Barry Porter phantom powering schem

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rafafredd

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Jun 3, 2004
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This is a circuit used by Barry Porter on the mixer he started designing for me just before he was gone...

porter_phantom.GIF


What do you think about this? Will it effectively ramp the phantom? I was thinking about using it on my new mic preamp, as I´m using 1:20 input transformers... Ramped phantom would be a must, I think, with that big of a ratio at the input.

R2 (3.4k) simulates the mic load.
 
If R2 is your two 6.8K phantom sourcing resistors, then it looks right. Will depend somewhat on the FET though - I don't know that type.

I'm not sure at all that ramped phantom is really needed anywhere - I would prefer matched resistors and always-balanced inputs...

Jakob E.
 
The circuit looks good to me. It's pretty straightforward. The MOSFET is biased into conduction using an R-C network with a long time constant.

How is the phantom on/off switching handled? You didn't show that in your schematic.
 
I sent a few people here a schematic like that some time ago. You need to remember a power mosfet often needs >4V to turn on for the lowest on resistance (RDSon)

To make sure that happens use a 9 to 12V zener gate to source. Attach the gate RC network to the higher voltage unregulated supply. This will make sure the gate to source is >4V and under the 15 to 20 volts that breaks down the gate oxide layer of alot of mosfets.

I first saw this circuit many years ago used for hot swap cards in a sat TDMA system. The 15V was used to ramp up the gate on the 5V line.
 
Thanks for all the inputs. The mosfet part number is wrong in the schem. It´s actually VN10LM, and the PDF can be found here:

http://pdf.searchdatasheets.com/pdf/559/559316.pdf

How is the phantom on/off switching handled? You didn't show that in your schematic.

It´s just a regular switch. Here is the updated schem:

porter_phantom_2.GIF


To make sure that happens use a 9 to 12V zener gate to source. Attach the gate RC network to the higher voltage unregulated supply

I don´t know if that would change the thongs, but this mosfet have an internal diode from drain to source, as you can see at the datasheet.

I'm not sure at all that ramped phantom is really needed anywhere

:?: Strangely, that´s ALL I want everywhere! I´m looking for a simple enought circuit to use at all my units new units as well as mod the existing ones..
 
Yes, you are right, it´s supposed to be 10mega ohms. I thought 10m=10mega for circuit maker... :oops:

But even after changing the resistor value, the circuit won´t simulate nice. It´s in fact making the oposite. The voltage falls instead of rising. Probably because I don´t have the spice model for the ZVN10LM. I´ll try to get it at zetex site...
 
I would not build it like drawn.

Move the 10meg to the higher voltage unregulated point. The 48 still at the drain. Switch the connection to the unregulated higher voltage supply also add a 100meg or greater from gate to ground to discharge the 1 uf cap to shut the channel off.

Now cathode of a 9 Volt zener to the gate and 9V zener anode to the drain, Now the gate to drain voltage will not go over 9V and the gate overvoltage failure mode should be taken care of. The mosfet will be at its lowest RDSon. The internal drain to source diodes does nothing for this. The gate oxide will break down at 15 to 20V DC in most MOSFETs like this one. The zener stops this.
 
[quote author="Gus"]I would not build it like drawn.

Move the 10meg to the higher voltage unregulated point. The 48 still at the drain. Switch the connection to the unregulated higher voltage supply also add a 100meg or greater from gate to ground to discharge the 1 uf cap to shut the channel off.

Now cathode of a 9 Volt zener to the gate and 9V zener anode to the drain, Now the gate to drain voltage will not go over 9V and the gate overvoltage failure mode should be taken care of. The mosfet will be at its lowest RDSon. The internal drain to source diodes does nothing for this. The gate oxide will break down at 15 to 20V DC in most MOSFETs like this one. The zener stops this.[/quote]

Gus sent me this picture to illustrate the idea:

slowstartpp.GIF


Hope this helps
Stewart
 
[quote author="Gus"]I sent a few people here a schematic like that some time ago...[/quote]I was just getting ready to post that Gus sent me a similar schematic as I scrolled down and saw this, haha. I have not tried it yet, tho. If it works, I'm putting it in the next rev of my power supply layout (with Gus's permission, of course!)
 
[quote author="thomasholley"]Turn the diode around.[/quote]

No. The diode is there to bleed off the charge on the MOSFET gate when the switch is in the "off" position. It's drawn correctly.
 
The spec sheet I found does not have a gate to source voltage when the device is ON (I would guess about 4V from the last time I was building stuff with mosfets) mosfet RDSon is low in this case about 5 Ohms.

What we need is to test one. Use a drain R and a voltage source on the gate. Connect the source to ground measure the drain voltage at different gate voltages.

This will give the Voltage range on the gate that the device changes its resistance. We then want the RC network to charge in this voltage range for the ramp up time we want. OR we could change the 10 Meg and the gate to ground cap until it ramps up nice.
 
Any thoughts on the circuit? People have been wanting soft starts.

I need to build it to test it but looking at this page

http://ec.irf.com/v6/en/US/adirect/ir?cmd=catNavigateFrame

It looks like the 9V zener will need to be 12V if the device VN10LN is like the IRF 60 volt ones. The RDS on is speced at 10V at the IRF page.

You can make the 1uf bigger and scale down the resistor values for the same time constant.

If you get the resistor values down you might be able to use a 4n25 opto to switch the gate drive on and off. Why did I post that? I was thinking easy remote control of phantom maybe a 8 to 3 and 3 to 8 (for you digital people)for 8 channels etc.
 
Gus,

I´ll be trying this circuit in my new units. I am still thinking about mosfet changes. This VN10LM is not a easy to get part, I think. At least here in Brazil...

I was thinking of trying a TO92 FET also, like 2n5457, as phantom powered mics sucks so little current and I will use separate soft starts for each channel, so no more than 1mA, and this is with hungry mics, as far as I know... Do you think it would be OK?
 
This is designed for N channel enhancement mode MOSFETs. Look for IRF or Mot(on semi?) Drill down in the above link N channel mosfets and check for 60V to 100V numbers.

The RDSon of the VN10M is listed at 5 ohms and the drain source is rated at 60V.

Many 48V phantom use a 100 ohm resistor and cap between the 48V and 6.82K resistors. I would think a RDSon of 1 ohm to 100 ohm would be fine.
 
the thing that worries me about these type of soft start circuits is what is the circuit doing to the audio when the phantom is off! I know that its is effectively biased to 0V of audio by the two 6K8 resistors but in an imperfect world.....I always think simple and clean is best and therefore would choose to have some unbiased fets and diodes hanging off the inputs to a mic-pre. If you want ultimate phantom power switch then you need either a multipole switch or relay. Then you can connect / disconnect the 6K8 resistors as well as ramp up the phantom ( the ramping can be done quite easily with a standard LM317 regulator ).
 
it should be acting like all the other phantom that use 1 switch to disconnect the top of the 6.82Ks connection to +48.

squib I think what you might be thinking of is a dpst to disconnect the bottoms of the 6.82Ks from pins 2 and 3. This can be a problem if one of the switch sides goes bad or switches faster than the other side. then you have unbalenced power Some microphones will not like that.

Price a 317 circuit for each channel and a a 317 +48V master supply and soft starts for each channel.
 
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