Mic. phantom power resistor values. - Page 1
Forum section: electronics / beginners

author: aneevuser
author_rank: Frequent Contributor
timestamp: April 08, 2020, 12:16:27 pm
content: What determines the value of the specced resistors for the various microphone phantom power voltages? I'm having difficulty finding anything that explains this at all.Would I be right in assuming that the values are highly non-critical, and are purely to limit maximum current to the capsule?I've read somewhere or other that they should be well matched to maximise CMRR, but I can't see why - they seem to be on the wrong side of the isolating capacitors to be able to affect CMRR at all. Is this CMRR aspect BS, or am I confused?

author: bob91343
author_rank: Super Contributor
timestamp: April 08, 2020, 05:24:21 pm
content: The only purpose of the polarizing voltage on a condenser microphone is to insure monotonic operation and to give the capsule something to work against. Other than that, it has no function and no current is drawn so the higher the resistance the better. Other factors come into play such as source loading and fault currents and voltage breakdown.You are correct that the resistor value is not critical. Of course it should be low noise, and an order of magnitude greater than the desired impedance level.CMRR needs to be defined before one can comment on it. The ground system can be a source of noise so needs to be designed carefully.

author: TimFox
author_rank: Super Contributor
timestamp: April 08, 2020, 07:04:08 pm
content: Some condenser microphones also have an amplifier (e.g. jfet source follower) that is powered from the phantom power supply.

author: themadhippy
author_rank: Super Contributor
timestamp: April 08, 2020, 07:38:13 pm
content: Phantom isnt just for condenser mics,DI boxes often like 48v up em.IEC 61938 is the rule book for phantom power and says 10mA is the max current

author: Cerebus
author_rank: Super Contributor
timestamp: April 08, 2020, 09:04:47 pm
content: Which flavour of phantom power? P12V, P24V or P48V (the commonest)?The standard resistors for 48V are 6.81k. Theexactvalue isn't too critical, butmatchingis. The resistors should be matched to 0.1% or better. The matching is necessary to avoid unbalancing the loading on the mike outputs and thus degrading the CMMR of the balanced signalling setup. Resistors for P24V are 1.2k, and P12V are 680R. I have never encountered anything other than 48V in professional gear, I've seen 12V in a few bits of cheap semi-pro gear and I don't think I've ever seen a P24 system face to face.As themadhippy said, the IEC rules say the maximum current draw should be 10mA or less for P48V (10mA for P24, 15mA for P12V). 10mA would be unusually high, 2mA or so is closer to what most mikes will typically draw. At 10mA a mike on 48V would actually only see 14V available after the drop across the supply resistors.As well as supplying current limiting, the resistors (in series) provide isolation between the hot and cold halves of the differential balanced signalling used. If the value dips too low relative to the mike's output impedance the differential load that they represent will reduce the signal level.

author: aneevuser
author_rank: Frequent Contributor
timestamp: April 10, 2020, 10:18:14 am
content: Quote from: bob91343 on April 08, 2020, 05:24:21 pm1. I don't know what you mean by "monotonic operation" - which variable is monotonic in which variable?2. "give the capsule something to work against" - I don't understand this either - you mean that it provides a bias voltage for a high impedance input?3. "no current is drawn so the higher the resistance the better" - this is contradicted by the page below, which quotes currents of up to 10 mA:https://www.sounddevices.com/phantom-powering-basics/

author: aneevuser
author_rank: Frequent Contributor
timestamp: April 10, 2020, 10:24:25 am
content: Quote from: Cerebus on April 08, 2020, 09:04:47 pmAll of them - I have no idea of the design process that starts with 48V or whatever and ends up with 6.81K - that's what I'm interested in. In general, if we have V volts phantom power, how do we get to the correct R ohms for those resistors?QuoteYeah, after my OP, I realised that I was wrong about matching - noob error.

author: Cerebus
author_rank: Super Contributor
timestamp: April 10, 2020, 01:23:26 pm
content: Quote from: aneevuser on April 10, 2020, 10:18:14 amThat's because the fella you're replying to has gone off piste and is talking about thepolarizing voltagefor a condenser microphone capsule not about phantom power iteself. The polarising voltage for a condenser microphone capsule might ultimately be derived from the phantom power supply, but it is not the same thing. Just forget about it for the time being.

author: Cerebus
author_rank: Super Contributor
timestamp: April 10, 2020, 02:29:46 pm
content: Quote from: aneevuser on April 10, 2020, 10:24:25 amYou've got two choices here:Take the standard values as holy writ and just use them.Go through the whole process of figuring out how the standard values came to be chosen and end up using them anyway.I suspect that you want to do the latter, which is fine, but it going to take some time*. There are two aspects to this, one is pure engineering and the other is the rather arbitrary process of how standards (that ipso facto have to serve many different people's requirements) get made.We can go some way to starting you on the engineering side of that. I'm not gonna touch the standards side with a barge pole.First off, let's go back in time to when phantom power was first used. Any microphone that might require phantom power wasguaranteedto have an audio output transformer - this was well before the point in time where a solid state differential output amplifier with adequate performance would have been practicable or affordable. There would have been a corresponding transformer at the amplifier end.So we end up with a system that looks like this:Microphone <=> transformer <=> cable <=> transformer <=> amplifier.So why do we have a transformer there? It provides:Galvanic isolationDifferential signalsFloating signalsImpedance transformationWhy are these desirable?Galvanic isolation - protects the microphone and amplifier (and their users) from supply voltage (mains and DC) that might be introduced under fault conditions.Differential signals - reject common mode noise picked up by cables e.g. mains hum, Radio 2 (i.e. RF interference, for some reason in the UK it always seemed to be the national Radio 2 LW service that broke through unless there was a taxicab company transmitter nearby)Floating signals - by dint of the differential signals being referenced off each other they need no ground reference. This mitigates problems from ground loops, especially on long cable runs when they are more likely to occur.Impedance transformation - permits a standard impedance to be presented to the microphone by the cable and by the cable to the amplifier. There's a whole bunch of other reasons, all the ones associated with impedance matching in general, but interoperability is the most important factor in this application.Phantom power rides on the back of the transformers. One winding of the transformer will be connected to the two signal wires in cable, the other will be connected to the microphone (or amplifier). One signal wire is known as 'hot', the other as 'cold'; there is also an overall electrostatic screen in the cable wrapped around the outside, containing both hot and cold wires. The positive side of the phantom power supply is connected to a centre tap on the transformer winding, the negative side of the phantom power supply is connected to the electrostatic screen. The current from the phantom power supply rides on both the hot and cold wires as an identical positive DC offset on both of them. On the other side of the transformers this DC offset cannot be seen, so it is as if we have created an invisible ghost wire to carry the power, thusphantompower.Here's what it looks like as a schematic:Image from Eargle's "The Microphone Book" p147In this setup a DC current is going to flow through the transformer windings. This is important because in general transformers don't like DC, it pushes the transformer core towards the point where it becomes magnetically saturated. When the transformer core is saturated it will distort the signal it is trying to transform, or in the worst case just stop acting as a transformer and pass no signal at all.So theprincipalrequirement for resistors in series with a phantom power supply is to limit the DC current through the transformer windings. Here's where we get into engineering trade-offs. Ideally we want the DC current through the transformers to be as little as possible, the greater it is the physically larger and more expensive the transformer will have to be. On the other hand, we want the supply current available to a microphone's built-in preamplifier to be as high as possible, the more current available the better we can make this preamplifiers signal-to-noise ratio. Two opposite requirements.So we have to pick a compromise maximum current. Arguing through the trade-offs here probably wouldn't shed too much more light, so let's skip to the compromise settled on which is 10mA maximum for a 48V supply. Calculating resistors to provide this current limit is, of course, just Ohm's law. For an output short circuit, that would be 48V/10mA = 4.8k. There are two legs to this, so two resistors in parallel, which would each have to be twice that value = 9.6k.But of course we're not intending to drive a short circuit, we're intending to drive a preamplifier, which is going to have a minimum supply voltage requirement. What you choose that to be is somewhat arbitrary - you need enough for the maximum signal you want to push down the wire, which will be relatively low, a volt or two maximum, plus biasing overheads and also you need enough to generate a polarizing voltage for a condenser capsule via some step up arrangement. Compromises again, discussion skipped again, the value picked for a 48V phantom supply was 14V to be left available to the preamplifier. With a 48V supply and a 14V load at the preamplifier we have to drop 34V in our current limiting resistors. Again, there are two in parallel so each will have to carry 5mA. We go and see Herr Ohm again and get 34V/5mA = 6800. The nearest standard (E96) series value is 6.81k. Job done, rinse and repeat for P24 and P12.* It did. Took me about an hour to write up, so you can see why people give terse answers on here.

author: drussell
author_rank: Super Contributor
timestamp: April 10, 2020, 02:40:02 pm
content: Cerebussaved me the time with the excellent explanation above...

author: Sam Spastic
author_rank: Regular Contributor
timestamp: April 10, 2020, 11:07:35 pm
content: Looks to me like a connection from the shield to the amplifier ground (-) is missing in the diagram.

author: The Soulman
author_rank: Super Contributor
timestamp: April 10, 2020, 11:16:34 pm
content: Quote from: Sam Spastic on April 10, 2020, 11:07:35 pmmeh, back in the day that connection would have been made by the connector and the microphone housing.

author: Cerebus
author_rank: Super Contributor
timestamp: April 11, 2020, 01:12:46 am
content: Quote from: Sam Spastic on April 10, 2020, 11:07:35 pmAs long as you mean the mic preamp, yes. There's a connection dot there in the schematic but it looks like somebody didn't draw the wire in

author: aneevuser
author_rank: Frequent Contributor
timestamp: April 11, 2020, 09:19:47 am
content: Quote from: Cerebus on April 10, 2020, 02:29:46 pmThe latter is indeed my goal, but it's not just out of idle curiosity, with which I am overly blessed. I have bought a condenser mic. which claims to run off 9-48V phantom power - I won't be using a standard voltage, so I want to understand the requirements which constrain the choice of resistor.QuoteRight. I see what's going on, more or less. In my scenario, the mic. claims to work off 9-48V phantom power, and draws 3 mA typically. No transformers enter the picture.So if I understand aright, I will merely need to choose the resistors to limit current to a bit more than 3 mA, based on my chosen voltage, which will probably be a nominal 18V - two 9V batteries. And match them as close as possible.Or have I missed some important point?Regardless, that was a very useful summary of the overall picture. Chapeau.

author: Cerebus
author_rank: Super Contributor
timestamp: April 11, 2020, 10:08:26 am
content: That sounds fine. Just remember that your resistors (in series) will appear across the hot and cold, so will appear in parallel with the load impedance. That is why you want them as high a value as possible so that they don't drag your signal down too much. Also, don't forget to add suitable DC blocking capacitors on the amplifier side, if they are not already there.

author: aneevuser
author_rank: Frequent Contributor
timestamp: April 11, 2020, 10:53:58 am
content: Quote from: Cerebus on April 11, 2020, 10:08:26 amThanks. However, I've just realised I'm not sure what voltage the mic. capsule needs to see. I'll have to know that to calculate the resistor sizes. Is there some rule of thumb here? I've been unable to find this kind of detailed info. about the mic. on the net - it's a Marantz MPM1000.QuoteStand by for further noobish questions about this aspect - I've discovered it may not be as trivial as I first thought...