Cheapest way to provide Phantom

Noiz2 said:
If you can solder and read a very simple schematic you could wire it up yourself.
Click to expand...

Daisy-chaining five 9V batteries together will provide a dead-quiet 45V that will last for the better part of a year in P48 applications. I would be surprised if the NTG3 would have a problem at 45V.
 
For you DIY'ers, this may be helpful

Deleted - see Noiz2 and Richard's posts below for better suggestions

For non-DIY'ers, this works (We have one in our kit for those rare occasions we're doing something wierd)

http://www.bhphotovideo.com/c/product/133794-REG/AKG_2198_Z_00020_B18E_Portable_Battery.html

These can often be had used - check the usual online sources.


Edit - Another thought. Duct tape that Rolls PB23 to a 12v lantern battery - a plug cut off an old wall wart (or $3 worth of cord & plug at radio shack) and you're in business.
 
Last edited:
Richard Crowley said:
Daisy-chaining five 9V batteries together will provide a dead-quiet 45V that will last for the better part of a year in P48 applications. I would be surprised if the NTG3 would have a problem at 45V.
Click to expand...

Yes but you have to add resistors and blocking capacitors so it's not just daisy chaining the 9V.

EDIT: no capacitors.
 
Last edited:
pixelated said:
For you DIY'ers, this may be helpful

http://www.tangible-technology.com/power/Phantom_Power_connect.html

(just add Dick's 5 9v batteries. )
Click to expand...

Interesting. Eddies good so this must work but the one's I've seen have blocking capacitors on the two phantom leads to block crosstalk, this one just counts on the two resistors knocking the crosstalk down to an insignificant level. I think I would still use the caps though.

Ignore the bits in RED, I was having a rectal cranial inversion.
 
Last edited:
Noiz2 said:
Interesting. Eddies good so this must work but the one's I've seen have blocking capacitors on the two phantom leads to block crosstalk, this one just counts on the two resistors knocking the crosstalk down to an insignificant level. I think I would still use the caps though.
Click to expand...

Eddie's circuit makes the assumption that the mic input circuit can tolerate the 48V DC on pins 2 and 3. There are too many examples of that NOT being a safe assumption. The info on that web page appears to be misleading and potentially damaging to equipment.

You are correct that one typically sees DC blocking capacitors in the balanced lines (pins 2 and 3). They do not "block crosstalk", they keep out ("block") the 48V from the mic input circuit. The resistors don't have any ability to "block crosstalk" either. The differential nature of the mic input circuit is what does the common-mode noise rejection. But if the 6K81 resistors are not very well matched (1% or better, typically) then they can degrade the (common-mode) noise rejection performance of the mic preamp.

You make an excellent point that I should not have made the assumption that people would automatically know that there is more to a P48 phantom power supply than just the DC source. At minimum, the matched-pair 6K81 resistors must be used to introduce the power into the balanced mic line. And then DC blocking capacitors may be necessary depending on the nature of the subsequent mic input circuit. Some circuits (such as those which use transformers) won't care about the 48V phantom power.

A good tutorial with practical circuits can be found here...
http://sound.westhost.com/project96.htm
That information includes a circuit for a 48V power supply, the phantom circuit, and even a simple way to match 6K81 resistors. Rod Elliot has a great DIY audio website.
 
Eddie's circuit makes the assumption that the mic input circuit can tolerate the 48V DC on pins 2 and 3. There are too many examples of that NOT being a safe assumption. The info on that web page appears to be misleading and potentially damaging to equipment.
Click to expand...

Well any true 48V phantom supply would have the same issue.

You are correct that one typically sees DC blocking capacitors in the balanced lines (pins 2 and 3). They do not "block crosstalk", they keep out ("block") the 48V from the mic input circuit. The resistors don't have any ability to "block crosstalk" either. The differential nature of the mic input circuit is what does the common-mode noise rejection. But if the 6K81 resistors are not very well matched (1% or better, typically) then they can degrade the (common-mode) noise rejection performance of the mic preamp.
Click to expand...

Your right I'm feeling dumb. The last time I was looking at this I was working on adding 48V Phantom power to a non phantom powered mic., hence the blocking capacitors. This is what happens when you used to use certain knowledge all the time and then you don't for a bunch of years and you open your mouth because "you know this stuff" and make a stupid mistake. Your absolute correct capacitors block DC, not AC.

Eddie specs 1% or .1% resistors. The bigger danger is having unbalanced phantom voltage and running a current through your mic (should only be an issue with a dynamic mic though since the phantom powered mic will have the DC blocking capacitors I erroneously referred to earlier.
 
Noiz2 said:
Eddie specs 1% or .1% resistors. The bigger danger is having unbalanced phantom voltage and running a current through your mic (should only be an issue with a dynamic mic though since the phantom powered mic will have the DC blocking capacitors I erroneously referred to earlier.
Click to expand...

Mismatched resistors don't cause any current to flow through a dynamic mic because both sides (pin 2 and pin 3) are at the same voltage (sourced from the same point) regardless of their actual resistance value. The problem with unbalanced resistors is upsetting the AC balance so that one can't reject the common-mode noise.
 
Some people may disagree with this... (based on an argument of SN, but the SN in this Rolls unit is acceptably high at 90db, especially for the most likely user and audio chain... or put another way... if you have a problem with the clarity of this unit then don't buy a Sanken CS3e over an MKH60, because that would also be unacceptable "noise"... and in case you're sarcastically challenged... I mean the TYPICAL user will never hear the difference in SN.)

The Rolls PM50sOB (MUST be "sOB" version) is a pretty decent solution for "cheapest way to provide phantom"... while offering a serious benefit of inline monitoring for a boom op... as well as talk-back to the boom op... and also (in my opinion another benefit) does NOT allow the boom op to change the mic level... only HIS personal volume (of both the mic pick-up and the talk-back).

END of Short Answer... READ on if you need to better understand "why?"...

If you have an inexperienced boom op... not screwing up the levels is a big advantage. You will most likely set levels for the shot and be better off if nobody touches them through the take. (See all the threads of unusable low audio... very common, because either nobody is watching it... or the guy who was had plenty of headphone volume while the actual recording did not... Taking that control away from "your buddy that's booming" saves you from either that or completely blown audio.)

When somebody asks for the cheapest solution to something I think it's safe to assume this is the likely scenario... sound will probably turn out best on set levels back to the [lower-end] camera or recorder... (saying "lower-end" because otherwise the cheap phantom question wouldn't have come up... although it could be a 5DmkII or something, but then I would question "cheapest"...)

The Rolls PM50sOB is discontinued... and I don't see that the replacement supplies phantom, but you can still pick one up (new) on ebay for $65 shipped.

The Denecke product Glen linked is probably a quality product with (perhaps) a cleaner PS... but the Rolls is clean enough that you'd never know if it was in the audio chain... even feeding a $5K camera... and the Rolls offers monitoring and talk-back... which are 2 very big advantages over any of the 48v solutions below a real mixer.

Being able to sit at a monitor and listen to your boom op, while telling him "you can get TWO FEET CLOSER dude!"... or "watch out, you're dipping into the shot!"... is pretty damn handy.

I honestly can't believe that one of the higher-end companies doesn't make their own version of this... and if Rolls discontinues this w/o offering a replacement (that also supplies phantom) then I think it leaves a vacuum in the audio market because this product takes control AWAY from somebody that you're better off taking control away from... while simultaneously forcing that same person to listen to instruction DURING the take, while not affecting the recorded audio.

Sounds like an amazing little product for under $99 doesn't it?

If I'm on a shoot and I'm not the boom op... it either means I'm lighting/rigging/gripping on a shoot that already hired a soundguy... (MtV Cribs, for example)... or I'm doing too much of everything else to do it all... but it's been the RARE exception that I'm hired for sound and not also booming... but in those cases... I can tell you that 100% of the time, I've been given a totally inexperienced boom op... even on some pretty serious shoots... I've been surprised at who they assign as the boom op... and being able to do "on the fly" instruction was priceless.
 
Last edited:
Well, from all of this I have gathered that maybe the cheapest way is not the smartest, I will probably end up getting the sound devices MM-1 as it provides phantom and allows for boom op monitoring.

@ Mattin: I checked the Rolls and it appears that it does not PROVIDE phantom but rather allows phantom from the mixer or camera to pass THROUGH the unit. At least that is what the description said when I looked it up.

Thank you everyone for the help :)
 
jaredgiese said:
Well, from all of this I have gathered that maybe the cheapest way is not the smartest, I will probably end up getting the sound devices MM-1 as it provides phantom and allows for boom op monitoring.

@ Mattin: I checked the Rolls and it appears that it does not PROVIDE phantom but rather allows phantom from the mixer or camera to pass THROUGH the unit. At least that is what the description said when I looked it up.

Thank you everyone for the help :)
Click to expand...


I'm a huge fan of Sound Devices gear... so I'd never go against an SD gear decision... however please note my attempt to be very clear on the Rolls PM50... sOB.

MattinSTL said:
The Rolls PM50sOB (MUST be "sOB" version)...

The Rolls PM50sOB is discontinued... and I don't see that the replacement supplies phantom, but you can still pick one up (new) on ebay for $65 shipped.
Click to expand...


To repeat myself... so that the thread doesn't end with misinformation... if you want PHANTOM available... you must get the PM50 S*O*B... named "PM50sOB" <- like that.

And yeah... it DOES provide phantom. When I go to that much trouble to explain something technical... I take pride in getting my facts straight :)

Okay... on to the MM-1... for those interested...

The MM-1 is a nice unit that does have the advantage (disadvantage? see my previous post) of allowing the boom op to change the mic level... and on that note, ironically... through a STEPPED... gain adjustment... no "trim" just steps of gain... so, in another words... not intended to allow riding levels... yet allowing somebody to attempt that... in STEPS of gain(?)

Why have those two nice level knobs for mic volume, and mix volume... which does nothing to address/effect the recording... (see don't forget you're now paying a premium for a PRE-AMP... we're "graduating" from simple monitoring + phantom... and paying significantly for that...) and yet the little stepped gain control is tucked up there in the corner? Shouldn't that be a big, usable TRIM knob switching places with one of the other big knobs on the face?

That unit... at that price... should be a 1-channel "mixer"... but it isn't... OR... a smarter pre-amp design based on SD's own (other) well-thought-out products.

SD... when you read this and realize I'm right... I'll be happy to review one of these for you... Read on...

They should have a 442 type knob for trim in the phone knob's location (unless you don't want to make this unit look too good... then use the same type of knob it is currently)... then put the little stepped gain knob below it just like one channel of a 442 (push-down/pop-up style like other SD products) and the have the phone volume knob and mix knob as the same, small black/silver push-downs in the left over space to the left... maintaining a clean, organized face... providing a point to actually CONTROL the included gain on the unit... and making the whole thing dramatically more valuable and sensible for a sub-$400 NEW option (yes, I'm saying do all this and raise the price to $399).

This design uses the same cheaper stepped gain pot from the MM-1... and only adds the parts cost of one extra pot (same pot as Mixpre uses)... on the same circuit path as the stepped "coarse gain"... and of course a new face plate, but everything else fits the current case... and dramatically increases the value of this unit with basically one additional pot... and if you want to be generous, a nice 442 knob... (I'd give it a peak/clipping LED as well... and don't tell me that's a big deal because the MM-1 already HAS a peak/clip circuit).



OP... all I'm saying is (from EXPERIENCE with THIS unit)... you'll see... having that stepped gain control on that thing will probably eventually bite you... unless it's YOU using this... and if that's the case I'd definitely suggest you hold out for a used Mixpre... (they pop up on ebay or in our own Marketplace every month or two... usually in great condition for under $500... I've seen excellent condition @ close to $400... and often @ $450 shipped... there's one RIGHT NOW in the Marketplace!)... OR get some (new) mid-quality mixer that does everything the MM-1 does... with more room to grow... for (most likely) nearly the same $.

The MM-1 is a nice product... but an oddly positioned product. It's in purgatory between affordable solutions that do nearly the same thing... and slightly higher $ solutions that do a whole lot more.

I think SD needs to address my above points about it... because otherwise it's a pricey (way less capable at nearly the same $) alternative to a used Mixpre... and more importantly... for it's fantastic specs (yes, SD gear specs very high)... you'll most likely never hear the difference in that... and the $65 Rolls sOB... unless it's a levels problem caused by somebody that had control of that... when they shouldn't have.
 
Last edited:
Rolls says that the PM55 has replaced the PM50 model. There is no mention of phantom power on the website, and there is no phantom powering circuit shown on the schematics (online at Rolls website). But the manual (PDF online) shows a jumper to switch phantom power on and off. Curiously, there is no other mention of phantom power, including what the voltage is, etc. MattinSTL, do you know what the phantom voltage on the Rolls PM units are? Rolls doesn't seem to know.
 
Last edited:
Just get a reliable P48 source and test it with a voltmeter. Even testing two identical units you sometimes have voltage problems. It is a real common issue in audio--if you need it, get the full 48 volts with reasonable load. You don't need anything fancy, you just need something that works. I used to carry one around as a spare, but I never used it since all my preamps have P48, so maybe just get a quality preamp. It takes only a minute to test it.
 
Richard and Dave (and everyone else)... I tried a cheapy little meter that I keep in my sound bag (for continuity... testing cables, batteries, etc.) and it's not giving me a good reading... it isn't registering anything on the DVX mic-in either (blinking)... so... I'll edit this post when I can answer the actual voltage question... all I know is I have a PM50sOB and it runs all my phantom mics.
 
Richard Crowley said:
Mismatched resistors don't cause any current to flow through a dynamic mic because both sides (pin 2 and pin 3) are at the same voltage (sourced from the same point) regardless of their actual resistance value.
Click to expand...

Actually that isn't nec. true. The voltage is the same at the point source but the resistance will drop a voltage and different values will create a potential difference between the two pins. Since the ends are tied to each other through the inductor I suspecting that your right that once the connections are made the flow will find an equilibrium and you will end up with balanced voltages. But not because the resistors don't make a difference but because they are shorting through the inductor.

BTW If there is no voltage difference it wouldn't affect the common mode rejection. It's the difference in potential that would throw the balanced signal out of balance.
 
Noiz2 said:
Actually that isn't nec. true. The voltage is the same at the point source but the resistance will drop a voltage and different values will create a potential difference between the two pins. Since the ends are tied to each other through the inductor I suspecting that your right that once the connections are made the flow will find an equilibrium and you will end up with balanced voltages. But not because the resistors don't make a difference but because they are shorting through the inductor.
Click to expand...

There can never be any current flow even if one resistor is zero ohms and the other is 10K. Because they are both at the same voltage, and the inductor of the mic voice-coil (or transformer secondary) is at zero. It is a simple application of Kirchhoff's Law.

Noiz2 said:
BTW If there is no voltage difference it wouldn't affect the common mode rejection. It's the difference in potential that would throw the balanced signal out of balance.
Click to expand...

Right. The common mode rejection doesn't care about the DC voltage. But if the two 6K81 resistors are too much different, it makes the AC input impedance of the mic preamp DIFFERENT for the + and - sides, and THAT causes the noise voltage to be different between +/- and the mic preamp can no longer cancel the noise.
 
You must log in or register to reply here.
Back
Top