boji

mu metal in the well...
« on: June 30, 2020, 01:40:21 PM »
In your standard, non-broadcast studio environment, would lining a backplane/power encasement with thin (due to costs) mu shielding be of any help, or would the 'returns' (pun) on such an investment be negligible?

« Last Edit: June 30, 2020, 01:46:41 PM by boji »


JohnRoberts

Re: mu metal in the well...
« Reply #1 on: June 30, 2020, 02:58:33 PM »
Generally aluminum and/or steel on 4 sides is enough.  Perhaps take more care if you plan to put a large power transformer inside the same box. High frequency switching supplies not only have smaller transformers but the magnetic fields could be easier to manage when done right.

I've used mu metal inside microphone transformer cans but they are unusually sensitive to magnetic fields.
 
JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

rackmonkey

Re: mu metal in the well...
« Reply #2 on: June 30, 2020, 05:36:10 PM »
The cost of the mu metal alone (thin or not) would probably be prohibitive. And thin mu metal isn't necessarily better shielding than thicker steel. I have a stack of very thin mu metal sheets, and typically have to use 4 layers - each separated by very thin tape - to get to 25 - 30 dB noise attenuation.
Whether you think you can or you think you can't, you're probably right.

boji

Re: mu metal in the well...
« Reply #3 on: June 30, 2020, 06:31:19 PM »
Quote
Generally aluminum and/or steel on 4 sides is enough.

...thin mu metal isn't necessarily better shielding than thicker steel

Ok thanks 'yall. If rolling the desk around the control room has an effect on noise levels, I'll make extra shielding a concern.

abbey road d enfer

Re: mu metal in the well...
« Reply #4 on: July 01, 2020, 07:45:31 AM »
Ok thanks 'yall. If rolling the desk around the control room has an effect on noise levels, I'll make extra shielding a concern.
Shielding is very uneffective when it comes to protect a mixer (or any kind of electronic equipment that extznds over several meters. Sensitivity to external magnetic fields depends directly on the inductance, which is also a direct function of length.
The best bus protection resides in the mixing scheme, in particular in balanced or ground-sensing bus, that actually cancelled noise differentially.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

boji

Re: mu metal in the well...
« Reply #5 on: July 01, 2020, 11:43:32 AM »
I suppose this hard-to-block noise, due to length-of-frame, is another way of saying longitudinal noise?  Thank you!

JohnRoberts

Re: mu metal in the well...
« Reply #6 on: July 01, 2020, 12:47:00 PM »
Shielding is very uneffective when it comes to protect a mixer (or any kind of electronic equipment that extznds over several meters.
I have never seen a bus several meters long. My last big un console was close to 2 meters wide, but a split with center master section meant signal buses were not that total length.
Quote
Sensitivity to external magnetic fields depends directly on the inductance, which is also a direct function of length.
I thought loop area was important for magnetic pickup. Longer bus length also increases capacitance. Bus resistance is generally insignificant.

Fairly standard practice is to put console PS (and transformer) in a separate steel chassis, so hum field has to penetrate through two separate layers of metal chassis to reach buses.

Bus length can also mimic an antenna, but the only mixer that I ever experienced RF rectification in the bus amp was a small 6x4 bus mixer I inherited in the mid 80s. Rather than stop production for an expensive and time consuming redesign, I simply dropped a bifet op amp in place of the bipolar summing amp and the RF disappeared.  8)

A decent, cheap bifet delivers about 3uV or so ein. Not a problem even boosted by the noise gain from combining 6 inputs. 
Quote

The best bus protection resides in the mixing scheme, in particular in balanced or ground-sensing bus, that actually cancelled noise differentially.
The differential bus can accurately transfer signals from a channel trip to master section feet away. Signal ground is a concept not a reference voltage, so there will always be tiny voltage potentials between all the sundry local grounds spread around inside a console. Differential buses allow us to subtract out those "ground" potential differences maintaining signal integrity.

JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

abbey road d enfer

Re: mu metal in the well...
« Reply #7 on: July 01, 2020, 01:09:54 PM »
I have never seen a bus several meters long.
Should I have written feet long?

Quote
I thought loop area was important for magnetic pickup.
If only loop area counted, a straight wire would have no inductance. There is nothing like a half-turn in a coil. That's how current sense transformers work.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

boji

Re: mu metal in the well...
« Reply #8 on: July 01, 2020, 01:46:27 PM »
Quote
Fairly standard practice is to put console PS (and transformer) in a separate steel chassis
Definitely part of the plan.  I imagine the length of the PSU cable matters as well. Every OEM cable I've come across is meters in length, and usually not shielded.  I'd like to make this cable as short as possible, and mount the linear power supply with its gigantic AC transformer directly under the console. 

Is a ~30ft, unshielded power interconnecting cable worse to have than a short interconnect, with the non-switching rack PSU very close to desk?  Edit: Going to guess if the psu's emf is low, is mechanically quiet, and doesn't get crazy hot, a shorter, under-console mount is better.



Quote
Differential buses allow us to subtract out those "ground" potential differences maintaining signal integrity.
Thank you Abby and JR.  Squarewave posted a section of a mack1e mixer that used this technique.  I added another three distribution rails to try this out on the summing PCB's at a later time--when I learn how to properly implement it!
« Last Edit: July 01, 2020, 02:02:54 PM by boji »

abbey road d enfer

Re: mu metal in the well...
« Reply #9 on: July 01, 2020, 02:10:48 PM »
I'd like to make this cable as short as possible, and mount the linear power supply with its gigantic AC transformer directly under the console. 
I wouldn't recommand that. Having a huge magnetic field generator close to the sensitive bus is a receipe for disaster.

Quote
Is a ~30ft, unshielded power interconnecting cable worse to have than a short interconnect, with the non-switching rack PSU very close to desk?
A well designed mixer circuitry should not be dependant on stiff incoming rails. It's the internal rails that want to be stiff enough. Most mixers have resistors in their rails. Stiffness is provided by local decoupling or local regulators*. You certainly don't want to transfer current spikes to a remote PSU via meters of cable. Current spikes must remain in the shortest possible loop.

* Local regulators can be more trouble than benefits. They tend to transfer current peaks to the rest of teh circuitry, when they should stay within, and under control.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.


boji

Re: mu metal in the well...
« Reply #10 on: July 03, 2020, 04:26:27 PM »
Quote
Having a huge magnetic field generator close to the sensitive bus is a recipe for disaster
Ok thanks Abby for helping me avert disaster. :)   Guess that's why console PSU's always come with a crazy long cable...  ::) ::)

Not to go on about it, but certainly I'll test to see how close is too close.  Imagine there's an inverse square law at play.


JohnRoberts

Re: mu metal in the well...
« Reply #11 on: July 03, 2020, 09:44:05 PM »
Definitely part of the plan.  I imagine the length of the PSU cable matters as well. Every OEM cable I've come across is meters in length, and usually not shielded.  I'd like to make this cable as short as possible, and mount the linear power supply with its gigantic AC transformer directly under the console. 
often a couple feet under the console...

 I've seen HF switching supplies built inside a console chassis, but HF switchers are a different animal to shield.
Quote
Is a ~30ft, unshielded power interconnecting cable worse to have than a short interconnect, with the non-switching rack PSU very close to desk?  Edit: Going to guess if the psu's emf is low, is mechanically quiet, and doesn't get crazy hot, a shorter, under-console mount is better.
long console power cords can experience IxR voltage drops. Designers can compensate for this in original design with extra rail voltage.

JR 
Quote

Thank you Abby and JR.  Squarewave posted a section of a mack1e mixer that used this technique.  I added another three distribution rails to try this out on the summing PCB's at a later time--when I learn how to properly implement it!
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

boji

Re: mu metal in the well...
« Reply #12 on: July 08, 2020, 07:37:18 PM »
Off initial thread topic, but related:  I'm about to run some 5ft interconnections from amphenol 10pin connector to bus bars with some shielded 18/3 wire, however I figured if bumping up to 14 or even 12ga would be better/safer (however slight),  now'd be the time to ask.   

Appreciate knowing that 'stiffness' needn't be retained from source to sink, but larger wires right after bus seems appropriate as a step down, especially since I have the space for it, and some nice 10A copper ring terminals. 

Thanks y'all.  No need to reply if you think 18/3 is more than sufficient- I'll take it as a sign to keep on truckin'.   8)

abbey road d enfer

Re: mu metal in the well...
« Reply #13 on: July 09, 2020, 12:28:13 AM »
Off initial thread topic, but related:  I'm about to run some 5ft interconnections from amphenol 10pin connector to bus bars with some shielded 18/3 wire, however I figured if bumping up to 14 or even 12ga would be better/safer (however slight),  now'd be the time to ask.   
Can you explain the aim? Any length added to bus makes me shiver. The potential for picking magnetically induced garbage is frightening.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

boji

Re: mu metal in the well...
« Reply #14 on: July 09, 2020, 02:11:01 AM »
Quote
Can you explain the aim? Any length added to bus makes me shiver.
Happy to, thanks for asking.  Aim is to deliver PSU's power to long, previously discussed cable, then from the cable to back of console via an amphenol connector, which is a 10 pin screw-on type, solder cups at the back. 
Need to get the power from that connector to the bus rails, somehow.

Should I instead consider bypassing the connector altogether where bipolar supply is concerned, and go straight to copper bars from the PSU's cable? I could still use the fancy connector to bring in power for relays, led's and digital stuff, is my guess.

Appreciate the shiver-lry


« Last Edit: July 09, 2020, 02:41:11 AM by boji »

abbey road d enfer

Re: mu metal in the well...
« Reply #15 on: July 09, 2020, 04:46:14 AM »
Happy to, thanks for asking.  Aim is to deliver PSU's power to long, previously discussed cable, then from the cable to back of console via an amphenol connector, which is a 10 pin screw-on type, solder cups at the back. 
Need to get the power from that connector to the bus rails, somehow.

Should I instead consider bypassing the connector altogether where bipolar supply is concerned, and go straight to copper bars from the PSU's cable? I could still use the fancy connector to bring in power for relays, led's and digital stuff, is my guess.
You shouldn't have to worry too much about resistance in the power lines, within limits of course.
As I said earlier, your modules should not rely on any stiffness from your power distro.
As long as your modules have proper decoupling, the dominant limit is the voltage drop; dropping even 1V in the wires is not significant. For a typical DOA, dropping the rails from 24 to 23V has no operational consequence.
It would take 40 meters of AWG18 to drop 1V at 10 amps.
The other limit is connector rating and cable heating. AWG18 is conservatively rated at 5A. According to IEC, it would be good up to 12A for continuous use.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Pusch3l

Re: mu metal in the well...
« Reply #16 on: July 09, 2020, 07:26:08 AM »
Maybe it's my inner German-safety-feeling but with 5-12A going through the cable i definetly would take another cable. AWG15 is what suites our 1,5mm2 the most. And that can take up to 16A without getting any headaches of a burning cable.


abbey road d enfer

Re: mu metal in the well...
« Reply #17 on: July 09, 2020, 07:35:34 AM »
It is important not to roll the cable on itself, like a solenoid, because heat builds up more than when the cable is laid flat. Another good reason for not making power cables too long.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

fazer

Re: mu metal in the well...
« Reply #18 on: July 11, 2020, 09:41:45 AM »
When I brought my api home years ago I put the power supplies under the board in a small rack case.  I noticed when using a ribbon mic (lots of gain). Some 60 cycle noise .  I moved it just 2 feet to the side and the noise was gone or at lease extremely quieter.  The buses all run under with layers of aluminum for shield but the 4 powersuppies were inductive when under the console.  Wire from the supplies are 16ga except for the ground wires that are 8ga.   There are bypass caps on the modules but no regulators in the board , just the 605 supplies.   I mostly use directs for routing.  The 2mix bus of course is used all the time and is the important one. 

Newmarket

Re: mu metal in the well...
« Reply #19 on: July 11, 2020, 04:06:58 PM »
When I brought my api home years ago I put the power supplies under the board in a small rack case.  I noticed when using a ribbon mic (lots of gain). Some 60 cycle noise .  I moved it just 2 feet to the side and the noise was gone or at lease extremely quieter.  The buses all run under with layers of aluminum for shield but the 4 powersuppies were inductive when under the console.  Wire from the supplies are 16ga except for the ground wires that are 8ga.   There are bypass caps on the modules but no regulators in the board , just the 605 supplies.   I mostly use directs for routing.  The 2mix bus of course is used all the time and is the important one.

"60 Cycle Noise" (or 50 Hz here in UK) and low order harmonics of will be mainly H-field and not bothered by aluminium or copper shielding to a significant degree. Need ferrous for shielding - "mumetal" preferably but it's not really practicable or affordable. Also note that it comes in different 'grades' that trade off shielding against saturation.
Practically distance is your friend - as you found - and twisted pair / differential electronics for good CMMR.


 

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