EEVblog #616 - How Microphone Phantom Powering Works

I think that the next thing we'll have a look at is some real microphone designs some mine some legacy designs in order to do this we'll probably want to have a look at phantom powering found empowering of microphones has been used for 60 years or more and it's a well known technique for running power to a microphone down the same wise as you're putting the audio back up now let's do it over on this side we have the the mixer or the microphone preamp or whatever it is that's going to receive the microphone signal down over on this side we have the microphone itself and in between we have a twisted pair it's got to be twisted pair yes with shield with typically XLR connectors yes okay in preferences Anthony yeah okay why do we go to all of this effort of using twisted pair with screen instead of just plain coax or even just twisted pair well first of all the screen keeps out electrostatic noise electrostatic noise as any noise which is capacitively coupled from say a high voltage wire down he remains wire into these conductors so the screen keeps out the electrostatic stuff the fact that they are balanced twisted pairs keeps out any of the noise that can be coupled by magnetic fields 50 Hertz transformers adjacent transformers current-carrying conductors that kind of thing so typically what we've got over here in its simplest format might be say a dynamic microphone which is a coil and over here we've got effectively an instrumentation amplifier whose gain we vary and that might wrap around there is a again a Faraday shield electrostatic screen and that's grounded over there so that any capacitive current that's induced into there has somewhere to drain to that's a mistake that some of the Americans tend to make in their hundred ten volt here they kind of actually forget to connect that ground to real ground as a result of which any mains leakage entering there makes the shell of the microphone effectively electrically hot so grab hold of a microphone grab hold of a grounded microphone stand and see the shaking yeah all right now the whole purpose of an instrumentation amplifier is that it will respond to differential variations in the voltage between those two conductors the differential ant yep but it won't respond to common mode changes that means that we can induce quite a lot of common mode noise on that and it's completely ignored at the output over there that's all fine and good while you're using a dynamic mic all falls over when you want to use a circuit over here that draws some power this is where we adopt phantom powering whereby in its original incarnation going back six seven decades however long we would put a transformer in there mmm instead of having our solid state differential amplifier that would typically then go off to a valve stage sender tap that connect that via resistor to a supply voltage over here we'd have a transformer from which we could extract a voltage so between there and there we've got a DC voltage that powers the circuitry which is in turn driving that transformer okay the DC path because it's flowing through both halves of that transformer gives a net zero magnetic or magnetization of the core so how'd you put the the core and this transformer are not being stressed by any DC magnetization same thing applies over there the how'd you put it the standard p48 phantom powering uses a 48 volt supply a three-point 3k resistor excuse me which limits your had to put it your short-circuit current over here or something like about I think it's 1416 milliamps something about there mostly though you might be using about eight or ten milliamps consumption there if you're pulling ten milliamps here well that causes 33 volts drop across there at 48 volts drops to about what 15 volts and so you're getting 15 volts at 10 milliamps there yeah that's enough to power a fair bit of analog circuitry absolutely my plus may be light ups immediate leads or something like that if you really want it a variant on that if you don't like transformers and let's face it these days who does because they're expensive and their bandwidth limited and the only kinds of people who really like transformers are the ones who also like tunes ok let's take that away instead of using a Senate center-tapped transformer a pair of 3 k3 resistors will feed each leg from something around about double that resistance if you want to round off cut let's call it 6 k-8 each from the 48 48 volt supply so far so good we're still feeding them balanced oK we've loaded the line by a total of what 13 K but a big deal because we've generally got something with fairly low source impedance down there capacitor couple off that into our differential amplifier beauty that's what we do down that end okay what do we do up here hmm we can do a few things the easiest one is to pick up from both of those some DC voltage okay which we can then come over and we can do things like zina regulate those with reference to that ground there and then use that voltage there to run our circuitry which we then use to drive back into these fellows here capacitively coupled now if you've got a balanced source over here yeah then beauty you don't have to go any further now is differential driver yeah and incidentally an example of that might be a very simple cat circuit consisting of resistor say FET and resistor any voltage that you put in there appears there and in anti-phase there so you can quite happily couple those straight over there nice not so nice because the source impedance there is low yeah the source impedance there is equal to that resistor there so we haven't maintained a balanced impedance and it makes the thing susceptible to noise one of the cleverest circuits I think was come up with by the company shopes microphone company SCH o EP yes I'm not certain my fact to you but I think this is who came up the circuit whereby anything you get one of those and now this is going to be tricky to draw these are going to be triggered at all okay round okay over here basically what we're doing is feeding into the emitter of a PNP transistor and into the emitter of the PNP transistor those collectors go into the thing that forms our positive supply which is actually used there those bases there are starting to look like a differential yeah yeah are biased down of the collectors with high values esters keep you the about 100k and we capacitor couple of signals on to those bases oh that's an elegant circuit diagram now it's just a different and it's like a differential front and honor pop in it is rather the only unusual features are the fact that this Xena here has kind of locked the collector voltage which in turn locks the base voltage of being maybe yeah half a volt higher you have any on the bias currents flowing up here into those bases through those resistors it's very close the emitters are only about a volt in a bit away from the collectors yeah and fixes those at pretty much that voltage nice low output impedance because there any of the followers exactly these resistors act as the load resistors for those emitters it's it's elegant it's nice I like it yeah and is it still used aa half a Brazilian Chinese studio condenser microphones can't be wrong they're already using this this kind of type topology most and it doesn't have a lot of downsides one of the few downsides is that I'm just trying to think if for some reason you get a short from one of those lines to ground ah yes you've got enough capacitance there that and that's TBD sitting in 15 volts you've reverse avalanche those transistors by shorting that to ground you can basically cure that by putting in a couple of reverse bias diodes they're going few sensible enough to do so and that fixes that problem the other problem involves this phase splitter over the frontier which on a good day from there to there you get a gun if about 0.7 or 0.8 it's not a particularly good follower JJ FETs they're a follower but they're not as solid as a bipolar so typically from there to there it'll have a gain of about 0.7 and therefore from there to there it'll have a gain of about minus 0.7 total gain from there to there about 1.4 1.5 okay so far beauty or good the only problem is oK you've got this very small source capacitance feeding that I've run out of board space here Oh Keith um okay that's being fed by yeah maybe fifty puffs depending on the v that you're selected there it's going to have capacitance here well and it's going to have capacitance there typical order of magnitude let's call that fifty puffs and let's call that about say 10 puffs its benefits generally yeah no course these are using these aren't tiny tiny fits these are next level up in Junction size etc a reason because you're after fairly low noise there might be ten puffs and that might be say about thirty puffs okay now here's where mr. Miller are you ever seen okay from that point there to that point there we said we had a gain of minus 0.7 mm-hmm so the total voltage from there to there if you like is 1.7 the input voltage that 10 puffs looks like a about a 17 puffs capped out of there on the other hand that 30 puff cap well we've got one unit of voltage there point seven there point three across it point three times 30 is about nine puffs so basically our fifty puffs capsule and he is loaded by an additional twenty six puffs of capacitance that's you it's fairly large well 26 puffs against 50 puffs you're looking at probably three DB down three to be attenuation compared to the open circuit voltage there so it's a bit of a downside ah the other downsides are limited linearity not so much due to that but due to the fact that these can only swing so much remember I was saying that they're only yeah volt and a bit away from there so it can only scream down so much and they can only swing down so much consuella as far as you feel like yep so limited output Headroom on the outer circuit but still immensely successful developed I think probably three four decades ago now back back in the dawn of semiconductors almost bloody good circuit next line okay but let's improve that little okay we still phantom powering over there redo my termination that's still the typical configuration used today absolutely by everyone yep everything from your little cheap Mackie or behringer right X's through to your monsters and through to some of your multi-thousand dollar single channel studio preamps they'll be doing that once you go beyond you a couple of thousand dollar into your $10,000 plus stuff you're probably looking at a transformer again because that's the wanker market that you do exactly okay hmm oh I'm gonna get shot down a flame I'm going to hell now on that last circuit this shop circuit I did point out the fact that the impedances there were both low and balanced yes okay so we get all that benefit of always rejection common Mon note and comment common mode noise rejection in the system did the transistor pair have to be matched over here not really a lead but year ten or twenty percent of the driveway that's mainly more about DC bias conditions and about anything else okay let's once again and we matched no it doesn't matter and not hugely it helps right but it's not a decider let's once again extract power off those with a couple of resistors whack that into and do two great big electrolytic etc absolutely whatever regulation you feel like and again we're coupling signal into those the main criteria that we're after is to get the impedance on those two matched yeah now what happens if we've got say a lovely single ended my on what about the mismatch on the caps that's on the caps yes it does come into the sleeve of things yeah you have to make those caps large enough yep that over the frequency range of interest exactly and especially down to the magic fifty Hertz mark mm-hmm that their contribution to really let let's just say that's the impedance of those two or the overall system over here you might be looking at system impedances of well say 6k eight say hmm you want that to be maintained down to well below 60 Hertz before they decide to start absolutely going up at different break points yep so you want the air you want the reactants to be low enough at 50 Hertz compared to the system reactants that'd be too mismatch there doesn't hurt and it only really takes effect down in the some Hertz region nice and easy so aintry typically looking at using electrolytic yes so hence why I said because there you know you'll be plus minus 20% or some horrible I plus 50 yep yeah so they're going to be plus minus 50% yeah just put them down at such a low friends here the plus minus 50 doesn't matter matter yep so you've come up with this circuit you've got a lovely preamp there at single-ended mmm-hmm what do you do that's real easy we've got as far as here we've developed the world's brilliant asingle ended preamp what the hell do we do okay how about we add more why did you dye design a single-ended to begin with why didn't you do a different ID differentially up basically because defense are going to be intrinsically always noisier than a good single ended hmm there's another video yes let's couple that on onto the virus Easter so we get a defined output impedance this is typically got a near zero output impedance but just for the hell of it will might give that yeah maybe 50 ohms or hundred ohms or 200 ohms or something like that and let's simply match that impedance and connect it to ground and all of a sudden we've created a microphone which only uses a single-ended gamify but still has completely balanced impedances so we get all the benefits of system noise reduction and system immunity from induced noise I used to do just with simplicity right because we don't need a balanced apology out far anymore got it let's have a look at a balanced topology and fire let's do it you you you

Channel: EEVblog

Views: 68,732

Rating: 4.9335642 out of 5

Keywords: Phantom Power, microphone, Schoeps amplifier, amplifier, circuit, design, preamplifier, tutorial, transformer, common mode noise, xlr connector, jfet, transistor, 48v phamtom power

Id: e5xenXTwAzo

Length: 20min 12sec (1212 seconds)

Published: Wed May 14 2014

Reddit Comments

[u/czdl]

Everyone should watch this video and it's ilk. It's a beautiful demonstration of how gear designers (who are good) actually think about their designs.

Seeing him reminisce about the Schoeps design for a bit before then showing what's wrong with it was awesome.

We need more videos with this guy. Anyone got any?

Edit: it starts here: http://youtu.be/ihAG6cMpUlY

[u/RandomMandarin]

The "wanker market" heheheeh.

Wait, I think that includes me.

[u/jaymz168]

He's clearly in the "accuracy is everything" camp. And you don't need to spend 10k+ to get a preamp with a transformer in it.