Should phantom power be supplied by a linear supply?

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moamps

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The mixer I commented on was designed in 2007. Greg left Mackie in 2006. So surely some novice engineer designed this one ;).
 

abbey road d enfer

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Unfortunately I don't have a schematic of that mixer but I have one from his younger brother (402VLZ3) and here's what grounding looks like. This one was obviously designed by some novice engineer.
Would you care to elaborate? I don't see anything gross in here. Maybe I'm not concentrating on the right part of the circuit...
Of course I wouldn't extend this to a large mixer.
 

JohnRoberts

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The mixer I commented on was designed in 2007. Greg left Mackie in 2006. So surely some novice engineer designed this one ;).
I thought the 402 was an old model from last century... rebranding it with extra letters after the numbers does not make it a scratch design.

I used to manage a mixer engineering department and even junior engineers working for me did not have free license to start with a blank sheet...

That said, Mackie did not have a great reputation for RFI rejection back last century when I was still paying attention to the competitors in the market.

JR
 

moamps

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Would you care to elaborate? I don't see anything gross in here. Maybe I'm not concentrating on the right part of the circuit...
Of course I wouldn't extend this to a large mixer.

I don't see anything gross in here either.

That post of mine was written with a certain dose of irony. In my post # 100, I asked why a tripler would not be used instead of SMPS for phantom power, and I was criticized for not being able to make phantom power 0V in the only correct way. After a long discussion that followed, we went back to square one, where in a similar way Mackie did the grounding as I suggested, and was promoted by my critic that he did it the right way. BTW, I analyzed more Mackie power supply designs, and everywhere I found that the phantom power 0V is connected directly to the audio ground in the power supply itself and not connected separately to the chassis.
So, if I suggest something, it’s not good, if it’s done by Mackie or any other manufacturer (Soundcraft or whoever) then that’s just fine. This is not the first time such a situation has happened to me here, and I must admit that it is nothing new to me.

To be completely clear I will write a few words about the grounding design of small mixers, from my perspective. Small mixers are generally designed in such a way that all the parts (most are in SMD technique nowadays) are mounted on one large PCB located just below the front panel. The front panel is made of one piece of bent iron sheet. Mutual mechanical fastening is made with potentiometer mechanics, XLRs, TRS connectors and distance bolts. The TRS connectors on Mackie mixers (and Behringer, etc.) do not have an insulated sleeve contact so the audio ground is connected to the front panel / housing at many points. Additionally, the electrical integrity of the housing is increased by using bolts that are typically made of brass that can be electrically connected to the audio ground on a PCB. That is why it is quite difficult to distinguish between audio ground and chassis 0V reference, and this design cannot be used as a school example of how best to perform ground wiring. This is done similarly to older versions of the vacuum tube gear where the whole mounting chassis is used as a large audio and power ground at the same time.
 

abbey road d enfer

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After all these discussions, why not create a new thread: "Should phantom power be supplied by batteries? After all, that's how the concept started. :)
In somewhat exoteric circles, there are discussions about providing phantom individually via photocells lit by high-efficiency LED's.
 

JohnRoberts

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I really hate being forced to defend Mackie, and by extension Behringer (cough) since they borrowed liberally from Mackie's successful SKUs (I don't feel left out as they copied one of mine too).

It is fairly common to use the chassis as shield ground. Note that "pin 1 problem" advice is to bond pin 1 to chassis ground at inputs. Mixers don't routinely use chassis ground for carrying power ground (while routinely connected). I have seen that strategy used inside power amps to deal with the large currents involved. Not as the only power ground path but in parallel with wires to lower resistance.

Judicious use of differential circuitry helps maintain signal integrity coming and going despite different local chassis ground potentials.

I look forward to reading your small mixer design advice, but I have been out of those trenches for decades.

JR
 

Bo Deadly

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The TRS connectors on Mackie mixers (and Behringer, etc.) do not have an insulated sleeve contact so the audio ground is connected to the front panel / housing at many points. Additionally, the electrical integrity of the housing is increased by using bolts that are typically made of brass that can be electrically connected to the audio ground on a PCB. That is why it is quite difficult to distinguish between audio ground and chassis 0V reference, and this design cannot be used as a school example of how best to perform ground wiring.

moamps, you are paranoid and you misrepresent the truth. I'm not going to go back and fourth with you about this just because your knowledge has been challenged. The older Mackie schematics from like 20+ years ago clearly had a simpler grounding scheme. Yes. And then they changed it to isolating the chassis from 0V and using the chassis for phantom return. The VLZ3 and presumably VLZ4 do in fact isolate the chassis from 0V and so your statement is simply false. Look at the schematic fragment from the VLZ3 input I posted in #131. The sleeves are NOT connected to 0V and there's a very prominent comment about GROUND "SWEET SPOT" where all the grounds are connected together. Does the fact that they made that change not tell you something? Mackie has sold a helluva lot of mixers over the years. The ones designed within this century are actually well designed mixers. The VLZ3-4 have ground cancelling sum buses. And Behringer also makes a lot of perfectly good products. Your claims are not founded in reality.

Now I know you're going to reply and say I misrepresent what you said or I don't understand E at all or you're offended or whatever. I don't care and I'm not going to reply to your posts because it's just not constructive. I did find this thread interesting though so I'm not annoyed. You can learn a lot from a good argument. But I just don't think this is going anywhere anymore.
 

moamps

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I will just say that I am not criticizing the design decisions in Mackie etc. (Soundcraft did it in a similar way) but I am curious why I am criticized when I propose a similar solution?
.... And then they changed it to isolating the chassis from 0V and using the chassis for phantom return. The VLZ3 and presumably VLZ4 do in fact isolate the chassis from 0V and so your statement is simply false...
Is it?
 

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Paul Wolff
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A few of us have designed and sold many consoles. A few of us haven't. I would suggest that taking advice from the ones that have would probably be the shortest path to success...
 

moamps

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..The sleeves are NOT connected to 0V and there's a very prominent comment about GROUND "SWEET SPOT" where all the grounds are connected together...
You're right, but can you tell me what is audio 0V reference for a connected compressor to an insert point? Does he "see" the difference between mixer chassis reference and audio 0V? That's why I said "That's why it's quite difficult to distinguish between audio ground and chassis 0V reference, and this design cannot be used as a school example of how best to perform ground wiring."
 

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abbey road d enfer

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It's not terribly difficult to engineer the grounding of a mixer where ALL connections are balanced.
The challenge is when there are unbalanced connections.
Good performance can be achieved when the connections are ground-sensing; it implies the ancillary equipment has balanced or differential inputs and floating or ground-sensing outputs, in conjunction with an adequate wiring, that use 2 cond. + shield cable, with the cold point joined to the sleeve at the unbalanced side.
Then the responsibility of the mixer designer is to provide clean unbalanced signals.
Failure to obey these constraints in the ancillary equipment results in compromised performance (noise).
 

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So again, the details are in the implementation. Don't roll your own. Use some MeanWell modules with CMs and input filters. It can be done. Whoever did that supply just whiffed. It happens. One data point is not conclusive.
Myself and Atomic use the MeanWell supplies, they are very well made.
 

abbey road d enfer

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Do you know of any MeanWell module suitable for a couple of mic channels (ie very light load)?
No. The only remotely possible soluttion would be using two IRM-01 24V in series, for a max current of 42mA. If you add an LED that draws 10mA, you still have enough for 3 P48 mics.
I can't vouch for these anyway. I've had much disappointment with PC mount smps.

EDIT: actually, it seems there is a 48V version of the IRM-01, which can deliver a max of 21mA, so should be capable of delivering a few mA for an LED and enough for two P48 mics, albeit with current slightly limited in regard to the norm.
BTW, the datasheet says "no minimum load".
 
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Bo Deadly

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Do you know of any MeanWell module suitable for a couple of mic channels (ie very light load)?
Maybe. For a "very light load" you are going to run into a problem because you have to load an SMPS enough to stop it from modulating the output. I think I've already posted my Rules for SMPS in Audio about 50 times here but look at #2. So just about the smallest you'll find is something like APC-16-350 which I have used before and works very well. But it puts out 350mA which is enough for about 30+ phantom supplies. If you only have say 2 or 3, that means you have to basically burn 100mA just to keep the output from modulating. You could just slap a resistor across the output but 48V * 0.1A = 4.8W which means you probably need one of those aluminum cased resistors which, altogether, starts to make for a pretty chunky and clumsy solution. So it's not really right but it could be made to work if this is just for fun or a bench sort of project. The smaller ones like IRM-01 24V that Abbey mentioned might work but I have found that the output voltage of the smaller DC/DC converters can droop a bit under load.
 
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