th April 2013
Old 7th April 2013
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Right now the grounding is a little poorly done. I did not finish the grounding the XLRs, I was only playing around with alligator clips. I was planning to connect them to the circuit ground.

I was thinking about adding a grounding mesh sleeve to the signal wiring, and connecting one end to the circuit ground as well, but didnt because it was so quiet and nice sounding when I was testing with only one pre connected.

I did a little more testing tonight, and tried another power trafo (same 2nd testing one as before, from the PSU of a pair of scullys), but this time connected to a different regulator circuit, all outside of the pre enclosure. The last time I tried something like this, I was using an identical regulator circuit as is in the pre enclosure, which resulted in hum when both pres connected.

This time, I used the upgrade model that gives + and - DC rails (317 and 337 regs). I tested it using the - rail and NO HUM
I tested it also using the + rail, but with the wires reversed, and again NO HUM ...

I would like to proceed by looking at the power that is coming off of the two regulators to see if I can see the difference, and then change things until that difference disappears. Right now, the only obvious differences that I observe are:

1) the regulator circuit is different (from the same manufacturer)
2) the regulator circuit was far away from the power trafo that was feeding it, also far away from the two pres (as far as the wires would allow)

Ideally, I would like to use my scope to look at the power, Im not so sure I know how to do this properly. If that fails, I will use the same physical setup as tonights successful test but swap regulator circuits and observe the results.
#18
7th April 2013
Old 7th April 2013
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Set the vertical input on the scope to "AC" so you can look at the rails after cranking up the vertical gain on that channel. You should just see some "fuzz" on the scope's trace, indicating just low level noise.
#19
7th April 2013
Old 7th April 2013
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Richard Crowley's Avatar
 

Quote:
Originally Posted by enginefire View Post
....
This time, I used the upgrade model that gives + and - DC rails (317 and 337 regs). I tested it using the - rail and NO HUM
I tested it also using the + rail, but with the wires reversed, and again NO HUM ...
All's well that ends well. However those statements don't make any sense to me. You say that the "upgrade model gives both DC rails", but does that mean that the original model DIDN'T? And when you tried it "with the wires reversed", what wires are you talking about?

Quote:
I would like to proceed by looking at the power that is coming off of the two regulators to see if I can see the difference, and then change things until that difference disappears. Right now, the only obvious differences that I observe are:

1) the regulator circuit is different (from the same manufacturer)
2) the regulator circuit was far away from the power trafo that was feeding it, also far away from the two pres (as far as the wires would allow)
Neither of those things should have any effect on the symptoms you are describing.
enginefire
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#20
7th April 2013
Old 7th April 2013
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Thread Starter
Sorry I will clarify :
The upgraded model has bi polar dc output and ground. Two independent regulator ICs, one for positive one for negative.

The lesser model only has one regulator, and outputs positive DC and ground. I connected the pres ground to the PSU +24 output and the -v input to the PSU ground , to make the voltage appear to be -24 rather than +24 in relation to ground.

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#21
7th April 2013
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Richard Crowley's Avatar
 

Uni-polar and bi-polar power supplies cannot generally be exchanged. Do your preamp modules require uni-polar or bi-polar power? It is not clear from what you have written here that you even have the proper power supply for your preamps.
#22
7th April 2013
Old 7th April 2013
  #22
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Quote:
Originally Posted by enginefire View Post
My problem is this :
When I have both units connected to the power source, I get hum. When I have either unit connected on its own, I get no hum at all! Just great sound. I would really like to use these as a pair
If the hum happens only when both pre's are grounded then the way to fix it is to first; make sure the pin ones of the IN/OUT XLR's are connected directly at the chassis via their grounding tabs. To do this you will have to solder a bridge between the grounding tab and pin one. DO NOT connect the pre grounds to the IN/OUT XLR's! Second: connect both pre's chassis TOGETHER(!) and connect that mutual wire to the PS ground. That should solve the problem. If it doesn't solve the problem then you've done something wrong somewhere else!
enginefire
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#23
7th April 2013
Old 7th April 2013
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Richard,
It is functional supply for the pres, it works great as mono.

The power requirements are uni polar, wanting -24v dc. When using the bi polar supply, I was only using the negative side, as a test to isolate the offending variable.

Vitalis, I will try out the wiring suggestion you proposed and see... However when I switch PSU regulator circuits everything is fine as is. There are a few other variables that were involved with that test that I will also try and isolate.

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enginefire
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#24
8th April 2013
Old 8th April 2013
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It seems like the AC wiring was the culprit. I have eliminated the power switch (which was a lighted switch) and have kept the AC wires well away from the regulator circuit, and now the hum is gone.

I will wire a basic switch in the back next to the fuse tomorrow and retest, hopefully all will be good!!

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#25
8th April 2013
Old 8th April 2013
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Richard Crowley's Avatar
 

The mains power wiring needs to be twisted tightly for the "complimentary" reason that you twisted the audio wiring. (i.e. to reduce the transmission of the 60Hz magnetic field just as twisting the audio wiring was to reduce "reception" of ambient magnetic fields).
#26
8th April 2013
Old 8th April 2013
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Quote:
Originally Posted by enginefire View Post
It seems like the AC wiring was the culprit. I have eliminated the power switch (which was a lighted switch) and have kept the AC wires well away from the regulator circuit, and now the hum is gone.

I will wire a basic switch in the back next to the fuse tomorrow and retest, hopefully all will be good!!

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Hi

Although it fixed the issue, it doesn't sound right to me... The switch aspect in the first instance and inducing interference into the dc regulator board from the cabling.

You might certainly have sorted it but it was something else you did and may not have noticed, like firming up the ground connection that came in with the power.

I don't know.... It's just that the fixes didn't sound right for the nature of your problem.

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enginefire
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#27
9th April 2013
Old 9th April 2013
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I hear you... But its the only thing I have changed. I didnt touch the grounding at all.

What I dont get is why the ac inductance was a problem only when both pres were connected.

Thinking about it, I did route the -24v and ground wires a little differently. Just to know, I will reroute them as they were before and see if the hum returns.

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enginefire
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#28
9th April 2013
Old 9th April 2013
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I have come to the conclusion that this must be haunted, because I plugged it back in and the hum returned

My favorite type of probkems, intermittent ones!

For now I am just going to use one pre, it sounded great on vocals this morning with an ev635. When I get more play time I will try reworking the grounding to be "ideal" to see if that helps, otherwise it's off to see the old man for an hour of his time!

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#29
9th April 2013
Old 9th April 2013
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Quote:
Originally Posted by enginefire View Post
I have come to the conclusion that this must be haunted, because I plugged it back in and the hum returned

My favorite type of probkems, intermittent ones!
Hi

That's what I thought would be the next post as I could not see why the two original items you mentioned would make a darned bit of difference.

It's not haunted and there's no such thing as magic. Magicians are just smart illusionists.

Back to the case in point, I have always felt that we are only in possession of part of the facts and environment concerning your rack.

I would have given up and used an external power supply ages ago rather than fight this with one hand tied behind one's back due to not being there and seeing what's what first hand.

For all I know you could be moving your Weller soldering iron around in close proximity to the rack and at various times during the testing and that has been radiating and affecting things.

Or someone is turning on a lamp dimmer or your neighbour is doing some arc welding in his garage or whatever.

You need to take notes of every variable and every condition.

And maybe rewire that rack? Bad joints?

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#30
9th April 2013
Old 9th April 2013
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Ike Zimbel's Avatar
 

Good point, Geoff. Brings back not-so-fond memories of working on an AMEK Media 5.1 with some noise problems...all day and far into the night. Towards the end of the day, it felt like we were winning. Then, as darkness fell in the already poorly lit control room, it suddenly got worse, a lot worse. I can't remember how many hours later I figured out that the "worse" was coming from the transformer in the base of a high intensity quartz desk lamp that the owner had helpfully placed on the meter bridge!
Cheers,
Ike

Last edited by Ike Zimbel; 10th April 2013 at 03:30 AM.. Reason: punktuation %#@%#!
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