Seventh Circle preamp hum

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bradb

Well-known member
Joined
May 5, 2005
Messages
523
Location
Brooklyn, NYC
Hello everyone,

I've got some 7th circle preamps (A12, N72, J99) that have been giving me hum problems when I use them with dynamic and ribbon microphones. With condensers, the preamps are quiet as can be. Here are some observations:

1. Pre-amps CRANKED with no input mic cable, the pre's are quiet, typical low hiss noise signature. This is what I would expect.

2. UNTERMINATED (no mic plugged in) mic cable plugged into the pres' input gives a loud hum.

3. condenser mic plugged in, totally quiet, no hum. This is what I would expect.

4. ribbon mic plugged in, i get a hum that is the same as the unterminated hum, but its quieter, the hum changes in intensity when I move the mic and cable around.

5. dynamic mic plugged in, i get a hum that is the same as the unterminated hum, but its quieter, same as the ribbon mic hum, the hum changes in intensity when I move the mic and cable around.

The pre's have an internally mounted powersupply. This will give you an idea of what they look like: The pre cards and power supply are mounted horizontally.

http://www.seventhcirclestudios.com/SCA/CH01/ch01_about.htm

Some suggest that an externally mounted power supply would help but observation 1 suggests that this is not necessary. Does anyone have any advice on this or any more test that I could do?

Thank you
 
Good question, i THINK its 60Hz, but i'm not sure.

its definitly not a straight "hum", its kind of a hum/buzz. your "Zzzzzzzzzzzz" makes me thing that it is 60Hz fund with some harmonics in it.

what are you getting at?

thanks!
 
What do you get if you bridge the input XLR pins 2&3 with a 150 ohm resistor?

How are you connecting the output?

Is phantom power on or off (or even switchable)?

Is there continuity between all pin 1s (XLR) and the mains earth?

Observation 2 is troubling....are your XLRs (both connectors and cables) wired correctly?

Check phantom power voltages on pin 2 and 3 relative to pin 1....are they both 48V. Any chance you've stuffed some incorrect resistors in place of the 6k81 phantom current limiting resistors....or maybe mismatched?

Cheers,

Kris
 
If the hum is 60Hz with lots of buzzy harmonics, given your other descriptions, I'd bet on there being a TV station nearby, probably UHF. The fact that moving the mic and cable around the room makes the problem worse is evidence in favor of that theory. Other possibilities include a fluorescent light ballast that's putting out buzz, or a dimmer.

In all these cases, the problem is RFI -- radio frequency interference, modulated at 60Hz. RFI filtering is the cure.

Peace,
Paul
 
some other data points..

My hamptone and sytek pre's exhibit none of these behaviors, even with gains cranked, with the same cables, mics, etc.

The output is always going into my 002Rack... i can also connect the output to my mackie powered monitors (i haven't done this in awhile)

Phantom power is either on or off, it makes no difference.. I'll try the 150 Ohm resistor idea and investigate your other recommondations Kris.

I'm not sure if this is a tv station... since its not exhibited with my other pre's... i'm thinking its the power supply or design.

thanks guys,
 
"mmmmmmm", too many ground references (loop);
"zzzzzzzzzzzz", no ground reference(open). also, if it were not grounded, moving the cable around could act as an antenna.
 
Condenser mic working indicates it is grounded to something or else nothing.
Make sure you dont have a short from pin 1 to pin 2 or 3, I think it may be possible to have one side of the phantom shorted and a condenser mic still work.
Double check the values of the resistors too.
 
I have the same issues when using ribbons with these pres. Funny thing is, by turning the mic I can bring the noise in our out.

Facing east the noise is gone, facing west the noise is greatest. This is in the control room, only a foot or two from the pre.

When the mic is in the booth, attached to the snake (30ft) the same rules apply as far as turning the mic, but the noise never fully goes away.

I doubt the problem is a radio station, etc. since a few people are having this issue with ribbons.
 
UPDATE!
(by the way, this thread is cross posted on the 7th circle site)

I did some PSU tricks (removing some caps) that Tim from 7th circle recommened I try... In short... it didn't help, I observed the same phenemonon with and without the caps..

but I did notice something else during my test...

After removing the caps, I did a quick hook up on one of the A12s, into protools with an SM57 and the hum was gone! WOW. but wait... whats changed? I realized in my haste to hook everything up, I had used a 4' patch cable made from mogami 2552 and neutriks. This was the one piece in the signal chain that was different. SO I switched the patch cable with one of the original cables I always get the hum with. The hum was back and as normal. The missing caps didn't make a difference, it was mic-cable related. I switched back to the mogami patch cable and the hum was gone. Even without the cable being terminated, there WAS NO HUM with the mogami!!

I then went thru ALL of my microphone cables. All but 2 of the canare/neutriks hummed like mad. My el-cheapo's didn't hum. But why?

Now at this point i became a little confused. NONE of my cables hum with my other preamps. But with the 7th circles, SOME of my cables hum.

So i did some measurements on two identical cables, one hums, the other is silent. Both cables are 20' Canare, L-2T2S. With standard Neutrik XLRs.

-------------No-hum Cable -------------
Capacitance
1-2 1.00 nF
1-3 1.01 nF
2-3 .566 nF

Resistance
1 0.4ohm
2 0.7ohm
3 0.7ohm

------------Humming Cable -----------
Capacitance
1-2 0.566 nF
1-3 0.993 nF
2-3 0.982 nF

Resistance
1 0.7ohms
2 0.7ohms
3 0.4ohms

Notice anything?

It seems like there is an imbalance between the 2 and 3 lines on the humming cable and more of a balance on the non-humming cable. I would say that the fault is with my cables, but this "humming", "unbalanced" cable doesn't hum with other preamps.

Could the inputs on the 7th circle pres be such that a slight difference in input impedance could cause a hum... or incomplete cancellation?

Perhaps the condensers are quiet because their output impedances are large making the "imbalance" less consequencial?


Any ideas?

Thanks!
 
Also, I have complete continuity between pin 1's on pre's input and outputs and chassis ground and the 3rd prong on the IEC...

sorry didn't try the 150 ohm trick, will do it soon!
 
[quote author="bradb"]UPDATE!


-------------No-hum Cable -------------
Capacitance
1-2 1.00 nF
1-3 1.01 nF
2-3 .566 nF

Resistance
1 0.4ohm
2 0.7ohm
3 0.7ohm

------------Humming Cable -----------
Capacitance
1-2 0.566 nF
1-3 0.993 nF
2-3 0.982 nF

Resistance
1 0.7ohms
2 0.7ohms
3 0.4ohms

Notice anything?

Any ideas?

Thanks![/quote]

Is it me or does the resistance seem to be exactly opposite from pins 1,2,3, on one cable to the other? I am have always been a big mogami fan as there cable seems to be best cable for cost issue. a hell of a lot better then munster cheese cable which IMO is way way over priced.
I suggest opening up each cable and checking conections for continuity as well as connection to the shield/pin 1. I bet that there is a cold solder connection in there on the one with hum. could be floating a ground which could be your hum...
 
> SOME of my cables hum.

Who made the cables, and what have you not pulled the covers off and LOOKed what-connects-to-what?

Sure smells like mis-wiring.
 
[quote author="PRR"]> SOME of my cables hum.

Who made the cables, and what have you not pulled the covers off and LOOKed what-connects-to-what?

Sure smells like mis-wiring.[/quote]
wow we were looking in the same direction. I must be getting better at this. :shock:
 
Yeah, it makes sense that the shield would have the least resistance, since there should be more wire there (and maybe a foil too). If your shield is wired to pin 3 (- signal), that probably explains your problem. The capacitance numbers agree with this guess too. I would expect coaxial lines to have more capacitance than parallel or twisted lines. The 1-n lines should have the the highest capacitance. As PRR and Pucho said, check your cable wiring.
 
Hey guys,

1. thanks for all the replies and advice! i really appreciate it.

2. i made the cables, but they are wired correctly... there MAY be cold solder joints... I'll check that tommorow... (i just got home and need to sleep) I just did another check... For both cables, pin 1 is connected to pin 1, pin 2 to pin 2, pin 3 to pin 3, EXCLUSIVELY. there are no weird wiring problems.

If two lines are not balanced and they go into a cancelling input, then they will not completely cancel... correct? Even though the differences in impedances we are seeing are so minimal, couldn't they be the cause?

sleep now, email tommorow! and thanks for all of your continued help.



oh here's a dumb question... when you guys wire to neutrik XLRS with the solder cups, do you crimp the cup to the wire first or just put the wire into the cup, clamp the two into your handsfree thingy and heat and flow lots of solder in? Is this my recipe for disaster?
 
[quote author="bradb"]
oh here's a dumb question... when you guys wire to neutrik XLRS with the solder cups, do you crimp the cup to the wire first or just put the wire into the cup, clamp the two into your handsfree thingy and heat and flow lots of solder in? Is this my recipe for disaster?[/quote]
unless specified by neutrik those should not be crimped at all at least not with the ones I use. you should but the connector into a handsfree thingie in my case my panavise. The tin the connector by heating it and adding solder. Then tin my wire same manor. Last put them together. works 100% of the time. as for the wires testing out could be a short somewhere in the wire. I once ran into a mix cable that would work but would not pass phantom power. outside of that the cable was fine well as fine as it could be.
 
[quote author="bradb"]For both cables, pin 1 is connected to pin 1, pin 2 to pin 2, pin 3 to pin 3, EXCLUSIVELY.[/quote]

Yes, but you can still have all pin to pin connections OK, with no shorts yet still have problems if the shield wire isnt the one connecting pins number 1. If all you did was check for continuity and shorts without opening the cable up that wouldn't find the problem. For example:

Option A
Pin 1 <red wire> Pin 1
Pin 2 <black wire> Pin 2
Pin 3 <shield wire> Pin 3

Option B
Pin 1 <shield wire> Pin 1
Pin 2 <red wire> Pin 2
Pin 3 <black wire> Pin 3

Both test OK for continuity on the multimeter, but option A picks up Mexican Radio, option B does not.

-Chris
 
HOLY FUCK..... i almost want to cancel my account here and never post again...

Chris, you unmasked me as an idoit. My "good" cables are wired right.. my "bad" cables have the shield carrying signal.. I KNOW THIS! arrg. Oh well, we're all buddies here right? you'll keep my secret.

What the fuck is wrong with me? Looks like i've got some nights of cable fixing coming up..

seriously, what the fuck?! blame it on the flux... blame it on the flux..

idiot out..
 
[quote author="bradb"]HOLY FUCK..... i almost want to cancel my account here and never post again...

Chris, you unmasked me as an idoit. My "good" cables are wired right.. my "bad" cables have the shield carrying signal.. I KNOW THIS! arrg. Oh well, we're all buddies here right? you'll keep my secret.

What the fuck is wrong with me? Looks like i've got some nights of cable fixing coming up..

seriously, what the fuck?! blame it on the flux... blame it on the flux..

idiot out..[/quote]

Get over it :green:

I loved to watch how the measurements fed the reasoning process to the ultimate conclusion...

I laid out of this one but I am proud of the participants :grin:
 

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