15V phantom power

[wesm]

Picking up an old 80's 16 channel mixer (Biamp 1683) for next to nothing tomorrow. Doesn't come with phantom power... Luckily due to the fact that its old i have access to complete detailed schematics, which provides info for incorporating phantom power from each channel's 15V rail. From research it seems that using 15V instead of 48V may work fine, may not work, or may work with a loss of mic sensitivity, depending on what mic is hooked up. I was wondering if to combat this i could alter the value of the standard pp 6.8k resistors to allow more current, and have a (more) reliable phantom power setup?

Clearly I'm not so knowledgeable in the field of electronics, could someone who is help me out??

 

[abbey road d enfer]

You would have to alter the value of the resistors.
The DIN 45596 norm for phantom powering makes it absolutely clear. It specifies three DC voltage levels of 48 volts, 24 volts and 12 volts, delivered through 6.8k ohms, 1.2k ohms, and 680 ohms. For 15 volts you should use 820-1k.
However, that will just guarantee that mics conforming to the DIN norm will work, which is fortunately the case of most modern mics.
As to other mics, some wouldn't work at all and some would exhibit a loss of sensitivity.
IIRC, a U47fet wouldn't work, but a U87 would.

 

[JohnRoberts]

I have used a lot of low voltage phantom (12-15V) in fixed install and low cost/low performance markets.

be aware as you drop down the phantom resistors, these are in parallel with your input termination so it will be difficult to maintain bridging (10x) termination to typical mic source impedances.

JR

 

[wesm]

thanks for your replies so far.

JR, if the impedance matching is off as your saying it would be, what disadvantages will i face?

 

[owel]

Try a DC-DC converter. 15Volts in, step up to 40V. 
http://cgi.ebay.com/3-5-40V-Out-5-40V-Adjustable-DC-DC-Step-up-Converter-/110557173211

If you'll be adding phantom power to that mixer, check that there are DC blocking caps in your inputs.
Otherwise, you'll be feeding DC to the input of your mixer when PP is turned on.

 

[JohnRoberts]

thanks for your replies so far.

JR, if the impedance matching is off as your saying it would be, what disadvantages will i face?

In most phantom powered mics you will just lose a little level. With dynamic mics it can change the sound character of some mics.

JR

 

[wesm]

I'm planning on only incorporating pp into some of the channels, so I will have unmodified channels set for working with dynamic, ribbons, etc. If i get motivated enough i may just add a switch for the pp.

Owel, thanks for your suggestion. I was considering stepping up the 15V, but it appears I may be alright without. If I notice issues, I will try stepping it up and see what i find. And yes DC blocking caps will be added, thanks for the reminder. Luckily the manual describes well the components that need to be added/swapped, and all will fit well into the existing pcbs ;D

 

[Rossi]

IMO it makes little sense to add "some kind of phantom power" to your mixer. A lot of condenser mics won't work well or not at all with anything less than P48. And that's not only old mics but lots of newer ones, too; cheapo ones in particular.

Whenever a mic doesn't work or is noisy, you're going to yourself, if it's because of your sub-standard phantom powering. In other words, you're adding another source of insecurity and worry. Also, your PSU may run out of steam if it has to power more stuff than what it was designed for.

 

[pucho812]

fwiw, the easier solution would be to buy an external phantom power. It may not be diy but it would have up and running in a more timely manor plus how often do you need phantom anyway?  can get a nice 4 channel external phantom and be covered for little money.

 

[JohnRoberts]

I don't know what the majority of mics out there use,  but I have seen a few low end Phantom mics with 5-7V zener diode clamps inside. Some cheap capsules can't handle much more voltage than that.

This seems pretty harmless (as a learning experience) to try on one or two channels. You might even build a simple cap doubler-tripler circuit into the standard PS to generate a low current phantom supply... It only needs several mA for one or two mics .

Of course everybody knows you really need to build a kick ass dedicated stand alone mic preamp, from scratch...  ;D

JR

edit- come to think of it.. Peavey used to sell an accessory phantom supply that used 1x9V battery. I also designed one battery powered mixer that stacked 2x 9V for phantom...  while I did get some complaints from our japanese distributor over that one not being 48V /edit

 

[mad.ax]

@ Owel
the product you linked sure looks tempting for 10 bucks!

Did you tried it personally?
I had a look at the LM2577 datasheet, and feel a bit concerned by the frequency of the internal oscillator, which can run as low as 42KHz...

Axel

 

[owel]

@ Owel
the product you linked sure looks tempting for 10 bucks!

Did you tried it personally?
I had a look at the LM2577 datasheet, and feel a bit concerned by the frequency of the internal oscillator, which can run as low as 42KHz...

Axel

I haven't tried that one particularly.

But I tested a ready-made DC-DC converter (potted module pulled from an equipment), with only a 10Khz switching freq. and surprisingly I didn't hear any audible noise even when used for 48V phantom.  (Maybe the CMRR of the preamp input stage also helped.)

Re: noise.  I rolled my own dc converter from discrete parts and found that the choice of inductor has more to do whether you'll hear noise in your preamp circuit or not, rather than the effect of the switching freq. of the dc converter.

You need to use a well shielded inductor, and design the PCB such that the inductor is as far away as possible from your preamp's input stage/XLR jacks. If you used a plain unshielded inductor (cheap $), you'll hear lots of "fuzzy/fritzing/high freq RF" noise that gets louder or softer depending on the distance of the inductor from the input stage. So always use shielded inductors.

 

[sodderboy]

External phantom is the way to go.  Don't go messing with the modules inside.  The original power supply is rated to power the modules, and likely not many extra bells and whistles.  You can build a phantom box or buy them from Stewart.  Would you put a toggle switch on each channel strip?

Phantom is spec'ed to be 48 VDC +/- 5% or so.  You will get no where plugging a 414 or Chinese FET into P15 unless you mod the mic.

If it is one of those small metal "console" consoles, you could consider adding an extra 48 VDC power supply and the necessary components (6K8 resistors, DC blocking caps, etc.) onto the FXLR's in the back panel and putting a row of ON/OFF switches back there.  I usually find it is better for me to work at my career and use the resulting $$ to buy what I otherwise would DIY.  You have to judge which side of the fine line will benefit you.  For a few mics better to go off the shelf.  And the supplies will stay as you upgrade your consoles.
Mike

 

[zebra50]

Try a DC-DC converter. 15Volts in, step up to 40V. 
http://cgi.ebay.com/3-5-40V-Out-5-40V-Adjustable-DC-DC-Step-up-Converter-/110557173211

Owel,

Thanks for this - looks useful as a problem-solver for other things! Have you tried this? In other words, is it quiet?

Stewart

 

[wesm]

thanks for everyones time put in to help me out!

JR- As a student overpaying for college, I unfortunately have a fairly sad and humble setup here...no high end mics residing here. and yes i most likely will try it on a couple channels- will be a good learning experience. Will either work well or not. +1 about having the killer outboard preamp- already built the green pre discussed heavily here(obviously with solid PP) and plan on building more high-end pres next time i have some spare time. Not that the Green is bad, by any means.

Rossi and Sodderboy- I'm assuming the designers designed the PSU to handle every channel running simultaneously with at least mid-gain, but hopefully at full gain. Maybe I shouldn't assume that- either way, PP doesn't require use many mA, and I can't imagine myself using close to all the channels at that much gain all at once. By that time, hopefully I'll have a nicer console ;D

In the end...this is more of a good learning experience. Using external 48V makes the most sense, but thats no fun is it ;D
As well as trying out the PP, I may rehouse some of the channels with higher quality components- swap the tl072's with ne5532's(any other suggestions?) 1% metal film resistors(schematic specs them at 5%, doesn't spec material :-\), maybe better caps, etc...and maybe only change one type of component per channel, and in another channel change them all. Would set up for a great controlled experiment of what affects the sound in what way. Assuming the channels aren't very consistent with each other already...i'll have to do before and after recordings from each channel. I'll be sure to post the results possibly with sound samples if this happens. 

Thanks again!

 

[Rossi]

I don't know what the majority of mics out there use,  but I have seen a few low end Phantom mics with 5-7V zener diode clamps inside. Some cheap capsules can't handle much more voltage than that.

Nah, most condenser capsules, even the cheap ones, are designed for about 60V. If the mic has a 5-7V zener, then it uses a dc/dc converter (e.g. MXL 603) or it is an electret condenser which uses no external voltage for the capsule. But there are lots of mics, even some respectable ones, that use P48 for capsule bias. As the mic draws some current through the 6.8k resistors, those mics are usually left with 35-44V for capsule bias. That's already lower than optimum, and if you drop that to 15V, you lose a lot of sensitivity. Plus, the mic electronics are out of spec. A KM84, for instance, uses a 24V zener. Bad phantom power can turn a beautiful mic into a noisy POS.

So, anything below true P48 is asking for trouble. Personally, I would never buy a mic pre that doesn't comply with P48 requirements (48V +/-4V and 10mA max current). The appeal of P48 is that it is a very convenient way of powering microphones. A sub-standard P48 supply is a source of worry and thus defeats its purpose.

 

[JohnRoberts]

I don't know what the majority of mics out there use,  but I have seen a few low end Phantom mics with 5-7V zener diode clamps inside. Some cheap capsules can't handle much more voltage than that.

Nah, most condenser capsules, even the cheap ones, are designed for about 60V. If the mic has a 5-7V zener, then it uses a dc/dc converter (e.g. MXL 603) or it is an electret condenser which uses no external voltage for the capsule. But there are lots of mics, even some respectable ones, that use P48 for capsule bias. As the mic draws some current through the 6.8k resistors, those mics are usually left with 35-44V for capsule bias. That's already lower than optimum, and if you drop that to 15V, you lose a lot of sensitivity. Plus, the mic electronics are out of spec. A KM84, for instance, uses a 24V zener. Bad phantom power can turn a beautiful mic into a noisy POS.

So, anything below true P48 is asking for trouble. Personally, I would never buy a mic pre that doesn't comply with P48 requirements (48V +/-4V and 10mA max current). The appeal of P48 is that it is a very convenient way of powering microphones. A sub-standard P48 supply is a source of worry and thus defeats its purpose.

I made a point of stating what I personally observed... while I don't disagree that the popular sentiment these days is strongly in favor of 48V. Actually it has been for a while.. I recall some 15 years ago upgrading the phantom voltage inside the popular XR600 topbox powered mixer from 15V (that was fine for decades), to 48V, because the customer is always right, even when gigging in bar bands they wanted 48V.

I also spoke with some of the microphone design engineers, working inside Peavey's transducer engineering group when I was engineering director over the recording products division (20+ years ago) and even looked at some of the mic schematics. Admittedly Peavey was not highly regarded for their "recording" (cough) microphones ;D  but I'm not making this stuff up. I saw what I saw.

For the poster it won't hurt to experiment, especially if on a budget (I was only kidding about building a kick ass stand alone preamp.. I was making making a little fun of the vibe around here). The problem with most modern recordings I hear is not the preamp.  :-[

I stand by my suggestion to try a simple cap doubler/tripler to get a few mA at 50V from the mixer PS. I know this works because i've done it too many times to count inside commercial products.

That said any mixer sold in recent history without any phantom power capability may have cut other corners, too. I put phantom power inside topbox powered mixers and even fixed install products (those weren't 48V either BTW).

Have fun... try not to release any magic smoke, and learn something.

JR

 

[Rossi]

You know, the thing is most customers don't speak electronics. They say, "oh that mic pre doesn't sound right" or "why is my mic noisy?" For instance I know some engineers who absolutely hate  Universal Audio 610 preamps, because they "don't sound right" and "overload easily" . Apparently quite a number of them have a faulty/weak P48 supply, and that's probably why. I bought an LA-610 a while ago, and had to send it back because the P48 did not deliver enough current and the voltage dropped ever so slowly (even unloaded). Got another unit that works and sounds pretty good.

If you look at various (studio) mic schematics, you'll find that the majority won't work well with less than real P48. That may be different for stage mics, because stage condensers tend to be electrets (actually, that's changing, too). Weren't the Peavey mics, electrets, too? Ten years ago, lots of entry level studio mics were electrets, but that has changed.

A voltage tripler may be a solution, at least if only a few channels require phantom power. And yes, the vast majority of new products has correct P48 (except USB audio interfaces that are bus powered). It's one of the things I always measure when I review gear.

 

[JohnRoberts]

You know, the thing is most customers don't speak electronics. They say, "oh that mic pre doesn't sound right" or "why is my mic noisy?" For instance I know some engineers who absolutely hate  Universal Audio 610 preamps, because they "don't sound right" and "overload easily" . Apparently quite a number of them have a faulty/weak P48 supply, and that's probably why. I bought an LA-610 a while ago, and had to send it back because the P48 did not deliver enough current and the voltage dropped ever so slowly (even unloaded). Got another unit that works and sounds pretty good.

If you look at various (studio) mic schematics, you'll find that the majority won't work well with less than real P48. That may be different for stage mics, because stage condensers tend to be electrets (actually, that's changing, too). Weren't the Peavey mics, electrets, too? Ten years ago, lots of entry level studio mics were electrets, but that has changed.

A voltage tripler may be a solution, at least if only a few channels require phantom power. And yes, the vast majority of new products has correct P48 (except USB audio interfaces that are bus powered). It's one of the things I always measure when I review gear.

I recall talking with at least one guy who used a switcher to make 48V phantom from a USB (5V) drop. As I recall it was difficult because of limited voltage and current but he found a solution that worked. 

I don't advocate using less than 48V for serious recording, and cap tripler circuits were pretty common in my experience, due to cost of extra windings for modest current.

JR

 

[wesm]

Understood. Just opened up the console and i do see some room for adding a tripler...i may hook up a couple channels with the +48 and maybe try one with the 15, and see what I find for differences with different mics. In the end though, I'm gonna always be worrying about the 15V messing things up, and in session the last thing i should be thinking about is whether my PP works...so this will for the most part be an experiment to help gain a bit more knowledge and experience.

Thanks everyone for all your advice.