Phantom regulator?

[PRR]

Phantom regulator?

http://www.supertex.com/pdf/datasheets/LR8.pdf

"...regulator. It has a wide operating input voltage range of 13.2 - 450V. The output voltage can be adjusted from 1.20 - 440V provided that the input voltage is at least 12V greater than the output voltage. ...The output current limit is typically 20mA"

http://www.mouser.com/Search/Refine.aspx?Keyword=LR8
Mouser Part #  689-LR8N3-G
LR8N3-G
TO-92
Supertex  Linear Regulator Integrated Circuits 450V Adj 3 Termnl  Page 334
Stock:  2,634 Can Ship Immediately
$0.72 

 

[mad.ax]

At first sight, this looks like quite the solution to hack phantom power in tube preamps...

But the 0.75 W dissipation of the TO92 package limits my enthusiasm...
Sure, one can add an RC stage to bring down the input voltage to, say, 70 V, but in this case, why not using a good ole 317 which have much better specs?

Axel

 

[abbey road d enfer]

Clearly, this part has been designed for one specific application, and, although I have nothing against using parts out of their intended field, I feel that some of the performance aspects we're after in audio apps are not really taken care of, such as noise. The 20mA current limit restricts it to individual phantom powering (14mA short-circuit current). The necessity of 12V min. diff voltage may be a problem in some cases, particularly when unreg voltage is derived from a voltage multiplier. The low price makes it attractive though...
But I still think nothing beats the TL783 in this application.

 

[JohnRoberts]

+1.. not a slam dunk for phantom...

JR

 

[buschfsu]

i considered the 317 for my MILA build but to keep the circuit simple why not serial 24v zenners?  is there a disadvantage over a 317 and its related parts?

 

[JohnRoberts]

I've used 2x24V zeners several times.. usually with a cap and simple pass transistor.

JR

 

[sahib]

I played with supertex parts about five-six years back. I still have the samples and data sheets stuffed somewhere in my Alaaddin's cave.  As far as I remember they were quite safe parts for stepping down straight from mains for low current output applications.

 

[buschfsu]

same q as above.

can the transistor be left out.  whats it doing? 

 

[mshilarious]

A zener by itself isn't the greatest regulator because it is relatively high impedance and noisy, and plus it regulates by dumping current until the required voltage drop is achieved.  This is much less than ideal if you're trying to regulate a plate supply down to +48V.

My usual phantom reg circuit has a high-value resistor feeding the zener, and another large resistor to base/gate with a filter cap.  Use a Darlington or MOSFET to keep the zener current as low as possible.  Remember to compensate for the diode drop (~1.4V) with your zener value.  Sure, 46V is within spec, but it's not like a 50V zener costs more than a 48V . . .

If you are using a plate supply, make sure your transistor can handle the volts and watts!

 

[skipwave]

same q as above.

can the transistor be left out.  whats it doing? 

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

 

[Svart]

http://www.groupdiy.com/index.php?topic=33653.0

The transistor is the power handling device.  The Zener sets the operating current for that transistor.

Built this out of parts I scrounged from power supplies and junk.  Works great.

 

[user 37518]

same q as above.

can the transistor be left out.  whats it doing? 

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

Very good link thanks for that, but using that approach i was thinking, wouldnt it be easier, to use a  7824 regulator, another 7924 negative regulator, use the output of the positive regulator (24 v) as "+" and the output of the negative regulator (-24v) as ground, for a total of 48v?, isnt this design more stable than the one with the zeners? resistors must be added to limit the output current to the mic?, what about adding 2 big caps to both outputs (+ and - of the output regulators) to physical ground, so it works as a slow ramp to feed the mic?

 

[JohnRoberts]

I've used 2x24V zeners several times.. usually with a cap and simple pass transistor.

JR

How do you use the cap, you mean after the diodes to block DC to go into the circuitry?, whats the transistor for?

Others have already answered this but if still curious. The transistor is used as a simple emitter follower to buffer the source impedance of the zener. The cap across the zener reduces noise and ripple.

I wouldn't lose any sleep over hitting 48V exactly, since it is supplied through 6.8k resistors that drop several volts before it gets to the mic.

JR

 

[mshilarious]

Well, I went to fix a phantom supply for a two-channel amp that was ailing (for other reasons).  It had the 48V zener right at the base of a TIP112 with no filtration at that point.  After the emitter was 330R then a cap.  Never mind filtering the zener noise where it was high-impedance and thus easier.

Darlington drop of 1.4V + 330R * 10mA for one mic = 4.7V drop to less than 44V and thus noncompliant for two mics.  Do most mics care?  No, but still the marginal cost to build a fully compliant supply would have been zero.

 

[skipwave]

Well, I went to fix a phantom supply for a two-channel amp that was ailing (for other reasons).  It had the 48V zener right at the base of a TIP112 with no filtration at that point.  After the emitter was 330R then a cap.  Never mind filtering the zener noise where it was high-impedance and thus easier.

Darlington drop of 1.4V + 330R * 10mA for one mic = 4.7V drop to less than 44V and thus noncompliant for two mics.  Do most mics care?  No, but still the marginal cost to build a fully compliant supply would have been zero.

The same issue with the design in the link I posted. Zener-referenced pass transistor followed by an RC filter stage that knocks the voltage below the target 48V. A smaller resistor makes the RC filter less effective, but how much is it really needed at that point? Why not just a small C as would be seen following a standard regulator IC like the LM317?

 

[JohnRoberts]

Maybe I'm too old to give a sh.. but the 48V target is pretty much arbitrary and not specified to be held very tight (44-52V). It wasn't always expected to deliver more than a few mA per input, but in recent decades people have started powering direct boxes and even lamps off the phantom power so console makers have had to beef them up.

I would suggest paying more attention on how well the 6.81k build out resistors match each other than the exact phantom voltage.

At my old day job we sold a 9V battery powered external phantom supply that was ummm  9V.  Many install (background music) products routinely operated with 12-18V phantom.  I did one battery powered small mixer with 18v or whatever was had from two stacked 9v batteries. So many mics are more than happy with modest levels of phantom power. I've looked inside several mic designs that zener down the phantom to 5 or 6V for use. (Note: for high end studio use you want to still work for that one mic in a thousand that will crap out at 42V).

In recent decades even inexpensive top boxes (like XR600 powered mixers) shifted to 48V nominal phantom, because the boys and girls think it's better, and the customer is always right.  :

One concern with operating on the hot side of nominal 48 output is that more than 50V input blocking caps get larger and more expensive, so 48v + nothing, minus a little... is the real target.

Of course making your phantom 48.0V will not hurt anything, but like I said don't scrimp on matching the phantom build out resistors to each other.

JR

 

[mshilarious]

I agree with most of that and I think I owned that little mixer, it was clever.  But the world has changed from those days.  I do battery-powered stuff so I wish the entire world switched to P12 because P48 is incredibly expensive for a 9V battery. Then somebody will hook up a couple of Earthworks and bitch when the battery dies in two hours.  I have honestly no idea why a mic really needs 10mA; the ones I do these days drain less than 2mA and the amp distortion is plenty low.  But some do, and if you can't supply them your product gets a bad rep.

Anyway, for DIY isn't the idea to build a proper design, especially when it costs nothing extra?  Of course you match resistors, but why not do the other stuff the right way too?  Although I do use 63V caps where phantom is present, that's a bit extra.

 

[JohnRoberts]

Well "proper" is between 44 and 52V. The mics don't need more than a mA or two but some mic designers use the extra current available to power active buffers for lower output impedance.

Even before they upped the current in the phantom power spec, it was conservative practice to tolerate all inputs shorted to ground without the phantom dropping out of regulation or releasing smoke. The phantom power supply is probably the most over designed part of most consoles.

I'm working with some inexpensive mic capsules I use in a product that only draw around 250 uA and run off a couple volts. 48V is an archaic and arbitrary standard, but I don't see it changing for open designs. OTOH self contained designs have long since abandoned it.

JR

 

[Svart]

Zener-referenced pass transistor followed by an RC filter stage that knocks the voltage below the target 48V.

If you look at the schematic in the link I posted you'll see that I included a regular silicon diode in the zener string.  I used this to increase the voltage seen at the base slightly, bringing the output closer to the intended 48V.  Use as many as you need to get the output closer.  I found that I only needed around 1v so I stuck with the silicon diode drop of .7v.  I figured .3v wasn't a big deal..