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Hello,
A while back I made a simple 4 into 1 mic pre/mixer using SSM2019 mic pre chips and an LM833 op amp mixer. It worked pretty well but didn't have phantom power. I recently purchased a mic requiring phantom power, so I want to add phantom power to at least one of the inputs.In looking at how best to do this, some questions came to mind:
First off, I see most designs use some sort of regulator to get a ripple free regulated 48 volts out. The question I have about this is why do you need a regulator when A) the mic only draws maybe a milliamp or two, and B) you lose all your load regulation across the two matched 6.8K resistors that feed the +48 volts to the mic.
This leads me to think that the main advantage might be in reducing ripple, but wouldn't a few stages of R-C filtering do pretty well at this for such a low current draw?
This also makes me wonder about the specs for various mics. The mic I am planning to use (Shure Beta 87A) has a phantom power range of 11 to 52 volts DC. It says nothing about 'ripple rejection' so I am wondering how much ripple on the lines is acceptable? Maybe you do need that regulator after all?
Also, is that 11 to 52 volts *after* you drop a good 4 or 5 volts across the two 6.8k resistors? In which case, one could start with, say, a 55 volt supply and after the mic is drawing it's 1.2 mA, be feeding the capsule with 50 or 51 volts (for maximum headroom)?
Is there a standard spec document for phantom power that everyone designs to?
Thanks in advance to anyone who could shed light on this subject!
DF
Follow Ups:
not a power supply decoupling resistor. I would recommend going to the Jensen TransformerWebsite.
http://www.jensen-transformers.com/apps_sc.html
Look in SCHEMATICS
APP AS016 and a bunch of other schematics have directions on how to implement it. You need to match the 6.8k resistors to better than .01 % with a digital VOM so buy more than two. Buy like a 100 and go through them. You can also buy a Power One or Condor 48 volt power supply that has an adjustment pot on it. Yes, you need a voltage regulator as a ripple filter. Some other schematics have LEDs and a charging cap and a switch. You just have to search and look around. Good luck! Ray
"Polarities are seldom aware of their polarness." Prospero
FYI - I have a Power One power supply to power 4 mic pre cards, and it's been working like a champ for about 25 years. (Although I'm using outboard Studer 48v for phantom.)
nt
"Polarities are seldom aware of their polarness." Prospero
(1) It is necessary to get rid of any ripple: since the 2 branches which carry the phantom power (the mic balanced outputs) cannot have exactly the same DC source resistance (that is the 2*6.8kohms resistors and the xformer windings), the power current would give way to a differential voltage between the 2 branches. If it is DC, it's OK. If it is 100 or 120 hz, it is much more nasty. An imbalance of 0.1percent would give a 1.1mv hum from a 0.1v ripple, which would be unacceptable.
(2) The regulator is the simplest solution in terms of components, cost and reliability. Why don't you like it?
(3)> Also, is that 11 to 52 volts *after* you drop a good 4 or 5 volts across the two 6.8k resistors?
Yes.
> In which case, one could start with, say, a 55 volt supply and after the mic is drawing it's 1.2 mA,
> be feeding the capsule with 50 or 51 volts (for maximum headroom)?
No.
(1) You are at risk of a failur.e, if for some reason, the oscillator takes longer to start than usual. Current would be very low, and voltage above the specs. Maybe this voltage above the specs would make the oscillatior stalling, so, so, so...
(2) anyway, you don't need it.
(3) nothing to do with headroom. 2 uses for phantom power: polarization for electrostatic mikes (comes from a rectified stepped-up output of a stabilized self-sustaining oscillator buffer, so voltage doesn't change with phantom power) and feeding the output buffer (and since the buffer FET is fed through another FET used as a current regulator, there is no changes with phantom power.
So, stay between 30 and 48v and you are done...
You might be just the guy to answer this: If headroom isn't the issue, why do we always hear that if the polarizing voltage is "too low", that it affects the mic's ability to handle higher SPLs?Thanks.
Frankly, it sounds to me an urban legend.
Maybe it can ( could likelier) be true of one specially bad designed mike which gave birth to the motto?
All mikes with phantom power are to be built to work from 11 to 52 volts as specified, which includes the SPL at a given distorsion. Or it's just poorly designed. Which I don't believe from all manufacturers who want to stay on the market...Maybe some will disagree, in this case, I would be curious to have them ask and read the manufacturer's explanation on this one (they would be admitting their mikes does not work as specified!)
BTW, this large span of voltages is typical of a standard built after many manufacturers had their proprietary solution having worked on field for some time. So, the standard is build in order to accept all the oddities and choices everybody did.
A standard built from scratch would specify for example 22V minimum, 28 volts maximum, drop-outs shorter than 10 ms...In the same vein of late standard, ISO7816 (smart cards) is a marvel: each reader shall accept data with parity, without, inverse or direct, msb first or lsb first, intervall between bytes in 4 versions, etc etc... Such diversity brings nothing, but it's the way things are...
Thanks, Jacques! I will look back at some likely sources where I saw this. I'll let you know when I find it in writing.
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